NWI-2229.180.001
Revision 1
Evaluation of the Radon-222
Deficit Technique for Delineating
DNAPL Distribution and Assessing
DNAPL Removal at the OK Tool
Source Area, Operable Unit 1,
Savage Municipal Well Superfund Site,
Milford, New Hampshire: Final Report
January 2007
NWI-2229.180.001
Revision 1
Evaluation of the Radon-222 Deficit Technique for Delineating DNAPL Distribution and Assessing DNAPL Removal at the OK Tool Source Area, Operable Unit 1, Savage Municipal Well Superfund Site, Milford, New Hampshire: Final Report Robert C. Starr
Prepared for:
U.S. Environmental Protection Agency
National Exposure Research Laboratory
Technology Support Center
Characterization and Monitoring Branch
U.S. Environmental Protection Agency
Region 1
Office of Site Remediation & Restoration
U.S. Environmental Protection Agency
Office of Solid Waste and Emergency Response
Measurement and Monitoring Technologies for the 21st Century (21M2) Initiative
and
U.S. Department of Energy
Under DOE Idaho Operations Office
Contract DE-AC07-05ID14517
North Wind, Inc. 1425 Higham Street Idaho Falls, ID 83402
January 2007
ii
EXECUTIVE SUMMARY Radon-222 (Rn-222) is a natural constituent of groundwater and its concentration is affected by the
presence of non-aqueous phase liquids (NAPL) and other factors. Radon readily partitions from water
into NAPL, and therefore radon concentrations in groundwater are lower near a NAPL body than far from
NAPL. The reduction of dissolved radon concentrations led to the name “radon deficit technique” for the
approach of using radon concentrations to make inferences about the presence of NAPL. Previous
research suggests that spatial variations in dissolved Rn-222 concentrations may be useful for delineating
the spatial extent of NAPL in the subsurface, while temporal variations may be useful for evaluating
effectiveness of NAPL remediation. The OK Tool site in Milford, New Hampshire has NAPL
contamination in a sand and gravel aquifer, and in situ chemical oxidation (ISCO) is being used to
remediate NAPL. Measurements of Rn-222 concentrations in groundwater before ISCO began suggested
that Rn-222 concentrations were lower in NAPL-bearing areas than in NAPL-free areas, in agreement
with lab and field studies performed previously by others, although the preliminary results were not
definitive. The study described here evaluated two applications of the radon deficit technique. First, the
utility of the radon deficit technique for delineating the spatial extent of NAPL at a multi-acre field site
was evaluated. Second, the ability of the technique to evaluate the effectiveness of NAPL removal was
examined. This study was performed in conjunction with an ISCO project performed by others. ISCO
was performed by injecting a potassium permanganate (a strong oxidizer) solution into the subsurface in
an area where NAPL was known to be present. Two injections were made, separated by approximately
1 year.
Groundwater samples for Rn-222 analysis were collected prior to the first oxidant injection and
approximately 1 year after the first and second oxidant injections. Wells in areas where NAPL was
present and wells in background areas remote from NAPL were sampled. Rn-222 concentrations at the
OK Tool site are highly spatially variable, and concentrations measured in background areas cannot be
discriminated from those measured in NAPL-contaminated areas. The large spatial variability in Rn-222
concentrations in NAPL-free areas is thought to result from heterogeneity in sediment properties. The
inability to discriminate NAPL-free from NAPL-bearing areas based on measured Rn-222 concentrations
at this site indicates that the radon deficit technique cannot be used to delineate the spatial extent of
NAPL in the subsurface at the OK Tool site. However, the technique may be useful at sites where
background Rn-222 concentrations are more uniform.
The persistence of permanganate in groundwater for approximately 1 year after injection, the decline in
volatile organic compound concentrations, and increase in chloride concentrations collectively suggest
that ISCO reduced NAPL saturation in the subsurface. Based on this, it was anticipated that
concentrations of radon in groundwater would increase over time as a result of reduced partitioning from
water into NAPL. Contrary to this expectation, temporal trends in Rn-222 concentrations were highly
variable. It is not known how much of the difference in measured Rn-222 concentrations is due to
changes in NAPL saturation and how much is due to other factors. Nevertheless, the lack of a trend of
increasing Rn-222 concentrations over time is inconsistent with the expected response of the system and
is interpreted as indicating that the radon deficit technique cannot be reliably used to evaluate NAPL
remediation at the OK Tool site. Factors that may affect dissolved Rn-222 concentrations include that the
relationship between NAPL saturation and radon partitioning may be more complicated than assumed and
that oxidation reaction products (i.e., MnO2) may affect radon partitioning or emanation from minerals.
Variation in sampling and analysis and the hydrogeologic system may also contribute to variability in
measured radon concentrations. Investigation of these topics may account for unexpected trends observed
in this study and support application of the technique at other sites.
iii
CONTENTS 1
INTRODUCTION............................................................................................................................... 1
2
BACKGROUND ................................................................................................................................. 3
2.1
O
K Tool Source Area................................................................................................................ 3 2.1.1
2.1.2
3
4
Site Description .......................................................................................................... 3
Previous Remediation................................................................................................. 5
2.2
Evaluation of the Radon Deficit Technique in Conjunction with In Situ Chemical Oxidation ................................................................................................................................... 5 2.3 Theory of the Radon Deficit Technique.................................................................................... 6 EXPERIMENTAL METHODS......................................................................................................... 9
3.1 Groundwater Purging and Sampling ......................................................................................... 9 3.2
R
adon-222 Analysis .................................................................................................................. 9 RESULTS AND INTERPRETATION ........................................................................................... 11
4.1 Baseline Data Set..................................................................................................................... 11 4.2 Spatial Variability in Background Radon Concentrations at OK Tool and Use of Radon Deficit Technique for Delineating Non-aqueous Phase Liquid............................................... 14 4.3
Temporal Trends in Radon and Use of Radon Deficit Technique for Assessing Non-aqueous Phase Liquid Remediation ................................................................................ 17 5
SUMMARY AND CONCLUSIONS ............................................................................................... 20
6
ACKNOWLEDGEMENTS.............................................................................................................. 21
7
REFERENCES.................................................................................................................................. 22
iv
FIGURES 2-1.
DNAPL-bearing areas at the OK Tool site (from NHDES, 2003). ................................................. 4
4-1.
Wells sampled for radon-222 analysis in one or more sampling events (Starr, 2005). ................. 12
4-2.
Rn-222 in baseline (before ISCO) groundwater samples (adapted from Starr, 2004)................... 13
4-3.
Correlation between Rn-222 and dissolved PCE concentrations, baseline conditions at the OK Tool site (adapted from Starr, 2004). ...................................................................................... 14
4-4.
PCE concentration in selected wells at OK Tool (Starr, 2005)...................................................... 15
4-5.
