Guidance Document:
DESIGN, IMPLEMENTATION, AND
APPROVAL OF EVAPOTRANSPIRATION
COVERS IN PUERTO RICO
EPA/600/R-21/269 | February 2022 | www.epa.gov/research
v»EPA
United States
Environmental
Protection Agency
Pc — P — R — Et — ASw
Evapotranspiration
Office of Research and Development
Center for Environmental Solutions and
Emergency Response
Land Remediation and Technology Division
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Guidance Document: Evapotranspiration Covers in Puerto Rico
Design, Implementation, and Approval of
Evapotranspiration Covers in Puerto Rico
by
Emmie McCleary, Environmental Engineer
US EPA Region 5
Chicago, Illinois 60604
Dr. Tarek Abichou, Professor
FAMU-FSU College of Engineering
Department of Civil and Environmental Engineering
Tallahassee, Florida 32310
Steve Rock, Environmental Engineer
US EPA Office of Research and Development
Center for Environmental Solutions
and Emergency Response
Land Remediation and Technology Division
Cincinnati, Ohio 45628
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Guidance Document: Evapotranspiration Covers in Puerto Rico
Purpose of this Guidance
This guidance is intended to serve as technical and regulatory assistance as it relates to
the allowance of evapotranspiration covers (ET Covers) as final covers under 40 Code of
Federal Regulations (CFR) Part (§)258 and any other applicable regulations and policies.
This guidance shows how to demonstrate technical equivalence to accepted compacted
clay covers.
The document aims to give regulators, municipal representatives, and contractors in
Puerto Rico a way to achieve regulatory acceptance and closure at landfills for which an
ET Cover is appropriate. These ET Cover design guidelines are provided for regulatory
agencies, consultants, contractors, local governments, citizens, and owners and operators
who are involved in the permitting, design, operation, monitoring, and closure of solid
waste facilities. These guidelines are designed to help ensure the protection of public
health and the environment by providing a protective, low-cost, and low-maintenance
final alternative cover solution throughout Puerto Rico.
The prototype ET Cover designs described in this guidance document were designed
specifically for different Ecozones delineated for Puerto Rico considering the island's
geography and vegetation. The principles of defining a set of Ecozones and using
prototypical landfill covers are applicable in other localities. For example, other Caribbean
islands, Pacific islands, Central American nations, and other locations that have similar
Ecozones to those defined for Puerto Rico may be able to use the prototype ET Cover
design included in this guidance document with minimal modification. Any location that
can be divided into Ecozones with consistent climate and soil can adapt this format to
develop a simple guide to designing and approving ET Covers for landfills.
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Guidance Document: Evapotranspiration Covers in Puerto Rico
Notice/Disclaimer
The U.S. Environmental Protection Agency (US EPA), through its Office of Research and
Development, funded and conducted the research described herein under an approved
Quality Assurance Project Plan (Quality Assurance Identification Number K-LRTD-
0019792-QP-1-6). It has been subjected to the Agency's peer and administrative review
and has been approved for publication as a US EPA document. Mention of trade names or
commercial products does not constitute endorsement or recommendation for use.
Foreword
The United States Environmental Protection Agency (US EPA) is charged by Congress with
protecting the nation's land, air, and water resources. Under a mandate of national
environmental laws, the Agency strives to formulate and implement actions leading to a
compatible balance between human activities and the ability of natural systems to support
and nurture life. To meet this mandate, US EPA's research program is providing data and
technical support for solving environmental problems today and building the science
knowledge base necessary to manage our ecological resources wisely, understand how
pollutants affect our health, and prevent or reduce environmental risks in the future.
The Center for Environmental Solutions and Emergency Response (CESER) within the
Office of Research and Development (ORD) conducts applied, stakeholder-driven research
and provides responsive technical support to help solve the nation's environmental
challenges. The Center's research focuses on innovative approaches to address
environmental challenges associated with the built environment. We develop technologies
and decision-support tools to help safeguard public water systems and groundwater, guide
sustainable materials management, remediate sites from traditional contamination
sources and emerging environmental stressors, and address potential threats from
terrorism and natural disasters. CESER collaborates with both public and private sector
partners to foster technologies that improve the effectiveness and reduce the cost of
compliance, while anticipating emerging problems. We provide technical support to US
EPA regions and programs, states, tribal nations, and federal partners, and serve as the
interagency liaison for US EPA in homeland security research and technology. The Center
is a leader in providing scientific solutions to protect human health and the environment.
Gregory Sayles, Director
Center for Environmental Solutions and Emergency Response
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Guidance Document: Evapotranspiration Covers in Puerto Rico
Acknowledgments
This work was funded in part by the United States Environmental Protection Agency (US
EPA) Office of Research and Development (ORD), the Center for Environmental Solutions
and Emergency Response (CESER), Engineering Technical Support Center, Director, David
Gwisdalla.
The document was edited and formatted by Katherine Bronstein, Environmental Engineer,
RTI International under US EPA contract number: EP-C-16-021, WA#4-26.
Additional editing and formatting were performed by Cathy Steen, Environmental
Scientist.
This project was suggested and encouraged by Carl Plossl, US EPA, a long-time advocate
for safe and sustainable environmental technology for Puerto Rico.
Thank you to Felipe Nazario Muniz, P.E., and Juan Carlos Mercado, P.E., for the Puerto
Rico closure cap cost comparison calculations and other assistance.
Thank you to the Puerto Rico Department of Natural and Environmental Resources for
help and support, specifically to Maria Victoria Rodriguez, Carmelo Vazquez Fernandez,
and Pedro Guevara Lopez, for reviewing this document.
Thank you to everyone who submitted reviews, comments, and otherwise assisted,
especially those from Puerto Rico, including Michael Tilchin, P.E. (ASCE); Nivia I. Ayala,
P.E. (ASCE); Hector J. Colon De La Cruz (ASCE); Jose A. Cabrera, Esq. (USDA); Rene R.
Rodriguez; Maribelle Marrero, Agronomist; Felix Lopez, Ecologist (USFWS); and Jose C.
Zayas, P.E.
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Guidance Document: Evapotranspiration Covers in Puerto Rico
Executive Summary
Puerto Rico has a combination of waste disposal facilities that include both historical open
dumps and modern lined landfills. As of 2021, approximately 29 landfills remain in
operation throughout Puerto Rico, more than half of which are unlined open dumps that
continue to release significant volumes of contaminants that may threaten human health
and the environment. In addition to currently operating open dumps and modern lined
landfills that are at capacity, historical landfills have left a legacy of uncontrolled waste
facilities that are overdue for proper closure. While any new system for closing a landfill
can be difficult to assess and approve, evapotranspiration covers (ET Covers) are an
established alternative to conventional closures, have been approved for hundreds of
landfill closures, and may offer a cost-effective solution compared to clay covers that are
predominantly prescribed in Puerto Rico.
This guidance document was developed to allow landfill owners and operators, regulators,
and design firms to actively utilize the 40 Code of Federal Regulations (CFR) Part (§)
258.60(b) closure criteria's final cover alternative in lieu of a prescribed clay cover. This
document provides prototype ET Cover designs that were modeled for various Ecozones of
Puerto Rico. These prototype designs will meet the 40 CFR § 258.60(b) closure criteria's
final clay cover equivalency requirements and are expected to have lower long-term
maintenance and initial construction costs compared to clay covers, as well as improved
final vegetative results.
Rapid and cost-effective final design of an ET Cover is enabled through the initial design
steps available in this guidance.
Estimated ET Cover construction costs in Puerto Rico range from approximately $29,847
per acre to $120,803 per acre depending on the soil type and Ecozone in which the solid
waste facility is located. The overall average cost savings per acre by using an ET Cover
compared to a clay cover is $57,716, with cost savings ranging from 19 percent
(Mountains) to 76 percent (South Shore) across the Ecozones.
While ET Covers are often a more economically feasible final cover option, funding
remains a significant issue for municipal governments in Puerto Rico. There are several
opportunities for municipalities to apply for financial assistance to construct and maintain
a final cover. Some funding mechanisms are specifically for improvements to solid waste
infrastructure, and many are exclusively for Puerto Rico. Financial assistance is available
through government programs associated with the Commonwealth as well as federal
departments and agencies.
All landfills seeking site closure require a well-maintained cover, as prescribed by 40 CFR
§ 258.60, to "minimize infiltration and erosion" by reducing infiltration and leachate
generation. The regulations require the use of an 18-inch (in) layer consisting of material
meeting a prescribed permeability topped by a final 6-in soil layer to allow plant growth
and reduce erosion. These "compacted clay covers" often require significant ongoing
maintenance, many have required major repairs, and some covers have shown a high
failure rate (Albright, 2006).
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Guidance Document: Evapotranspiration Covers in Puerto Rico
Hundreds of ET Covers have been approved by state environmental agencies using
waivers or demonstrated performance equivalency to prescribed covers (i.e., clay covers).
The states of Texas and Colorado have adopted guidance specifically for accepting ET
Covers as an allowable final cover under 40 CFR § 258.60. Colorado designed their ET
Cover guidance for designers and regulators, including prototype designs that are
validated through soil testing. Prototype designs eliminate the need to develop new
conceptual designs for each landfill site, saving initial design costs and speeding up the
regulatory permitting process. This guidance follows a similar path.
The guidance presented in this document aims to ensure the protection of public health
and the environment by providing an effective, low-cost, and low-maintenance final cover
solution, where appropriate, to accelerate the closure of open dumps throughout Puerto
Rico and at other landfills as needed and appropriate. An affordable and regulatory
acceptable cover system will help build regulatory compliance and responsible solid waste
management throughout Puerto Rico.
Solid waste facilities across the world have successfully used ET Covers as an alternative
to traditional clay covers for site closure. ET Covers are:
• An earthen cover placed over waste material to reduce water percolation into the
underlying waste by using the water storage capacity of soils and the water
removal capabilities of vegetation.
• Allowable for use as a final landfill cover under U.S. and Puerto Rico regulations
when designed to be equivalent to conventional, compacted clay covers.
• Straight-forward to design and review using this guidance document.
• Ecologically self-sustainable when properly designed, built carefully, and planted
with locally appropriate vegetation.
• Low cost when compared to conventional, compacted clay covers.
This guidance relates to the allowance of ET Covers as alternative final covers under 40
CFR § 258 and other applicable regulations and policies. This document aims to give
regulators, municipalities, and contractors a way to achieve regulatory acceptance and
closure at landfills for which ET Covers are appropriate. Engineering guidance and the
results of modeled simulations are provided for those involved in the permitting, design,
operation, monitoring, and closure of solid waste facilities.
This ET Cover guidance also provides regulators with options to consider in terms of
beginning to permit ET Covers in Puerto Rico. The design requirements presented may
also assist regulators in identifying landfills that have already stopped operating, have
sufficient cover, and have well-developed vegetation. The effectiveness of the existing
cover at such sites may warrant the consideration for the legal authority to categorize the
site as "adequately closed" using this guidance with proper soil testing. This guidance
anticipates that the implementation of ET Covers for final closure at landfills would
positively affect regulators' resources by easing monitoring, tracking, and enforcement
responsibilities through allowing historical open dumps and illegally operating open dumps
to take advantage of ET Covers as a feasible closure and post-closure care option. The
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Guidance Document: Evapotranspiration Covers in Puerto Rico
permitting process for ET Covers in Puerto Rico is managed by and under the authority of
the Puerto Rico Department of Natural and Environmental Resources (PRDNER).
The process of using this guidance for design and/or approval of an ET Cover is presented
in a straight-forward process for the designer and the regulator to follow:
1. Find the Ecozone in which a solid waste facility (landfill or open dump) is located.
2. Review the ET Cover prototype design for the appropriate Ecozone.
3. Confirm the ET Cover prototype design is appropriate by testing the proposed soil.
4. If the proposed soil meets the requirements of the prototype ET Cover design,
choose the appropriate vegetation for planting.
5. Finalize the closure plan, incorporating all other considerations that must be
addressed in a final cover design (e.g., access restriction, seismic slope stability).
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Guidance Document: Evapotranspiration Covers in Puerto Rico
Table of Contents
Purpose of this Guidance iv
Notice/Disclaimer v
Foreword v
Acknowledgments vi
Executive Summary vii
Table of Contents xi
Figures xiii
Tables xiv
Acronyms and Abbreviations xv
Glossary xvii
1 Introduction 19
1.1 How to Use this Guidance 20
1.2 Regulatory Acceptance of ET Covers in the United States 21
1.3 Status of Municipal Solid Waste Facilities in Puerto Rico 22
1.4 Regulatory Acceptance for Conventional and Alternative Final Covers 24
1.5 ET Cover Basics 27
1.6 Field Performance of ET Covers 31
1.7 Ecozones of Puerto Rico 33
2 Performance Equivalency of Alternative Final Covers to Compacted Clay
Covers 36
2.1 Performance of Prescribed Clay Covers in Puerto Rico (Baseline Equivalency) .36
2.2 ET Cover Equivalence and Feasibility in Puerto Rico 37
3 Prototype ET Cover Design and Preliminary Site Characterization for Puerto
Rico 42
3.1 Acceptable Soil Types 44
3.2 Soil Cover Thickness 45
3.3 Borrow Source Analysis 46
3.3.1 Preliminary Soil Characterization Testing 46
3.3.2 Standard Soil Index Property Testing 46
3.3.3 Preliminary Soil Screening for Vegetative Properties 47
3.3.4 Soil Evaluation for Supporting Appropriate Vegetation 48
3.4 Revegetation Plan 49
4 Construction Guidance for ET Covers 53
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Guidance Document: Evapotranspiration Covers in Puerto Rico
4.1 Subgrade Preparation 53
4.2 Soil Bulk Density 53
4.3 Soil Moisture Content 54
4.4 Lift Thickness 54
5 Relative Cost of Design and Construction for ET Covers 57
6 Financial Assistance 59
6.1 Funding Programs and Applicability 59
6.2 Tips for Applying 63
7 Methodology and Supporting Documentation 64
7.1 Delineation of Ecozones for Puerto Rico 64
7.1.1 Physiography of Puerto Rico 65
7.1.2 Required Amount of Water to be Stored 69
7.2 Soils of Puerto Rico 71
7.3 Minimum Plant-Available Water Storage Capacity of Local Soils 73
7.4 Water Balance Modeling of Covers in Selected Areas of Puerto Rico 77
7.4.1 HYDRUS-1D Overview 77
7.4.2 Design Year and Initial Conditions 79
7.4.3 Unsaturated Soil Properties 80
7.4.4 PET, PE, and PT for Puerto Rico 81
7.5 Equivalency Criteria: Percolation Through Compacted Clay Covers 85
7.5.1 Previous Studies on Field Performance of Compacted Clay Covers in Humid
Sites 85
7.5.2 Compacted Clay Cover Performance Under Puerto Rico Conditions 86
7.6 Preliminary Design Criteria for ET Covers in Puerto Rico 88
8 References 91
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Guidance Document: Evapotranspiration Covers in Puerto Rico
Figures
Figure 1. Soil Cover Application at the Landfill in Culebra, Puerto Rico 19
Figure 2. Example of an Open Dump in Puerto Rico 22
Figure 3. US EPA Inspecting a Well-vegetated Historic Landfill in Puerto Rico 23
Figure 4. Horses Stand on a Closed Section of a Landfill in Puerto Rico 24
Figure 5. Horses Stand on an Old Landfill Cell With Shrubs and Tall Grasses 27
Figure 6. Runoff Along a Mountain in El Yunque National Forest 29
Figure 7. Schematic of the Water Balance Equation for an ET Cover at an Open Dump .. 30
Figure 8. The Understory and Overstory of Plants Both Use Water 32
Figure 9. View of the Sierra de Cayey From the Top of a Landfill 33
Figure 10. Five Defined Ecozones Developed for Puerto Rico 34
Figure 11. Example of a Maintained, Closed Landfill in Puerto Rico, Compared to the
Adjacent Local Vegetation 35
Figure 12. A Heavily Revegetated Slope of a Landfill 41
Figure 13. A Steep Landfill Slope in Puerto Rico With Applied Soil Cover 43
Figure 14. Grain Size Distributions for USDA Textural Soil Classifications Acceptable for ET
Covers in Puerto Rico 44
Figure 15. Map of Puerto Rico With Delineated Ecozones, Their Corresponding
Recommended ET Cover Design Thicknesses, and Modeled Landfill Locations 45
Figure 16. Established Vegetation at a Landfill in Puerto Rico 49
Figure 17. A Horse Stands on a Revegetated Slope of a Landfill 51
Figure 18. Growth-limiting Bulk Density of Soil Compaction for Balancing Mechanical
Stability and Plant Growth Capacity Across USDA Textural Soil Classifications 54
Figure 19. Closed Section Covered with Local Grasses at the Carolina Landfill in Puerto
Rico 55
Figure 20. Borrow Source Area at a Landfill in Puerto Rico 56
Figure 21. Potential Funding Sources and Services Available for Solid Waste Activities ..60
Figure 22. Potential Funding Sources for Solid Waste Infrastructure Investments 61
Figure 23. Agency Points of Contact for More Detailed Information and Eligible Activities 62
Figure 24. Delineated Ecozones for Puerto Rico 64
Figure 25. Physiography of Puerto Rico 65
Figure 26. 30-year Normal Precipitation Distribution Maps for Puerto Rico from 1971-2000
and 1981-2010 67
Figure 27. Soils of Puerto Rico 72
Figure 28. Schematic Showing the Relationship Between Water Storage Capacity and
USDA Textural Soil Classification 74
Figure 29. Water Balance Parameters Solved in the Richards Equation 79
Figure 30. Rainfall at a Landfill Located in Karst 79
Figure 31. Graphs of the Plant Water Stress Response Functions 83
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Guidance Document: Evapotranspiration Covers in Puerto Rico
Tables
Table 1. Characteristics of the Five Ecozones of Puerto Rico 35
Table 2. Summary of Clay Cover Simulations for the Ecozones of Puerto Rico 37
Table 3. Performance Goals (Sr) and Feasibility of ET Covers in Puerto Rico by Ecozone .39
Table 4. Water Balance Criteria Used to Model Clay and ET Covers in Puerto Rico Ecozones
40
Table 5. Average Percolation Rates of an Acceptable ET Cover (Any Soil) and Clay Cover
40
Table 6. Preliminary Borrow Soil Characterization Testing 46
Table 7. Standard Soil Index Property Tests to be Completed Prior to Construction 47
Table 8. Required Preliminary Screening for Soil Vegetative Properties Per Borrow Source
47
Table 9. Primary Forest Types and Examples of Common Tree Species in Puerto Rico ...52
Table 10. Comparison of Estimated Construction Cost Per Acre for ET and Clay Cover
Installation by Ecozone 58
Table 11. P/PET Ratio in Different Areas of Puerto Rico 68
Table 12. Modified P/PET Threshold, ET/PET Ratio ((3), and Runoff and Other Losses (A) by
Climate Type and Season 70
Table 13. Example Calculation of Sr for Arecibo Puerto Rico 70
Table 14. Average Annual Sr (mm/yr) for the Five Puerto Rico ET Cover Ecozones 71
Table 15. Soil Water Unit Storage Capacity of Soils of Puerto Rico 75
Table 16. Average Water Storage Capacity, Soil Cover Design Thickness, and Feasibility of
ET Covers by Ecozone 76
Table 17. Water Balance Criteria for Compacted Clay and ET Covers in Puerto Rico 77
Table 18. Unsaturated Soil Properties for Soils Used in ET Cover Simulations With Grasses
and Shrubs for All Ecozones (Rosetta Estimated Soil Parameters) 81
Table 19. Water Uptake Parameters for Feddes et al. (1978) Model 84
Table 20. Water Uptake Parameters for the S-Shaped van Genuchten Function 84
Table 21. Unsaturated Soil Properties of Compacted Clay Covers for All Ecozones 87
Table 22. Summary of Compacted Clay Cover Simulations 88
Table 23. Performance of Different Soil Types Used as Cover Designs 90
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Guidance Document: Evapotranspiration Covers in Puerto Rico
Acronyms and Abbreviations
°F degrees Fahrenheit
ACAP Alternative Cover Assessment Program
ASCE American Society of Civil Engineers
ASTM American Society for Testing Materials
CaC03 calcium carbonate
CDPHE Colorado Department of Public Health and Environment
CESER Center for Environmental Solutions and Emergency Response
CFR Code of Federal Regulations
CH4 methane
cm centimeter
C02 carbon dioxide
CQA Construction Quality Assurance
CY cubic yards
ENSO El Nino-Southern Oscillation
ET evapotranspiration
FEMA Federal Emergency Management Agency
ft feet
GCL geosynthetic clay liner
HUD Housing and Urban Development
HYDRUS A model for water movement in soil
in inch
in/yr inches per year
K hydraulic conductivity
LAI leaf area index
LGP low ground pressure
Mm Millimeter
m Meter
MSW municipal solid waste
N number
NASIS National Soil Information System
NCASI National Council for Air and Stream Improvement
NOAA National Oceanic and Atmospheric Administration
NRCS Natural Resources Conservation Service
ORD Office of Research and Development
P Local annual precipitation
Pc Percolation
pcf pounds per cubic foot
PE potential evaporation
PET potential evapotranspiration
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Guidance Document: Evapotranspiration Covers in Puerto Rico
ppm parts per million
PRDNER Puerto Rico Department of Natural and Environmental Resources
PT potential transpiration
QA quality assurance
QC quality control
R Runoff
RCRA Resource Conservation and Recovery Act
SARE Sustainable Agricultural Research and Education
sec second
SLS Sanitary Landfill System
Sr required soil water storage
Sw Soil water storage
SSURGO Soil Survey Geographic Database
T Transpiration
TCEQ Texas Commission on Environmental Quality
US EPA United States Environmental Protection Agency
USCS Unified Soil Classification System
USDA United States Department of Agriculture
USFWS United States Fish and Wildlife Service
USGS United States Geological Survey
UTV utility task vehicle
yr year
0r residual water content
0s saturated water content
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Guidance Document: Evapotranspiration Covers in Puerto Rico
Glossary
Alternative cover - An allowable final cover design option, per Puerto Rico and Resource
Conservation and Recovery Act (RCRA) regulations, that is equivalently protective as an
allowed conventional cover such as compacted clay or geomembrane.
