Push-pull partitioning tracer tests using radon-222 to quantify non-aqueous phase liquid contamination

Using groundwater monitoring wells for rapid application of soil gas radon deficit technique to evaluate residual LNAPL

The application of the 222Radon (Rn) deficit technique using subsurface soil gas probes for the identification and quantification of light non-aqueous phase liquids (LNAPL) has provided positive outcomes in recent years. This study presents an alternative method for applying this technique in the headspace of groundwater monitoring wells. The developed protocol, designed for groundwater monitoring wells with a portion of their screen in the vadose zone, is based on the use of portable equipment that allows rapid measurement of the Rn soil gas activity in the vadose zone close to the water table (i.e., smear zone) where LNAPL is typically expected. The paper first describes the step-by-step procedure to be followed for the application of this method. Then, a preliminary assessment of the potential of the method was carried out at two Italian sites characterized by accidental gasoline and diesel spills into the subsurface from underground storage tanks. Although the number of tests conducted does not allow for definitive conclusions, the results obtained suggest that, from a qualitative point of view, Rn monitoring in the headspace of monitoring wells is a promising, fast, and minimally invasive screening method that could also potentially reduce the costs associated with field data acquisition. This method proves to be suitable for detecting the presence of LNAPL in both the mobile and residual phases with results consistent with the other lines of evidence available at the sites, such as groundwater and soil gas monitoring. Future efforts should be directed toward evaluating the accuracy of this method for a quantitative assessment of residual LNAPL saturations.

Modeling of soil gas radon as an in situ partitioning tracer for quantifying LNAPL contamination

In the last decades radon (Rn) has been widely proposed as a naturally occurring tracer for non-aqueous phase liquids (NAPL) in the soil. This work examines the feasibility of using soil gas data collected at some distance from the source zone for the application of the Rn deficit technique for the identification and quantification of NAPL contamination. To this end, we used a steady-state 1-D analytical solution that is based on a 3-layer model that allows to simulate the transport and distribution of Rn in the source zone, capillary fringe and overlying unsaturated soil. The analytical solution was first validated against a more detailed numerical model available in the literature. Then, a series of simulations were carried out to evaluate the vertical concentration profiles of Rn in soil gas above the source zone and in background location not impacted by NAPL. Simulation results showed that the parameters that most influence the migration and distribution of Rn in the subsurface are the distance of the soil gas probe from the source zone and, to a lower extent, the type of contamination (e.g. diesel or gasoline) and the soil type. On the basis of these results, we developed some easy-to-use nomographs to estimate the residual NAPL phase based on the observed radon deficit in soil gas and on the probe to source distance and soil and NAPL characteristics. According to the obtained results, the radon deficit technique results a feasible method for a qualitative identification of residual NAPL when radon in soil gas is measured at distances lower than 2 m from the contaminated zone. However, for an accurate quantitative estimation of the NAPL phase content, soil gas probes should be preferably located at distances lower than 1 m from the source zone.

Characterization of a NAPL-contaminated site using the partitioning behavior of noble gases

Noble gases have been used for oil field exploration due to their partitioning behavior in oil-water systems. However, their application to study sites contaminated with non-aqueous phase liquids (NAPL) has been limited, except for ²²²Rn, which has been traditionally used as a partitioning tracer for contaminated sites. This study applied natural noble gas components such as ²²²Rn, He, Ne, Ar, Kr, and Xe to the characterization of a field site contaminated with trichloroethylene (TCE) located in Wonju, Korea. Groundwater at the site showed a maximum level of TCE that exceeded 1000 μg/L, with an approximate average of 400 μg/L, indicating the presence of residual TCE in the subsurface system even after remediation. The traditional tracer (i.e., ²²²Rn) was first used to characterize residual TCE. However, its heterogeneous distribution throughout the fractured bedrock aquifer negated its usefulness as a TCE indicator. The use of radiogenic ⁴He was also limited by the wide distribution of radiogenic sources on the site. By contrast, changes in the TCE level had clear effects on the conditions of other noble gases, such as Ne, Ar, and Xe, making them useful for characterization of the TCE-contaminated site. Furthermore, calculation of the TCE/water ratio including residual TCE was achieved, but identification of the TCE originating from the vadose zone was relatively hard. The results of this study indicate that based on their partitioning behavior, naturally-occurring noble gases can be used to delineate and quantify residual TCE.