Radon concentrations in groundwater, September 2004 (Starr, 2005). ......................................... 15
4-6.
Mean radon concentrations in background and non-background areas in three sampling events. One-standard deviation error bars are shown (Starr, 2006). .......................................................... 17
4-7.
Mean radon concentrations and 1-standard deviation uncertainty measured in 2003, 2004, and 2005 (Starr, 2006). .................................................................................................................. 18
TABLES 2-1.
Schedule of radon sampling and oxidant injections......................................................................... 6
4-1.
Descriptive statistics for the 2004 radon dataset (Starr, 2005). ..................................................... 16
4-2.
Temporal trends in radon concentration in non-background wells in the INEL Well Array......... 19
v
ACRONYMS 21M2
Measurement and Monitoring Technologies for the 21st Century
DNAPL
dense, non-aqueous phase liquid
EPA
Environmental Protection Agency
ISCO
in situ chemical oxidation
NAPL
non-aqueous phase liquid
NHDES
New Hampshire Department of Environmental Services
NWI
North Wind, Incorporated
PCE
tetrachloroethene
RDT
radon deficit technique
Rn-222
radon-222
SVE
soil vapor extraction
TCE
trichloroethene
VOC
volatile organic compound
vi
Evaluation of the Radon-222 Deficit Technique for Delineating DNAPL Distribution and Assessing DNAPL Removal at the OK Tool Source Area, Operable Unit 1, Savage Municipal Well Superfund Site, Milford, New Hampshire: Final Report 1 INTRODUCTION
Laboratory and small scale field studies performed by other researchers have suggested that the radon
deficit technique (RDT) may be used to identify areas where non-aqueous phase liquid (NAPL) is present
in the subsurface, and to assess the effectiveness of remedial techniques that remove NAPL from the
subsurface. Hunkler et al., (1997) observed that dissolved radon concentrations declined by
approximately 50% as diesel saturation increased from 0 to 2.3% in laboratory batch experiments.
Semprini et al., (1998, 2000) describe laboratory column and physical aquifer model experiments,
mathematical modeling studies, and measurements of dissolved radon concentrations at a field site where
NAPL is present in the subsurface. Davis et al. (2002, 2003) describe studies of static and push-pull tests
using dissolved radon-222 to assess NAPL saturation in a model aquifer. These studies show that
concentrations of radon in groundwater are lower in the vicinity of NAPL than they would be if NAPL
was absent, and suggest that spatial and temporal trends in dissolved radon concentrations may provide
information on the spatial distribution of NAPL and changes in NAPL saturation over time, respectively.
Historical manufacturing and disposal activities at the OK Tool site in Milford, New Hampshire, resulted
in tetrachloroethene (also known as perchloroethylene [PCE]) being present in a sand and gravel aquifer
as a dense, non-aqueous phase liquid (DNAPL) (i.e., a NAPL that is denser than water and therefore can
readily sink below the water table). Dissolution of DNAPL created a plume of dissolved PCE that
migrated to a nearby municipal water supply well in the town of Savage, New Hampshire. Investigation
of the source of contamination at this well led to characterization and remediation of subsurface
contamination at the OK Tool site. Previous characterization activities have delineated the spatial extent
of several DNAPL-bearing zones and created an experimental infrastructure that includes numerous
monitoring wells and a closely-spaced array of wells in an area where a substantial volume of DNAPL is
present. In addition, aggressive remediation of dissolved and DNAPL PCE using in situ chemical
oxidation (ISCO) is being conducted at the site. The combination of extensive subsurface
characterization data, ongoing groundwater monitoring, and aggressive subsurface remediation provided
an opportunity to evaluate the RDT under conditions of an active remediation site and at a larger scale
than in previous studies.
A baseline data set collected before ISCO was initiated suggested that the RDT could be used to
discriminate between zones where DNAPL was present from zones where it was absent at the OK Tool
site, although the data were not conclusive. Based on this baseline data, the U.S. Environmental
Protection Agency (EPA) provided funding for a more extensive evaluation of the RDT at the OK Tool
site, in conjunction with a pilot test of ISCO. The evaluation addressed two applications of the RDT.
First, the use of the RDT to delineate the spatial extent of NAPL-bearing zones by using measured
concentrations of radon in groundwater to discriminate areas where NAPL is present from areas where it
is absent. Second, the use of the technique to assess the effectiveness of NAPL remediation and to
provide an indicator that NAPL has been removed from an area.
1
The overall approach included measuring concentrations of radon in groundwater in both background
areas (areas where DNAPL was absent) and non-background areas (areas where DNAPL was present)
following each of two phases of oxidant injection. Radon concentrations measured in background areas
were compared to those measured in non-background areas to evaluate the ability to discriminate between
these types of areas based on radon concentrations. The baseline data set and two additional data sets
collected in this study were used in this evaluation. To evaluate the utility of the RDT for assessing
NAPL remediation effectiveness, temporal trends in radon concentrations in groundwater were evaluated
in areas affected by the ISCO pilot test and subsequent larger-scale implementation of ISCO.
The baseline data set and concentrations measured after two phases of oxidant injection were used in this
evaluation. The baseline data set was collected just before the first injection of oxidant, and two
post-ISCO data sets were collected approximately 1 year after the first and second oxidant injections,
which allowed a long period for injected oxidant to react with NAPL in the subsurface.
This document summarizes the results of this evaluation of the RDT at the OK Tool site. More detailed
information is presented in the following data reports that describe the baseline, first, and second
post-ISCO data sets:
• Baseline sampling: Data Report: Radon in Groundwater at the OK Tool Site, Milford, New
Hampshire, September 2003 (Starr, 2004).
• Post ISCO Pilot test sampling: Data Report: Radon in Groundwater at the OK Tool Site, Milford,
New Hampshire, September 2004 (Starr, 2005).
• Post Large-Scale ISCO sampling: Data Report: Radon in Groundwater at the OK Tool Site, Milford,
New Hampshire, October 2005 (Starr, 2006).
This summary report includes a description of background information on the OK Tool site and the RDT
(Section 2), the methods utilized (Section 3), presents and interprets the data generated (Section 4), and
summarizes the results and draws conclusions (Section 5).
2
2 BACKGROUND This section provides information about site conditions at the OK Tool site (Section 2.1), the approach for
evaluating the RDT at that site (Section 2.2), and background information about the RDT (Section 2.3).