Borrow source - The source of soil cover material taken from a location near the open
dump or landfill. The borrow area soil will be used for construction of the ET Cover.
Conventional cover - Any cover system commonly allowed under current regulations.
Puerto Rico and RCRA regulations allow membrane and compacted clay covers. May also
be referred to as a prescribed cover.
Ecozone - Geographic areas with similar climate, vegetation, and soil characteristics.
Equivalency - Demonstration, usually through a validated water balance model, that an
ET Cover will work as well or better than a conventional cover in preventing the
percolation of water into the waste mass. May be referred to as performance equivalency.
Evapotranspiration (ET) - The process by which water is transferred from the land to
the atmosphere by evaporation from the soil and other surfaces and by transpiration from
plants.
Evapotranspiration cover (ET Cover) - A cover system that stores precipitation in a
designed soil layer for removal by evaporation and transpiration. An ET Cover may be
referred to using one of the following terms: phytocap, water balance cover, store and
release cover, sponge and pump cover, or vegetated soil landfill cover.
Field capacity - At field capacity, the soil is wet and contains all the water it can hold
against gravity.
Hydraulic conductivity (K) - An indication of the permeability of a material to water. It
is a measure of the ability of vascular plants and porous material (e.g., soil, waste) to
transmit fluid (water) through pore spaces or fractures. It is correlated to soil properties
such as pore size, particle (grain) size distributions, and soil texture.
Infiltration - The movement of water from the atmosphere into the top of the soil cover.
Landfill - Solid waste land disposal facility. Generally expected to comply with current
regulations.
Leachate - Liquids that have percolated through disposed wastes that contain soluble,
suspended, or miscible materials removed (through leaching) from the waste.
Leaf area index (LAI) - Area of leaf cover per area of soil. The LAI correlates to the
amount of evapotranspiration by vegetation on the ET Cover.
Open dump - A landfill that does not meet modern, sanitary landfill standards and is
typically unlined and without leachate, stormwater, and landfill gas management systems.
Often an older facility that may not have been required to comply with current
regulations.
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Guidance Document: Evapotranspiration Covers in Puerto Rico
Percolation - The downward movement of water through the pores in soils or permeable
rock into the underlying waste mass.
Potential evapotranspiration (PET) - The combined loss of water through a plant's
process of transpiration via its vascular system and evaporation of water from the earth's
surface. PET is influenced by temperature, humidity, sunlight, and wind. PET values
indicate the amount of water that potentially can be lost from the soil. PET values will be
lower on cloudy, cool, or rainy days and higher on sunny, warm days with low humidity.
Wind increases PET values because it increases evapotranspiration. PET data used in this
document were sourced from the computer program PR-ET version 1.031 developed at the
University of Puerto Rico in Mayaguez.
Precipitation - Rain, snow, stormwater, hail, sleet, etc. that falls to the ground.
Prototype - For the purposes of this guidance, prototype is used to describe preliminary
ET Cover design requirements developed from Puerto Rico physiography (climate, rainfall,
soils, vegetation) and water balance modeling. The prototype ET Cover design includes a
list of acceptable soil types and preliminary minimum soil cover thickness by Ecozone.
Required soil water storage (Sr) - The required amount of water that needs to be
stored by a landfill ET Cover to minimize percolation of precipitation into the waste mass.
Runoff - The amount of precipitation that falls to the ground but does not infiltrate into
the soil cover.
Saturated hydraulic conductivity (Ksat) - A measure of the ease with which the pores
of a saturated soil permit water movement. It is correlated with soil properties such as
pore size, particle (grain) size distributions, and soil texture.
Unit soil water storage capacity - The amount of plant-available water in the soil; the
difference between the field capacity and the wilting point.
Wilting point - At the wilting point, the soil is dry, and the plant can no longer extract
water from the soil, leading to wilting of the plant.
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Guidance Document: Evapotranspiration Covers in Puerto Rico
1 Introduction
This guidance is intended to provide technical and regulatory assistance relating to the
allowance of evapotranspiration covers (ET Covers) as alternative final covers under 40
Code of Federal Regulations (CFR) Part (§) 258.60, "Criteria for Municipal Solid Waste
Landfills," and any other applicable regulations and policies. The document aims to give
regulators, municipalities, and contractors in Puerto Rico, or other similar jurisdictions, a
way to achieve regulatory acceptance and closure at landfills for which an ET Cover is
appropriate. Engineering guidance and modeled results are provided for regulatory
agencies, consultants, contractors, local governments, citizens, and owners and operators
who are involved in the permitting, design, operation, monitoring, and closure of solid
waste facilities. These guidelines are designed to help ensure the protection of public
health and the environment by providing an effective, low-cost, and low-maintenance final
cover solution, where appropriate, to accelerate the final closure of open dumps
throughout Puerto Rico.
ET Covers rely on the water storage capacity of soils and the water removal capabilities of
vegetation to limit percolation of water through the cover and into the waste mass. ET
Covers are designed and engineered based on site-specific criteria, matching local rainfall,
prevailing vegetation, and available soils and soil amendments to create a balanced water
capture, storage, and release system.
All waste cover systems must be designed to contain the waste from physical contact (via
exposed waste), be protective of groundwater, and be durable. ET Covers must meet
these requirements as well as or better than the prescriptive cover systems allowed in the
United States (U.S.) and under Puerto Rico laws and regulations. Properly designed and
built ET Cover systems can achieve those protective criteria [Colorado Department Public
Health and Environment (CDPHE, 2013); Texas Commission on Environmental Quality
(TCEQ, 2017) as evidenced by the computer modeling of site-specific climate conditions,
soil properties, and vegetation characteristics provided herein. This guidance also provides
key information related to designing, engineering, and obtaining regulatory acceptance for
an ET Cover under Puerto Rico's laws and regulations.
Figure 1. Soil Cover Application at the Landfill in Culebra, Puerto Rico.
Source: US EPA Region 2.
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EVAPOTRANSPIRATION (ET) COVER COST COMPARISONS
ET Covers cost less than clay or membrane covers because they are:
¦ Simpler to build.
¦ Easier to maintain (no mowing).
¦ Sustainable: stronger and more resilient as the cover vegetation mix establishes
and adapts.
¦ Self-repairing and improving: as vegetation goes through its life cycle, vegetation
biomass increases the organic matter content and naturally adds to the existing
soil.
ET Covers may cost more to design than conventional covers because they are
site-specific and it may take time to understand a site's unique characteristics. The
initial trees and shrubs may also cost more to purchase and establish compared to
the grass seed used for conventional erosion control.
1.1 How to Use this Guidance
For the purposes of this guidance, the island of Puerto Rico is divided into five areas of
similar rainfall, elevation, and vegetation patterns referred to as Ecozones. Within each
Ecozone, ET and compacted clay covers were designed and modeled using the HYDRUS-
1D Model,1 which simulates water flow, solute transport, and root water uptake in variably
saturated soil. A compacted clay cover, as described in U.S. and Puerto Rico regulations,
was modeled to determine the amount of water that would be likely to penetrate a clay
cover system in each Ecozone. A prototype ET Cover was then designed for each Ecozone
and modeled. Modeling confirmed that there is an ET Cover design that will perform
equivalently to or more efficiently than a compacted clay cover in each Puerto Rico
Ecozone.
The ET Cover prototypes presented in this guidance can be used as the basis for any
landfill in the appropriate Ecozone. The prototype designs were modeled extensively; ET
Covers intended for the same Ecozone as those modeled will not need to be modeled or
field tested further when designed to use the same soil type and thickness. Each site may
have slight variations in soil availability for building the cover and may have variations in
predominant vegetation. Therefore, each ET Cover design must account for site-specific
conditions.
Regulatory acceptance can be granted to a submitted design that is the same or better
than the ET Cover prototype from the same Ecozone.
1 See Section 7 for additional details on the modeling. Visit this link to learn more about HYDRUS-
1D and how to download the model at no charge.
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1.2 Regulatory Acceptance of ET Covers in the United States
ET Covers have been implemented in place of conventional covers for decades, including
at Superfund sites and at hundreds of Resource Conservation and Recovery Act (RCRA)
Subtitle C and D landfills throughout the U.S. ET Covers have most commonly been
implemented west of the Mississippi River in arid and semiarid regions. In some cases,
states have adopted guidance for their solid waste programs, inclusive of design criteria
for ET Covers to guide stakeholders.
One instance of a regulatory acceptance pathway was developed by the CDPHE in 2013
(CDPHE, 2013). In Colorado, many municipal solid waste (MSW) landfills requested the
use of alternative ET Covers instead of the typical compacted clay or geomembrane
barrier. State staff also had direct experience with ET Covers that were tested successfully
in its hazardous waste program. The requests the State received came in various levels of
detail and technical bases, which prompted the need for the department to help facilities
and its staff by developing formal guidance. The Colorado guidance document (CDPHE,
2013) provides a technically sound approach that is straight-forward to understand and
implement. The approach eliminates the need for numerical modeling and test plots at
each facility by providing modeled prototype cover designs in each Ecozone throughout
the state. This guidance follows the example of the Colorado guidance.
TCEQ also developed regulatory guidance for requesting ET Covers as an alternative final
cover at MSW landfills in 2017 (TCEQ, 2017). The Texas guidance document provides
stakeholders with several design and permitting options, one of which involves a
statewide design table adopted from Khire (Khire, 2016). Users can identify their
geoclimatic region to determine the appropriate cover soils, storage layer thickness, and
expected cover performance. This guidance document contains similar data and models
for Puerto Rico and intends to function as a user-friendly design and permitting tool for
regulators, municipalities, and engineers.
Regulators, landfill owners and managers, and engineers throughout the Caribbean or in
other tropical or subtropical zones may find the characteristics of Puerto Rico's Ecozones
similar to their local conditions and may utilize the data and models herein to help develop
their own guidance for ET Cover designs that are appropriate for their site conditions.
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1.3 Status of Municipal Solid Waste Facilities in Puerto Rico
As of 2021, approximately 29 landfills remain in operation throughout Puerto Rico, more
than half of which are unlined open dumps that continue to release significant volumes of
contaminants that may threaten human health and the environment. In addition to
currently operating open dumps and modern, lined landfills that are at capacity, historical
landfills have left a legacy of uncontrolled waste facilities that need permanent closure.
Figure 2. Example of an Open Dump in Puerto Rico.
Source: US EPA Region 2.
More than 40 historical open dumps were closed prior to 2000. Three of the closed, pre-
RCRA open dumps became Superfund sites. Today, some sections of modern, lined
landfills, historical landfills, and operating open dumps have been untended for a
considerable amount of time, during which the originally applied daily covers have
developed substantial vegetation. Some of these sites may be suitable for approval as
final ET Covers based on the soil thickness and vegetation.
In 2018, the US EPA estimated that there were fewer than 5 years of remaining capacity
across all the active landfills in Puerto Rico given the capacity at existing cells (US EPA,
2018). Approximately 22 of the 29 landfills are expected to have reached or exceeded
their capacities by 2021. Thirteen open dumps are under legal agreements with the US
EPA that require closure. The Puerto Rico Department of Natural and Environmental
Resources (PRDNER) has also pursued closure orders since its 1994 solid waste program
approval that subsequently granted the PRDNER the primary responsibility for regulating
solid waste landfills in the Commonwealth and made the PRDNER the only agency with
permitting and enforcement authority over landfills on the island (US EPA, 2016).
22
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Open dumps throughout Puerto Rico raise potential environmental justice concerns for
surrounding communities and may endanger human health and the environment by
releasing leachate into sensitive ecosystems, surface water, and groundwater resources.
Landfill gas emissions also create a potential explosion risk and contribute to climate
change.
Figure 3. US EPA Inspecting a Well-vegetated Historic Landfill in Puerto Rico.
Source: US EPA ORD.
The limited solid waste capacity throughout the island has encouraged the continued
operation of the remaining unlined open dumps. The continued operation of inexpensive
and non-compliant dumps discourages investment in compliant landfill cells in Puerto Rico.
Moreover, standard prescribed landfill closure designs have often seemed financially and
technically infeasible to many landfill owners and operators due to the high initial capital
investment and ongoing maintenance. Determining a low-cost and sustainable solution for
the swift final closures of open dumps is a critical step toward a successful solid waste
management plan for Puerto Rico.
LANDFILL COVERS THAT HAVE GROWN ON THEIR OWN
It is possible for an unclosed and uncontrolled landfill or dump to have grown
vegetation on the final daily cover or intermediate cover. In parts of Puerto Rico,
mature forests have grown on waste sites.
It is possible to officially close some of these sites by declaring the natural forest an
ET Cover. Following a site survey, an exploration of soil depth and type, and possibly
some calculations and modeling, it may be possible to approve a site closure if the
natural ET Cover is deemed equally protective as a designed cover.
23
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1.4 Regulatory Acceptance for Conventional and Alternative Final
Covers
All solid waste management facilities (e.g., landfills, open dumps) will eventually undergo
closure and must have a final cover capable of achieving regulatory acceptance. Final
cover systems are intended to control moisture and percolation, promote surface water
runoff, minimize erosion, prevent direct exposure of the waste, control landfill gas
emissions and odor, prevent the occurrence of disease vectors, and meet aesthetic and
other end-use purposes. RCRA describes the requirements for conventional, prescriptive
landfill cover and alternative cover designs (40 CFR § 258.60).
Per 40 CFR § 258.60(a), an acceptable final cover consists of the following:
• An infiltration layer containing a minimum of IS inches of earthen material.
• An erosion control layer of earthen material with a minimum of 6 inches in
thickness capable of sustaining native plant growth.
• A permeability less than or equal to the permeability of any bottom liner system or
natural sub-soils present, or a permeability no greater than 1 x 10"5 centimeters per
second (cm/sec) whichever is less.
Conventional final cover systems are designed to reduce water infiltration into the
underlying waste using resistive principles (e.g., layers with low saturated hydraulic
conductivity like compacted clay barriers) and geosynthetic clay liners with or without a
geomembrane. Because resistive principles provide the hydraulic impedance that limits
flow into and through underlying contaminated materials or waste, this design philosophy
is often referred to as an umbrella approach (Abichou, et al., 2015). This approach works
against nature because it prevents water from moving downward into the soil.
Conventional compacted clay barriers are prone to problems such as increasing
permeability overtime (Benson et al., 2007; Henken-Mellies and Schweizer, 2011),
preferential flow path development within the soil barrier (Albright et al., 2006), and
desiccation cracking (Albrecht and Benson, 2001). Additionally, the structural integrity
and performance of conventional covers may become compromised by earthquakes, as all
of Puerto Rico is a seismic zone under RCRA Subtitle D (40 CFR § 258.14 and USGS,
n.d.).
Figure 4. Horses Stand on a Closed Section of a Landfill in Puerto Rico.
Source: US EPA Region 2.
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Alternative final cover systems are allowable under RCRA Subtitle D, which stipulates that
the US EPA (or a US EPA-approved state agency responsible for implementing the state
permit program pursuant to RCRA) may approve an alternative final cover design. An
alternative cover must consist of the following components (40 CFR § 258.60(b)):
• An infiltration layer that provides equivalent reduction in infiltration to that of the
prescribed cover.
• An erosion control layer that provides equivalent protection from wind and water
erosion as the prescribed cover.
An alternative final cover is hydrologically equivalent to a prescribed conventional cover if
the percolation rate for the alternative cover is less than or equal to the percolation rate
for the prescribed cover and it provides at least equivalent protection from erosion.
Puerto Rico's Regulations for Non-Hazardous Solid Waste Management, Chapter 4, Part D,
Rule 5652 (PR Rule 565; PRDNER, 2016) establishes the same minimum criteria as 40 CFR
§ 258.60 for final covers, conventional and alternative, as excerpted below:
A. Sanitary Landfill System (SLS) owners or operators will install a final cover system
designed and built to:
1. Have a permeability less than or equal to that of any system of lining on the bottom
or natural subsoil present, or a permeability not greater than 1 x 10"5 cm / sec,
whichever is less.
