Arthur L. Baehr

Removing volatile contaminants from the unsaturated zone by inducing advective air-phase transport

Abstract Organic liquids inadvertently spilled and then distributed in the unsaturated zone can pose a long-term threat to ground water. Many of these substances have significant volatility, and thereby establish a premise for contaminant removal from the unsaturated zone by inducing advective air-phase transport with wells screened in the unsaturated zone. In order to focus attention on the rates of mass transfer from liquid to vapour phases, sand columns were partially saturated with gasoline and vented under steady air-flow conditions. The ability of an equilibrium-based transport model to predict the hydrocarbon vapor flux from the columns implies an efficient rate of local phase transfer for reasonably high air-phase velocities. Thus the success of venting remediations will depend primarily on the ability to induce an air-flow field in a heterogeneous unsaturated zone that will intersect the distributed contaminant. To analyze this aspect of the technique, a mathematical model was developed to predict radially symmetric air flow induced by venting from a single well. This model allows for in-situ determinations of air-phase permeability, which is the fundamental design parameter, and for the analysis of the limitations of a single well design. A successful application of the technique at a site once contaminated by gasoline supports the optimism derived from the experimental and modeliing phases of this study, and illustrates the well construction and field methods used to document the volatile contaminant recovery.

Evaluation of Unsaturated Zone Air Permeability Through Pneumatic Tests

Predicting the steady state distribution of air pressure in the unsaturated zone resulting from a pneumatic test provides a method for determining air-phase permeability. This technique is analogous to the inverse problem of well hydraulics; however, air flow is more complicated than ground water flow because of air compressibility, the Klinkenberg effect, variations in air density and viscosity that result from temperature fluctuations in the unsaturated zone and the possibility of inducing water movement during the pneumatic test. An analysis of these complicating factors reveals that, when induced water movement can be neglected, a linear version of the airflow equation can provide an appropriate approximation for the purpose of determining air-phase permeability. Two analytical solutions for steady state, two-dimensional, axisymmetric airflow to a single well partially screened in the unsaturated zone are developed. One solution applies where there is a stratum of relatively low air permeability, separating the stratum in which the well is completed, from the atmosphere. The other solution applies where there is no separating stratum between the domain and atmosphere. In both situations the water table forms the lower horizontal boundary. Applications of both solutions to determine air permeability from data collected during pneumatic tests are presented.

Quantification of aerobic biodegradation and volatilization rates of gasoline hydrocarbons near the water table under natural attenuation conditions

Aerobic biodegradation and volatilization near the water table constitute a coupled pathway that contributes significantly to the natural attenuation of hydrocarbons at gasoline spill sites. Rates of hydrocarbon biodegradation and volatilization were quantified by analyzing vapor transport in the unsaturated zone at a gasoline spill site in Beaufort, South Carolina. Aerobic biodgradation rates decreased with distance above the water table, ranging from 0.20 to 1.5 g m−3 d−1 for toluene, from 0.24 to 0.38 g m−3 d−1 for xylene, from 0.09 to 0.24 g m−3 d−1 for cyclohexene, from 0.05 to 0.22 g m−3 d−1 for ethylbenzene, and from 0.02 to 0.08 g m−3 d−1 for benzene. Rates were highest in the capillary zone, where 68% of the total hydrocarbon mass that volatilized from the water table was estimated to have been biodegraded. Hydrocarbons were nearly completely degraded within 1m above the water table. This large loss underscores the importance of aerobic biodradation in limiting the transport of hydrocarbon vapors in the unsaturated zone and implies that vapor-plume migration to basements and other points of contact may only be significant if a source of free product is present. Furthermore, because transport of the hydrocarbon in the unsaturated zone can be limited relative to that of oxygen and carbon dioxide, soil-gas surveys conducted at hydrocarbon-spill sites would benefit by the inclusion of oxygen- and carbon-dioxide-gas concentration measurements. Aerobic degradation kinetics in the unsaturated zone were approximately first-order. First-order rate constants near the water table were highest for cyclohexene (0.21–0.65 d−1) and nearly equivalent for ethylbenzene (0.11–0.31 d−1), xylenes (0.10–0.31 d−1), toluene (0.09–0.30 d−1), and benzene (0.07–0.31 d−1). Hydrocarbon mass loss rates at the water table resulting from the coupled aerobic biodgradation and volatilization process were determined by extrapolating gas transport rates through the capillary zone. Mass loss rates from groundwater were highest for toluene (0.20–0.84 g m−2 d−1), followed by xylenes (0.12–0.69 g m−2 d−1), cyclohexene (0.05–0.15 g m−2 d−1), ethylbenzene (0.02–0.12 g m−2 d−1), and benzene (0.01–0.04 g m−2 d−1). These rates exceed predicted rates of solubilization to groundwater, demonstrating the effectiveness of aerobic biodgradation and volatilization as a combined natural attenuation pathway.