2.1 OK Tool Source Area
2.1.1
Site Description
The OK Tool Source Area is Operable Unit 1 of the Savage Municipal Well Superfund Site. Site
characterization is described in the Remedial Investigation, Savage Municipal Water Supply Site, Milford,
NH (HMM, 1991) and subsequent documents. The site is located within the Milford-Souhegan glacial
drift aquifer of south-central New Hampshire. The subsurface materials include alluvial, glacial, and
bedrock units. The surficial deposit is alluvium. Underlying the surficial alluvium is a boulder/cobble
zone (5 to 20 ft below ground surface), which is underlain by stratified sand and gravel glacial outwash
deposits approximately 40 ft thick. Beneath the sand and gravel deposits is a discontinuous till unit
(typically 20 to 40 ft thick where it is present) that consists of gravel, sand, silt, and clay. Fractured
bedrock lies beneath the till unit. A highly permeable unconfined aquifer is located in the sand and gravel
deposit. The water table typically is less than 10 ft below ground surface.
Volatile organic compound (VOC) contaminants were discovered in the Savage Municipal Well near the
site in 1983. An inspection of the OK Tool Company by the New Hampshire Department of
Environmental Services (NHDES) revealed potential releases of contaminants to a floor drain and the
ground. A remedial investigation identified a dissolved chlorinated ethene (mainly PCE) plume in
groundwater that extended approximately 6,000 ft downgradient from the site. The site was listed as
Operable Unit 1 (the OK Tool Source Area) of the Savage Municipal Well Superfund Site; the dissolved
plume downgradient of the site is Operable Unit 2.
The OK Tool Company was a machine tool and machine tool parts manufacturing facility that operated
from the 1940s to 1987 and used PCE in the manufacturing process. Site characterization activities
indicate that PCE is present as a DNAPL in the subsurface, primarily in the vicinity of the former
manufacturing facility. A vertical profiling investigation performed in 1995 (CDM, 1995) delineated the
spatial extent of DNAPL in the subsurface (Figure 2-1).
3
Figure 2-1. DNAPL-bearing areas at the OK Tool site (from NHDES, 2003).
4
2.1.2
Previous Remediation
The major remedial activities at the OK Tool site were construction (in 1998) of a low permeability cutoff
wall that extends from ground surface to the top of bedrock and encircles the site, and operation of a
pump-and-treat system that extracts groundwater from within the cutoff wall enclosure to cause
groundwater to flow into the enclosed area, thereby cutting off the source of contaminants to the
dissolved plume downgradient of the site.
Although pump and treat is an effective technique for maintaining hydraulic control of a groundwater
plume, it is typically not effective for remediating contaminant sources, particularly DNAPL sources. In
recognition of this, a variety of techniques for remediating the DNAPL source material have been
considered for application at the site. On technique considered was a co-solvent flushing method. A
network of wells (known as the INEL Well Array or the Test Array) was installed in the area of the
former manufacturing building where a DNAPL-bearing zone had been delineated during previous site
characterization activities. Although this project did not proceed to a field test of the technique, it did
leave the legacy of a network of wells in a contaminant source area.
Two iterations of ISCO have been used to remediate PCE in the Test Array area. ISCO is performed by
injecting an oxidant solution (potassium permanganate in this case) into the subsurface to oxidize
dissolved contaminants, such as PCE, to non-hazardous reaction products. Destruction of dissolved phase
PCE accelerates dissolution of PCE from DNAPL, and the newly-dissolved PCE can then be destroyed
by oxidation. Hence, although oxidation occurs only in the dissolved phase, the net result of ISCO is
enhanced removal of DNAPL.
2.2 Evaluation of the Radon Deficit Technique in Conjunction with
In Situ Chemical Oxidation
Two iterations of ISCO have been performed using the Test Array wells. The first iteration (a pilot test),
and the associated data interpretation, are described in the Annual Report of the Groundwater
Remediation Progress at the Savage Municipal Water Supply Well Superfund Site, OU-1, The OK Tool
Site, July 2003 – June 2004 (NHDES, 2005). The pilot test began September 23-27, 2003, with injection
of approximately 8,400 lb of potassium permanganate as a 2.7% by weight solution into wells CI and WI,
with concurrent extraction from wells NCP and NWP in the first few hours of the injection. The area near
these four wells was selected for the ISCO pilot test because previous vertical profiling data indicated that
it contained a substantial portion of the DNAPL present in the entire test array area. Water quality
monitoring data generated approximately 1 year after oxidant injection was interpreted as indicating that
chemical oxidation destroyed approximately 145 lb of VOCs, which is approximately 12% of the DNAPL
mass estimated to have been present in the Test Array area at the beginning of the ISCO pilot test. The
majority of the VOC mass destroyed during the ISCO pilot test was in either the DNAPL or sorbed phase,
not the dissolved phase, at the start of the test. The mass in the DNAPL phase is probably much larger
than the sorbed mass. The estimated amount of VOCs destroyed is likely less than the amount actually
destroyed because the estimate does not account for oxidant or degradation products that were transported
beyond the test well array.
The second iteration of ISCO and the interpretation of the data generated are described in the In-Situ
Chemical Oxidation – Phase II Program at the OK Tool Source Area Savage Municipal Well Superfund
Site – Draft Copy (NHDES, 2006). The second iteration involved more wells and a larger mass of
oxidant than the pilot test. Approximately 24,000 lb of potassium permanganate was injected as a 2.7%
by weight solution into four Test Array wells (CI, WI, NWP, and SWP) and in four wells installed as soil
vapor extraction (SVE) wells (SVE-1, SVE-2, SVE-3, and SVE-5). Injection was accomplished
5
October 4-15, 2004. Water quality monitoring data generated during the year following the second
oxidant injection was interpreted as indicating that an additional 98 lb of VOC was destroyed in the Test
Array area; the mass destroyed outside the test array, including the areas near the SVE wells that were
used as injection points in the second ISCO iteration, was not estimated. As in the ISCO pilot test, it is
thought that most of the VOC destroyed was in the non-aqueous phase (DNAPL and sorbed phases) at the
beginning of ISCO. The mass of VOC destroyed was estimated to be approximately 8% of mass initially
present in the Test Array area prior to the ISCO pilot test. Thus, a conservative (low) estimate of the
amount of VOC destroyed during two iterations of ISCO is approximately 20% of the amount initially
present, which suggests that a significant fraction of the VOC mass in the Test Array area remained after
both iterations of ISCO. Rebound in VOC concentrations supports the interpretation that some DNAPL
remains in the Test Array area. Although mass balance calculations suggest that substantial amounts of
DNAPL remain in the Test Array area, the persistence of permanganate in that area for 1 year suggests
that DNAPL has been largely removed from the horizons flushed by oxidant solution, but remains in
horizons that were either not permeated by oxidant solution or where too little oxidant solution was
provided to oxidize all of the PCE present.
In summary, two iterations of ISCO appear to have destroyed at least 20% of the amount of VOC present
in the Test Array area. The majority of the VOC was initially present in a non-aqueous phase (probably
almost entirely as DNAPL), and some DNAPL appears to remain in the Test Array area.