2. Minimize infiltration through the closed SLS by using a layer which, after being
compacted, has at least eighteen (18) inches of fill cover material.
3. Minimize erosion of the final cover by means of a layer which, after being
compacted, has at least six (6) inches of fill cover material that is capable of
allowing for the growth of vegetation in the area.
B. The Environmental Quality Board may approve an alternate cover design that includes
the following:
1. A lining that achieves an equivalent reduction in infiltration specified in Paragraphs
A(l) and (2).
2. An erosion control liner that provides equal protection from wind and water erosion
as specified in Paragraph A(3).
2 The 2020 Puerto Rico Draft SLS Regulations do not affect these closure requirements.
25
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Only the PRDNER has permitting and solid waste enforcement authority over landfills
throughout the Commonwealth. In Puerto Rico, the permitting process for alternative
covers, including ET Covers, requires the submission of an operational permit
modification. The modification must include the following as set forth, in part, by the
PRDNER in Rule 649 of the Non-Hazardous Solid Waste Regulations, Modification or
Revocation of a Permit:
1. Fill out a permit application form for an Operation Permit Modification.
2. An executive summary addressing the modification.
3. A modified Operations Plan, Closure Plan, and Post Closure Plan.
4. Compliance with the Puerto Rico Non-Hazardous Solid Waste Regulations.
EQUIVALENCE
Approved final landfill covers typically include an impermeable membrane, an 18-
inch compacted clay layer, or the equivalent, in order to exclude water from
reaching waste.
ET Covers are not designed to exclude water like a prescriptive cover. Effective ET
Covers are shown to be equally protective at containing waste and preventing water
contamination by storing rain and releasing it back into the air via evaporation and
transpiration. This is an entirely different mechanism from exclusion and
equivalently protective.
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1.5 ET Cover Basics
ET Covers are a common type of alternative cover comprised of an earthen cover that
uses the water storage capacity of soils and the water removal capability of vegetation. ET
Covers are also known as a phytocap, water balance cover, store and release cover,
sponge and pump cover, or vegetated soil landfill cover. For simplicity, this guidance uses
the term ET Cover.
The primary purpose of an ET Cover is to control the percolation of precipitation into the
waste zone through water balance mechanisms instead of the resistive mechanisms
employed by conventional cover technologies (Albright et al., 2010). Water that infiltrates
into the ET Cover is stored by the soil and subsequently removed and returned to the
atmosphere either by vegetation (via transpiration) or through direct evaporation from the
soil, thereby limiting the percolation of water into the waste mass below.
l\i\ "v; .
Y..,' ' /
v i «"r ,• t i' 1
"'ft** ' * t ' ' \ /. 'M
Figure 5. Horses Stand on an Old Landfill Cell With Shrubs and Tall Grasses.
Source: US EPA Region 2.
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Several key terms are defined here as they relate to the functioning of an ET Cover and
are referenced later in this guidance:
• Percolation (Pc) - The movement of water through the pores in soils or
permeable rock. Precipitation that moves from the ground surface into the soil
cover is referred to as infiltration, which is then referred to as percolation
when it moves into the waste mass. Higher rates of percolation indicate more
water is reaching the waste mass.
• Precipitation (P) - Rain, snow, stormwater, hail, or sleet that falls to the
ground.
• Runoff (R) - The amount of precipitation that falls to the ground but does not
move into the soil cover; it is transported, usually by gravity or engineering
design to move away from the location where it falls.
• Leaf area index (LAI) - The area of leaf cover per area of soil. The LAI
correlates to the amount of evapotranspiration by vegetation on the ET Cover. A
higher LAI typically increases evapotranspiration.
• Evapotranspiration (Et)3 - The process by which water is transferred from the
land to the atmosphere by evaporation from the soil and other surfaces and by
transpiration from plants. Evapotranspiration decreases when PET is higher
relative to precipitation. Evapotranspiration can also increase with increasing LAI
(if more water is available) as more leaves are available for transpiration.
• Potential Evapotranspiration (PET) - PET is the combined loss of water
through the plant's process of transpiration via its vascular system, and
evaporation of water from the earth's surface. PET is influenced by temperature,
humidity, sunlight, and wind. PET values indicate the amount of water that has
been lost from the soil. PET values will be lower on cloudy, cool, or rainy days
and higher on sunny, warm days with low humidity. Wind increases PET values
because evapotranspiration rates are higher. PET data were sourced from the
computer program PR-ET version 1.03 developed at the University of Puerto
Rico in Mayaguez.
• Hydraulic conductivity (K) - K indicates the permeability of a material. It is a
measure of the ability of vascular plants and porous material (e.g., soil, waste)
to transmit fluid (water) through interconnected pore spaces or fractures. K is
correlated to soil properties such as pore size, effective porosity, particle (grain)
size distributions, and soil texture.
3 The process of evapotranspiration is spelled out in this guidance instead of using the
abbreviation, Et, to avoid confusion with the abbreviation of ET Cover.
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Figure 6. Runoff Along a Mountain in EI Yunque National Forest.
Source: Emmie McCleary.
Figure 7 shows a schematic of an ET Cover in the context of the water balance equation
(Equation 1). Percolation (Pc) is the net effect of the interactions of precipitation (P),
runoff (R), evapotranspiration (Et), and the change in soil cover water storage (ASw).
Evapotranspiration is the largest component of ET Covers and surface runoff is the
smallest fraction of the water balance on average (Apiwantrogoon, 2007).
Evapotranspiration is affected by water availability from precipitation, energy demand
from PET, and LAI (Apiwantrogoon, 2007),
29
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Pc = P — R — Et — ASw
(Equation 1)
Evapotranspiration (Et)^
Precipitation (P)
¦¦
* ¦ ¦ ¦
WASTE
>¦.? iv.v ;v.5 ;v.? '¦VjyVjijw'
¦¦¦¦¦¦
\ ¦*«¦% ¦*¦¦/. ;¦
ijij
. .
¦¦ m *. ¦¦¦¦¦¦ i ¦ ¦ • ¦ ¦¦ i '¦ ¦¦¦«
¦- ¦- ¦- ¦- ¦- ¦- vv.'/yyfi
Percolation (Pc)
¦ ¦"¦¦¦¦¦ S ¦¦¦¦
Leachate
Groundwater
Figure 7. Schematic of the Water Balance Equation for an ET Cover at an Open
Dump.
While an ET Cover's core purpose is to function as a final cover, it is a living system that
also mitigates hazards to human health and the environment by decreasing stormwater
runoff and leachate generation, providing vector control (specifically for open dumps),
reducing landfill gas methane emissions through increased rates of methane biological
oxidation (as a result of a thicker soil layer compared to prescribed clay covers; Abichou
et al., 2015), and restoring wildlife habitat, all of which become more efficient overtime.
When designing ET Covers, engineers often use models to simulate water flux through the
design cover under certain climatic input scenarios, which is performed to develop the
prototype ET Covers in this guidance. There are two basic steps to designing an ET Cover:
• Selecting a soil profile that has sufficient capacity to store the precipitation while
ensuring that percolation from the base of the soil cover is maintained below an
acceptable maximum value. This maximum value is calculated using meteorological
data (e.g., maximum annual precipitation), which is typically agrees with the
percolation through a conventional cover in the same location.
30
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• Selecting vegetation to be planted on the cover that will efficiently remove the
stored water from the soil profile.
ET Covers are selected and designed to maximize runoff, evapotranspiration, and soil
water storage. The design is very site-specific as it involves assessing local climatic
conditions (e.g., precipitation, PET), local soil properties (e.g., hydraulic conductivity
functions, storage capacity), and local cover vegetation. Organic amendments such as
compost, woodchips, or other local fine-textured organic materials can be integrated with
the cover soil to improve fertility and water storage capacity.
In general, ET Covers are not designed to a level of impermeability that can be measured
in the field. Instead, they are designed based on performance to meet applicable
percolation criteria. This type of design relies heavily on computer modeling to simulate
the water balance through the soil profile under given climate and vegetation conditions.
As a result, although the life-cycle cost of such covers is lower than a conventional cover,
the design cost can be higher, making it difficult for many smaller landfills to afford the
design (CDPHE, 2013). This guidance document has been prepared to reduce the design
cost of ET Covers in Puerto Rico and to encourage more landfill owners and operators to
choose ET Covers as an effective alternative to prescribed compacted clay covers.
TREES, ET COVERS, AND CLIMATE CHANGE
There is an area of soil around plant roots, known as the rhizosphere, that is an
excellent habitat for the soil microbial community, which degrades (oxidizes)
methane (CH4) in landfill gas. Microbes supported in rhizospheres by various root
functions are 40 times more abundant than in unplanted soils. ChUthat may be
released from a conventional landfill cover has been shown to not escape from ET
Covers.
In addition, trees remove carbon dioxide (CO2) from the air and store it in their
woody parts and the roots. The CO2 removed by trees is estimated to be 2 tons of
C02 per acre of trees per year.
1.6 Field Performance of ET Covers
Several field studies of ET Covers have measured percolation through the covers,
documented how they work, and measured their overall performance. Together, they
have provided useful and practical engineering design guidance that has been
incorporated into this and the guidance developed by the states of Texas and Colorado.
These field studies include Nyhan et al. (1990, 1997, and 2005); Anderson et al. (1993);
Gee et al. (1993); Hakonson et al. (1994); Khire et al., (1999); Chadwick et al. (1999);
Wagner and Schnatmeyer (2002); Dwyer (2003); Forman and Anderson (2005); Scanlon
et al. (2005); Ward et al. (2005); Albright et al., (2004); and Abichou et al. (2005). A
literature summary of field studies of ET Covers is presented in Section 7, Methodology
and Supporting Documentation (specifically Section 7.5).
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IMPACTS OF USING TREES
ON ET COVERS
ET Covers may use grasses, trees, or other
vegetation appropriate for the location of the
cover system. Several studies have shown
that various species of trees intercept rain
and absorb it directly through leaves and
needles. Measured amounts of this
interception range from 25% to 45% of
precipitation depending on the duration and
intensity of rain. This effect decreases the
amount of water that an ET Cover system
needs to exclude from the waste.
Tree-, shrub-, and grass-mixed systems also
increase ET Cover effectiveness through
maximizing plant transpiration by utilizing
both the overstory and the understory layers
of vegetation.
Figure 8. The Understory and Overstory of Plants Both Use Water.
Source: US EPA ORD.
A large field study conducted by the US EPA, the Alternative Cover Assessment Program
(ACAP), tested different ET Cover designs in a variety of arid, semiarid, and high
precipitation-high humidity regions at 11 locations over several years in the United States
(Albright et al., 2004). This milestone study resulted in the development of key variables
that are used in computational modeling for the design of ET Covers today. The ACAP
team found that monolithic4 ET Covers were able to achieve comparable or better
performance than conventional covers in the arid and semi-arid locations tested.
Conversely, the monolithic ET Covers did not control percolation as well in high
precipitation-high humidity climates. High precipitation is a bigger adverse factor of
potential effectiveness of an ET Cover compared to a high humidity-low precipitation
region. In addition to monolithic ET Covers, a variety of granular media were effectively
demonstrated for the construction of ET Covers. The variety of granular media allows for
ease of construction and cost savings using local soils (Zornberg et al., 2003). As noted in
this guidance, the chosen media must encourage robust plant growth first and foremost
(Hauser et al., 2001) because sustained vegetation is a key factor in ET Cover
performance.
A review of field hydrology of ET Covers (Apiwantrogoon, 2007) confirms
evapotranspiration is the largest component of ET Covers (approximately 71% to 107% of
precipitation) and surface runoff is the smallest fraction of water balance (less than 10%
4 A monolithic cover is also referred to a monofil cover, which uses a single fine-grained soil layer
to retain water and support vegetation (Albright et al. 2010).
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of precipitation) on average. Evapotranspiration is affected by water availability from
precipitation and energy demand from PET and LAI (Apiwantrogoon, 2007).
Evapotranspiration decreases when PET is higher relative to precipitation and can also
increase with increasing LAI (if more water is available) as more leaves are available for
transpiration.
The fraction of the water balance that becomes percolation represents less than 20% of
precipitation (Apiwantrogoon, 2007). Percolation is generally transmitted from the cover
to the waste when the storage capacity is exceeded (Apiwantrogoon, 2007). Preferential
flow or thermal gradients also influence percolation. Percolation can occur when the peak
storage capacity is less than the storage capacity of the cover soils due to scaling effects
in soil hydraulic properties and preferential flow. Percolation that occurs due to thermal
gradient is typically small (<1% of precipitation).
Based on the above and other unpublished ET Cover construction and monitoring
assessments, several states have issued permits to implement ET Cover designs at
landfills, especially west of the Mississippi River. Therefore, the implementation of ET
Covers in this area is widespread.
1.7 Ecozones of Puerto Rico
Puerto Rico's geography is defined by a series of central ridges of mountains and hills,
which run mainly from east to west and divide the island into northern and southern
sections (USDA, 2008). The northern and southern sections are further divided, by
elevation and proximity to the sea, into foothills and shore regions. These physical
distinctions influence the climate and seasonally-specific variations in rainfall and
temperature, which impact important water balance parameters and the overall
performance of an ET Cover.
Figure 9. View of the Sierra de Cayey From the Top of a Landfill.
Source: US EPA Region 2.
This guidance used physiological data for Puerto Rico (e.g., climate, topography,
precipitation) to generate five Ecozones. Ecozones are geographical areas with similar
climate, vegetation, and soil types. The Ecozones are referenced throughout this guidance
33
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and used to estimate the baseline performance equivalency of compacted clay covers,
generate prototype ET Cover designs, and scientifically demonstrate the performance
equivalency of each prototype ET Cover design in each Ecozone. Methodological details
are provided in Section 7,
The five Ecozones delineated for Puerto Rico, as depicted in Figure 10, include:
• North Shore.
• Northern Foothills.
• Mountains.
• Southern Foothills.
• South Shore,
Atlantic Ocean
AA
North Shore Ecozone
Northern Foothills Ecozone
Mountains Ecozone
Caribbean Sea
\ \ \ Southern Foothills Ecozone
) | South Shore Ecozone
Landfill Sites
Figure 10. Five Defined Ecozones Developed for Puerto Rico.
Figure note: The locations of the landfill sites are approximate.
Table 1 presents key characteristics for each Ecozone. These Ecozones were developed
considering geophysical maps, historical monthly precipitation data, and temperature data
from the US Geological Survey (USGS, 2011), and aerial imagery. The Ecozone
boundaries are not sharply defined. Areas adjacent to other Ecozones share similar
characteristics and vegetation.
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Table 1. Characteristics of the Five Ecozones of Puerto Rico.
Ecozone and
Approximate Area
Elevation
Annual
Precipitation
Primary Forest Types
North Shore
80 miles by 2-6 miles
Near sea
level
1500-1750 mm
59-69 in
Moist coastal forest
Moist limestone forest
Northern Foothills
110 miles by 4 miles
100-1000 ft
1016-1651 mm
40-65 in
Moist coastal forest
Moist limestone forest
Mountains
60% of island
Up 4390 ft
1524-2286 mm
60-90 in
Tropical moist forest
Moist coastal forest
Moist limestone forest
Southern Foothills
110 miles by 4-10 miles
100-1000 ft
1500-1750 mm
59-69 in
Lower cordillera forest
Dry coastal forest
Moist coastal forest
South Shore
75 miles by 8-12 miles
Near sea
level
800-1000 mm
31-39 in
Dry limestone forest
Dry coastal forest
Site owners, contractors, and regulators seeking landfill closure using an ET Cover should
confirm the Ecozone of the landfill, review the prototype ET Cover design for the selected
Ecozone (see Section 3), and submit the prototype ET Cover design, with design changes
as appropriate, for regulatory acceptance. ET Covers for sites on the border of Ecozones
should consider a design that supports vegetation in the more conservative Ecozone with
respect to soil cover design thickness.
Figure 11. Example of a Maintained, Closed Landfill in Puerto Rico, Compared to
the Adjacent Local Vegetation.
Source: US EPA Region 2.
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2 Performance Equivalency of Alternative Final
Covers to Compacted Clay Covers in Puerto
Rico
Long-term performance of MSW landfill closure cover systems is a critical issue for
landfills because the proper functioning of these covers is important with respect to
reducing post-closure impacts to human health and the environment. While 40 CFR §
258.60 and PR Rule 565 allow for the use of a conventional cover or alternative cover (as
discussed in Section 1.4), the regulations provide few details regarding how performance
equivalency should be formulated for an alternative cover.
This section shows how a performance equivalency determination can be conducted within
the example area of Puerto Rico. Section 2.1 presents the results from a modeling
exercise to estimate the performance of prescribed (conventional) compacted clay covers
and percolation rates in Puerto Rico where field data are lacking. Section 2.2 presents
similarly modeled results for ET Covers and demonstrates how this allows for the
determination of equivalency as specified in the RCRA and Puerto Rico regulations.
Additional details about the model used and input parameters are included in Section 7.
2.1 Performance of Prescribed Clay Covers in Puerto Rico (Baseline
Equivalency)
Conventional covers permitted by PR Rule 565 are based on a barrier concept that
employs resistive principles. This barrier concept is achieved with a compacted, low-
hydraulic conductivity layer- a geosynthetic clay liner (GCL) with or without a
geomembrane. In the U.S. and Puerto Rico regulations, an alternative cover is considered
hydrologically equivalent to a prescribed conventional cover if the percolation rate for the
alternative cover is less than or equal to the percolation rate for the prescribed cover.
To determine whether an ET Cover is equivalent to a conventional cover for a specific
climate or site, a direct comparison of percolation rates between the two covers is needed.
However, a direct comparison of percolation rates is only possible at locations where side-
by-side testing of ET Covers and conventional covers can be conducted. Compacted clay
covers have been studied extensively and their rates of infiltration are well documented in
many locations, however, there are no measured data available for compacted clay cover
effectiveness in Puerto Rico.
In this guidance, evidenced-based values from peer-reviewed literature and a modeling
approach are used to estimate the field performance of compacted clay covers assumed to
be subjected to the different climates of Puerto Rico. This modeled approach estimates the
long-term percolation rates in millimeters per year (mm/yr) as an index of performance of
compacted clay covers under the different climatic conditions in Puerto Rico. The modeled
simulated percolation rates through these compacted clay covers are assumed to be
baseline percolation rates used in the assessment of the equivalency of any proposed ET
Cover design for the different Ecozones of Puerto Rico.
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Table 2 shows the modeled percolation rates of compacted clay covers subjected to
different climatic conditions. These percolation rates represent the baseline rate for
comparing ET Cover performance to that of compacted clay covers (equivalency criteria).
Table 2. Summary of Clay Cover Simulations for the Ecozones of Puerto Rico.