Evaluation of the atmosphere as a source of volatile organic compounds in shallow groundwater

The atmosphere as a source of volatile organic compounds (VOCs) in shallow groundwater was evaluated over an area in southern New Jersey. Chloroform, methyl tertbutyl ether (MTBE), 1,1,1-trichloroethane, tetrachloroethylene (PCE), and carbon disulfide (not a VOC) were detected frequently at low-level concentrations in a network of 78 shallow wells in the surficial Kirkwood-Cohansey aquifer system. The atmosphere was sampled for these compounds and only MTBE concentrations were high enough to potentially explain frequent detection in shallow groundwater. A mathematical model of reactive transport through the unsaturated zone is presented to explain how variations in unsaturated properties across the study area could explain differences in MTBE concentrations in shallow groundwater given the atmosphere as the source. Even when concentrations of VOCs in groundwater are low compared to regulatory concentration limits, it is critical to know the source. If the VOCs originate from a point source((), concentrations in groundwater could potentially increase over time to levels of concern as groundwater plumes evolve, whereas if the atmosphere is the source, then groundwater concentrations would be expected to remain at low-level concentrations not exceeding those in equilibrium with atmospheric concentrations. This is the first analysis of VOC occurrence in shallow groundwater involving colocated atmosphere data.

Estimation of Rates of Aerobic Hydrocarbon Biodegradation by Simulation of Gas Transport in the Unsaturated Zone

The distribution of oxygen and carbon dioxide gases in the unsaturated zone provides a geochemical signature of aerobic hydrocarbon degradation at petroleum product spill sites. The fluxes of these gases are proportional to the rate of aerobic biodegradation and are quantified by calibrating a mathematical transport model to the oxygen and carbon dioxide gas concentration data. Reaction stoichiometry is assumed to convert the gas fluxes to a corresponding rate of hydrocarbon degradation. The method is applied at a gasoline spill site in Galloway Township, New Jersey, to determine the rate of aerobic degradation of hydrocarbons associated with passive and bioventing remediation field experiments. At the site, microbial degradation of hydrocarbons near the water table limits the migration of hydrocarbon solutes in groundwater and prevents hydrocarbon volatilization into the unsaturated zone. In the passive remediation experiment a site-wide degradation rate estimate of 34,400 g yr−1 (11.7 gal. yr−1) of hydrocarbon was obtained by model calibration to carbon dioxide gas concentration data collected in December 1989. In the bioventing experiment, degradation rate estimates of 46.0 and 47.9 g m−2 yr−1(1.45 × 10−3 and 1.51 × 10−3 gal. ft.−2yr−1) of hydrocarbon were obtained by model calibration to oxygen and carbon dioxide gas concentration data, respectively. Method application was successful in quantifying the significance of a naturally occurring process that can effectively contribute to plume stabilization.

An Updated Model of Induced Airflow in the Unsaturated Zone

Simulation of induced movement of air in the unsaturated zone provides a method to determine permeability and to design vapor extraction remediation systems. A previously published solution to the airflow equation for the case in which the unsaturated zone is separated from the atmosphere by a layer of lower permeability (such as a clay layer) has been superseded. The new solution simulates airflow through the layer of lower permeability more rigorously by defining the leakage in terms of the upper boundary condition rather than by adding a leakage term to the governing airflow equation. This note presents the derivation of the new solution. Formulas for steady state pressure, specific discharge, and mass flow in the domain are obtained for the new model and for the case in which the unsaturated zone is in direct contact with the atmosphere.