An evaluation of the RDT (see Section 2.3) was performed in conjunction with two applications of ISCO.
Groundwater samples for radon analysis were collected before the ISCO pilot test, approximately 1 year
after oxidant was injected in the pilot test, and approximately 1 year after a second injection of oxidant in
a larger-scale application of ISCO. The dates of radon sampling and oxidant injections are shown in
Table 2-1.
Table 2-1. Schedule of radon sampling and oxidant injections.
Activity
Date
Baseline radon sampling September 8-12, 2003 ISCO Injection #1 September 23-16, 2003 Post ISCO radon sampling #1 September 13-16, 2004 ISCO Injection #2 October 4-15, 2004 Post ISCO radon sampling #2 October 17-19, 2005 2.3 Theory of the Radon Deficit Technique
Radon-222 (Rn-222) is a naturally occurring constituent of groundwater that is continually produced in
the subsurface by radioactive decay of radium-226 (Ra-226). Ra-226 and Rn-222 are members of the
uranium-238 (U-238) decay series (Friedlander et al., 1981), and hence the abundance of Ra-226 and
Rn-222 in groundwater depends on the concentration of U-238 in rocks or sediment that host the aquifer
and the rate at which radon is released from minerals (known as the “emanation rate”) (Hall et al., 1987;
Wathen, 1987; Cecil and Green, 2000). For a given rate of Rn-222 production in a mineral, Rn-222
concentrations in groundwater tend to be higher in finer-textured porous media than in coarser textured
media because a greater proportion of the radon travels far enough to exit the mineral grain, and hence
6
heterogeneity in texture may cause heterogeneity in dissolved Rn-222 concentrations (LeGrand, 1987;
Cecil and Green, 2000). Radon concentrations reach secular equilibrium because the radioactive decay
half-life of Rn-222 (3.8 days) is short relative to that of U-238 (4.5 × 109 years) and Ra-226 (1,600 years).
Hence, under steady-state flow conditions in the absence of sinks or sources for radon, other than
production by Ra-226 decay, Rn-222 concentrations in groundwater would vary from place to place due
to spatial variations in the rate of radon emanation from rocks and sediments along a groundwater
flowpath; temporal variations would be negligible. However, the presence of isolated sinks for Rn-222
would cause spatial variations in concentrations in addition to those due to heterogeneity in radon
emanation; temporal variation in sink strength would cause temporal variation in radon concentrations.
If the spatial variations in the background Rn-222 concentration could be accounted for, then the
additional spatial variation could be used to identify the location of radon sinks. Even without knowing
the natural heterogeneity in background concentrations, temporal variations in radon concentrations could
be used to assess variations in source strength over time.
Sinks for radon in groundwater include partitioning from water into other phases (Hunkler et al., 1997),
but not chemical reactions. Radon (a noble element) is chemically inert, and thus concentrations of radon
in groundwater are not affected by reactions in the dissolved phase or by reactions between the dissolved
and solid phases. However, radon readily partitions from groundwater to other phases (i.e., gas bubbles
and NAPLs). Gas bubbles are typically present in groundwater only near the water table, and thus
partitioning into gas bubbles is generally not an important process in groundwater except near the water
table. DNAPLs can occur at any depth in an aquifer, and thus partitioning into NAPL as a radon sink can
also occur at any depth in an aquifer. Laboratory studies (Semprini et al., 2000) demonstrated that radon
readily partitions into some NAPLs, including PCE. Partitioning from water to NAPL locally reduces the
aqueous concentration, and hence partitioning is a sink for radon in groundwater. The radioactive decay
half-life for radon is sufficiently short that decay of Rn-222 in NAPL prevents equilibrium between radon
in the dissolved and NAPL phases. This results in a continuous mass flux of radon from the aqueous
phase to the NAPL phase, and thus partitioning into NAPL is a long-term sink for aqueous radon. Hence,
the occurrence – spatially and temporally – of depressed radon concentrations can be used as an indicator
of the presence of a NAPL.
The use of radon concentrations in groundwater that are depressed relative to background concentrations
leads to “the radon deficit technique” as the name for this approach, which was suggested by Semprini et
al., (1998). Laboratory and modeling studies (Semprini et al., 2000; Davis et al., 2002; 2003) show that
the amount of partitioning, and hence the reduction of Rn-222 concentrations in groundwater from
background, increases with increasing NAPL saturation. Trichloroethene (TCE) saturations in excess of
approximately 1% within 1 to 2 m of the sampling point cause easily detectable reductions in Rn-222
concentrations (Semprini et al., 2000).
Two applications of the RDT have been tested in the lab and in small-scale field studies: 1) delineating
the spatial extent of NAPL and 2) assessing NAPL removal. The spatial extent of NAPL-contaminated
zones can be delineated by generating “snapshots” of Rn-222 concentrations. Rn-222 concentrations
measured in a suspected NAPL-contaminated zone are compared to background concentrations measured
in a NAPL-free area (the background area). A sample whose Rn-222 concentration is significantly less
than the background concentration indicates that NAPL is present in the immediate vicinity (typically
within approximately 1 to 2 m) of the sample location. Background concentrations are measured in
samples collected from the same geologic setting as the suspected NAPL-contaminated zone, but outside
the area where NAPL is present.
7
The second application, evaluating NAPL removal, relies on time series data instead of snapshot data.
To generate a time series data set, samples are collected repeatedly from the same locations. Changes in
Rn-222 concentrations over time indicate that the rate of partitioning into the NAPL has varied,
presumably as a result of NAPL removal during remedial activities. If radon concentrations increased, it
could be interpreted that NAPL had been at least partially removed from the vicinity of the sample point.
If the background radon concentration is known and the radon concentration stabilized at the background
value, then it could be concluded that all NAPL had been removed from the close to the sample point.
The radon method was previously tested in two controlled field studies at the Canadian Forces Base
Borden test site in Ontario, Canada (Semprini et al., 2000). In the first study, a fairly uniform NAPL
source composed of a mixture of chloroform, TCE, and PCE was emplaced in the shallow sand aquifer
and slowly dissolved under natural groundwater flow conditions. Groundwater samples for radon analysis
were obtained upgradient, within, and downgradient of the source. Within the NAPL zone, dissolved
radon concentrations decreased to 0.3 to 0.5 of the upgradient (background) concentration, and rebounded
within a few meters to the background value. This data set indicates that the radon deficit occurs locally
near the NAPL contamination. Simulated radon concentrations along the flowpath are in good agreement
with the field results. A NAPL residual saturation of 4.5%, estimated based on the model simulations, is
similar to the value of 3.8% provided by the researchers responsible for creating the emplaced source.