Climate
Ecozone
Site Name
Percolation Through Clay Cover
Percolation
(mm/yr)
% of
Precipitation
Average
(mm/yr)
Isabela
174
11
North Shore
Toa Baja
223
13
189
Vega Baja
169
11
Northern
Florida
196
13
203
Foothills
Fajardo
210
13
Moca
560
29
Mountains
Jayuya
686
35
455
Juncos
350
20
Ba rranquitas
225
17
Cayey
223
16
Guayama
242
16
Southern
Foothills
Hormigueros
329
20
Humacao
638
31
377
Mayagiiez
616
32
Cabo Rojo
82
7
Yabucoa
508
25
Juana Diaz
83
8
Lajas
66
6
Ponce
6
1
South Shore
Santa Isabel
31
3
73
Yauco
15
2
Penuelas
237
16
Note: The thickness of the compacted clay and topsoil cover is 600 mm.
2.2 ET Cover Equivalence and Feasibility in Puerto Rico
The major objective of ET Cover systems is to minimize percolation into the underlying
waste mass. The likelihood of acceptable performance (i.e., minimal percolation rate into
the waste mass below the cover) is correlated with the local annual precipitation (P) and
annual PET at a given location. Albright et al. (2010) combined monthly PET and P to
estimate the required soil water storage (Sr) for a given ET Cover design to be
effective. The P/PET ratio and Sr were calculated for each Ecozone in accordance with
Albright et al. (2010). Most available soils in Puerto Rico are classified as: Loam, Silty Clay
Loam, Silty Clay, Clay Loam, and Sandy Clay Loam. These soil types are appropriate to
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use for an ET Cover soil layer in Puerto Rico. The unit soil water storage capacity for
Puerto Rico soils was also estimated. These key terms are defined below.
• P/PET ratio - A measure of the amount of water managed relative to the energy
available to manage water through evapotranspiration. A lower P/PET ratio
corresponds to a higher confidence in the landfill cover managing precipitation with
minimal percolation.
• Sr - The required amount of soil water storage that an ET Cover needs to be
designed to achieve in order to be approved as equivalent to a prescribed clay
cover under 40 CFR Part 258.60. Sr is a good index of the annual amount of water
that needs to be stored by the soil layer of the ET Cover to minimize the percolation
rate of rainwater into the waste mass. Sr could be calculated at any given location
using site-specific P and PET data.
• Unit soil water storage capacity - The amount of plant-available water in the
soil; the difference between the field capacity and the wilting point.
• Field capacity - At field capacity, the soil is wet and contains all the water it can
hold against gravity.
• Wilting point - At the wilting point, the soil is dry, and the plant can no longer
extract water, leading to wilting of the plant.
The required thickness of the ET Cover was then calculated by dividing Sr by the unit soil
water storage capacity of the soil to be used for an ET Cover. A feasibility score was then
determined for using ET Covers by Ecozone. Feasibility in this context was defined when
the required thickness of an effective ET Cover is no more than 1.5 m. See Section 7 for
more details on the methodology and modeling inputs.
Table 3 shows the feasibility of ET Covers in the different Ecozones of Puerto Rico for the
acceptable soil types, as classified by the US Department of Agriculture (USDA), all of
which are available in the delineated Ecozones. ET Covers are feasible to varying degrees
across the island. ET Covers intended for sites in the Mountains Ecozone will require site-
specific assessments. ET Covers in the Mountains Ecozone were first modeled with shrubs
and grasses, which was found to be a challenging climate for ET Covers, and then
modeled with trees and grasses as the cover, which resulted in a feasible classification.
The modeling indicated that the only acceptable soils for ET Covers in Puerto Rico
include: Loam, Silty Clay Loam, Silty Clay, Clay Loam, and Sandy Clay Loam.
Combinations of these soils are also acceptable. Combination soil layers and soil
amendments (e.g., compost, wood chips) may be added to improve the water-holding
capacity or plant-supporting ability of the ET Cover soil.
38
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Table 3. Performance Goals (Sr) and Feasibility of ET Covers in Puerto Rico by
Ecozone.
Ecozone
Performance Goal,
Sr,
(Required Water
Storage)1
(mm/yr)
USDA Soil
Classification
Available in
Ecozone2
Avg. Unit
Water
Storage
(cm3/cm3)
ET Cover
Feasibility
North Shore
173
Loam
Silty Clay Loam
Silty Clay
Clay Loam
Sandy Clay Loam
0.2289
Very feasible
Northern
Foothills
387
Loam
Silty Clay Loam
Silty Clay
Clay Loam
Sandy Clay Loam
0.2289
Feasible
Mountains
502
Loam
Silty Clay Loam
Silty Clay
Clay Loam
Sandy Clay Loam
0.2289
Feasible with
tress and
grasses.
Challenging with
shrubs and
grasses.
Southern
Foothills
307
Loam
Silty Clay Loam
Silty Clay
Clay Loam
Sandy Clay Loam
0.2289
Feasible
South Shore
67
Loam
Silty Clay Loam
Silty Clay
Clay Loam
Sandy Clay Loam
0.2289
Very feasible
^he Sr, required amount of water to be stored by the soil cover, is the performance goal for a
prototype ET Cover in each Ecozone.
2The soils that are acceptable for ET Covers in Puerto Rico are: Loam, Silty Clay Loam, Silty Clay,
Clay Loam, and Sandy Clay Loam. All Ecozones contain these five soil classes.
A modeling approach (described in Section 7) was conducted to estimate percolation into
the waste mass at representative landfills in two locations within each Ecozone. A water
balance model was also used to create hypothetical compacted clay covers and ET Covers
consisting of local soils and local vegetation (local grasses with shrubs). Two cover types
were modeled for the Mountains Ecozone: local grasses with shrubs and local grasses with
trees. The key input parameters modeled are presented in Table 4.
39
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Table 4. Water Balance Criteria Used to Model Clay and ET Covers in Puerto Rico
Ecozones.
Cover
Type
Cover
Thickness
Erosion Control
Layer Thickness
Cover Vegetation
Soil Type
Clay1
450 mm
150 mm
Local grasses
Loam
Silty Clay Loam
Silty Clay
Clay Loam
Sandy Clay Loam
ET2
Erosion
control layer
is part of the
cover
300 mm
Local grasses and shrubs (all
Ecozones)
Local trees and grasses (only for
the Mountains Ecozone)
Loam
Silty Clay Loam
Silty Clay
Clay Loam
Sandy Clay
Loam
1 The compacted clay layer is overlaid with an erosion control layer for 600 mm of total cover
thickness.
2 The erosion control layer of the ET Cover is part of the ET Cover and rooting zone.
Table 5 presents modeled results of the average percolation rates through ET Covers that
are equivalent to a clay cover, incorporated with a design factor of safety, resulting in the
prototype ET Cover design thicknesses. For example, an ET Cover with a soil thickness of
900 mm and an average percolation rate of 133 mm/yr in the Northern Shore Ecozone is
presumed equivalent to a compacted clay cover with an average percolation rate of 196
mm/yr.
Table 5. Average Percolation Rates of an Acceptable ET Cover (Any Soil) and Clay
Cover.
Climate
Ecozone
Site (City)
P
(mm/yr)
PET
(mm/yr)
Avg.
Percolation
(mm/yr)
Prototype
ET Cover
Soil
Factor of
Safety1
Clay
Cover
ET
Cover
Thickness
(mm)
North Shore
Toa Baja
1725.4
1651
196
133
900
1 c:
Vega Baja
1586.2
1667
1. D
Northern
Florida
1542.1
1632
203
133
1000
1.5
Foothills
Fajardo
1585.0
1615
Mountains
Juncos
1750.7
1544
(grasses and
shrubs)
Barranquitas
1334.9
1392
288
189
1500
1.5
Mountains
Juncos
1312
1544
288
169
1200
1.7
(trees and
grasses)2
Barranquitas
1001
1392
Southern
Guayama
1484.0
1542
355
242
1200
1.5
Foothills
Hormigueros
1670.7
1591
South Shore
Lajas
1145.2
1745
73
44
600
1.66
Santa Isabel
914.5
1771
^he ratio of the percolation rate of the clay cover to that of the ET Cover.
2 Trees with 25% tree canopy interception were modeled.
40
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Modeling provides an estimate of percolation through ET Covers but is one of many
factors that must be considered in their design. Other factors, including maintenance
and use of appropriate vegetation also affect the performance of ET Covers and should
be considered during design, construction, and post-construction maintenance.
TREES AND SHRUBS AS LANDFILL COVERS
For many years, standard practice has been to exclude trees and other deep-rooted
plants from landfill cover systems. This is based on the fear that roots will disrupt
the water excluding nature of the cover system.
In an ET Cover, the plants are not just on the cover - they are the cover. The
deeper the roots, the better the plants are able to draw water from the soil layer,
which acts as a water-holding sponge. Deeper rooted plants also tend to survive
better during times of higher-than-normal precipitation and drought.
Figure 12. A Heavily Revegetated Slope of a Landfill.
Source: US EPA Region 2.
41
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3 Prototype ET Cover Design and Preliminary
Site Characterization for Puerto Rico
This section provides guidance for a prototype ET Cover design approach considering
available cover soil characteristics, vegetation properties, climate, and construction
considerations. The prototype approach is aimed at achieving regulatory acceptance and
can be used at sites with comparable conditions to those of the Puerto Rico Ecozones.
A preliminary site characterization is required to better understand the site soils and
vegetation in order to develop the engineering design. The site characterization must
include a borrow source analysis and an evaluation of the local vegetation, which can be
done concurrently. The preliminary site characterization and engineering design steps for
the prototype ET Cover design consider the performance goals by Ecozone (Sr, as
presented in Table 3). These steps are presented below and described throughout the
rest of this section.
1. Identify acceptable soil types (Section 3.1)
• This step has been completed as part of this guidance and is included as part
of the prototype ET Cover design.
• Acceptable soil types for ET Covers in Puerto Rico: Loam, Silty Clay Loam,
Silty Clay, Clay Loam, and Sandy Clay Loam.
2. Determine recommended soil cover thickness (Section 3.2)
• This step has been completed as part of this guidance and is included as part
of the prototype ET Cover design.
• Normally, this step would be done after the preliminary site characterization
has been completed (e.g., borrow source analysis, vegetation evaluation).
For this guidance, preliminary design computations have been completed to
determine the required water storage of the soil (Sr), the available water
storage, and required soil thickness. Water balance modeling has also been
completed as part of this guidance to determine performance equivalency to
a prescribed clay cover.
3. Complete borrow source analyses (Section 3.3)
• Volume of soils available.
• Uniformity of soils.
• Particle size and soil type.
• Water content.
• Soil screening for vegetative properties [e.g., hydrogen ion concentration
(pH), calcium carbonate (CaCOs)].
• Soil evaluation to support appropriate vegetation (helps determine whether
amendments are needed to support vegetation).
4. Evaluate local vegetation and develop the revegetation plan (Section 3.4)
42
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• Plant species.
• Phenology (how species are affected by seasonal variations in climate).
• Planting locations.
• Schedule for planting.
• Additional activities as needed.
5. Finalize the ET Cover design (specifics not discussed in this guidance)
• Additional water balance modeling, if needed (e.g., if conditions differ from
those used in developing the prototype ET Cover in this guidance's
scenarios).
• Geometric design.
• Surface water and leachate management strategies.
• Landfill gas management.
• Erosion control strategies.
• Specification preparation.
6. Obtain regulatory approval. Basic regulatory references regarding equivalency and
permit modifications are provided in Section 1.4, specifics are not discussed in this
guidance.
Figure 13. A Steep Landfill Slope in Puerto Rico With Applied Soil Cover.
Source: US EPA Region 2.
43
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3.1 Acceptable Soil Types
Soil texture and other physical properties of the water storage layer (e.g., preferential
flow features) contribute to a successful ET Cover. Minimizing preferential flow and
maximizing the ability of the soil to store water are design criteria that are enhanced by
specifying the soil's physical properties. It is paramount to use the USDA Textural Soil
Classification as shown in Figure 14. The only soil types (i.e., textures) that are
acceptable for ET Covers in Puerto Rico are: Loam, Silty Clay Loam, Silty Clay, Clay Loam,
and Sandy Clay Loam (shaded in Figure 14).
Figure 14. Grain Size Distributions for USDA Textural Soil Classifications
Acceptable for ET Covers in Puerto Rico (Shaded).
Source: USDA, n.d.
Soils can be amended by blending in organic materials such as compost. Strategic
composting of vegetative debris, especially from tropical storms, can provide amendments
to enhance degraded soils and provide benefits to local soil properties. Compost
incorporation into locally available soils generally reduces bulk density, enhances
infiltration and hydraulic conductivity, increases water content and plant-available water,
and often results in enhanced grass growth and reduced sediment loss. Kranz et al.
(2020) provides a comprehensive review on the effects of compost incorporation on soil
physical properties in urban soils and how incorporation of compost into soil can
significantly enhance soil physical properties, nutrient dynamics, and vegetation
establishment.
100
% ^ 'o
Percent sand
44
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3.2 Soil Cover Thickness
ET Cover thickness is a function of site-specific precipitation and the selected granular
material (Jacobson et al., 2005). The granular material has been modeled in this guidance
as part of the ET Cover prototype design, along with the defined performance goals (Sr)
by Ecozone, vegetation type, and other parameters to determine the minimum soil cover
thickness that should perform equivalently to a compacted clay cover.
Figure 15 presents the recommended minimum thickness of soil cover by Ecozone. These
soil cover thicknesses were calculated using models that assumed the vegetation of the ET
Covers consisted of local grasses and shrubs for all Ecozones, except for the Mountains
Ecozone, which was also modeled with trees and grasses as the vegetative cover.
Atlantic Ocean
Caribbean Sea
[ 1 South Shore (600 mm. Grass and shrubs)
® «. North Shore (900 mm. Grass and shrubs)
Northern Foothills (1000 mm. Grass and shrubs)
v A Mountains (1500 mm. Grass and shrubs)
1200 mm. Grass and Trees) J
Southern Foothills (1200 mm. Grass and shrubs)
Landfill Sites
Figure 15. Map of Puerto Rico With Delineated Ecozones, Their Corresponding
Recommended ET Cover Design Thicknesses, and Modeled Landfill Locations
(Designated by Black Dots).
In summary, for performance equivalent to a compacted clay cover in the same Ecozone,
the following minimum ET Cover thicknesses are recommended:
• North Shore - 900 mm (3 ft).
• Northern Foothills - 1000 mm (3.25 ft).
• Mountains - 1500 mm (5 ft) when using grasses and shrubs, 1200 mm (4 ft) when
using grasses and trees.
• Southern Foothills - 1200 mm (4 ft).
• South Shore - 600 mm (2 ft).
45
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Additional modeling can be performed if different scenarios (i.e., different soil cover
thickness, soil type, vegetation) are needed.
3.3 Borrow Source Analysis
3.3.1 Preliminary Soil Characterization Testing
The first step in defining the specific design of an ET Cover is to collect soil samples from
borrow sources to demonstrate that the correct type, quantity, and uniformity of soils are
available at the site to achieve the design cover area and thickness. A preliminary analysis
using standard index testing for soil properties (summarized in Table 6) should be
performed. The water content of the soil is important because it directly influences the
construction of the cover by providing a lower density fill and informs the selection of
appropriate vegetation for the cover. Similarly, the grain size analysis will inform the
optimal degree of soil compaction for cover stability and plant growth capacity (discussed
in Section 4.2).
The results should confirm which borrow source(s) is acceptable for use in the engineering
design. Further analysis is required if soils other than those listed in Table 6 are intended
to be used for the ET Cover because different soils may require a different ET Cover
design thickness.
Table 6. Preliminary Borrow Soil Characterization Testing.
Property
Method
Requirement
Water Content
American Society for
Testing and Materials
(ASTM) D2216
See construction guidance in Section 4 and
vegetation requirements in Section 3.3.3
Grain-Size Analysis (Soil
Texture)
ASTM D422 (with full
hydrometer)
For Puerto Rico, soil must be classified as
Loam, Silty Clay Loam, Silty Clay, Clay Loam,
Sandy Clay Loam (see the USDA Textural Soil
Classification chart, Figure 14)
3.3.2 Standard Soil Index Property Testing
Once the borrow source(s) is selected, additional standard index testing is needed to
supplement the preliminary testing for each selected borrow source to confirm the
engineering design. This additional testing should be performed at the frequencies
summarized in Table 7. The testing frequency may be adjusted based on the uniformity
of the soil and the facility's previous experience with the same materials. Representative
samples should also be tested for vegetative soil properties (as discussed in Section
3.3.3).
In general, soils used for the water storage layer of the water balance cover should meet
the following gradation requirements:
• Contain 15% gravel or less (> 2.00 mm [0.0787 in], retained on the No. 10 sieve).
• Limit maximum particle size to less than 25.4 mm (1 in) in longest dimension.
• Limit maximum clod size to less than 101.6 mm (4 in) in longest dimension, with a
clod defined as a soil aggregation that does not break down by hand.
46
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• Should not contain foreign debris or deleterious materials.
All gradation data provided above are in accordance with the USDA Textural Soil
Classification.
Table 7. Standard Soil Index Property Tests to be Completed Prior to
Construction.
Property
Method
Selected
Frequency
(per Method)
ET Cover Feature
Water Content
ASTM D2216
1 per 6,500 cubic
yards (CY)
See vegetation requirement in
Section 3.3.3
Grain-Size Analysis
ASTM D422 (with
full hydrometer)
1 per 6,500 CY
Soil should be classified as Loam,
Silty Clay Loam, Silty Clay, Clay
Loam, Sandy Clay Loam (see
USDA Textural Soil Classification
chart, Figure 14)
Laboratory Compaction
(Standard Proctor)
ASTM D698
1 per 6,500 CY
Proper soil
placement/compaction (soil
density, moisture content, and
lift thickness)
3.3.3 Preliminary Soil Screening for Vegetative Properties
While representative soil samples from each borrow source are analyzed for the standard
soil index properties, an initial screening for the soil vegetative properties, as summarized
in Table 8, should be performed at the recommended frequency (1 analysis per 6,500
cubic yards [CY]) for each borrow source. The testing frequency may be adjusted based
on the homogeneity of the soil and the facility's previous experience with the same
materials. It is important to verify that when soils are placed in the ET Cover, suitable pH
and CaCCh content are attained throughout the entire depth of the water storage layer as
opposed to just near the surface.
Table 8. Required Preliminary Screening for Soil Vegetative Properties Per
Borrow Source.
Property
Method
Requirement
QA/QC
Frequency
PH
ASTM D4972
6.0-8.4 standard units (s.u.)
1 per 6,500 CY
CaCCb
ASTM D4373
< 15%, by weight
1 per 6,500 CY
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3.3.4 Soil Evaluation for Supporting Appropriate Vegetation
In general, soils used for the water storage layer of the ET Cover should be suitable for
establishing vegetation. The role of vegetation in an ET Cover is essentially to remove
moisture through evapotranspiration. Therefore, soils proposed for the construction of an
ET Cover should support long-term vegetation growth. In that context, soils should be
tested for:
• Salt content
High concentrations (e.g., greater than 2%) of ionic salts (sodium,
potassium, calcium, etc.) can inhibit vegetation growth.
