science BRIEF
INNOVATIVE RESEARCH FOR A SUSTAINABLE FUTURE
www.epa.gov/research
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Cost Effective Tools for Assessment of Infiltration at Green Infrastructure
Stormwater Management Sites
Motivation:
impermeable surfaces such as
buildings arid roads limit natural
infiltration of precipitation into
underlying soil. Reduced
infiltration limits natural
replenishment of groundwater,
increases stormwater runoff and
flooding, and can overload water-
treatment systems. Green
infrastructure (Gl) is one potential
method for offsetting reduced
infiltration. Gl encompasses a
range of purpose-built landscape
features that provide storage
capacity for stormwater runoff
and increase infiltration into the
subsurface. Successful
management of stormwater
infiltration is achieved by
optimizing the dimensions and
characteristics of the Gl system for
compatibility with the infiltration
capacity of the existing soil profile.
The long-term performance of the
Gl system will be governed by
changes to the water transmission
characteristics of the Gl system
materials and the soil profile
resulting from potential
reductions in porosity and
permeability overtime, i.e.,
clogging. Direct measurements of
water flux and the physical
properties of subsurface materials
during the lifetime of the Gl
system can be expensive and
labor-intensive and therefore
impractical. These limitations can
be addressed using robust, low
cost subsurface monitoring
Figure 1.
Installation of a
temperature
profiler into a
cased borehole
adjacent to the
infiltration
gallery.
devices that track surrogate
indicators of subsurface water
movement.
Pilot Field Studies to Test Gl-
performance Monitoring Devices:
Two pilot studies were initiated to
test the performance of
subsurface monitoring devices and
their ability to track movement of
infiltrating stormwater into the
subsurface. In the first study, EPA
is collaborating with the U.S. Army
as part of their Net Zero Initiative
at a site in Kansas. This project
studied an instrumented Gl
system that includes a section of
permeable pavement coupled to a
subsurface infiltration gallery to
manage stormwater runoff and
enhance recharge to groundwater.
Soil temperature monitoring was
demonstrated for tracking
movement of captured
stormwater runoff from the
infiltration gallery into the soil
profile. In the second study, EPA
is collaborating with the U.S.
Geological Survey to develop and
evaluate performance of
autonomous and remote
geophysical sensing systems for
monitoring infiltration. This
project entails development of a
sensing device and associated
telemetry system to allow remote
operation and data acquisition.
Evaluations for remote monitoring
of infiltration are being conducted
for a permeable pavement test
system and a natural soil control
site in Connecticut.
Monitoring Soil Temperature in
Kansas:
The use of soil temperature to
monitor water flow is based on
the principles of heat transfer that
are in play when there is a
contrast in the temperature of
infiltrating stormwater and the
ambient temperature of the soil
profile. Heat transfer occurs
through soil particles that are in
contact with each other
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U.S. Environmental Protection Agency
Office of Research and Development

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(conduction) and via flowing water
as it moves through connected
porosity in the soil (convection).
The observed rate of temperature
change can be used as a diagnostic
to isolate the influence of flowing
water on the overall heat transfer
process. Research is being
conducted to develop data
analysis protocols for extracting
information on water flow directly
from the transient temperature
signals. At the study site in
Kansas, this exploratory work can
be verified through comparison to
independent measures of the
movement of stormwater
drainage through and out of the
infiltration gallery. A view of the
Gl system and the physical
locations of some of the
monitoring devices are shown in
Figure 1. Stormwater runoff is
routed from the parking lot to the
permeable pavers section, which
is the entry point for flow into the
infiltration gallery. Flow of
stormwater through the
infiltration gallery is monitored
near the surface and near the
gallery bottom.
One performance issue under
investigation is the potential for
infiltrated stormwater to exit the
gallery through its sidewalis as
well as its bottom. Evaluation of
transient temperature signals
following stormwater infiltration
events has shown that there is
lateral flow near the bottom of the
Porous Pavers
southern sidewall of the
infiltration gallery, but stormwater
primarily exits through its bottom
(Figure 2). Further evaluations are
underway to optimize data
acquisition and analysis protocols
for using subsurface temperature
to monitor stormwater infiltration.
