United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-96/031 March 1996
EPA Project Summary
LIST Corrective Action
Technologies: Engineering
Design of Free Product Recovery
Systems
Jack C. Parker, Dan W. Waddill, and Jeffrey A. Johnson
The objective of the report summa-
rized here is to develop and evaluate
the applicability of improved technolo-
gies for assessing subsurface liquid
fuel spills and for evaluating effects of
well placement and pumping rates on
separate phase plume control and on
free product recovery. Procedures are
described for estimating hydrocarbon
spill volume from soil core data and
monitoring well data. The first method
involves vertical integration of soil con-
centration measurements to yield oil
volume or species mass-per-unit area
within the measurement zone. This
method is especially well suited to de-
termine the amount of residual prod-
uct in the unsaturated zone. The sec-
ond method uses a physically based
model for vertically hydrostatic three-
phase fluid distributions that converts
well product thickness to soil product
thickness, followed by area! integra-
tion to estimate the volume of free prod-
uct floating on the water table. A pro-
cedure is presented to evaluate the ef-
fects of water pumping on the oil flow
gradients to evaluate if plume spread-
ing will be hydraulically controlled for
a selected system. Procedures are also
described to estimate the volume of
recoverable product as influenced by
well placement and operation. Practi-
cal examples and case studies are pre-
sented to illustrate the methodology
and to demonstrate how various fac-
tors interact to affect the free product
recovery system. The applicability of
trenches and vacuum-enhanced prod-
uct recovery to hydrocarbon spills is
also discussed.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory, Cincinnati, OH,
to announce key findings of the re-
search project that is fully documented
in a separate report of the same title
(see Project Report ordering informa-
tion at back).
Introduction
The characterization and remediation of
petroleum-contaminated groundwater is
currently one of the most challenging prob-
lems. Hydrocarbon fluids that are immis-
cible with water are referred to as
nonaqueous phase liquids (NAPLs). In
general, most hydrocarbon compounds are
less dense than water and are termed
light nonaqueous phase liquids (LNAPLs).
When released in the subsurface, LNAPLs
remain as distinct fluids and flow sepa-
rately from the water phase. The down-
ward migration of LNAPLs in the vadose
zone is generally rapid and, depending
upon the complexity of the heterogene-
ities in the soil, may form an intricate
network of pathways. Once in the vicinity
of the capillary fringe, LNAPLs will spread
horizontally with limited penetration below
the water table due to buoyancy. Contact
with groundwater and with infiltrating soil
pore water causes chemical constituents
to dissolve from the LNAPLs into the
groundwater and to contaminate the aqui-
fer. Further, volatile constituents may par-
tition into and move in the soil vapor.
Through this complex array of physical
and chemical processes, the hydrocarbon
continually changes. The characterization,
containment, and remediation of hydro-
carbons pose unique and difficult prob-
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lems because of the various processes
that may occur at a given site.
The first step in assessing a hydrocar-
bon spill generally involves delineating the
vertical and horizontal extent of soil and
groundwater contamination. Characteriza-
tion may include visual observations of
soil borings, in situ vapor readings, labo-
ratory analysis of soil concentrations, mea-
surements of fluid levels and dissolved
and vapor concentrations in monitoring
wells or probes, and surface or subsur-
face geophysical methods.
Measurements of soil concentrations
(e.g., total petroleum hydrocarbons or in-
dividual species) provide the most reliable
quantitative information on the actual vol-
ume or mass of hydrocarbon in the sub-
surface. Since laboratory analyses of the
soil samples are costly and are not practi-
cally amenable in monitoring temporal
changes, estimating spill volume from fluid
level measurements in monitoring wells is
more practical. Unfortunately, estimation
of hydrocarbon volume from well fluid level
data is less straightforward than estima-
tion from soil concentration data. A gen-
eral lack of understanding in this area,
compounded by promulgation of methods
of doubtful validity and poor accuracy, has
resulted in widespread misunderstanding.
A theoretically based method for esti-
mating oil-specific volume from well prod-
uct thickness was developed based on
the assumption of vertical equilibrium pres-
sure distributions near the water table,
which can be inferred from well fluid lev-
els. From the fluid pressure distributions
and a general model for three-phase cap-
illary pressure relations, vertical oil satu-
ration distributions are computed and in-
tegrated to yield oil-specific volume (hy-
drocarbon volume per unit soil area).
In addition to "free" product that is suffi-
ciently mobile to enter a monitoring (or
recovery) well, a significant portion of the
total spill volume may occur as "residual
product," confined as hydraulically isolated
blobs or thin films of oil that are effectively
prevented from moving by capillary forces.
Changes in water table elevations will gen-
erally result in increased residual volumes
over time. These fluctuations may result
from natural recharge variations, draw-
down, or injection as well as from air-oil
and oil-water fluid interface elevation
changes resulting from plume spreading
or recovery operations. The key to maxi-
mizing product recovery from spill sites
involves minimizing the volume of residual
product.