High gypsum content is an indicator that vegetative growth may be
inhibited.
- Amendments with composted manure can increase salt content and may not
be appropriate to use.
• pH
- Soil pH should be between 6.0 and 8.4 s.u.
- pH greater 8.0 (usually due to high sodium content) can cause soils to
disperse, resulting in drainage problems that may inhibit vegetative growth.
pH greater than 8.4 can inhibit vegetative growth.
- pH less than 6.0 can inhibit vegetative growth (suggestion: raise pH by
adding lime, calcitic limestone, or dolomitic limestone to the soil).
• Nitrogen
- The recommended soil nitrogen content is 5-30 parts per million (ppm).
Nitrogen readily leaches from soil and may require additional monitoring.
Nitrogen is very important for initial growth and vegetative health, but native
and many naturalized plants are often adapted to low-nitrogen conditions.
Nitrate-nitrogen as low as 5 ppm in conjunction with 1.5-2.0% soil organic
matter will be satisfactory for most major dryland native plants likely to be
used on covers.
• Phosphorus
- The recommended soil phosphorus content is 30-70 ppm.
Phosphorus has a moderate leaching potential from soil.
• Potassium
- The recommended soil potassium content is approximately 75-200 ppm.
Potassium has a low leaching potential and generally stays in place until used
by the vegetation.
• Electrical conductivity
48
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- The electrical conductivity of soils should be less than 400 millisiemens per
meter (mS/m). This conductivity is a good indicator of a soil capable of
sustaining healthy vegetation, with higher values indicating higher salt
content. Optimal electrical conductivity levels in the soil can range from 110
to 570 mS/m.
3.4 Revegetation Plan
Successful revegetation is critically important to the establishment of a functional ET
Cover. Native vegetation is generally preferable, as it is already adapted to the local
climate and soil conditions (Rock, 2003). Non-native vegetation tends to be naturally
supplanted by better-adapted native vegetation overtime. An evaluation of the local
vegetation is typically performed to document the following information, which can inform
the Revegetation Plan:
• Species distribution.
• Species phenology (how they react to seasonal stressors such as drought).
• Species coverage (how much of an area the local vegetation covers).
A practical way to conduct the evaluation is to mark off a plot near where the ET Cover is
intended to be installed, visually assess the plot and document findings. The plot should
be an undisturbed area with similar characteristics (e.g., borrow source, grade, length,
slope) as the planned ET Cover area. A vegetation evaluation may be conducted more
than once to capture species phenology if there are distinct seasonal differences in rainfall
and temperature that may impact plant growth in an Ecozone.
Figure 16. Established Vegetation at a Landfill in Puerto Rico.
Source: US EPA Region 2.
49
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Engaging with a local revegetation expert/agronomist is recommended. Other resources
that may be able to assist with or provide information pertaining to local vegetation
include the NRCS, Sustainable Agricultural Research and Education (SARE), local farmers,
and academic institutions. The report entitled An Introduction to Using Native Plants in
Restoration Projects (Dorner, 2006) is recommended as a comprehensive resource for
preparing the site vegetation planning activities.
The main purpose of a Revegetation Plan is to define criteria such as target values for
species composition and abundance (Albright et al., 2010). These criteria will allow for an
evaluation of the revegetation's success after the plants are established at year one and
year three to four. The criteria in the Revegetation Plan should be developed from a
baseline vegetation evaluation (similar to a baseline ecological survey, but not as
rigorous) and should include: the ecological basis for the criteria, time steps for the target
values, and vegetation sampling designs, instrumentation, and statistical methods for field
data collection and analysis (Albright et al., 2010). Findings from the soil screening and
evaluation should also be incorporated into the Plan.
Best practice general guidelines for revegetation are listed below:
• Plant at the appropriate time (e.g., season, time of day) to increase plant survival.
• Amend the soil as needed depending on borrow source analysis results. Soil
amendment application of compost and woodchips may be necessary to address
soil organic matter content, soil structure, or nutrient deficiencies. Using inorganic
fertilizers is generally not advisable because it might promote weed growth, off-site
nitrate migration, and generally requires prolonged use for maintenance.
• Prepare a firm but not overly compacted seed bed and perform seeding in a manner
that is appropriate for the plants chosen for the site.
• Irrigate the revegetated areas if necessary to maintain soil water content during
initial growing phase. This will assist in the establishment of the new vegetation.
Factors to be considered include: proposed soil moisture content, water application
rate, weather conditions, and other site-specific criteria. Maintenance of a moist
seed bed is best accomplished by frequent and light application of water.
• Evaluate the revegetated areas after the first growing season (one year). After
approximately three to four years, plants should be well-established and ready for
comparison to performance standards established in the approved Revegetation
Plan.
50
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TREES, SHRUBS, AND GRASS ROOTS ON AND IN WASTE
Standard practice has been to exclude trees and other deep-rooted plants from
waste containment systems for fear that the deep roots will absorb and then release
contaminants from the waste into the air or on the ground via leaf litter.
Phytoremediation is the study of plant and contaminant interactions and how plants
can clean up contaminants in soil and water. While some plant species can take up
certain contaminants, there are hundreds of installations of plants on waste sites
and landfills where the source contamination has never spread because of the
plants.
Testing the leaves, branches, and tree cores to confirm they are not taking up
contaminants is relatively simple, inexpensive, and recommended.
Table 9 serves as a general starting point in selecting tree species during the
revegetation process and as a supplemental resource to the recommendations of suppliers
and revegetation consultants.
Figure 17. A Horse Stands on a Revegetated Slope of a Landfill.
Source: US EPA Region 2.
51
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Table 9. Primary Forest Types and Examples of
Common Tree Species in Puerto Rico.
Forest
Type
Common Tree Species Examples1
Moist
Coastal
Forest
• Sideroxylon foetidissimum
(Tabloncillo, tortugo) • Tabebuia heterophylla
• Andira inermis (Moca) (Roble)
• Citharexylum fruticosum (Pendula) • Guettarda sea bra
• Zanthoxylum martinicense (espino • Randia aculeata (Tintillo)
rubio o espino rubial)
Moist
Limestone
Forest
„ , . • Bursera simaruba
• Ochroma pyramidale (Balsa) (Almacigo)
• Bucida buceras (Ucar) _ , . ,
v 1 • Cedrela odorata
• Zanthoxylum martinicense ,
7 • Guettarda scabra
• Dipholis salicifolia (Almendron) _ .
• Sapium laurocerasus
• Sideroxylon foetidissimum . .
' • Randia aculeata
Tropical
Moist Forest
• Cyathea arborea (Helecho gigante)
• Matayba domingensis (Tea Cimarrona, Doncella)
• All the Lower Cordillera Forest Species listed below.
Lower
Cordillera
Forest
• Ocotea leucoxylon (Cacaillo) * Ormosia krugii
. Casearia arborea ' Lin°ciera domingensis
• Ocotea moschata (Nuez moscada) * Cedrela odorata (Cedro)
• Hirtella rugosa (Hicaquillo, Icaquillo, * Guarea trichilioides
Jicaquillo) • Byrsonima coriacea
• Buchenavia capitata (Maricao)
. Inga laurina (Guama) ' Dry petes glauca (Palo
bianco, palo de aceituna)
• Myrcia deflexa
• Tabebuia heterophylla
• Andira inermis (Moca) (Roble)
Dry
Limestone
Forest
• Capparis cynophallophora • Bucida buceras
• Pictetia aculeata (Tachuelo) • Bursera simaruba
• Guaiacum officinale (Guayacan) • Dipholis salicifolia
Dry Coastal
Forest
• Capparis cynophallophora
• Citharexylum fruticosum
• Bucida buceras
• Guaiocum officinale
• Pictetia aculeata (Tachuelo)
1 All species listed appear in at least one other forest region in Puerto Rico to facilitate sites with
mixed forest types. Sources: USDA, 1974; USDA, 1964.
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4 Construction Guidance for ET Covers
4.1 Subgrade Preparation
For MSW landfills, subgrade is typically defined as a minimum 6-inch-thick foundation
layer composed of earthen material (e.g., typically derived from intermediate or daily
cover soils) that is situated between the disposed material and the ET Cover. For non-
MSW landfills with homogeneous wastes (e.g., ash monofills), subgrade may be defined
as the top of the waste surface. Best practices for subgrade preparation include the
following:
• Proof-roll the subgrade and make repairs as needed to achieve a stable surface.
• Grade the subgrade to achieve a surface consistent with the approved design
contours for ET Cover construction.
• Roughen relatively steeper side slopes (e.g., > 5%) using appropriate equipment
prior to placement of cover soil.
• Survey the prepared subgrade surface prior to ET Cover construction to establish a
basis for the lines, grades, and total soil cover thickness to be achieved during
construction.
4.2 Soil Bulk Density
To function properly, an ET Cover relies on a water storage layer to retain precipitation
and support vegetation until the water is transpired or evaporated, thus reducing deep
percolation. To take full advantage of the transpiration process, a well-developed and
sustainable vegetative cover is desired. Research performed by Goldsmith et al. (2001)
has shown that when soil compaction levels are high, there is a threshold soil bulk density
beyond which roots have difficulty penetrating due to the high physical resistance of the
soil. This threshold density is called the growth-limiting bulk density and varies depending
on soil texture and plant type. Typical growth-limiting bulk density values range from
approximately 90 pounds per cubic foot (pcf) for predominately clayey soils to
approximately 106 pcf for sandy soils (Goldsmith et al., 2001; Schenk and Jackson,
2002). Typical growth-limiting bulk density iso-density line values are shown in Figure 18
for USDA soil types.
53
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Legend:
Isodensity line
87.4 = Pounds per cubic
foot
(1.40) = Grams per cubic
centimeter
% S a n d
Figure 18. Growth-limiting Bulk Density of Soil Compaction for Balancing
Mechanical Stability and Plant Growth Capacity Across USDA Textural Soil
Classifications.
Source: Goldsmith et al., 2001.
In theory, the growth-limiting bulk density should range from 83% to 88% of the
maximum standard Proctor density (ASTM D698). In practice, the standard Proctor
density specified for the water storage layer should be greater than or equal to 80% and
less than or equal to 90% of the standard Proctor density for that soil type (Albright et al.,
2010). Additional design details might include measures to control erosion on side slopes
(e.g., armored down-slope channel chutes, terraces, berms) and to enhance slope
stability.
4.3 Soil Moisture Content
During soil placement for an ET Cover, keeping the moisture below the soil's optimum
moisture content facilitates a lower density fill. Therefore, incorporation of this
requirement in the construction specifications is considered best practice. Soil for the
water storage layer should be placed in loose lifts greater than 18 inches thick to avoid
overcom paction.
4.4 Lift Thickness
The thickness of any existing interim cover should be determined using a grid pattern as
specified by the site engineer and documented in the ET Cover permit application. Soil
samples should be obtained to verify that the existing interim cover consists of Loam,
54
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Silty Clay Loam, Silty Clay, Clay Loam, and/or Sandy Clay Loam, as required for soils from
the borrow area. Additional modeling should be performed using the classification of
available soils to verify the percolation rates if the interim cover will be included as part of
the ET Cover, and if it consists of different soils than the prototype ET Cover in the Puerto
Rico Ecozone. The total required thickness as shown in Figure 15 should include the
thickness of the interim cover, or, depending on the age and vegetation, both the daily
and interim covers. Regulatory approval will be required for the thickness and composition
of all ET Cover soil layers.
Figure 19. Closed Section Covered with Local Grasses at the Carolina Landfill in
Puerto Rico.
Source: US EPA Region 2.
Best practices to achieve the placement of a minimally compacted water storage layer and
topsoil layer, as applicable, are listed below.
• Excavate the soil from previously approved borrow sources. Each water storage
layer borrow source should meet the soil gradation requirements as well as the
standard soil index property requirements.
• To minimize overcompaction, place the water storage layer in thick lifts
(approximately 1.5 ft, which is preferred over multiple, thinner lifts). Placement of
the water storage layer as a single lift has proven successful.
• Place the soil using non-wheeled equipment to minimize overcompaction.
• Use soil that is at less than (or "drier" than) its optimum moisture content because
"dry" soil is relatively more difficult to over-compact.
55
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• Spread, level, and track-walk the soil using equipment (for example, a D6 low
ground pressure [LGP] or D7 LGP bulldozer).
• Track-walk the soil during placement only enough to place and rough-grade the
soil.
• If overcompaction occurs at locations, such as beneath haul roads, the soil might
need to be ripped or disked and then recompacted within the appropriate growth-
limiting bulk density range (Albright et al., 2010). This practice is only intended to
alleviate overcompaction and should not be used as a standard operating procedure
for cover soil placement.
• Survey the prepared water storage layer/topsoil surface to verify that the designed
lines, grades, and thickness have been achieved during ET Cover construction.
Water balance cover soil component thickness also may be determined by field
measurements. In-place soil thickness field measurements should be documented
under the supervision of the Construction Quality Assurance (CQA) Engineer and as
specified by a preapproved CQA project plan.
• Perform revegetation activities while the placed soil still is "dry" of optimum
moisture content and when suitable for planting.
Figure 20. Borrow Source Area at a Landfill in Puerto Rico.
Source: US EPA Region 2.
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5 Relative Cost of Design and Construction for
ET Covers
Limited cost data are available for the construction of ET Cover systems. The available
construction cost data (ITRC, 2003) indicate that these cover systems have the potential
to be 30-50% less expensive to construct than conventional cover systems requiring
compacted clay and/or geomembranes. Factors affecting the cost of construction include
soil layer thickness, availability of materials, placement methods, and project size.
The relative costs for the construction of ET Covers in Puerto Rico consider the following:
• Ecozones into which the island was divided.
• Thickness and recommended soils for each Ecozone.
• Local availability of these soils.
• Whether intermediate cover is already in place at the site.
• Hauling costs.
• Proper placement procedures for this type of landfill cover.
• A 1.25 shrinkage factor for ET Cover soil placement (total change from bulk-mined
soil to 80% compaction in place).
Work activities that must be carried out on site prior to the placement of the ET Cover,
such as clearing, grubbing, and subgrade preparation, are needed for any cover and are
not unique to installing ET Covers. Therefore, the costs of such activities are not included
in the relative cost comparison presented. These activities will need to be accomplished
before any cover system is constructed. Significant work may be required that involves
slope preparation and grading to achieve a surface consistent with the approved design
contours. These must be evaluated in detail and the cost estimate must be adjusted
considering the characteristics of the landfill for which a cost estimate is being prepared.
The relative cost estimates for installing an ET Cover compared to a clay cover are
presented by Ecozone in Table 10. The average cost per acre for a clay cap across the
Ecozones is approximately $136,575/acre. The average cost per acre for an ET Cover
across the Ecozones is approximately $78,859/acre. The overall average cost savings per
acre by using an ET Cover is $57,716 compared to a clay cover, with cost savings ranging
from 19 percent (Mountains) to 76 percent (South Shore) across the Ecozones.
57
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Table 10. Comparison of Estimated Construction Cost Per Acre for ET and Clay
Cover Installation by Ecozone.
Ecozone
and
Cover
Type
Depth
of Final
Cover
(m)
Depth Over
Intermediate
Cover (m)
Volume
Cover
Soil
(m3)
Soil,
Transport1,
and
Placement
Cost Per m3
Cost Per
Acre2
ET Cover Cost
Savings Per Acre3
North Shore
ET
0.90
0.60
3,009
$21
$63,193
$67,312
52%
Clay4
0.60
0.60
3,035
$43
$130,505
Northern Foothills
ET
1.00
0.70
3,515
$23
$80,844
$61,801
43%
Clay
0.60
0.60
3,035
$47
$142,645
Mountains
ET
1.30
1.00
5,033
$24
$120,803
$27,912
19%
Clay
0.60
0.60
3,035
$49
$148,715
Southern Foothills
ET
1.20
0.90
4,528
$22
$99,607
$36,968
27%
Clay
0.60
0.60
3,035
$45
$136,575
South Shore
ET
0.60
0.30
1,492
$20
$29,847
$94,588
76%
Clay
0.60
0.60
3,035
$41
$124,435
1 Some areas of Puerto Rico have local, naturally occurring soils that may be suitable and available
for ET Covers, rather than commercially quarried soil, as used in this cost analysis. As a result,
soil and transport costs may be lower than the projected costs in this table.
2 ET Cover construction assumes that the placement of soil is completed in loose lifts (greater than
18 in [0.45 m]) with a compaction greater than or equal to 80%, or less than or equal to 90% of
the Standard Proctor density for the type of soil used.
3 Note that ET Cover costs have the potential to have much greater cost-savings than the projected
amounts above, as many aspects of ET cover construction and maintenance can be conducted
using municipal resources and amortized over time on a municipal budget.
4 Clay cover construction assumes the clay cap is placed on compacted soil with wet lifts to achieve
a permeability no greater than lxlO 5 cm/sec and a thickness of at least 18 inches (0.45 m).
58
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6 Financial Assistance
6.1 Funding Programs and Applicability
While ET Covers are often a more economically feasible final cover option, funding
remains a significant issue for municipal governments in Puerto Rico. There are several
opportunities for municipalities to apply for financial assistance to construct and maintain
a final cover. Some funding mechanisms are specifically for improvements to solid waste
infrastructure, and many are exclusively for Puerto Rico. The financial assistance
mechanisms discussed in this section are all available through government programs
associated with the Commonwealth or federal departments or agencies.
The US EPA and Federal Emergency Management Agency (FEMA), in collaboration with
Housing and Urban Development (HUD), USDA, and PRDNER, have developed an
infographic guide that outlines funding sources. The infographic guide also describes how
improvements to solid waste infrastructure and management in Puerto Rico can mitigate
potential hazards that result from disaster debris and municipal solid waste generated
from disasters. The infographic guide highlights the public health and environmental
concerns associated with Puerto Rico's open dumps and municipal solid waste landfills, as
described in Section 1.3 of this guidance document.
According to the infographic guide, "[t]he existing conflicts/conditions at open dumps and
landfills may hinder the re-establishment of community lifelines after disasters. For this
reason, incorporating solid waste mitigation strategies into hazard mitigation planning will
help to protect lifelines and prevent and mitigate potential impacts to them." Examples of
community lifelines at risk of disruption given the current conditions of landfills in Puerto
Rico include public health, safety, drinking water sources, and government services.
Applying for funding to close and place a final cover on a landfill may seem
counterintuitive as it relates to having capacity for disaster debris and additional municipal
solid waste. However, as discussed in Section 1.3, the continued operation of open dumps
in Puerto Rico discourages investment in compliant, strong, and resilient solid waste
infrastructure and management, which is the main priority of the government entities that
are offering funding sources. In addition to slowing solid waste infrastructure
development, open dumps without final covers will, for example, generate significantly
greater volumes of leachate, especially during storms, that may contaminate drinking
water sources and surface water. The infographic guide addresses the potential for
contamination to impact community lifelines such as public health and drinking water
sources.