The second experiment at Borden, referred to as the “Free-Release” experiment, involved releasing 5 L of
NAPL into a sand aquifer within a steel sheet pile test cell. The NAPL was permitted to distribute itself
in the aquifer and form an irregular NAPL zone. Groundwater flow (10.1 cm/day) was induced by
injecting and pumping groundwater at opposite ends of the cell. Radon-stripped recharge water (20 pCi/L
aqueous radon) was injected upstream of the NAPL spill. Radon concentrations were measured
downgradient at selected locations. Modeling of a residual NAPL zone (0.5 m in length with a residual
saturation of 7.8%) produced a predicted radon concentration profile that matched the field observations.
The modeling and field results indicate NAPL contamination exists at this location. This zone of NAPL
contamination was confirmed when the contaminated zone was excavated.
Increases in radon concentration during NAPL remediation has also been recently reported in large
physical aquifer model experiments conducted by Davis et al., (2003). Detailed contour analysis of radon
concentrations showed concentration increases were associated with the remediation of the NAPL by
alcohol cosolvent flushing.
These investigations indicate that the RDT has the potential for being a very useful tool for delineating
the spatial extent and amount of NAPL present and for assessing NAPL removal activities, at least in the
relatively small physical systems examined. The evaluation at the OK Tool site described here examined
the performance of the method at a much larger and more heterogeneous site where active groundwater
remediation is in progress.
8
3 EXPERIMENTAL METHODS The techniques used for purging and sampling groundwater (Section 3.1) and analyzing samples for
Rn-222 (Section 3.2) are described in this section.
3.1 Groundwater Purging and Sampling
This section provides an overview of the techniques utilized. Detailed descriptions are provided in the
following sampling plans and information about purging and sample collection is documented in the
corresponding data reports:
• Baseline Sampling in 2003: Protocol for Collection of Samples in Support of Bioremediation
Polishing Step and for Assessing the Effectiveness of In Situ chemical Oxidation (ISCO) for Source
Removal, OK Tool Site, Milford, NH – Pre-ISCO Baseline Sampling (Starr et al., 2003).
• Sampling in 2004 after the ISCO pilot test: Protocol for Collection of Samples in Support of
Bioremediation Polishing Step and for Assessing the Effectiveness of In Situ chemical Oxidation
(ISCO) for Source Removal, OK Tool Site, Milford, NH – Post-ISCO Sampling and Radon
Background Sampling – Revision 0 (Starr and Macbeth, 2004).
• Sampling in 2005 after larger-scale application of ISCO: Protocol for Collection of Samples in
Support of Bioremediation Polishing Step and for Assessing the Effectiveness of In Situ chemical
Oxidation (ISCO)for Source Removal, OK Tool Site, Milford, NH – Pre-ISCO Sampling and Radon
Background Sampling – Revision 1 (Starr and Macbeth, 2005).
The major differences between these documents are addition of information needed to meet EPA quality
assurance requirements in the 2004 document, modification of the locations sampled in the 2004 and
2005 documents, and minor editorial changes in the 2005 document.
Groundwater samples were collected using the low-flow purging and sampling approach. Purging was
carried out until water quality indicator parameters stabilized, and then samples for dissolved radon
analysis were collected. Samples were collected without exposure to the atmosphere and placed into
sample bottles that did not contain a headspace. Samples were stored on ice in an insulated cooler and
shipped to the analytical laboratory via overnight courier.
Details related to purging, sampling, and shipment are documented in a data report prepared for each
sampling event (Starr, 2004; 2005; 2006).
3.2 Radon-222 Analysis
Samples were analyzed via liquid scintillation counting at the Environmental Assessment Laboratory in
the Idaho State University Physics Department. The lab prepared subsamples for analysis by transferring
an aliquot of groundwater to a scintillation vial that was pre-loaded with scintillation cocktail using
techniques that minimized exposure to the atmosphere to prevent loss of radon via volatilization. The
activity of Rn-222 in each sample was determined by counting in a scintillation counter, correcting for
background, counting efficiency, and quench. The activity per unit volume of groundwater at the time of
analysis was calculated using the measured mass of groundwater present in the samples analyzed. The
activity per unit volume of groundwater at the time of sample collection was determined by correcting the
activity at time of analysis for radioactive decay between the time of sample collection and analysis. A
quench correction method, developed as part of this study to account for possible interference with
scintillation photon transmission that might occur as a result of the color imparted by permanganate in
samples, was applied to all samples.
9
Additional details on the sample collection and analysis procedures are provided in the sampling plans
and data reports for each sampling event. The analytical data and associated calculations are provided in
appendices to data reports prepared for each sampling event. A laboratory calculation error was
discovered in the baseline (pre-ISCO) radon data (2003) after the post-ISCO pilot test samples (2004)
were analyzed. Corrected baseline radon data are provided in the Data Report: Radon in Groundwater at
the OK Tool Site, Milford, New Hampshire, September 2004 (Starr, 2005).
10 4 RESULTS AND INTERPRETATION The results of limited sampling in the pre-ISCO baseline sampling event in 2003 (Section 4.1) suggested
that the RDT might be useful at the OK Tool site, although whether the background radon concentrations
could be differentiated from radon concentrations in DNAPL-contaminated areas was ambiguous. More
extensive sampling of background wells, which was performed in the post-ISCO pilot test sampling in
2004, was used to resolve this ambiguity (Section 4.2). Temporal trends in radon concentrations during
three sampling events (baseline, approximately 1 year after the first oxidant injection, and approximately
1 year after the second oxidant injection) were interpreted relative to inferring information about DNAPL
removal at the OK Tool site (Section 4.3).
4.1 Baseline Data Set
Baseline Rn-222 and VOC sampling was conducted during August and September 2003 immediately
before the initial oxidant injection of the ISCO pilot test. Samples were collected from the ISCO pilot test
area where DNAPL is present (wells CI, EI, NCP, NEMLS, NEP, MWMLS, and NWP), from an area
where DNAPL is present but outside the influence of the pilot test (PW6-S and PW6-M), and from three
wells in background areas (B95-8, PW5-M, and PW7-S). The locations of wells sampled in this sampling
round and in subsequent sampling rounds are shown in Figure 4-1.
Measured radon concentrations in triplicate samples from each well are shown with ±1 standard deviation
error bars (Figure 4-2). Note that the radon concentrations plotted in Figures 4-2 and 4-3 differ from
those reported in the baseline sampling event data report (Starr, 2004) because laboratory calculation
errors caused the concentrations previously reported to be too high, although the general trends in the data
were not affected. The calculation error is discussed and the revised data set is provided in the 2004
sampling event data report (Starr, 2005).
The following features are apparent in this data set:
• In general, analysis of triplicate groundwater samples for Rn-222 showed excellent reproducibility.