Solute Reactive Tracers for Hydrogeological Applications: A Short Review and Future Prospects

Tracer testing is a mature technology used for characterizing aquatic flow systems. To gain more insights from tracer tests a combination of conservative (non-reactive) tracers together with at least one reactive tracer is commonly applied. The reactive tracers can provide unique information about physical, chemical, and/or biological properties of aquatic systems. Although, previous review papers provide a wide coverage on conservative tracer compounds there is no systematic review on reactive tracers yet, despite their extensive development during the past decades. This review paper summarizes the recent development in compounds and compound classes that are exploitable and/or have been used as reactive tracers, including their systematization based on the underlying process types to be investigated. Reactive tracers can generally be categorized into three groups: (1) partitioning tracers, (2) kinetic tracers, and (3) reactive tracers for partitioning. The work also highlights the potential for future research directions. The recent advances from the development of new tailor-made tracers might overcome existing limitations.

In-situ determination of field-scale NAPL mass transfer coefficients: Performance, simulation and analysis

Better estimates of non-aqueous phase liquid (NAPL) mass, its persistence into the future, and the potential impact of source reduction are critical needs for determining the optimal path to clean up sites impacted by NAPLs. One impediment to constraining time estimates of source depletion is the uncertainty in the rate of mass transfer between NAPLs and groundwater. In this study, an innovative field test is demonstrated for the purpose of quantifying field-scale NAPL mass transfer coefficients (kl(N)) within a source zone of a fuel-contaminated site. Initial evaluation of the test concept using a numerical model revealed that the aqueous phase concentration response to the injection of clean groundwater within a source zone was a function of NAPL mass transfer. Under rate limited conditions, NAPL dissolution together with the injection flow rate and the radial distance to monitoring points directly controlled time of travel. Concentration responses observed in the field test were consistent with the hypothetical model results allowing field-scale NAPL mass transfer coefficients to be quantified. Site models for groundwater flow and solute transport were systematically calibrated and utilized for data analysis. Results show kl(N) for benzene varied from 0.022 to 0.60d(-1). Variability in results was attributed to a highly heterogeneous horizon consisting of layered media of varying physical properties.

Fluorescent dye imaging of the volume sampled by single well forced-gradient tracer tests evaluated in a laboratory-scale aquifer physical model

This study presents a new method to visualise forced-gradient tracer tests in 2-D using a laboratory-scale aquifer physical model. Experiments were designed to investigate the volume of aquifer sampled in vertical dipole flow tracer tests (DFTT) and push-pull tests (PPT), using a miniature monitoring well and straddle packer arrangement equipped with solute injection and recovery chambers. These tests have previously been used to estimate bulk aquifer hydraulic and transport properties for the evaluation of natural attenuation and other remediation approaches. Experiments were performed in a silica glass bead-filled box, using a fluorescent tracer (fluorescein) to deduce conservative solute transport paths. Digital images of fluorescein transport were captured under ultraviolet light and processed to analyse tracer plume geometry and obtain point-concentration breakthrough histories. Inorganic anion mixtures were also used to obtain conventional tracer breakthrough histories. Concentration data from the conservative tracer breakthrough curves was compared with the digital images and a well characterised numerical model. The results show that the peak tracer breakthrough response in dipole flow tracer tests samples a zone of aquifer close to the well screen, while the sampling volume of push-pull tests is limited by the length of the straddle packers used. The effective sampling volume of these single well forced-gradient tests in isotropic conditions can be estimated with simple equations. The experimental approach offers the opportunity to evaluate under controlled conditions the theoretical basis, design and performance of DFTTs and PPTs in porous media in relation to measured flow and transport properties.

Effect of soil organic carbon on the quantification of jet-fuels in soil using partitioning tracer method

Partitioning tracer method has been studied as an effective technique for estimating the light nonaqueous-phase liquid (LNAPL) contamination in the subsurface. This study is for investigating the effect of soil organic contents on the LNAPL quantification using partitioning tracer method. The sorption characteristics of alcohol tracers to the soils having different organic carbon contents were evaluated by sorption isotherm experiments. In the column tests, the soils were contaminated with jet-fuel and the average saturations of residual jet-fuel were estimated by partitioning tracer method and compared with that by the volume measurement. The sorption results indicated that considerable amount of 4-methyl-2-pentanol and hexanol could be sorbed to the soils and the sorption amount of 2-ethyl-1-butanol was relatively smaller than those of the other alcohol tracers. The column experiments demonstrated that the accuracy of quantification for the jet-fuel by partitioning tracer method should decrease with increasing the soil organic carbon contents. However, the accuracy could be enhanced by considering the sorption of tracer to the soils, especially for the tracer of 2-ethyl-1-butanol.