Figure 21, Figure 22, and Figure 23 break down the potential funding sources and
services available for solid waste infrastructure and management. Only a few of the
infographics from the infographic guide are provided in this section. The full document can
be found here in English and here in Spanish (FEMA 2021). Additional funding information
and sources may become available after the publication of this document, such as
handbooks expanding on the infographic guide and funds from the American Rescue Plan
Act. Contact the agencies listed in Figure 23 for current potential funding sources and
information.
59
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Possible Funding Sources or Services
for Solid Waste Activities by Category:
.......
Planning for
Solid Waste and
Disaster Debris
Management
0 SERVICES
A FUNDING
r
USDA-NRCS
(
Emergency Watershed
)
V
Program (EWP-Recovery)
J
USDA-NRCS Technical Assistance
Programs (Conservation
Technical Assistance,
State Technical Committee)
i
I
9
USDA-NRCS Landscape Planning
Programs (Emergency Watershed
Protection Program, Watershed
and Flood Prevention Operations
Program, Watershed Rehabilitation)
USDA-NRCS Financial Programs
(Conservation Stewardship
Program, Environmental
Quality Incentives Program)
For private landowners and entities
USDA-RD Solid Waste
Management Grants
HUD Community Development
Block Grant - Disaster Recovery
(CDBG-DR)
HUD Community Development
Block Grant - Mitigation (CDBG-MIT)
Figure 21. Potential Funding Sources and Services Available for Solid Waste
Activities.
Source: FEMA 2021.
60
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A
FEMA Hazard Mitigation
Grant Program (HMGP)
HUD Community Development
Block Grant - Mitigation (CDBG-MIT)
HUD Community Development
Block Grant - Disaster Recovery (CDBG-DR)
K
USDA-NRCS Emergency
Watershed Program (EWP-Recovery)
us DA Water and Waste Disposal
Loan and Grant Program
USDA Community Facilities
Loan and Grant Program
USDA Disaster Assistance Grant
W
w
s
USDA-NRCS Financial Programs
(Conservation Stewardship Program,
Environmental Quality Incentives Program)
For private landowners and entities
13
USDA-NRCS Landscape Planning Programs
(Emergency Watershed Protection Program,
Watershed and Flood Prevention
Operations Program,
Watershed Rehabilitation)
SERVICES
FUNDING
Figure 22. Potential Funding Sources for Solid Waste Infrastructure
Investments.
Source: FEMA 2021.
If disaster-related funding is not available or not applicable to a specific project, there are
several ongoing government funding assistance programs that can support solid waste
infrastructure improvements. One such program includes the USDA Rural Development
Water and Waste Direct Loan and Grant Program, This program can provide funding for
construction, repairs, expansions, and improvements at solid waste facilities. Final landfill
covers qualify as an eligible use of funds. Further eligibility depends on other components,
such as socioeconomic factors within the project service area. The USDA Rural
Development has additional programs, that are not described in this guidance document,
that assist applicants with funding predevelopment and planning expenses for landfill
projects, and additional funding for technical assistance with solid waste management
activities. To learn more about the USDA Rural Development grant and loan funding
mechanisms through its Water and Environmental Programs and eligibility requirements,
visit this link or contact Jose A. Cabrera, Esq. (contact information in Figure 23 for
USDA).
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For more detailed information on eligible solid waste activities,
please refer to the agency point of contact information below.
Possible Funding Sources for Solid Waste Activities
Federal Agency
Program
Point of Contact
U.S. Department of
Agriculture (USDA)
- Rural
Development
• USDA Disaster Assistance Grant
• USDA Solid Waste Management Grants
• USDA Water & Waste Disposal Loan and Grant Program
• USDA Community Facilities Loan and Grant Program
• USDA Rural Business Development Grant
• USDA Rural Community Development Grant
Jose A. Cabrera, Esq.
Community Programs Specialist and
Promise Zone Community liaison
787-756-5634
Jose.Cabrera (S) usda.eov
U.S. Department of
Agriculture (USDA)
- Natural Resources
Conservation
Service (NRCS)
« USDA-NRCS Landscape Planning Programs (Emergency
Watershed Protection Program, Watershed and Flood
• Prevention Operations Program, Watershed Rehabilitation)
• USDA-NRCS Emergency Watershed Program (EWP-Recovery)
• USDA-NRCS Technical Assistance Programs (Conservation
Technical Assistance, State Technical Committee)
• USDA-NRCS Financial Programs (Conservation Stewardship
Program, Environmental
• Quality Incentives Program) For private landowners and entities
Luis A. Cruz-Arroyo
Caribbean Area Director
Office: 737-281-4836
Cellphone: 787-405-7368
luis.cruz-arroyo (Susda.gov
Federal Emergency
Management
Agency (FEMA)
• FEMA Hazard Mitigation Grant Program (HMGP)
Region 2 HMGP POC:
Sharon Edwards
HMA Branch Chief
Sharon.edwa rdsiSfema.dhs.gov
917-561-2935
PR Mitigation Recovery HMGP POC:
Antonio Busquets Lopez
Hazard Mitigation Division Director
Puerto Rico Joint Recovery Office
FEMA DR 4339/4473-PR
Antonio.busquetslopezrsfema.dhs.eov
202-341-2607
U.S. Department of
Flousing and Urban
Development (HUD)
• Community Development Block Grant - Disaster
Recovery (CDBG - DR)
• Community Development Block Grant - Mitigation
(CDBG-MIT)
Laura 1. Rivera-Carrion
Coordinating Officer for Disaster
Recovery
Tel. 787-274-5817/ Cel. 202-258-3929
Laura.l.Rivera-Ca rrioniShud.gov
Puerto Rico Department of Housing is the
administering agency for CDBG-DR and
CDBG-MtT funds far the Government of Puerto Rko.
Department of
Natural and
Environmental
Resources (DNER)
• Clean Water State Revolving Fund (CWSRF) Program
Javier Verardi
Department of Natural and
Environmental Resources
Projects Division,
Water Quality Area
(787) 767-8181 ext. 3080.
javierverardi © jca. pr. go v
Figure 23. Agency Points of Contact for More Detailed Information and Eligible
Activities.
Source: FEMA 2021.
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6.2 Tips for Applying
Financial assistance applications should address how improvements to the solid waste
infrastructure of concern would align with the priorities of the government entity's solid
waste management strategies and financial assistance program(s). Certain features of
solid waste infrastructure improvements may align with the priorities of some programs
more than others and application language may need to be modified accordingly.
Government agencies reviewing applications may also be more likely to grant assistance
for final cover funding when proposals highlight an ET Cover's lower construction and
post-closure care costs, as it would be a protective and cost-effective final closure solution
without as much long-term oversight. Referencing this US EPA guidance in a proposal for
a final cover could help application reviewers better understand and feel confident in an
ET Cover design and its ability to mitigate hazards to public health and the environment
(e.g., significant reductions in leachate releases). This guidance may also assist in
expediting the preliminary design process required by some funding entities.
Some funding programs, such as the USDA Rural Development Water and Waste Direct
Loan and Grant Program, may require that the applicant's solid waste facility is providing
a service to the community. Applications must be clear and concise in describing that
service or planned service. Services to the community can include, but are not limited to:
• Plans to install controls, such as final covers, landfill gas controls, or leachate
collection and management systems, that protect human health and the
environment from ongoing environmental emissions at open dumps or landfills
without adequate emission controls.
• Continued solid waste management with plans for new, compliant landfill cells.
• Conversion of a solid waste facility that is no longer suitable for waste disposal to
other land uses such as waste transfer stations, recycling collection areas, or other
municipal operations.
Municipalities will have greater success in being approved for a more favorable ratio of
grant monies versus loans if their projects are required due to the risks the solid waste
facility poses to human health and the environment. To apply for a final landfill cover that
will serve the community by protecting human health and the environment, the applicant
must clearly explain the risks the landfill poses to human health and the environment
without the final cover. Environmental agencies have compliance evaluations based on
risk factors to human health and the environment that are public information and may be
provided upon request. These evaluations, in addition to the US EPA 2018 Landfill
Capacity Study for Puerto Rico (see reference: US EPA, 2018), can be used as references
by applicants to support risk claims.
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7 Methodology and Supporting Documentation
This section includes modeling data and supporting documentation for the ET Cover
design and construction guidance presented in earlier sections.
7.1 Delineation of Ecozones for Puerto Rico
The modeling approach and literature-based data relevant to compacted clay covers and
physiological data for Puerto Rico (e.g., climate, topography, precipitation) were used to
generate a prototype ET Cover that is feasible in each Ecozone of Puerto Rico. The
Ecozones were delineated considering the physiography of the island and meteorological
data such as precipitation, PET, and the required amount of water to be stored (Sr) in the
ET Cover. Supporting documentation for how these Ecozones were delineated is provided
in Section 7.1.1.
For the purposes of this guidance, Puerto RJco has been divided into five ET Cover
Ecozones as presented in Figure 24:
• North Shore.
• Northern Foothills.
• Mountains.
• Southern Foothills.
• South Shore.
Atlantic Ocean
AA
North Shore Ecozone
Northern Foothills Ecozone
Mountains Ecozone
Caribbean Sea
\ \ \ Southern Foothills Ecozone
South Shore Ecozone
~ Landfill Sites
M
Figure 24. Delineated Ecozones for Puerto Rico.
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7.1.1 Physiography of Puerto Rico
Puerto Rico is an archipelago formed by the main island and 143 small islands, islets, and
cays located 18° 15" N, 66° 30" W in the Caribbean, between the Caribbean Sea and the
North Atlantic Ocean. The main island is almost rectangular in shape, approximately 100
miles long by 35 miles wide, and is the smallest and the most eastern island of the
Greater Antilles (Cuba, Hispaniola, Jamaica, and Puerto Rico). With an area of 3,425
square miles, Puerto Rico is the 82nd largest island in the world and the third largest island
in the United States.
Topography. More than half of Puerto Rico's land is mountainous, except in the regional
coasts, which are plains (25%). The remaining area is covered with hills (20%), plateaus
(1%), and water bodies (river and lakes, 1%). The central core of Puerto Rico is
mountainous, transitioning to hills or slopes, and then plains or coastal areas as shown in
Figure 25. Gould et al. (2008a) divided the terrain of Puerto Rico into plains, plateaus,
slopes, ridges and summits, and wetlands. This figure was key in identifying the five
distinct Ecozones used for this guidance. Landfills and open dumps are not found in any
wetlands and the plateaus are not on the main island, thus these two categories from
Gould et al. (2008a) are not applicable, leaving the plains, slopes, and ridges and
summits.
Figure 25. Physiography of Puerto Rico.
Source: Gould et al. (2008a).
Geology. According to Ewel and Whitmore (1973) and Bawiec (2001), the most
widespread areas in Puerto Rico are the moist and dry alluvials, limestone areas, moist
and wet volcanic areas, and moist and dry serpentine areas. The geologic layers of Puerto
Rico are diverse and include limestone, volcanic, and serpentine bedrock and alluvial,
colluvial, and marine quaternary deposits.
Climate. The climate of the island is affected by the topography of the land. Puerto Rico
has tropical marine weather with an annual mean temperature of 75°F (Daly et al., 2003).
The climate condition of Puerto Rico is also affected by several global scale climate
patterns such as El Nino-Southern Oscillation and the North Atlantic Oscillation. The El
Nino-Southern Oscillation (ENSO) has a mean average return interval of around four
65
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years for either a moderate or high intensity El Nino incident (Quinn et al., 1987). Despite
the low rate of occurrence (every decade) of the North Atlantic Oscillation, it has a large
effect on the higher mean precipitation rate of Puerto Rico (Malmgren et al., 1998; Hurell
et al., 2003).
In general, the east-west trending Cordillera Central and Sierra de Cayey mountains
divide the island of Puerto Rico into two climatologically distinct regions:
• The northern two-thirds of the island has a relatively humid climate.
• The southern one-third of the island is semiarid (USGS), n.d.
Temperature. Mean monthly air temperatures on the main island and the outlying
islands are consistent throughout the year. In coastal areas, annual air temperatures
range from a mean maximum of 80.6°F to a mean minimum of 75.2°F. In the interior
mountainous areas, annual air temperatures range from a mean maximum of 77°F to a
mean minimum of 71.6°F. Rainfall tends to vary from 27 in (697 mm) in the subtropical
southern coast to more than 157 in (4000 mm) in the subtropical wet rainforest.
Precipitation. Annual precipitation across the island varies due to the climatologically
distinct regions mentioned above. Historical annual precipitation was reviewed to identify
the maximum annual amounts of rainfall. An example of the annual rainfall is presented in
Figure 26 from 2011, which shows the arid and semiarid areas in red and dark red. In
general, higher rainfall is observed on the northern side of the mountains, and less rainfall
is typically observed on the southern side of the mountains. Heavy precipitation across
the region has increased since the 1950s, with the largest number of extreme
precipitation events (days with precipitation greater than three inches) occurring in the
past decade (Runkle et al., 2018). In the coming years, projections associated with
climate change anticipate that Puerto Rico will continue trends of increased precipitation
rates during its rainy season and decreased precipitation rates during its dry season
(Harmsen, et al, 2009). Such trends should be taken into consideration when designing an
ET Cover, or any type of landfill cover, and other water controls for final landfill closures.
66
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Annual Rainfall (Inches)
145-150
150-155
155 -160
160 -165
165 -170
170-175
| 25 - 30 55 -60
| 30 - 35 HI 60 " 65 I
| 35 - 40 ~~ 65 - 70 |
| 40 - 45 ~~ 70 - 75 |
| 45 - 50 ~~ 75 - 80 |
| 50 - 55 ]80 - 85
85-90
90 -95 |
|95 -100 |
| 100 • 105 |
| 105 -110 |
I 110-115 I
| 115-120 |
|120-125|
| 125 -130 |
|130 -135|
|135 - 140 |
I 140 -145 1
Figure 26. 30-year Normal Precipitation Distribution Maps for Puerto Rico, (A)
1971-2000 and (B) 1981-2010.
Figure note: The dots on the map indicate National Weather Service rainfall observation stations
and numbers indicate Normal precipitation in inches; the ellipsoid marks the location of the Gurabo
Agricultural Experimental Station rain gage. Source: USGS, 2011.
P/PET Ratio. Precipitation by itself is not a good index of the performance of an ET Cover
in a certain Ecozone. Precipitation (P) must be combined with PET (i.e., the P/PET ratio)
for a better index of the feasibility of ET Covers. PET data were sourced from the
67
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computer program PR-ET version 1.035 developed at the University of Puerto Rico in
Mayaguez. PR-ET program inputs include the longitude, latitude, and the elevation of the
area, which was obtained using Google Earth, around select sites. Table 11 presents the
22 sites across the island where the P/PET ratio was calculated. The P/PET ratios vary
from 0.52 in cities located in the South Shore to 1.36 in cities located in the Mountains.
Table 11 also shows good ET Cover potential for many regions of Puerto Rico (i.e., areas
where the P/PET ratio is less than one are shaded).
Table 11. P/PET Ratio in Different Areas of Puerto Rico.
City with Landfill
Precipitation (P)
Potential
Evapotranspiration
(PET)
P/PET Ratio
Salinas
913
1770
0.52
Santa Isabel
913
1767
0.52
Ponce
906
1664
0.54
Juana Diaz
1040
1730
0.60
Lajas
1133
1773
0.64
Cabo Rojo
1231
1758
0.70
Culebra
1273
1603
0.79
Arecibo
1376
1725
0.80
Penuelas
1453
1761
0.83
Arroyo
1487
1621
0.92
Florida
1545
1631
0.95
Guayama
1484
1542
0.96
Juncos
1751
1714
1.02
Isabella
1652
1601
1.03
Hormigueros
1671
1591
1.05
Fajardo
1588
1482
1.07
Cayey
1769
1645
1.08
Carolina
1982
1665
1.19
Moca
1946
1617
1.20
Jayuya
2001
1578
1.27
Barranquitas
1864
1373
1.36
Humacao
2160
1590
1.36
5 PR-ET version 1.03 was developed through a grant by the University of Puerto Rico Experimental
Station SP347 and USDA HATCH Project H402.
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Additional references reviewed to understand the physiography of Puerto Rico include:
Gould et al., 2008a to 2008g; Martinuzzi et al., 2007; Gould et al., 2006; Jogler, 2005;
Helmer, 2004; Grau et al., 2003; Chinea, 2002; Helmer et al., 2002; Franco et al., 1997;
Birdsey and Weaver, 1987; NOAA, 1982.
7.1.2 Required Amount of Water to be Stored
To design a proper ET Cover, it is necessary to determine the maximum amount of water
the soil must be able to retain before percolation occurs (Sr) (Albright et al., 2010). Sr is
a slightly better index than the P/PET ratio. Equation 2 shows that seasonality influences
the monthly precipitation, the ratio of evapotranspiration to PET ((3), and runoff (A).
sr = Zm=i((Fm - PHMPETm) - Ahm} + T,m=i{(pm ~ PcPETm) - Ac) (Equation 2)
May-October November-April
(Hot/Muggy Season) (Cool Season)
Where:
Sr = required amount of water that needs to be stored by the soil
Pm = monthly precipitation
PETm = monthly PET
Phm = the ET/PET ratio in hot and muggy season
(3c = the ET/PET ration in cool season
Ahm = runoff and other losses in the hot and muggy season, assumed to be 0 mm as a
conservative measure
Ac = runoff and other losses in the cool season, assumed 0 mm as a conservative
measure (Albright et al. [2010] uses 184 mm)
The climate of Puerto Rico is tropical and relatively warm year-round, with a hot and
muggy season from May to October and a relatively cool season from December to March,
with November and April as intermediate months. Therefore, instead of fall-winter and
spring-summer months, the seasonality incorporated into modeling for this guidance are
the Hot and Muggy Season and Cool Season.
• Hot and Muggy Season: May to October.
• Cool Season: November to April.
Values for the (3 (ET/PET ratio) and A (runoff and other losses) parameters are provided
for the Puerto Rico seasons in Table 12 (as modified from Albright et al., 2010). The A
parameter, which subtracts runoff and other losses from the amount of water that needs
to be stored for climates without snow or frozen ground, was reported by Albright et al.
(2010) to vary from 27 mm during fall-winter to 168 mm during spring-summer.
However, for this modeling, the A parameter is modeled as zero to serve as a
conservative measure for the prototype ET Cover design.
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Table 12. Modified P/PET Threshold, ET/PET Ratio (P), and Runoff and Other
Losses (A) by Climate Type and Season.
Climate Type
Season
P/PET
Threshold
(unitless)
ET/PET Ratio, p
(unitless)
Runoff and Other
Losses, A
(mm)
No Snow or
Frozen Ground
Cool Season1
(November to April)
P/PET > 0.65
0.65
0 (Conservative)
Hot and Muggy
Season
(May to October)
P/PET > 0.97
1.00
Source: Albright et al., 2010.
^he cool season P/PET threshold is the average of 0.34 and 0.97. The A parameter is modeled as
zero as a conservative modeling measure for the prototype ET Cover design.