However, the results for three wells (PW7-S, NEMLS and NEP) each have two similar values and
one dissimilar value. The cause for the dissimilar (low) values is not known.
• Radon concentrations (~5,400 pCi/L) in two of the background wells (B95-8 and PW7-S) are
distinctly different from the remainder of the values (~1,500 to ~4,200 pCi/L), which is consistent
with partitioning into DNAPL, reducing the aqueous concentration of Rn-222.
• The measured concentrations in one well that is thought to be a background well (PW5-M) fall in the
range of values for wells in DNAPL areas.
11 PCE DNAPL Source Area
(INEL Well Array)
HCN
NCP
HCNW NWP
NW-MLS
SVE-4
HCW
NC-MLS
NEP
Cl
WI
HCNE
NE-MLS
El
HCE
P2 SG1
P2 SG3
SW-MLS
HCSW
SC-MLS
SWP
SCP
SEP
HCSE
P2
10
PW3D
PW12S
PW12M
PW3S
B95-3
PW9M
HCS
PW11D PW11M
Current P/T
Reinjection
(Gallery)
MI-22A
B95-4
TP-3
MW-27
PW8M
SVE-6
SVE-3
PW10M
B95-15
MI-22 PW10D
MI-23
SP-2
PW14M
B95-2
IW-2
PW13M
PW13S
HM-1
10
0
RW-2
P2 SG2
P2 SG4
SE-MLS
PW13D
PW12D
PW12R
MI-68
MOW-63
MI-63
TP-2
PW14D
PW14S
MI-47
EW-1
RW-1
B95-16 SVE-5
B95-10
PW7S
MI-24
PW6S
SVE-4
B95-1
MI-24A
SVE-1
B95-8
RW-2
MI-21A
PW5M
PW5R
PW6D
PW6R
PW6M
PW7M
B95-6
PW5D
B95-11
SP-1
Former location
of OK Tool
Building
PW2R PW2M
B95-17
SVE-2
PW4D
PW4M
B95-13
MI-32
PW2S
PW1D
EW-2
PW2D
MW16C
MW16B
IW-1
10
0
MW16R
MW16A
MI-26
B95-9
MI-25
B95-7
PW1S
10
B95-5
RW-3
OK Tool Site Well Infrastructure
TP-1
MI27
Savage Well OK Tool
Source Area
Treatment Plant
B95-12
Milford Police
Station
Explanation
GW Extraction Well
INEL Well
Air Sparge Well
Well (active)
Well (abandoned)
MI28
Soil Vapor
Extraction Well
Figure 4-1. Wells sampled for radon-222 analysis in one or more sampling events (Starr, 2005).
12 Sampling Wells
Slurry Wall LocationMI-31
OK Tool Area
September 2003
7,000
Background
Non-Background
6,000
Rn (pCi/L)
5,000
4,000
3,000
2,000
1,000
N
EP
P
C
N
P
W
N
N
EM
LS
EI
6M
6S
PW
M
W
N
PW
LS
I
C
7S
PW
5M
PW
B9
58
0
Well Identifier
Figure 4-2. Rn-222 in baseline (before ISCO) groundwater samples (adapted from Starr, 2004).
If only the two high (~5,400 pCi/L) values represent background conditions, this data set would indicate
that Rn-222 concentrations may be used to identify the spatial extent of DNAPL. On the other hand, if
the low Rn-222 concentrations at PW5-M are also background values, then the data set would indicate
that radon concentrations in background areas are similar to those in DNAPL-bearing areas. Determining
the variability in background concentrations was critical to the evaluation of the utility of the RDT at the
OK Tool site.
Comparison of dissolved PCE (the major dissolved VOC in groundwater at the site) and Rn-222
concentrations (Figure 4-3) shows that Rn-222 concentrations were depressed relative to background
where PCE concentrations were greater than approximately 60 µg/L, at least in this limited data set
(assuming that PW5-M does not represent background). This behavior is consistent with DNAPL,
causing both elevated dissolved VOC concentrations and depressed Rn-222 concentrations.
The baseline data set suggested that radon concentrations might be useful for delineating DNAPL-bearing
zones and assessing DNAPL remediation at the OK Tool site; however, the low concentration of Rn-222
in well PW-5M relative to other background wells was problematic. In order to further evaluate the
utility of the RDT for discriminating between NAPL-bearing and background areas, a more extensive
network of background wells was sampled approximately 1 year after the ISCO pilot test began
(Section 4.2).
13 September 2003
7,000
B95-8
6,000
PW-7S
Average Rn-222 (pCi/L)
5,000
Postulated
Relationship
4,000
3,000
2,000
PW5-M
1,000
0
1
10
100
1,000
10,000
PCE (ug/L)
Figure 4-3. Correlation between Rn-222 and dissolved PCE concentrations, baseline conditions at the OK
Tool site (adapted from Starr, 2004).
4.2 Spatial Variability in Background Radon Concentrations at OK
Tool and Use of Radon Deficit Technique for Delineating Non-aqueous Phase Liquid The fundamental requirement for applying the RDT to delineate DNAPL-bearing zones is that it must be
possible to discriminate radon concentrations in DNAPL-bearing zones from those in background areas.
The baseline data set (Section 4.1) suggested that the RDT could possibly be used at the OK Tool site to
identify areas where DNAPL was present, although the preliminary results were somewhat ambiguous.
The ambiguity arose because two background radon concentrations appeared to be distinctly different from
those in DNAPL-bearing areas, but a third background value was comparable to those in DNAPL-affected
areas. The RDT can be used to identify NAPL-bearing zones only if radon concentrations in these areas
can be unambiguously differentiated from those in background areas. The ambiguity in background radon
concentrations in the baseline data set led to a more extensive sampling of background wells to better
characterize background radon concentrations at the OK Tool site. Radon concentrations were measured
in 13 background wells in the 2004 sampling event.
Each of the 24 wells sampled in 2004 was assigned to either the background or non-background category
based primarily on its location relative to areas where DNAPL has been previously inferred to be present
and secondarily based on contemporaneous measurements of PCE concentrations (Figure 4-4). In the
background wells, PCE concentrations (determined in August and September 2003 and August and
September 2004 by NHDES and Veolia Water Systems [now NA WaterSystems]) were less than
100 µg/L. In contrast to the low concentrations in background wells, PCE concentrations in the
non-background wells were above 100 µg/L in one or both of the 2003 and 2004 sampling events. Lower
concentrations in 2004 at wells in the Test Array are thought to be a result of the ISCO Pilot Test
conducted in that area.
14 100000
2003
2004
Background
Non-Background
10000
PCE (ug/L)
1000
100
10
1
WI
PW-6S
PW-6M
NWP
NWMLS
NEP
NCP
NEMLS
NCMLS
EI
CI
PW-9M
PW-7S
PW-8M
PW-7M
PW-5M
PW-4M
PW-4D
PW-3D
MW-16B
MW-16C
B95-8
B95-5
B95-3
0.1
Well ID
Figure 4-4. PCE concentration in selected wells at OK Tool (Starr, 2005).