Monitoring Resistivity in
Connecticut:
The bulk electrical resistivity of
earth materials is dependent on
the resistivity of the fluid in the
pore space. The open pore space
in unsaturated or partially
saturated soil is filled by a
combination of air and water.
Infiltrating water that flows
through the connected pore space
modifies the bulk resistivity of the
Resistivity Sensor
Temperature
Thermistors
Resistivity
Ring Electrodes
Modular, Plug-and-
Play Sensor Design
Acquisition &
Communications
Hardware
Infiltration
v Gallery /
Figure 3. Sensor with resistivity
ring electrodes and temperature
thermistors. Modular design
allows customizable depth and
sensor geometry. Top of sensor
houses data acquisition and
telemetry system for remote
operation and acquisition
control.
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Impermeable
Permeable
Resistivity Sensors
Power Supply & Communications
Figure 4. Connecticut study site showing sensors (Fig. 3) installed in
permeable and impermeable pavement pads. Undisturbed reference soil
(not shown) plot instrumented with soil moisture probe and resistivity
sensor.
soil profile. This provides the basis
for the measurement of resistivity
to track the flow of stormwater
infiltration through porous
material. By conducting time-
lapse resistivity measurements, it
then becomes possible to track
water movement through a soil
profile. A dedicated, in-situ and
low-cost instrument for
continuous logging of resistivity
and temperature profiles is being
developed and tested for this
study (Figure 3).
In addition, a telemetry system is
being evaluated for remote
monitoring of the performance of
Gl systems to allow data collection
via an automated acquisition,
processing, modeling, and
visualization framework. At the
Connecticut study site, three
resistivity sensors, a soil moisture
probe, and weather station will be
deployed to conduct a year-long
recharge monitoring experiment
(Figure 4). Initial testing has
shown that resistivity decreases in
response to simulated recharge
events and corresponding
increases in water content.
The next stage of the research will
include long-term monitoring with
an updated version of the
resistivity sensors to demonstrate
remote control of data acquisition,
use of an on-line data viewing
platform, and model testing for
interpreting results. Observations
from this study will help to
validate the use of resistivity to
monitor recharge events and
demonstrate remote application
of the technology for monitoring
performance of Gl systems.
This document has been reviewed by
the U.S. Environmental Protection
Agency, Office of Research and
Development and approved for
publication.
REFERENCES:
Razzaghmanesh, M., and M. Borst.
2019. Monitoring the performance of
urban green infrastructure using a
tensiometer approach. Science of the
Total Environment 651(2): 2535-2545,
Sherrod, L, W. A. Sauck, D. D.
Werkema, Jr. 2012. A low-cost in situ
resistivity and temperature
monitoring system. Groundwater
Monitoring and Remediation 32(2):
31-39.
Terry, N., F. D. Day-Lewis, D
Werkema, J. W. Lane Jr. 2018,
MoisturEC: A new P. program for
moisture content estimation from
electrical conductivity data.
Groundwater 56(5): 823-831.
US EPA. 2018. The Influence of Green
Infrastructure Practices on
Groundwater Quality: The State of the
Science. U.S. Environmental
Protection Agency, Office of Research
and Development. EPA-600-R18-227
US EPA. 2019. Science in Action.
Demonstrating Net Zero Green
Infrastructure Technologies on Fort
Riley, KS.
CONTACT:
Kansas Pilot Study:
Steven Acree, 580-436-8609,
acree.steven@epa.gov
Randall Ross, 580-436-8611,
ross.randall@epa.gov
Robert Ford, 513-569-7501,
ford.robert@epa.gov
Connecticut Pilot Study:
Dale Werkema, 541-867-4048,
werkema.d@epa.gov
August 2019
EPA/600/F-19/141
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U.S. Environmental Protection Agency
Office of Research and Development
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