Environ. Sci. Technol. 1998, 32, 3472-3478
Temporal and Spatial Trends in Biogeochemical Conditions at a Groundwater-Surface Water Interface: Implications for Natural Bioattenuation JOHN M. LENDVAY, SEAN M. DEAN, AND PETER ADRIAENS* Environmental and Water Resources Engineering, University of Michigan, 181 EWRE Building, 1351 Beal Avenue, Ann Arbor, Michigan 48109-2125
The biogeochemical effects of a large surface water on a contaminated anaerobic groundwater at the groundwatersurface water interface (GSI) were evaluated using spatially discretized multilevel arrays and interpreted in light of natural bioattenuation mechanisms. Groundwater samples, collected during a 5-month evaluation period with increasing storm activity, were evaluated to determine the effect of lake activity on the oxidation capacity and contaminant distribution within the plume. Our analyses indicate that concentrations of methane and chloroethene decreased as the groundwater became increasingly oxidized along the GSI in shallow sample points impacted by infiltration of oxygenated lake water. cis-1,2Dichloroethene remained unchanged or slightly increased at the same locations, indicating that the decrease in methane and chloroethene was not due to dilution effects. Moreover, negative correlation of chloroethene and methane data with oxygen suggest that chloroethene is cooxidized by methane-oxidizing bacteria in the shallow zone of the plume. Contrary to oxidative processes in the shallow zone, reductive dechlorination of contaminants remained the predominant bio-transformation process in the deep zones of the GSI with chloroethene and ethene being the major contributors to total contaminant concentration. This study is the first to evaluate the effects of seasonal changes on a chlorinated ethenecontaminated plume at the GSI in spatial and temporal detail.
Introduction The St. Joseph, MI, National Priority List (NPL) site has been extensively characterized for contaminant distribution and biogeochemical conditions between the contaminant source and zone of emergence in Lake Michigan (1-8). The source of the contaminant plume at the site consists of trichloroethene (TCE) and minor contamination with hydrocarbons (9). The total chlorinated ethene contaminant flux is estimated at 310 kg/yr near the suspected source to 13 kg/yr near the beach (4). Contaminant hydrocarbons and natural organic matter have stimulated sufficient indigenous microbial activity in the groundwater to result in an anaerobic, predominantly sulfate-reducing and methanogenic, plume * Corresponding author telephone: (734)763-1464; fax: (734)7632275; e-mail:
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in near-source regions (3, 7). Under these terminal electronaccepting processes (TEAPs), TCE has been reductively dechlorinated to predominantly cis-1,2-dichloroethene (DCE), chloroethene (vinyl chloride or VC), and ethene with minor production of 1,1-DCE, trans-1,2-DCE, and ethane. Since the source of contamination is located approximately 750 m up gradient of Lake Michigan, the flow of groundwater toward the lake has raised concern about the potential contamination of the surface water (Lake Michigan) with reductive dechlorination products, particularly chloroethene. Between 1994 and 1996, three transects of temporary bore holes were established on the beach and approximately 100 m from shore under the lake bottom (1) to monitor the contaminant distribution and oxidationreduction conditions at the GSI (Figure 1A,B). This study of the GSI suggested that the predominant TEAP under Lake Michigan and in the deeper zone along the beach was sulfate reduction interspersed with zones of methanogenic conditions at the northern end of the study area. In addition, it was found that aquifer solids were increasingly depleted in bioavailable iron(III) toward the southwest, resulting in an accumulation of iron(II)-containing solids. Furthermore, Lendvay et al. (1) found evidence suggesting that reductive dechlorination processes were the predominant contaminant transformation processes in the deep zones of the plume under methanogenic conditions at the GSI. It was hypothesized that re-oxygenation of the aquifer to hypoxic conditions might provide a suitable environment for aerobic commensalic or cometabolic biodegradation processes in the shallow zone along the beach. The anaerobic transformation of chlorinated ethenes has been extensively studied. The main conclusions of laboratory studies are that DCE is the end product under iron- and sulfate-reducing conditions (10-12), and ethene or ethane is the end product under methanogenic conditions (13-15). These laboratory trends have been