Estimation of hydrocarbon biodegradation rates in gasoline-contaminated sediment from measured respiration rates

An open microcosm method for quantifying microbial respiration and estimating biodegradation rates of hydrocarbons in gasoline-contaminated sediment samples has been developed and validated. Stainless-steel bioreactors are filled with soil or sediment samples, and the vapor-phase composition (concentrations of oxygen (O2), nitrogen (N2), carbon dioxide (CO2), and selected hydrocarbons) is monitored over time. Replacement gas is added as the vapor sample is taken, and selection of the replacement gas composition facilitates real-time decision-making regarding environmental conditions within the bioreactor. This capability allows for maintenance of field conditions over time, which is not possible in closed microcosms. Reaction rates of CO2 and O2 are calculated from the vapor-phase composition time series. Rates of hydrocarbon biodegradation are either measured directly from the hydrocarbon mass balance, or estimated from CO2 and O2 reaction rates and assumed reaction stoichiometries. Open microcosm experiments using sediments spiked with toluene and p-xylene were conducted to validate the stoichiometric assumptions. Respiration rates calculated from O2 consumption and from CO2 production provide estimates of toluene and p-xylene degradation rates within about ±50% of measured values when complete mineralization stoichiometry is assumed. Measured values ranged from 851.1 to 965.1 g m−3 year−1 for toluene, and 407.2–942.3 g m−3 year−1 for p-xylene. Contaminated sediment samples from a gasoline-spill site were used in a second set of microcosm experiments. Here, reaction rates of O2 and CO2 were measured and used to estimate hydrocarbon respiration rates. Total hydrocarbon reaction rates ranged from 49.0 g m−3 year−1 in uncontaminated (background) to 1040.4 g m−3 year−1 for highly contaminated sediment, based on CO2 production data. These rate estimates were similar to those obtained independently from in situ CO2 vertical gradient and flux determinations at the field site. In these experiments, aerobic conditions were maintained in the microcosms by using air as the replacement gas, thus preserving the ambient aerobic environment of the subsurface near the capillary zone. This would not be possible with closed microcosms.

Methyl tert-butyl ether degradation in the unsaturated zone and the relation between MTBE in the atmosphere and shallow groundwater

Atmospheric methyl tert-butyl ether (MTBE) concentrations in southern New Jersey generally exceeded concentrations in samples taken from the unsaturated zone. A simple unsaturated zone transport model indicates that MTBE degradation can explain the attenuation with half-lives from a few months to a couple of years. Tert-butyl alcohol (TBA), a possible degradation product of MTBE, was detected in unsaturated-zone samples at concentrations exceeding atmospheric levels at some sites, suggesting the possible conversion of MTBE to TBA. At sites where MTBE was detected in shallow groundwater, the concentration was typically higher than the overlying unsaturated-zone concentration. This observation is consistent with outgassing from the aquifer and combined with the unsaturated-zone attenuation suggests some of the MTBE detections in shallow groundwater are nonatmospheric in origin, coming from leaking tanks, road runoff, or other sources. The identification of sources of MTBE in groundwater and attenuation mechanisms through the hydrologic cycle is critical in developing an understanding of the long-term effect of MTBE releases.

Spatial Variability of Groundwater Recharge and its Effect on Shallow Groundwater Quality in Southern New Jersey