Table 13 presents an example calculation for Sr at one selected ET Cover in the city of
Arecibo (North Shore Ecozone). When calculating Sr, only months where the P/PET
Threshold (shown in Table 12) is exceeded are summed in the calculation (Albright et al.,
2010). Percolation occurs when the P/PET ratio exceeds the P/PET Threshold (Albright et
al., 2010). Months where the P/PET ratio exceeds the P/PET Threshold are presented in
shaded rows in Table 13.
Table 13. Example Calculation of Sr for Arecibo Puerto Rico.
Month
Precipitation,
P
(mm)
Potential
Evapotrans-
piration, PET
(mm)
P/PET
Ratio
P/PET
Threshold1
Need to
Store
Water?
Sr, Monthly
Soil Water
Storage
(mm)
January
111
D.72
D.74
.65
Yes
8
February
115
.65
Yes
10
March
90
149
0.60
0.65
No
-
April
120
155
0.78
0.65
Yes
20
May
148
166
0.89
0.97
No
-
June
83
165
0.50
0.97
No
-
July
86
172
0.50
0.97
No
-
August
118
166
0.71
0.97
No
-
September
139
153
0.91
0.97
No
-
October
L54
L41
L.09
0
.97
Yes
13
November
L57
L20
L.31
0
.65
Yes
79
December
L16
L13
L.03
0
.65
Yes
42
Total
1,376
1,725
0.80
--
--
172
(Annual)
1 When calculating Sr, only months where the P/PET threshold (shown in Table 12) is exceeded
are summed in the calculation (Albright et al., 2010).
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Table 14 presents the annual Sr used in the prototype ET Cover design for each Puerto
Rico Ecozone. Two locations in each Ecozone were modeled and the final Sr for each
Ecozone is presented as the average of the two locations.
• ET Covers are a feasible alternative in the North Shore and South Shore Ecozones.
• The Sr for the Southern Foothills and Northern Foothills Ecozones averaged from
307 to 387 mm/yr, which indicates that ET Covers can be effectively used.
• The Sr for the Mountains Ecozone averaged 502 mm/yr. In this Ecozone, ET Covers
are more challenging to implement but could be considered on a case-by-case
basis.
Table 14. Average Annual Sr (mm/yr) for the Five Puerto Rico ET Cover Ecozones.
ET Cover Ecozone
Sr 1
(mm/yr)
Sr 2
(mm/yr)
Average Sr
(mm/yr)
Recommended Soil
Cover Design Thickness
(mm)
North Shore
173
172
173
900
Northern Foothills
485
288
387
1000
Mountains
465
538
502
1500 (with grasses +
shrubs)
1200 (with grasses +
trees)
Southern Foothills
275
339
307
1200
South Shore
95
38
67
600
7.2 Soils of Puerto Rico
Different classification schemes exist for the soils of Puerto Rico: the U.S. Natural
Resources Conservation Service's (NRCS, an agency of USDA) Soil Taxonomy, the official
system of soil classification for the United States National Cooperative Soil Survey, and
that of soil scientists at the University of Puerto Rico (Munoz et al., 2018). The soil orders
and different classification schemes are described below. These two soil classification
schemes were used in conjunction with the USDA Textural Soil Classification to determine
the appropriate soil types to use in developing this guidance and the prototype ET Cover
design.
The NRCS (USDA NRCS, 2015) has recognized 12 soil orders, 10 of which are found in
Puerto Rico. Figure 27 shows the following soil orders found in Puerto Rico: Alfisols,
Aridisols, Entisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, and
Vertisols. Andisols are not present in Puerto Rico; although largely of volcanic origin, the
island lacks the recent volcanic materials that are a prerequisite for Andisol formation. The
tropical climate precludes the formation of Gelisols, soils that must have permafrost.
71
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00.79 5
V i':-
Map Created by: Manuel Matos. State Soil Scientist,
USOA-NRCS Caribbean Area. 2018
Legend
Miscellaneous
I Alfisols
Mollisols
Aridisols
| Oxisols
| Entisols
Spodosols
Histosols
| Ultisols
| Inceptisols
Vertisols
Soil Orders of Puerto Rico
Figure 27. Soils of Puerto Rico.
Source: Munoz et al., 2018.
The locations of these soils are briefly described below:
• Inceotisols cover the largest part of Puerto Rico since they occur in wide variety of
climates from semiarid to humid environments that exhibit medium rates of soil
weathering and development.
• The most humid areas of the island are covered with Ultisols. These soils are
formed from intense weathering and leaching processes that resulted in clay
enriched minerals such as kaolite and quartz. Most of their nutrients are within the
upper few centimeters.
• Mollisols are common in the north and south part of the island where grassland is
found. This soil has a dark color surface due to the high content of organic matter
and is good for agriculture.
• Aridisols are common in the southwestern area of Puerto Rico, which is an arid
region.
• Oxisols, the most weathered soil in the tropical and subtropical regions, are also
common in the central part of the island. These soils are basically known by their
composition of kaolinite, quartz, and iron oxide, which are low activity minerals.
72
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•
part of the island. When they are drained or exposed to air, the microbial
decomposition is accelerated and can cause the soil to subside due to the high
content of organic matter in the soils.
• In the southern part of the island where there are plains and plateaus, Vertisols are
common. These possess a high content of expanding clay minerals thar can
undergo changes in volume with changes in moisture. They have cracks that open
and close periodically. Since they swell when wet, these soils can transmit water at
a slower pace and leach very little.
general soil types for Puerto Rico: humid coastal plains, semiarid coastal plains, humid
uplands, semiarid uplands, and humid upland valleys.
Munoz et al., 2018 classify the island's soils into coastal lowlands, alluvium, coastal plains,
alluvium in terraces, upland dark, and upland reddish-purple.
This soil classification information for Puerto Rico was matched with corresponding soil
textural groups under the USDA Textural Soil Classification, which is used in this guidance
for the prototype ET Cover designs. The soils of Puerto Rico are classified into five textural
groups listed below:
• Clay Loam.
• Silty Clay Loam.
• Silty Clay.
• Loam.
• Sandy Clay Loam.
7.3 Minimum Plant-Available Water Storage Capacity of Local Soils
The ability of a soil to retain water is defined as the unit soil water storage capacity
(referred to as the water storage capacity herein). Key terms in calculating this parameter
are defined below and presented in Figure 28.
• The water storage capacity for a specific soil is the difference between the field
capacity and wilting point of the planted vegetation.
• Field capacity is the amount of soil moisture or water content held in soil after
excess water has drained away by gravity and is defined as the water content at a
suction of 330 cm of water.
• The wilting point is the minimum point of soil moisture that plant requires to not
wilt and relates to the water content at a suction of 15,000 cm of water. The wilting
point occurs when the water content in the root zone drops (i.e., when soil water
suction increases) and the plant pulls too hard to extract water from the soil, which
collapses the conduits for flow (stem and roots), causing the plant to wilt.
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As shown in Figure 28, the water storage capacity is the portion of water that can be
stored by the soil and is available to plant. It is the amount of water available, stored, or
released between field capacity and the wilting point water contents. The soil types with
higher total available water content are generally more conducive to high biomass
productivity because they can supply adequate moisture to plants during times of low
rainfall.
LOAM LOAM LOAM CLAY
Figure 28. Schematic Showing the Relationship Between Water Storage Capacity
and USDA Textural Soil Classification.
Source: Modified from Schroeder et a I., 1994.
Sandy soils are more prone to drought and will quickly (within a few days) be depleted of
their available water when evapotranspiration rates are high. A plant growing on fine sand
has most of its roots in the top foot of soil where there is typically less than one inch of
readily available water; this plant will need consistent water replenishment to avoid
reaching its wilting point. It is important during ET Cover modeling to understand the
characteristics of available soil types because water storage capacity is critical to plant
survival.
74
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The water storage capacity by soil type is presented in Table 15.
Table 15. Soil Water Unit Storage Capacity of Soils of Puerto Rico.
Soil Type
Field Capacity1
(cm3 water/cm3 soil)
Wilting Point2
(cm3 water/cm3 soil)
Water Storage
Capacity
(cm3 water/cm3 soil)
Loam
0.3775
0.1497
0.2278
Silty Clay Loam
0.4646
0.1942
0.2704
Silty Clay
0.4496
0.2433
0.2063
Clay Loam
0.4079
0.1765
0.2314
Sandy Clay Loam
0.3770
0.1684
0.2086
Soil Average
0.2289
1 The field capacity at suction of 33 cm of water.
2 The wilting point at suction of 1500 cm of water.
Table 16 presents the Sr, available soil classes, average water storage capacity, and the
preliminary soil cover design thickness using the different soils available in Puerto Rico for
each Ecozone. To estimate the preliminary soil cover design thickness, the Sr is divided by
the water storage capacity of the soil. The thicknesses shown are theoretical thicknesses
for ET Covers that were assumed to have no percolation.
Table 16 shows that ET Covers planted with local grasses and shrubs are feasible in all
but one Ecozone, Mountains- where the recommended plantings should consist of local
trees and grasses.
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Table 16. Average Water Storage Capacity, Soil Cover Design Thickness, and
Feasibility of ET Covers by Ecozone.
Ecozone
Water
To be
Stored
(Sr)
USDA Textural Soil
Classification1
Avg. Water
Storage
Capacity
(cm3 water/
cm3 soil)2
Preliminary
Soil Cover
Design
Thickness
(mm)
ET Cover
Feasibility 2
North
Shore
173
Loam
Silty Clay Loam
Silty Clay
Clay Loam
Sandy Clay Loam
0.2289
900
Very feasible
Northern
Foothills
387
Loam
Silty Clay Loam
Silty Clay
Clay Loam
Sandy Clay Loam
0.2289
1000
Feasible
Mountains
502
Loam
Silty Clay Loam
Silty Clay
Clay Loam
Sandy Clay Loam
0.2289
1500
Feasible with
trees and
grasses only
Challenging
with shrubs and
grasses
Southern
Foothills
307
Loam
Silty Clay Loam
Silty Clay
Clay Loam
Sandy Clay Loam
0.2289
1200
Feasible
South
Shore
67
Loam
Silty Clay Loam
Silty Clay
Clay Loam
Sandy Clay Loam
0.2289
600
Very feasible
(minimum
thickness)
1 All USDA Soil Types of Loam, Silty Clay Loam, Silty Clay, Clay Loam, and Sandy Clay Loam are
found in every Ecozone of Puerto Rico.
2 All Ecozones were modeled with local grasses and shrubs. A second scenario was modeled for the
Mountains Ecozones to achieve ET Cover feasibility with local trees and shrubs.
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7.4 Water Balance Modeling of Covers in Selected Areas
of Puerto Rico
The Richards equation is required to conduct effective water balance modeling that
simulates the mechanisms that most greatly affect cover function (Albright et al., 2010;
ITRC, 2003). For this guidance, the objective of modeling was to estimate the percolation
of rainwater from the atmosphere into the waste mass at representative landfills in each
Ecozone for both a hypothetical compacted clay cover and an ET Cover consisting of local
soils and local vegetation. Water balance modeling incorporated the scenario details in
Section 7.4.2 and the criteria in Table 17. Section 7.4.3 presents the soil unsaturated
properties and Section 7.4.4 presents modeling details for PET, potential evaporation (PE),
and potential transpiration (PT).
Table 17. Water Balance Criteria for Compacted Clay and ET Covers in Puerto
Rico.
Cover Type
Cover Thickness
Erosion Control
Layer Thickness
Cover
Vegetation
Soil Type
Compacted clay1
450 mm
150 mm
Local grasses
All
ET2
NA
300 mm
Locally
appropriate
vegetation:
grasses, shrubs,
and trees in any
combination
1 The compacted clay cover is overlaid with an erosion control layer for a total cover thickness of
600-mm.
2 The erosion control layer of the ET Cover is underlaid by a rooting zone.
7.4.1 HYDRUS-1D Overview
This section provides information about the specific model used and the modeled
scenarios.
Computer models can determine the expected performance of ET Covers. Suitable models
account for the movement of water within the soil profile and define the appropriate upper
and lower boundaries for the cover. The movement of water through the soil profile
should be simulated in a detailed manner and should include the amount of water lost due
to uptake by the roots. To achieve this, the model should be based on a solution to the
Richards equation for unsaturated water flow (Richards, 1931). The surface boundary
conditions should simulate the interactions at the soil-atmosphere interface (i.e.,
precipitation, infiltration, evaporation, runoff) and should be driven by user-provided
climatic inputs. The lower boundary should account for the interactions that may occur
between the cover and the waste. There are several flow models available that meet these
criteria that can be used for ET Cover modeling.
Developed as a collaborative effort between the U.S. Salinity Laboratory and the
University of California at Riverside, HYDRUS-1D is computationally efficient, well-
supported, continually updated, and available free of charge. A graphical user interface
can be used for data input and to view simulation results. The code has been used to
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solve a wide variety of problems, such as water balance modeling, recharge estimation,
engineered cover performance, nitrate and pesticide leaching, and chlorinated
hydrocarbon transport. For these reasons, in addition to the modeler's experience,
HYDRUS-1D was chosen to model ET Covers for this guidance.
HYDRUS-1D is one of the most widely used models for unsaturated flow and solute
transport modeling. HYDRUS-1D is a finite element model that solves the Richards
equation (Equation 3) for unsaturated flow. The model has options for non-isothermal
liquid and vapor flow and heat transport. Constitutive relationships include the van
Genuchten and Brooks-Corey water retention functions. Information on soil texture can be
used with pedotransfer functions to determine water retention and hydraulic conductivity
parameters.
Richards Equation:
<90 _ dK d
dt dz dz
Where:
0 = water content of the soil
t = time
K = unsaturated hydraulic conductivity
Ksat = saturated hydraulic conductivity (this guidance uses Ksat instead)
z = elevation above a selected depth
V = soil water suction
S = sink term for root water uptake
The solution to the Richards equation is 0, as a function of space (z) and time (t). K and
are a function of 0. The HYRDUS-1D model uses the surface water characteristic curve
(field capacity, wilting point, plant-available water), K, and other factors to compute the
following water balance quantities: R, runoff; E, evaporation; P, precipitation; Pr,
percolation; and T, transpiration as shown in Figure 29, where the dots along the "z" line
represent discrete iterations of the model, and the curved, dotted line is the solution of
the equation, where the dots represent solutions for each discrete iteration that was
modeled.
Ct
Ksat
-s
(Equation 3)
78
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Finer-Grained Soil
Clean Coarse Soil
*0
Solution
is 0{z, t)
Figure 29. Water Balance Parameters Solved in the Richards Equation.
7.4.2 Design Year and Initial Conditions
In the simulations that were modeled, a meteorological boundary condition was used as
the upper boundary to simulate the interaction between the cover soil and the
atmosphere. Standard constant pressure and constant flux conditions, in addition to
meteorological forcing, were used in the upper boundary condition, Options for the lower
boundary condition include the unit gradient and seepage face. In this guidance, the unit
gradient was used for the lower boundary. All modeled simulations presented herein use
average yearly climatic data.
Figure 30. Rainfall at a Landfill Located in Karst.
Source: US EPA Region 2.
79
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The solution of the Richards equation (Richards, 1931) requires the specification of initial
condition 0 (z, t=0), where 0 is the initial water content. The initial water content of the
entire soil and waste profile layers was assumed to be 0.30 cm3 water/cm3 soil. This is a
somewhat arbitrary set of values that may influence the model output. Thus, to eliminate
the effect of the initial conditions on the modeling results, each simulation was conducted
for the five consecutive years following the design year. Only Year 5 results are reported
as the long-term performance of each cover design. This means that the simulations
reached steady state as early as the second year of all the modeled locations. The results
of the model simulations are for Year 5 and include the water balance results used to
assess ET Cover performance compared to compacted clay covers. Year 5 results are
assumed as the steady state or long-term behavior of the modeled covers.
7.4.3 Unsaturated Soil Properties
Required soil-related model input parameters for HYDRUS-1D include those in the
Richards equation: saturated hydraulic conductivity (Ksat), residual water content (0r),
saturated water content (0s) (equivalent to soil porosity); and a series of parameters
(a, m, n, I) used in the van Genuchten and Mualem functions that describe the
functional relationship between soil moisture, matric potential, and unsaturated
conductivity. Each of these is an empirical constant; a is inversely related to the air-entry
pressure value, m and n are related to the pore-size distribution, and I is a pore
interaction term that describes connectivity. In HYDRUS-1D, unsaturated hydraulic
functions are based on a combination of the van Genuchten (1980) function with the
Mualem (1976) pore-size distribution model. Some research on the conductivity
parameter (I) suggests that a value of I = - 2, recommended by Burdine (1953), fits soil
data reasonably well and is conservative for storage cover design (Albright et al., 2010).
Soil data for Puerto Rico were obtained from the Soil Survey Geographic (SSURGO)
Database. The SSURGO database is linked to a National Soil Information System
(NASIS) relational database, which provides the proportionate extent of component
soils and properties for polygonal units known as soil map units. Each map unit consists
of one to three soil components identified by the taxonomic classification listed in
SSURGO. Soil-related model input parameters were estimated for the USDA Textural Soil
Classification in Table 18 using the Rosetta program built into HYDRUS-1D (Schaap et
al., 2001). Rosetta employs hierarchical pedotransfer functions to obtain unsaturated
hydraulic conductivity parameter inputs using either soil particle-size distribution and bulk
density or soil textural class alone.
Rosetta implements pedotransfer functions to predict van Genuchten (1980) water
retention parameters and Ksat by using textural class, textural distribution, bulk density,
and one or two water retention points as inputs. Although the use of more input data
often leads to better predictions (Schaap and Bouten, 1996; Schaap et al., 1998), there
are many cases where only limited soil information is available. Rosetta follows a
hierarchical approach to estimate water retention and Ksat values using limited or more
extended sets of input data (Schaap et al., 1998; Schaap and Leij, 1998a).
80
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Using Rosetta for other climate zones and other pedogenic processes could lead to
inaccurate predictions. The reader is referred to Schaap et al. (1998) and Schaap and Leij
(1998a,b) for more information about the calibration of the pedotransfer functions in
Rosetta.
Table 18. Unsaturated Soil Properties for Soils Used in ET Cover Simulations With
Grasses and Shrubs for All Ecozones (Rosetta Estimated Soil Parameters).
Available Soil
Saturated
Hydraulic
Conductivity,
K (cm/sec)
Residual
Water Content
Or)
Saturated
Water
Content(6s)
Alpha, a
(1/mm)
n
Erosion Control Layer
(Sandy Loam 300 mm)
1.23 x 10"3
0.065
0.410
0.0075
1.89
Loam
1.39 x 10"4
0.0609
0.3991
0.00111
1.47
Silty Clay Loam
1.29 x 10"4
0.0901
0.482
0.00084
1.52
Silty Clay
1.11 x 10"4
0.1108
0.4808
0.00162
1.32
Clay Loam
9.47 x 10"5
0.0792
0.4418
0.00158
1.41
Sandy Clay Loam
1.53 x 10"4
0.0633
0.3837
0.00211
1.33
Compacted Clay
(weathered)
5 x 10"5
0.070
0.38
0.001
1.20
7.4.4 PET, PE, and PT for Puerto Rico
HYDRUS-1D requires separate values for potential transpiration (PT) and potential
evaporation (PE) for the atmospheric boundary conditions. The model calculates values of
transpiration and evaporation based on the availability of water in the soil profile. LAI
estimates are used to partition PET into PT and PE using Equations 4 and 5.