The background radon data set consists of concentrations measured in samples collected from 13 wells,
and the non-background data set consists of data from 11 wells (Figure 4-5). The measured value and
±1 standard deviation error bars are plotted. Visual examination of these data sets suggests that the
non-background data cannot be differentiated from the background data.
6,000
Background
Non-Background
5,000
Rn-222 (pCi/L)
4,000
3,000
2,000
1,000
0
-1,000
I EI S
5
3
8 B
P LS E P
P M
S
S
C
L
LS
5- 95- 95- 16 16C -3D -4D -4M -5M -7M -7 -8M -9M
W 6 -6
9
M N C EM N
M
N W- W
W
W
W
B W W P
B
B
W
W
W
W
W
C
W
P
P
P
P
P
P
P
P
P
N
N
N
M M
Well Identifier
Figure 4-5. Radon concentrations in groundwater, September 2004 (Starr, 2005).
15
W
I
Descriptive statistics for the 2004 data sets are summarized in Table 4-1. These statistics were computed
using the means of measured values for each well so that all wells would be weighted equally while
calculating population statistics, even though there are three measured values for some wells and four for
others. The overlap of the P=0.95 confidence interval for the means of the background and
non-background datasets indicates that the means are not significantly different. The same conclusion
can be drawn by comparing the 2003, 2004, and 2005 data sets (Figure 4-6).
Table 4-1. Descriptive statistics for the 2004 radon dataset (Starr, 2005).
Statistic
Background Dataset
(pCi/L)
Non-Background Dataset
(pCi/L)
Mean
2,260
2,497
Standard Deviation
1,354
1,242
0.95 Confidence Interval for the Mean
1,442 – 3,078
1,663 – 3,332
To further compare the 2004 data sets, two additional statistical techniques were used. First, the means of
the two data sets were compared. Second, data for each non-background well were individually
compared to the background data set.
The comparison of the means of the background and non-background data sets began by calculating the
mean radon concentration for each well and computing a global mean of background and non-background
values by averaging the means for background wells and non-background wells. This gives equal weight
to each well, in spite of there being three measurements for some wells and four for others. The global
means of the background and non-background data sets were compared using Student’s t-test via the
statistical analysis software package SigmaStat 3.1 (Systat Software, Inc.). The t-test indicates that the
means of the two data sets are not significantly different, which is consistent with the overlap between the
confidence intervals of the means.
Next, data for each non-background well were compared to the background data set. This comparison
used all three (or four) data values for an individual non-background well and the mean concentration for
each background well. In cases where the assumptions of normality and equal variance were met, the
t-test was used to compare the means. If these assumptions were not met, then the non-parametric
Mann-Whitney Rank Sum test was used to compare median concentrations, again using SigmaStat. The
radon concentration at only one of the 11 non-background wells (NCMLS) could be discriminated from
background radon concentrations.
Based on these comparisons, the background and non-background data sets are not statistically different.
Hence, it can be concluded that the RDT cannot be used at the OK Tool site to differentiate between
background and non-background areas, and therefore cannot be used to delineate the spatial extent of
DNAPL-bearing zones. This finding is specific for the OK Tool site and cannot be generalized to other
sites.
16 Figure 4-6. Mean radon concentrations in background and non-background areas in three sampling
events. One-standard deviation error bars are shown (Starr, 2006).
4.3 Temporal Trends in Radon and Use of Radon Deficit Technique
for Assessing Non-aqueous Phase Liquid Remediation
Lab tests and small-scale field trials at other locations suggest that dissolved radon concentrations
increase as NAPL saturation declines, and thus variations in radon concentrations in groundwater over
time may provide information about changes in NAPL saturation. In this study, concentrations of radon
in groundwater were monitored before and after two iterations of ISCO in a DNAPL-contaminated area.
It was anticipated that ISCO would reduce DNAPL saturation and that there would be a corresponding
increase in dissolved radon concentrations.
Monitoring data (permanganate, VOCs, and chloride) collected during two iterations of ISCO in the Test
Array area were interpreted as indicating that at least 20% of the VOC mass present in the Test Array area
prior to ISCO was removed during two iterations of ISCO (NHDES, 2005; 2006). The majority of the
mass removed was probably in the DNAPL phase at the beginning of ISCO. Although the majority of the
estimated DNAPL mass was thought to remain in the Test Array area after two iterations of ISCO, the
persistence of permanganate for approximately 1 year after injection suggests that DNAPL was largely
removed from the zones flushed with oxidant solution. If DNAPL remained in these horizons, then
permanganate would be expected to be completely consumed via oxidation reactions with VOCs.
17 Groundwater samples for radon analysis were collected from Test Array wells within the depth interval
where oxidant solution was injected. Many of the post-ISCO samples for radon analysis were pink or
purple, indicating that permanganate was present in groundwater at those locations. It is assumed that
DNAPL saturation declined in the vicinity of monitoring wells where radon samples were collected, and
hence it was expected that radon concentrations would increase between the baseline and post-ISCO
sampling events. If this expected behavior was observed, it would support the use of the RDT for
assessing DNAPL remediation at this site. On the other hand, if the expected trend was not observed,
then it would be concluded that the RDT does not provide definitive information about DNAPL
remediation at this site.
Radon concentrations measured in background and non-background (nine Test Array wells and one well,
PW-6D, in a DNAPL-bearing area outside the Test Array) areas are shown as the mean and standard
deviation of analysis of replicate samples (Figure 4-7). Here we focus on the Test Array wells because it
was expected that there would be a trend of increasing radon concentration over time in conjunction with
ISCO and the associated reduction in DNAPL saturation. Radon concentrations varied from place to
place and varied in time at most wells in the Test Array. The 2003 radon data represent pre-ISCO
conditions, and the 2004 and 2005 data represent conditions after approximately 1 and 2 years of
oxidation.
Figure 4-7. Mean radon concentrations and 1-standard deviation uncertainty measured in 2003, 2004, and
2005 (Starr, 2006).
18 In contrast to the expected trend of increasing radon concentrations with time at all wells in the Test
Array, only one (NEMLS) of the seven Test Array wells sampled all 3 years shows this trend (Figure 4-6
and Table 4-2). The expected trend of increasing radon concentrations over time, in conjunction with
DNAPL remediation, was not observed at the remaining six Test Array wells. One well showed stable
radon concentrations, one showed a decline in radon concentration, and the remaining four had
inconsistent trends over time. The failure for the anticipated trend to be observed indicates that either
ISCO was not effective for removing DNAPL from the immediate vicinity of the locations where radon
samples were collected, that radon concentrations are not related to DNAPL saturation in a simple
manner, or that other sources of variation in radon concentrations mask the changes caused by reduction
in DNAPL saturation. As a result of observing trends counter to the expected trends, our interpretation is
that changes in radon concentrations over time cannot be used to evaluate the effectiveness of DNAPL
remediation at the OK Tool site.