Push-pull tests to quantify in situ degradation rates at a phytoremediation site

Nine push-pull tests (PPTs) were performed to determine in-situ aerobic respiration rates at a creosote-contaminated site and to assess the contribution of hybrid poplar trees to the remediation of polynuclear aromatic hydrocarbons (PAH) in groundwater. PPTs were conducted by injecting a solution containing dissolved oxygen and naphthalene (reactive tracers) with bromide (nonreactive tracer) into wells constructed in a shallow unconfined aquifer. The objective of this study was to determine seasonal variation and spatial differences (contaminated versus uncontaminated areas and treed versus untreed areas) in the rate of consumption of dissolved oxygen. First-order aerobic respiration rates varied from 0.0 (control well) to 1.25 hr(-1), which occurred at a planted area in early summer (June). Rates measured in winter at treed areas were greater by a factor of 3-5 when compared to winter rates determined at nontreed areas of the site. Rates at treed regions were found to increase by over 4 times in summer relative to winter at the same location.

Numerical simulations of radon as an in situ partitioning tracer for quantifying NAPL contamination using push-pull tests

Presented here is a reanalysis of results previously presented by [Davis, B.M., Istok, J.D., Semprini, L., 2002. Push-pull partitioning tracer tests using radon-222 to quantify non-aqueous phase liquid contamination. J. Contam. Hydrol. 58, 129-146] of push-pull tests using radon as a naturally occurring partitioning tracer for evaluating NAPL contamination. In a push-pull test where radon-free water and bromide are injected, the presence of NAPL is manifested in greater dispersion of the radon breakthrough curve (BTC) relative to the bromide BTC during the extraction phase as a result of radon partitioning into the NAPL. Laboratory push-pull tests in a dense or DNAPL-contaminated physical aquifer model (PAM) indicated that the previously used modeling approach resulted in an overestimation of the DNAPL (trichloroethene) saturation (S(n)). The numerical simulations presented here investigated the influence of (1) initial radon concentrations, which vary as a function of S(n), and (2) heterogeneity in S(n) distribution within the radius of influence of the push-pull test. The simulations showed that these factors influence radon BTCs and resulting estimates of S(n). A revised method of interpreting radon BTCs is presented here, which takes into account initial radon concentrations and uses non-normalized radon BTCs. This revised method produces greater radon BTC sensitivity at small values of S(n) and was used to re-analyze the results from the PAM push-pull tests reported by Davis et al. The re-analysis resulted in a more accurate estimate of S(n) (1.8%) compared with the previously estimated value (7.4%). The revised method was then applied to results from a push-pull test conducted in a light or LNAPL-contaminated aquifer at a field site, resulting in a more accurate estimate of S(n) (4.1%) compared with a previously estimated value (13.6%). The revised method improves upon the efficacy of the radon push-pull test to estimate NAPL saturations. A limitation of the revised method is that 'background' radon concentrations from a non-contaminated well in the NAPL-contaminated aquifer are needed to accurately estimate NAPL saturation. The method has potential as a means of monitoring the progress of NAPL remediation.

Static and push-pull methods using radon-222 to characterize dense nonaqueous phase liquid saturations

Naturally occurring radon in ground water can potentially be used as an in situ partitioning tracer to characterize dense nonaqueous phase liquid (DNAPL) saturations. The static method involves comparing radon concentrations in water samples from DNAPL-contaminated and noncontaminated portions of an aquifer, while the push-pull method involves the injection (push) and extraction (pull) of a radon-free test solution from a single well. In the presence of DNAPL, radon concentrations during the pull phase are retarded, with retardation manifested in greater dispersion of radon concentrations relative to a conservative tracer. The utility of these methods was investigated in the laboratory using a physical aquifer model (PAM). Static and push-pull tests were performed before and after contamination of the PAM sediment pack with trichloroethene (TCE), and after alcohol cosolvent flushing and pump-and-treat remediation. Numerical simulations were used to estimate the retardation factor for radon in push-pull tests. Radon partitioning was observed in static and push-pull tests conducted after TCE contamination. Calculated TCE saturations ranged up to 1.4% (static test) and 14.1% (push-pull test). Post-remediation tests showed decreases in TCE saturations. The results show that radon is sensitive to changes in DNAPL saturation in space and time. However, the methods are sensitive to DNAPL saturation heterogeneity, test location, sample size, and test design. The influence of these factors on test results, as well as the apparent overestimation of the retardation factor in push-pull tests, warrant further investigation.