observed in the field, including at this site (16). Upgradient of the GSI, Semprini et al. (3) observed a correlation of isomer distribution reflective of reductive dechlorination with sulfate-reducing and methanogenic conditions, with DCE as the predominant product under the former and chloroethene or ethene as the final product under the latter. Lendvay et al. (1) corroborated the conclusions of Semprini et al. (3) for sampling locations at the beach and under Lake Michigan for this same site. While reductive dechlorination of chloroethene is relatively slow as compared to higher chlorinated ethenes (17), oxidative processes may result in faster and more complete transformation. (Co)-oxidation of chloroethene has been demonstrated to occur under heterotrophic (18-20), autotrophic (21), methanotrophic (22-24), and iron(III)-reducing (25) conditions. Additionally, the use of chloroethene as a sole source of carbon and energy was demonstrated using a Mycobacterium isolate from a chloroethene-contaminated soil (26, 27) and a Rhodococcus isolated from a TCE-degrading mixed laboratory culture (28, 29). Groundwater-surface water interfaces may provide conditions favorable for oxidative processes, as the flow field generated by wave run-up and infiltration with oxygenated surface water may reoxidize the aquifer. Coastal regions experiencing tidal action and beach erosion or accretion have GSIs that may extend several meters inland from shore (30, 31). These interfaces can be hydrologically extremely dynamic and are characterized by wave run-up and infiltration, groundwater seepage in to and out of the surface water body, groundwater-surface water mixing, and possible 10.1021/es980049t CCC: $15.00
1998 American Chemical Society Published on Web 09/24/1998
FIGURE 1. Conceptual cross-sectional view (A) and plan view (B) of the sample locations with inset showing geographical location of site in Michigan. ebullition processes as a result of gases produced from anaerobic microbial activity (32, 33). While the hydrodynamic effects of a GSI on a contaminant plume have been extensively described (34-37), little has been done to evaluate the result of this physical interaction on the aquifer oxidation-reduction (redox) capacities and potential for in-situ microbial activity (38). Recently, a groundwater-surface water simulator has been developed for this site to describe the hydrological component of GSI interactions (39). This model demonstrates that wave run-up on the beach during storm events results in re-oxygenation of the aquifer in shallow zones due to infiltration of surface water. This led to the suggestion that microbial processes might be stimulated to co-oxidize and thus remove chloroethene in the shallow zones of the plume (2). The goals of this study are (i) to determine the spatial extent of physical interactions (dilution effects) between surface water and the contaminant plume, (ii) to evaluate the temporal effects of increased wave activity on the TEAPs and contaminant distribution, and (iii) to determine the most likely microbial processes affecting intrinsic remediation of the contaminants at the GSI.
Experimental Section Array Installation and Field Sampling Procedures. Groundwater was sampled through stationary semipermanent multilevel sampling arrays installed at the GSI of the St. Joseph, MI, plume as previously described (40). The position of the sample arrays at the GSI was chosen based on the predicted location of elevated contaminant concentrations as previously determined (1, 5). The vertical spacing of sample points was selected to span the entire vertical profile of the plume based on our previous beach sampling results (1). Specifically, eight separate sample points were vertically spaced 61 cm apart in the shallow region (elevations above 170.5 m above mean sea level, MAMSL) and 91 cm apart in the deep aquifer (elevations below 170.5 MAMSL). The installed arrays were semipermanent to evaluate temporal effects of lake activity on contaminant distribution and TEAPs during a 6-month period encompassing the summer and fall seasonal weather and lake activity. Field sampling of multilevel arrays was completed for five separate time points with intervals spaced at 4-6 weeks between July and December 1996. The sampling period was selected to compare the contaminant distribution and TEAPs between the quiescent summer months and the stormy fall (40). Figure 2 presents information on wave height during this same period as collected from NOAA buoy 45007 located in the southern part of Lake Michigan. The summer and fall seasons are clearly separated by average wave height of 0.20.3 m versus 1.0-1.3 m, respectively.