Product recovery systems are often
based solely on containment consider-
ations. That is, trenches and/or wells are
located to prevent further plume migra-
tion. Although such an approach may be
effective in a limited sense, it disregards
an evaluation of efficiency because plume
containment can be achieved through the
use of many different well/trench configu-
rations and operating conditions. Depend-
ing on the regulatory requirements, risk
characterization, and hence the cleanup
objectives, "efficiency" may have different
meanings: total volume of product recov-
ered, ratio of product recovered per gallon
of water pumped, time to reach asymp-
totic recovery, capital and operating costs,
etc. Once specific objectives have been
defined, various design strategies may be
evaluated to obtain the desired "efficiency."
Recent advances in numerical models
for multi phase flow along with increases
in microcomputer speed and capability
have made it possible to perform sophisti-
cated analyses to assess the effects of
various design options and natural events
on spill migration and recovery system
performance. Although such analyses re-
quire significant computational effort and
personnel commitment, which can limit
their applicability to large or high-risk sites,
their use is essential to fully evaluate the
potential complexities of hydrocarbon as-
sessment and remediation.
Purpose
The purpose of the full report is to
present a set of accurate yet
computationally simple protocols for spill
site assessment and remedial design for
hydrocarbon spills. The methods are par-
ticularly suited to small spills, for which
more sophisticated analyses may not be
warranted, and as a preliminary modeling
tool for larger spills. This report discusses
the physical processes that control hydro-
carbon retention, movement, and recov-
ery. It describes algorithms for estimating
free and residual hydrocarbon volumes
from monitoring well and soil boring data
as well as for evaluating plume migration
and containment, and for evaluating prod-
uct recovery volume and time as affected
by well and/or trench placement and op-
eration. The methodologies are simple and,
although somewhat laborious for hand cal-
culations, require minimal computational
effort for desktop computers.
Document Contents
The document summarized consists of
five sections that provide a fundamental
understanding of free product behavior in
the subsurface and of spill assessment
methodologies that can be used to design
product recovery systems. Section 1 pro-
vides an introduction and background on
previous methodologies that have been
used for estimating free and residual hy-
drocarbon thickness and for designing re-
covery systems.
Section 2 provides the fundamental con-
cepts of hydrocarbon movement and re-
tention in the subsurface and the methods
for estimating fluid and soil properties. The
concepts presented include: two- and
three-phase (water, air, and NAPL) fluid
flow; capillary retention relations (i.e., cap-
illary pressure-saturation functions); verti-
cal equilibrium fluid distributions (i.e., fluid
"table" elevations); residual oil in satu-
rated and unsaturated zones; oil relative
permeability and transmissivity; and esti-
mation of fluid and soil properties.
Section 3 presents a methodology for
hydrocarbon spill assessment. The meth-
odology includes: interpreting soil contami-
nant data (TPH or BTEX) to estimate the
hydrocarbon volume and volume of con-
taminated soil; estimating free oil volume
from monitoring well data; and estimating
dissolved and free-phase transport. Ex-
ample calculations illustrate the use of the
methodology.
Section 4 builds on the concepts pre-
sented in the previous sections and pre-
sents a methodology for the design of
product recovery systems. Discussions
concern the criteria for system design, the
effects of well placement and operation,
evaluation of plume capture and travel
time analysis, and estimation of recover-
able product. Considerations are also given
on the use of trenches and vacuum-en-
hanced free product recovery systems.
Example problems illustrate the method-
ology presented in this section.
Section 5 applies the methodology pre-
sented in this report for assessment and
remediation of a hydrocarbon spill site in
a case study. As part of this case study, a
screening level model that employs this
methodology was used to calculate the
mobile hydrocarbon volume, contaminated
soil volume, recoverable and residual hy-
drocarbon volumes to estimate the flow
field configuration, time of recovery, opti-
mal recovery rates, and system efficiency.
The methodology described in this report
has been implemented in the program
SpillCAD.
The full report was submitted in fulfill-
ment of Contract No. 68-C2 0108 by IT
Corporation and its subcontractor Envi-
ronmental Systems & Technologies, Inc.
under the sponsorship of the U.S. Envi-
ronmental Protection Agency.
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Jack Parker, Dan Waddill, and Jeffrey Johnson are with Environmental Systems &
Technologies, Inc., Blacksburg, VA 24060.
Chi-Yuan Fan is the EPA Project Officer (see below).
The complete report, entitled "LIST Corrective Action Technologies: Engineering
Design of Free Product Recovery Systems," (Order No. PB96-153556; Cost:
$25.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Edison, NJ 08837
United States
Environmental Protection Agency
National Risk Management Research Laboratory (G-72)
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
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EPA/600/SR-96/031
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