Table 4-2. Temporal trends in radon concentration in non-background wells in the INEL Well Array.
Observed
Trend
Increasing
Decreasing
Stable
Wells
NEMLS
CI
NWP
1
1
1
Number of
Occurrences
Variable
EI
NCP
NEP
NWMLS
4
The finding that radon concentrations did not show an increasing trend over time in the INEL Well Array
is contrary to the results of laboratory studies and field studies performed by others (Semprini et al., 2000;
Davis et al., 2002; 2003). We speculate that the following factors may account for the expected trend not
occurring:
• The variability in measured radon concentrations due to factors other than changes in DNAPL
saturation may mask the expected trend. The temporal variation in radon concentrations measured in
background wells (left portion of Figure 4-7) may reflect differences in the groundwater flow system,
sampling technique, or analytical technique. Similar variability in the Test Array wells may be larger
than the changes caused by removal of DNAPL.
• Although ISCO was performed for approximately 2 years between the 2003 and 2005 sampling
events, it is possible that DNAPL saturations in the immediate vicinity of the radon sampling points
were not substantially affected. This could occur if DNAPL was initially absent from the immediate
vicinity of the sampled depth in each well or if DNAPL saturation near a sampled depth was not
affected by ISCO.
• The relationship between DNAPL saturation and radon partitioning may be more complex than
assumed. For example, partitioning between the aqueous and DNAPL phases is probably related to
the DNAPL-water interfacial area as well as DNAPL saturation, and the relationship between
interfacial area and saturation may be complicated, especially at high DNAPL saturation (i.e., as
DNAPL transitions from a high saturation “pool” configuration to a low saturation “residual”
configuration).
• Precipitation of MnO2 on mineral grain surfaces could reduce the radon emanation rate, which could
account for the reduced Rn-222 concentrations in groundwater sampled from well CI.
19 5 SUMMARY AND CONCLUSIONS The study described in this report, which was funded by the EPA, evaluated the RDT for delineating the
spatial extent of DNAPL in the subsurface at a multi-acre field site, and also evaluated the ability of the
technique to evaluate the progress of DNAPL remediation. This study was performed in conjunction with
ISCO, which was used to remediate DNAPL in a sand and gravel aquifer at the OK Tool site. Rn-222
concentrations measured in groundwater samples collected before the first oxidant injection suggested
that Rn-222 concentrations in background samples (i.e., samples from areas where DNAPL was not
present) were higher than concentrations in samples collected from areas where DNAPL was present.
This finding was consistent with the results of lab and field studies performed by others, and suggested albeit with some ambiguity - that the RDT could be used to delineate the spatial extent of DNAPLbearing zones at the OK Tool site.
Approximately 1 year after the first injection of oxidant, groundwater samples were collected from
numerous wells in background areas and in areas where DNAPL was present. Although measured radon
concentrations in replicate samples showed good agreement, radon concentrations showed considerable
variation between wells in background areas. Furthermore, Rn-222 concentrations in background
samples were similar to concentrations measured in samples collected from DNAPL-bearing areas. These
features prevent Rn-222 concentrations in background areas from being discriminated from
concentrations in NAPL-contaminated areas. The inability to discriminate background areas from
DNAPL-contaminated areas (based on Rn-222 concentrations) indicates that the RDT cannot be used to
delineate the spatial extent of NAPL in the subsurface at the OK Tool site. This conclusion does not
necessarily apply to other sites. Indeed, the technique may be feasible at sites that have more
homogeneous background Rn-222 concentrations. Examples of such sites include ones smaller than the
approximately 4-acre OK Tool site and sites with more homogeneous subsurface media.
Temporal trends in Rn-222 concentrations in groundwater were determined for selected wells in
background areas, in areas where NAPL was present and affected by ISCO. The expected trends included
no change in background areas and a monatomic increase in areas affected by ISCO. The observed trends
were not consistent with the expected trends and in particular, radon concentrations in the areas affected
by ISCO did not uniformly increase. The inconsistency between the observed and expected trends
suggests that the RDT does not provide reliable information about the extent of DNAPL remediation at
the OK Tool site. It is recognized that some of the variability in measured Rn-222 concentrations may
result from variability in the purging, sampling, and analytical processes. However, the procedures
employed were typical of those used for routine groundwater sampling at remediation sites, and therefore
we anticipate that a similar level of variability would be encountered at many sites. Additional variability
may be attributed to differences in hydrologic conditions caused by precipitation events, fluctuations in
river stage, and changes in groundwater flowrates and direction, which were affected by variations in
operation of groundwater extraction wells. These complicating factors may interfere with application of
this technique but are typical of active remediation sites.
In summary, the RDT at this site does not appear to be useful for delineating the spatial extent of NAPL
or for evaluating the effectiveness of remedial activities. Although the extent to which these findings can
be transferred to other sites is unknown, these findings do not encourage further application of the RDT at
active remediation sites outside the research arena.
20 6 ACKNOWLEDGEMENTS
The study described in this report was funded by the EPA Office of Solid Waste and Emergency
Response’s Measurement and Monitoring Technologies for the 21st Century (21M2) initiative, EPARegion 1, and the National Exposure Research Laboratory Technology Support Center. EPA Region 1
provided overall project management for the OK Tool site, coordinated activities among various
organizations, and provided review comments on project work control documents and data reports. The
United States Geological Survey provided natural gamma logs for selected boreholes at the site. The
New Hampshire Department of Environmental Services provided analytical data from their groundwater
monitoring program. NA WaterSystems performed the ISCO project and provided logistical support to
sampling activities. North Wind, Inc. (NWI) field teams led by Tamzen Macbeth collected groundwater
samples. Rn-222 analyses were preformed by the Environmental Assessment Laboratory at Idaho State
University, under the direction of Richard Brey. Discussions with Lewis Semprini of Oregon State
University provided useful information during formulation of research plans.
Funding for this project was provided by the EPA as a result of a proposal submitted by Richard Goehlert
of EPA Region 1. These funds were managed by EPA’s Technical Support Center in Las Vegas under
the direction of J. Gareth Pearson, Chris Sieborg, and, currently, Brian Schumacher. Funds were
transferred to the U.S. Department of Energy via an interagency agreement managed by Robert Jones of
the Idaho Field Office, to Battelle Energy Alliance via a project managed by Kenneth Moor, and thence to
NWI.
21 7 REFERENCES
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