FIGURE 2. Average wave height for Lake Michigan as measured by NOAA buoy 45007 during the sample time study. Error bars represent one standard deviation. Installed screens were sampled using a peristaltic pump at a flow rate of 500-600 mL/min to purge the volume of the screen and the attached tubing in less than 20 s. After purging the sample tubing for a minimum of 5 vol, the pump effluent was attached to a QED Purge-Saver flow cell (Ann Arbor, MI), which is equipped to measure temperature, pH, reduction (redox) potential, specific conductance, and dissolved oxygen. All probes were calibrated immediately prior to each sampling trip or daily to ensure proper operation and accurate measurement of each parameter. Flow cell readings were recorded on 5-min intervals, and the final values were recorded once all five parameters were stable for two consecutive readings, usually 20-30 min after initiation of pumping regime. After the flow cell readings, samples were collected and analyzed for contaminants (TCE, DCE, VC, and ethene), electron acceptors (dissolved oxygen and sulfate), electron acceptor products (ferrous iron, sulfide, and methane), and fermentation products (short-chain organic acids). Triplicate laboratory samples were collected in 25-mL borosilicate serum vials, preserved with sodium hydroxide (5 N) solution to raise the pH above 10, capped with Teflon-coated red rubber stoppers (The West Company, Lionville, PA), and sealed with aluminum crimps. Collected samples were immediately stored on ice until transport to the University of Michigan Environmental and Water Resources Engineering (EWRE) laboratories where they were kept at 4 °C until analysis. Following collection of three serum vials, dissolved oxygen, aqueous ferrous iron, and aqueous sulfide were determined by color formation using a Chemetrics (Calverton, VA) sampling kit. The detection limits for analytes were 0.1 µM for TCE, DCE isomers, and chloroethene; 1.0 µM for VOL. 32, NO. 22, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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ethene; 10 µM for methane, oxygen, and sulfate; 0.2 µM for aqueous ferrous iron; and 0.3 µM for sulfide. Analytical Procedures. Dissolved hydrogen gas concentrations were determined for each sample point using field sampling protocols developed earlier (41-44). Volatile organic carbon analysis was performed to quantify TCE, cis1,2-DCE, trans-1,2-DCE, 1,1-DCE, chloroethene, ethene, ethane, and methane in groundwater using a HewlettPackard 5890 series II gas chromatograph (GC) (Palo Alto, CA) equipped with a headspace analyzer. The analysis for TCE, cis-1,2-DCE, trans-1,2-DCE, and 1,1-DCE was performed using a DB-624 (J&W Scientific, Folsom, CA) column and an electron capture detector (ECD). Similarly, chloroethene, ethene, ethane, and methane were analyzed using a GS-Q (J&W Scientific, Folsom, CA) column with a flame ionization detector (FID). Sulfate was analyzed using a Dionex 4500i ion chromatograph (IC) (Sunnyvale, CA) equipped with an IonPac AS4A column (Dionex, Sunnyvale, CA) and a conductivity detector with an eluent consisting of both sodium bicarbonate (1.7 mM) and sodium carbonate (1.8 mM) pumped at 2 mL/min. Short-chain organic acids were analyzed on the same instrument using a Chemosphere C8 column (Alltech, Deerfield, IL) and an ultraviolet detector (λ ) 210 nm) with a phosphoric acid (20 mM, pH of 2.0) eluent.
Results and Discussion Spatial and Temporal Trends in Contaminant Distribution and TEAPs. Spatial comparisons of data for the two installed arrays (ML-2 corresponding to beach transect I and ML-3 corresponding to beach transect II) are shown in Table 1, (see paragraph at the end of the paper for Supporting Information). Contrary to previous results at these two locations (1), the difference in contaminant concentration between arrays lie within one standard deviation for any given time with only one exception (the August sampling of ethene). Therefore, the data suggest that horizontal spatial variations are within data analysis variations. The difference in results between both studies may in part be explained by the fact that the data from Lendvay et al. (1) were collected over a 2-year time period, and the trends observed were based on the entire width of the plume. Thus, the statistically defensible similarity in contaminant data with depth and the lack of differences in trends with time between both horizontally spaced multilevel clusters indicate that both locations were subjected to similar environmental variables and that temporal analysis of geochemical data based on one cluster is reflective of trends observed in the other. Selected vertical contaminant profiles are shown in Figure 3 for TCE, cis-DCE, chloroethene, and ethene for the July, September, and December sampling of the multilevel array ML-3, which corresponds to beach transect II (Figure 1A). The sample location and times were selected to illustrate vertical contaminant distribution and weather (wave) variations between July and December. Data for 1,1-DCE and trans-DCE are not shown because these contaminants were minor in concentration (less than 0.6 µM for all locations but mostly below detectable concentrations,