For most of the modeling in this guidance, only the maximum seasonal LAI was input
since daily LAI data were unavailable. The LAI will vary between the hot and muggy and
cool seasons, where maximum LAI and leaf duration are affected by environmental
stressors such as temperature and lack of water or nutrients. However, the effects of such
stressors were not considered in the reported values of LAI used in the simulations during
our study. It is important to note that given Puerto Rico's climate, the island's growing
season is year-round, with transpiration only slowing for three months of the year
(beginning of December through the beginning of March). Models for this time of year
assume that moisture removal from the soil profile occurs through evaporation to a
slightly greater extent.
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Local grasses were selected as the option for vegetating compacted clay covers in this
analysis. LAI estimates were used to partition PET into PT and PE using Equation 4, known
as the Ritchie-Burnett-Ankeny function, and Equation 5.
PT = 0.52 x PET x Vla!
PE = PET - PT
(Equation 4)
(Equation 5)
Where:
PT = potential transpiration
PE = potential evaporation
PET= potential evapotranspiration
LAI = leaf area index, a dimensionless quantity that defines the amount of area of leaf
to area of the ground. A higher LAI typically increases evapotranspiration
LAI estimates for grass were obtained by the method described by Allen et al. (1996) as
presented in Equation 6.
LAI(max) = the maximum leaf area index during the growing season.
The 0.5 multiplier in Equation 6 is supported by field data presented in Allen et al. (1996).
Allen et al. (1998) approximated LAI (max) as 24 times the crop height in meters. For
grasses, we assumed an average height of 0.15 m. A grass cover of 0.15 m height
corresponds to an LAI (max) of 3.6 and Average LAI of 1.8. "Grass only" vegetation was
used only for simulations of compacted clay covers. For ET Cover simulations, a different
simulation scenario was modeled: grass and shrubs. For grass and shrub vegetation
(during ET Cover simulations), we assumed a height of 0.25 meters to obtain an LAI(max)
of 6, leading to Average LAI of 3.02. The root density function was considered to vary
linearly with depth (from fully rooted at the top of the soil profile to zero roots at the
bottom of the cover). It was assumed that this root distribution is reasonable for modeling
purposes.
Once PE and PT values are determined, the model then calculates values for transpiration
and evaporation based on the availability of water in the soil profile. HYDRUS-1D
calculates evaporation and transpiration from the PET using root water uptake models.
Two modeling functions are available: that of Feddes et al. (1978) and an S-shaped
function (van Genuchten, 1985) model (Figure 31).
In the approach of Feddes et al. (1978), water uptake is assumed to be zero when the soil
is close to saturation (i.e., wetter than some arbitrary point, hi). Root water uptake is
also zero for pressure heads less than the wilting point (h4). The optimal pressure heads
for water uptake are between pressure heads h2 and h3, whereas for pressure heads
between h3 and h4 or hi and h2, water uptake decreases (or increases) linearly with
pressure head.
Average LAI = 0.5 X LAI(max), for grass (Equation 6)
Where:
82
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o
u
1.2
1.0
0.8
0.6
§ 0.4
"" 0.2
3! 0.0
O
Q.
V)
a)
as
(a) /
!
1.2
l.o
0.8
0.6
0.4
0.2
0.0
h4 h3 h2 hj 0
Soil Water Pressure Head, h
: (b)
- h50
2.5 2.0 1.5 1.0 0.5
Reduced Pressure Head, h/h50
o.o
Figure 31. Graphs of the Plant Water Stress Response Functions, a, as Used by a)
Feddes et al. (1978) and b) van Genuchten (1985).
Water uptake parameters used for local grasses were sourced from Feddes et al. (1978)
and are presented in Table 19.
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Table 19. Water Uptake Parameters for Feddes et al. (1978) Model.
Symbol
Definition
hi
Value of the soil water pressure head below which roots start to extract water from
the soil.
h2
Value of the soil water pressure head below which roots extract water at the
maximum possible rate.
h3H
Value of the limiting soil water pressure head below which roots cannot longer
extract water at the maximum rate assuming a potential transpiration rate of r2H.
h3L
Value of the limiting soil water pressure head below which roots cannot longer
extract water at the maximum rate assuming a potential transpiration (PT) rate of
r2L.
h4
Soil water pressure head below which root water uptake ceases (taken at the wilting
point).
r2H
Potential transpiration (PT) rate (LT 1).
r2L
Potential transpiration (PT) rate (LT1).
PT rates (LT"1) V2I and V2H permit one to make the variable h3 a function of the PT rate.
These values simply define the shape of the transpiration curve and are not related to the
PT values supplied to the model.
Alternatively, a simplified S-shaped function (van Genuchten, 1985) can also be
implemented to calculate actual transpiration. The parameters required for the S-shaped
function are p and h5o as presented in Equation 7.
a(Jl) — — ~p— (Equation 7)
1+(a/Aso)
Where:
a (7?) = dimensionless water stress response function
pand /?5o = parameters for the water stress function
Water uptake parameters for the S-shaped van Genuchten function are presented in
Table 20.
Table 20. Water Uptake Parameters for the S-Shaped van Genuchten Function.
Symbol
Definition
P
The exponent, p, in the root water uptake response function associated with water
stress (dimensionless).
h 50
The coefficient, hso, in the root water uptake response function associated with water
stress (L). Root uptake at this pressure head is reduced by 50%.
The calculated values of PT are then distributed in the root zone according to a function
defining the density of roots with depth. Albright et al. (2010) suggests the assumption of
a well-developed plant community with exponentially decreasing root density with depth.
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The root density distribution is assumed to vary linearly from fully rooted at the top of
each cover to no roots the bottom of the cover.
7.5 Equivalency Criteria: Percolation Through Compacted Clay Covers
in Puerto Rico
In 40 CFR § 258.60, an alternative cover is hydrologically equivalent to a prescribed
conventional cover if the percolation rate for the alternative cover is less than or equal to
the percolation rate for the prescribed cover (see 40 CFR, Subtitle D, Subpart F, §
258.60). A direct comparison of percolation rates is possible only at locations where side-
by-side testing of ET Covers and conventional covers was conducted. For other sites, the
use of a literature-based values for the field performance of compacted clay covers may
be permissible. This section includes a summary of the percolation rates measured on
compacted clay cover test sections constructed in humid climates closest to that of Puerto
Rico.
7.5.1 Previous Studies on Field Performance of Compacted Clay Covers
in Humid Sites
Montgomery and Parsons (1989) tested two clay covers near Milwaukee, Wisconsin, which
differed only in surface layer thickness. Both covers included a 1,220-mm-thick clay
barrier layer (CL in the Unified Soil Classification System, or USCS) and were overlaid with
an uncompacted surface layer. The surface layer of one cover was 152 mm thick, whereas
the other was 457 mm thick. Precipitation ranged from 578 mm to 896 mm during the
monitoring period. Over four years of monitoring, percolation from both covers increased
from 2 to 7 mm/yr (1% of precipitation) to more than 50 mm/yr (7% of precipitation) due
to weathering and a resulting blocky structure in the upper 250 mm of clay in test
sections. Montgomery and Parsons (1989) concluded that cracks in the clay barrier layers
were responsible for the increase in percolation rates, the cracks persisted regardless of
soil water status of the clay layer, and the thickness of the surface layer did not affect
percolation rate.
Melchior (1997) reported on eight years of field data from two test sections in Hamburg,
Germany, which simulated compacted clay covers. Annual precipitation at the site ranged
between 740 mm and 1,032 mm. Percolation started to increase 20 months after
construction, following a dry summer, and increased during the experiment from 10
mm/yr the first year (1% of precipitation) to as much as 174 mm/yr (17% of
precipitation) for the thicker cover and 202 mm/yr (26% of precipitation) for the thinner
cover. Excavation of the soil barrier layers eight years after construction revealed an
extensive network of cracks, some several millimeters in width, attributed to desiccation.
The National Council for Air and Stream Improvement (NCASI, 1997) field tested two
replicates of a soil barrier cover near Kalamazoo, Michigan, consisting of a 610 mm
surface layer over 610 mm of compacted clay (annual precipitation during the monitoring
period ranged between 795 mm/yr and 1,109 mm/yr). The measured percolation rates
through the compacted clay test sections were 16 mm/yr and 26 mm/yr (1.8% and 3.0%
of precipitation) during the first year but increased to as much as 70 mm and 56 mm/yr
(6.3% and 5.1% of precipitation) during nearly 7.5 years of observation. A dye tracer test
followed by excavation of both covers eight years after construction showed
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interconnected vertical preferential flow paths and horizontal interlift flow paths (Benson
and Wang, 1998).
Albright et al. (2006) described a study conducted in southern Georgia, USA, to evaluate
how the hydraulic properties of the compacted clay barrier layer in a final landfill cover
changed over a 4-year service life. The cover was part of a test section constructed in a
large drainage lysimeter that allowed continuous monitoring of the water balance.
Patterns in the percolation rate (i.e., flow through the bottom of the cover) record
suggested that preferential flow paths developed in the clay barrier soon after
construction, apparently in response to desiccation cracking. After four years, the clay
barrier was excavated and examined for changes in soil structure and hydraulic
conductivity. The in situ and laboratory tests indicated that the hydraulic conductivity
increased by approximately three orders of magnitude (from 10"7 to 10"4 cm/sec during
the service life of the test section). A dye tracer test and soil structure analysis showed
that extensive cracking and root development occurred throughout the entire depth of the
barrier layer. The findings also indicate that clay barriers undergo desiccation and root
intrusion even at sites in warm, humid locations.
Over the monitoring period, the percolation rate through the compacted clay cover was
608 mm/year (25.6% of the applied water, precipitation, and irrigation). The amount of
percolation, relative to precipitation, increased following one drought event, and at the
same time, there was a change in the temporal response of drainage (i.e., into the waste
layer) per individual precipitation event. Following the drought, percolation was often
observed within one hour of precipitation events and exhibited a "stair-step" pattern
indicative of preferential flow, whereas drainage was relatively equally distributed and
occurred at a steady rate through the soil barrier prior to the drought. During the period
prior to the drought (April 19, 2000, to September 23, 2000) percolation was 42.1 mm
(8.7% of applied water), which is equivalent to an annual rate of 102 mm/yr. During the
same period the following year, after the drought, percolation was 2.6 times higher (111
mm). The impact of desiccation on the percolation rate for the conventional cover is
dramatic. Prior to the dry period the percolation rate was approximately 100 mm/yr. After
the dry period, the percolation rate jumped to approximately 480 mm/yr. During the
entire monitoring period following the drought (628 days), 564 mm of percolation (30.3%
of applied water) was transmitted, which corresponds to an average annual drainage rate
of 327 mm/yr.
7.5.2 Compacted Clay Cover Performance Under Puerto Rico
Conditions
The preceding literature review indicates that the performance of compacted clay covers
decreases after construction due to the deterioration of the barrier layer. That is, the
percolation rate through the compacted clay cover increases by orders of magnitude from
initial performance and is also related to local climatic conditions. The long-term
percolation rate through compacted clay cover should be used when evaluating the
hydraulic equivalency of an ET Cover design. In the absence of a specific percolation value
(i.e., performance criteria) for a compacted clay cover in Puerto Rico, a modeling
approach (via HYDRUS, described earlier in the document) was used to develop the long-
term percolation rates in mm/yr as an index of the performance of compacted clay covers
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under climatic conditions in Puerto Rico. We hypothesized that the percolation through
these compacted clay cover simulations would yield the percolation rate necessary for the
equivalency assessment of any ET Cover design.
A total of 24 locations of actual landfills or cities near actual landfills were simulated. The
compacted clay covers were modeled to be a 450 mm-thick barrier of compacted clay
(hydraulic conductivity of 5 x 10"5 cm/sec) overlaid with a 150 mm erosion control layer.
The erosion control layer was modeled to be vegetated with local grasses. Table 21
shows the unsaturated hydraulic properties used for all the simulated compacted clay
covers at the different locations throughout Puerto Rico.
Table 21. Unsaturated Soil Properties of Compacted Clay Covers for All Ecozones.
Cover Type and
Thickness
Ksat
(cm/Sec)
Saturated
Water
Content
Residual
Water
Content
Alpha
(1/mm)
n
Vegetation
Compacted Clay Cover
(450 mm)
5 x 10"5*
0.38
0.07
0.001
1.20
Not
Applicable
Erosion control layer
(Sandy Loam,
150 mm)
1.2 x 10"3
0.41
0.07
0.001
1.89
Grasses
^Compacted clay layer assumed to go through wet-dry cycling (mm/yr).
Table 22 shows the percolation rate in mm per year for the hypothetical compacted clay
covers (consisting of 150 mm of erosion control layer and 450 mm of compacted clay
barrier). These percolation rates represent the baseline percolation rates that were used
to compare the performance of ET Covers to that of compacted clay covers (equivalency
criteria).
The percolation rate through a compacted clay cover varied from 169 mm/yr to
223 mm/yr and averaged 189 mm/yr in the North Shore climate, which represents 11%
to 13% of percolation.
In the Northern Foothills climate, the percolation rates associated with compacted clay
covers varied from 196 mm/yr to 210 mm/yr and averaged 203 mm/yr, which
corresponds to 13% of precipitation. In these two climates, the compacted clay covers
yield similar percolation rates.
For the South Shore Ecozone, the percolation rate through the simulated compacted clay
cover varied from 6 mm/yr to 83 mm/yr and averaged 47 mm/yr. These percolation rates
corresponded to 1% to 8% of precipitation for this Ecozone.
For the Southern Foothills Ecozone, the percolation rates varied from 223 mm/yr to
638 mm/yr and averaged 399 mm/yr. These percolation rates correspond to 16% to 32%
of precipitation.
For the Mountains Ecozone, the percolation rates were much higher and ranged from 225
to 686 mm/yr and averaged 455 mm/yr. These percolation rates represent 17% to 35%
of precipitation. For the islands of Culebra and Vieques, the percolation rates varied from
0.2% to 5% of precipitation. Vieques and Culebra are considered part of the South Shore
for the purpose of this ET Cover guidance document.
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Table 22. Summary of Compacted Clay Cover Simulations.
Climate
Ecozone
Precip.
(mm/yr)
PET
(mm/yr)
Percolation Though Clay Cover1
Site
(mm/yr)
% of
Precip.
Average
(mm/yr)
Isabela
1652
1654
174
11
North Shore
Toa Baja
1725
1651
223
13
189
Vega Baja
1586
1667
169
11
Northern
Florida
1542
1632
196
13
203
Foothills
Fajardo
1585
1615
210
13
Moca
1945
1611
560
29
Mountains
Jayuya
1981
1454
686
35
455
Juncos
1751
1544
350
20
Ba rranquitas
1335
1392
225
17
Cayey
1437
1452
223
16
Guayama
1484
1542
242
16
Southern
Foothills
Hormigueros
1671
1591
329
20
Humacao
2053
1524
638
31
377
Mayagiiez
1931
1611
616
32
Cabo Rojo
1231
1758
82
7
Yabucoa
2018
1605
508
25
Juana Diaz
1039
1761
83
8
Lajas
1145
1745
66
6
Ponce
683
1760
6
1
73
South Shore
Santa Isabel
915
1771
31
3
Yauco
914
1760
15
2
Penuelas
1451
1761
237
16
Culebra
1273
1603
64
5
34
^he thickness of the compacted clay cover is 600 mm.
7.6 Preliminary Design Criteria for ET Covers in Puerto Rico
The major objective of vegetated soil covers is to minimize percolation into the underlying
waste mass. The preliminary design to meet this objective was conducted to calculate the
P/PET ratio and the amount of water that needs to be stored (Sr) for each Ecozone using
the following steps, as described by Albright et al. (2010):
1. Use climatic data to determine the required amount of water to be stored by soil
cover design (Sr).
2. Estimate the in-field water storage capacity of the soils available at the location in
which the cover is proposed.
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3. Estimate a preliminary thickness of the cover capable of storing the required
amount of water that was determined in step 1 by using a water balance model
(conducted using HYDRUS-1D).
4. Choose vegetation capable of removing all the water stored in the soil and model
the unsaturated flow across the ET Cover design (conducted using HYDRUS-1D and
local grasses and shrubs for all Ecozones, and local grasses and trees for the
Mountains Ecozone).
For each Ecozone in Puerto Rico, the percolation of rainwater from the atmosphere into
the waste mass was modeled at two representative landfills for both compacted clay
covers and ET Covers consisting of local soils and local vegetation. Yearly average climatic
conditions were used for modeling. The compacted clay covers were modeled as a 450
mm-thick barrier of compacted clay overlaid with a 150 mm erosion control layer. The
erosion control layer on the compacted clay cover was modeled to be vegetated with local
grasses. For the ET Covers, the erosion control layer was modeled as a 300-mm-thick
layer underlaid by a rooting zone. The vegetation of ET Covers was modeled as local grass
and shrubs. Different soils (Loam, Silty Clay Loam, Silty Clay, Clay Loam, and Sandy Clay
Loam) were considered in all Ecozones.
Table 23 shows the unsaturated hydraulic properties of the simulated ET Covers.
The first set of simulations were performed to investigate the effects of soil type on the
percolation rates of ET Covers. Simulations of 900-mm-thick ET Covers for the Northern
Foothills, Mountains, Southern Foothills, and North Shore Ecozones were performed with
all five of the soils (Loam, Silty Clay Loam, Silty Clay, Clay Loam, Sandy Clay Loam). In
the South Shore Ecozone and in Culebra, the thickness of the ET Covers was limited to
600 mm, as a 600 mm ET Cover design in these regions is adequate. Simulations of
percolation through compacted clay covers were also performed.
For all simulations, including those with the highest percolation rates for all of the
modeled soil types, the simulations with Loam yielded the highest percolation rate.
Therefore, Loam was used as the worst-case scenario to determine the recommended
thicknesses of an ET Cover in each of the Puerto Rico Ecozones.
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Table 23. Performance of Different Soil Types Used as Cover Designs.
Climate
Ecozone
Site Name
Preliminary
Soil Cover
Design
Thickness
(mm)1
Percolation Though ET Cover
(mm/yr)
Loam
Soil
Silty
Clay
Loam
Silty
Clay
Clay
Loam
Sandy
Clay
Loam
North Shore
Toa Baja
900
133
125
122
121
133
Vega Baja
Northern Foothills
Florida
900
142
133
129
129
142
Fajardo
Mountains
Juncos
900
236
228
224
229
235
Barranquitas
Southern Foothills
Guayama
900
260
252
250
250
260
Hormigueros
South Shore
Lajas
900
44
41
17
17
*
Santa Isabel
1 The modeling compared the effect of soil types on ET Covers of equal thickness.
* Simulation did not converge (percolation was too low).
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