ity in the Glassboro, NJ area (Fig. 1). The Darcian method was used to estimate groundwater recharge Point estimates of groundwater recharge at 48 sediment-coring from water-retention parameters and unsaturated hylocations vary substantially ( 18.5–1840 cm yr 1) in a 930-km2 area of southern New Jersey. Darcian estimates of steady, long-term recharge draulic conductivity on the basis of sediment texture made at depth in the unsaturated zone were estimated using pedoand moisture content data obtained near the water table transfer functions of soil texture and interpolated (mapped) with at 48 locations in the study area. The recharge estimates nonparametric methods to assess aquifer vulnerability in the area. were geostatistically analyzed to evaluate the spatial The probability of exceeding the median recharge (29.1 cm yr 1) is variability of measured sediment properties, to map relow in the southwestern and northeastern portions of the study area charge with respect to land use, and to derive statistical and high in the eastern and southeastern portions. Estimated recharge distributions of recharge at specific locations in the study is inversely related to measured percentage clay and positively related area. The recharge estimates were compared with soils to the percentage of well-drained soils near wells. Spatial patterns of and topographic data to determine whether recharge recharge estimates, exceedance probabilities, and clay content indicate could be accurately predicted from landscape characterthat sediment texture controls recharge in the study area. Relations with land elevation and a topographic wetness index were statistically istics. Finally, recharge estimates were compared with insignificant. Nitrate concentration and atrazine (6-chloro-N2-ethylconcentrations of NO3 and atrazine to evaluate potential N4-isopropyl-1,3,5-triazine-2,4-diamine) percentage detection in sameffects on the quality of shallow, recently recharged ples of shallow groundwater (typically 10 m) are higher for low groundwater. In this study, “shallow groundwater” rerecharge sites ( 29.1 cm yr 1) than for high recharge sites ( 29.1 cm fers to depths 10 m. The depth of the screened interval yr 1) in agricultural and urban areas. Differences between high and below water in observation wells in the area is about low recharge sites in these areas are highly significant for NO3 concen3 m. The objectives of the study were to tration, but not for atrazine concentration. • evaluate the spatial variability of point estimates of ground-water recharge, • map recharge with respect to land use, and N contamination is considered the • compare recharge estimates with NO3 and atrazine single greatest threat to water quality (Corwin et concentrations in shallow groundwater. al., 1997). Preventing contamination of groundwater is crucial in areas where it is a major source of public and domestic supply. Knowing where an aquifer is vulneraMATERIALS AND METHODS ble to surface-derived contaminants would help managDescription of Study Area ers prioritize scarce resources for alternative management practices, monitoring, and cleanup. The study area (Fig. 1) comprises about 930 km within the Coastal Plain Physiographic province of southern New Jersey. Aquifer vulnerability studies at large spatial scales Population in the area has increased from about 50 000 people have used index methods, such as DRASTIC and SEEPin 1940 to about 250 000 in 2000. Groundwater withdrawals AGE (Navulur and Engel, 1996), or overlays made with from the surficial, Kirkwood-Cohansey aquifer system, which geographic information systems (GISs) (Nolan et al., consists of highly permeable unconsolidated sands and gravels, 1997). Both DRASTIC and SEEPAGE underestimated have recently increased to meet the growing demand for drinkcontamination potential by describing areas with high ing water. As of 1986, the Glassboro area comprised 21% NO3 concentration as low risk (Navulur and Engel, urban land, 26% agricultural land, and 39% undeveloped land 1996). Index and overlay methods provide only limited (Stackelberg et al., 1997). understanding of processes controlling the transport of The outcrop of the Kirkwood Formation, a confining unit water and chemicals in the unsaturated zone. Alternaabout 30 m thick, underlies the aquifer and forms the northwest boundary of the study area. Aquifer thickness increases tively, deterministic models can simulate water and to about 75 m at the southeastern boundary (Zapecza, 1989). chemical fluxes, but the spatial variability of sediment Unsaturated zone sediment in the study area consists mainly properties at field scales and above limits accuracy and of the Cohansey Sand, which was deposited during the Mioimposes large uncertainty on model predictions. cene Age on inner shelf, nearshore, and beach areas during In the current study, we used a combined determinisslow retreat of the sea. Sediments in the Cohansey Sand genertic–geostatistical approach to assess aquifer vulnerabilally are coarser at shallower depths, which is consistent with similarly deposited formations in the New Jersey Coastal Plain B.T. Nolan, U.S. Geological Survey, 413 National Center, Reston, (Zapecza, 1989). The Bridgeton Formation overlies the CoVA 20192; A.L. Baehr and L.J. Kauffman, U.S. Geological Survey, West Trenton, NJ. Received 27 Nov. 2002. Original Research Paper. Abbreviations: CCDF, conditional cumulative distribution function; *Corresponding author (btnolan@usgs.gov). DEM, digital elevation model; DO, dissolved oxygen; GIS, geographic information system; IGF, Indicative Goodness of Fit; IK, indicator Published in Vadose Zone Journal 2:677–691 (2003).  Soil Science Society of America kriging; KED, Kriging with external drift; MLR, multiple linear regression; PTF, pedotransfer function. 677 S. Segoe Rd., Madison, WI 53711 USA

Seasonal and daily variations in concentrations of methyl-tertiary-butyl ether (MTBE) at Cranberry Lake, New Jersey

Methyl-tertiary-butyl ether (MTBE), an additive used to oxygenate gasoline, has been detected in lakes in northwestern New Jersey. This occurrence has been attributed to the use of gasoline-powered watercraft. This paper documents and explains both seasonal and daily variations in MTBE concentrations at Cranberry Lake. During a recent boating season (late April to September 1999), concentrations of MTBE typically exceeded 20 microg/L. MTBE concentrations varied daily from 12 to 24 microg/L over a 2-week period that included the Labor Day holiday. Concentrations were highest on weekends when there is more boat traffic, which had an immediate effect on MTBE mass throughout the lake. MTBE concentrations decreased to about 2 microg/L shortly after the end of the summer recreational season. The loss of MTBE can be accounted for by volatilization, with a half-life on the order of 10 days. The volatilization rate was modeled with the daily decrease in MTBE then the modeled rate was validated using the data from the seasonal decline.