Long Term Fate of Organic Micro-pollutants in the Subsurface Environment, Cape Cod, Massachusetts - From Field Surveys to Experimentation

By Larry B. Barber
U.S. Geological Survey, 3215 Marine Street, Boulder, Colorado 80303

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Abstract

Understanding the processes controlling the fate of organic micro-pollutants in the subsurface environment has been the focus of research conducted at the Cape Cod Toxics Substances Hydrology Research Site (Barber, 1988; Barber and others, 1988; Thurman and others, 1986; Harvey and Barber, 1992), as well as other locations around the U.S (Barber and others, 1997; Flynn and Barber, 2000; Leenheer and others, 2001). In particular, this research focuses on the fate of organic contaminants associated with treated municipal wastewater. Although a hierarchical analytical approach is used to characterize both natural and contaminant organic matter (Barber, 1992; Leenheer and others, 2001) particular attention has been given to detergent derived compounds (Thurman and others, 1987; Field and others, 1992a, 1992b; Barber and others, 1995; Krueger and others, 1998a, 1998b). Much of what has been learned from these investigations about subsurface hydrogeochemistry and field experimental protocol also should be applicable to other types of emerging contaminants (Barber and others, 2000; Kolpin and others, 2002) that have not yet been studied in detail.

Initial surveys at the Cape Cod site involving installation of monitoring wells and conducting water quality sampling, identified an extensive plume of sewage contaminated groundwater extending more than 5 km from sand and gravel rapid infiltration beds used for disposal of secondary treated domestic wastewater (LeBlanc, 1984). Subsequent investigations to further characterize the hydrogeological conditions and organic geochemistry of the plume indicated that a variety of trace organic chemicals had persisted for more that thirty years in the subsurface environment (Barber, 1988; Barber and others, 1988).

Investigations at other aquifer recharge sites using different suites of trace organic compounds (Tomson and others, 1981; Bouwer and others, 1984; Leenheer and others, 2001) have typically focused on the removal of compounds during infiltration through the vadose zone. However, few studies have traced the resulting plumes of contamination as the recharged wastewater is incorporated into the groundwater system. Most trace organic contaminants undergo significant degradation in the biologically and chemically active zone known as the "schmutzdecke", and removal rates decrease with increasing depth. Once contaminants have been transported out of the zone of influence from the infiltration site and introduced into uncontaminated portions of the aquifer, additional attenuation in the core of the plume occurs, but at lower rates than in the infiltration zone. The fate of organic contaminants in the subsurface depends on geochemical and nutrient conditions, with low dissolved oxygen/low nutrient conditions favoring long-term persistence.

These findings led to the development of a conceptual model of processes governing the long-term fate of organic contaminants in the Cape Cod aquifer (Field and Barber, 1994). This model consists of biogeochemical reactors in series. Under the aerobic conditions existing in the infiltration basins, biodegradation can effectively attenuate the moderately degradable compounds that are present in the treated effluent. The rapidly degradable component of the dissolved organic matter (DOM) is removed during the wastewater treatment process. During infiltration, additional removal occurs during the first meter of transport through the unsaturated zone due to sorption and biodegradation. However, because of the high organic loading and aerobic biodegradation occurring in this zone, the aquifer beneath the infiltration beds rapidly becomes oxygen depleted, and aerobic processes decrease. The moderately degradable component of the DOM is removed during the initial infiltration, and compounds that persist through this zone are generally more recalcitrant in nature and less susceptible to further aerobic biodegradation. Once these compounds reach the saturated zone of the unconfined aquifer they are transported in the subsurface along the regional groundwater flow path. The saturated zone under the beds can become anoxic, and anaerobic biodegradation of compounds not degraded by aerobic processes (i.e. tetrachloroethene) can occur. The effect of sorption is diminished with distance from the infiltration beds because of the low organic carbon content of the aquifer sediments (Barber, 1994). Organic contaminants that persist through the near bed zones can be transported by the regional groundwater flow system with little additional attenuation because (1) the degradable and sorptive components have been removed near the beds, (2) the groundwater in the contaminant plume is anoxic, (3) the water becomes nutrient limited for microbial processes, and the (4) the subsurface microbial community structure changes and is significantly less active. Although this model is based on results from a rapid infiltration wastewater disposal site in a shallow unconfined aquifer, the primary hydrological and biogeochemical processes are similar to, and the understanding can be applied to aquifer recharge and recovery operations.

In addition to steady-state processes associated with the various reactive zones of the rapid infiltration system, there are dynamic factors such as loading rates, wetting/drying cycles, and groundwater hydrology. Within a given infiltration cycle, contaminants can be introduced into the aquifer as a series of pulses. When sediments are loaded after a period of drying, compounds stored in the shallow infiltration zone can be remobilized and released as a concentrated pulse. After this initial flushing, concentrations ambient to the effluent pass into the subsurface during the lag time for microbial activity to establish. Once aerobic biodegradation begins, concentrations of compounds in the effluent can be significantly decreased. This phase of efficient removal proceeds until the reactive zone becomes anoxic due to oxygen consumption resulting from the effluent organic loading, at which point aerobic degradation ceases and compounds present in the effluent again are introduced to the subsurface.

Of course, the groundwater hydrogeology plays a significant role in the subsurface fate and transport of organic micro-pollutants. Factors such as depth to water table, sediment porosity and permeability, and ground water velocity all control how rapidly and to what extent contaminants make their way from the point of recharge to point of recovery.

Because of the widespread use of surfactants in domestic and commercial applications, and their direct introduction into the hydrologic environment through disposal down the drain, they are ubiquitous in treated wastewater effluents throughout the United States. There are a variety of common surfactants in use including nonionic, anionic, and cationic compounds. Research at the Cape Cod site has focused primarily on the anionic surfactant class, alkylbenzene sulfonic acids. Linear alkylbenzene sulfonates (LAS) are the major anionic surfactants currently in use, whereas branched chained alkylbenzene sulfonates (ABS) were in use prior to 1964 after which they were replaced with LAS. Both LAS and ABS consist of a series of homologs with alkyl chain lengths ranging from 10-14 carbons. The ABS compounds have a chemical structure that prevents biological degradation whereas the LAS compounds are rapidly biodegraded under aerobic conditions. This mixture of homologs and positional isomers results in a complex suite of chemical structures, each with differing biodegradation and sorption characteristics, that can be used as intrinsic tracers in experiments to identify environmental processes.

The sewage plume resulting from wastewater recharge at the Cape Cod site consists of three distinct geochemical zones. Pristine freshwater recharge overlies the plume and is very low in dissolved ions and DOM and is fully oxygenated. The transition zone has an increase in specific conductance and DOM, and dissolved oxygen concentrations decrease. The plume core is primarily sewage effluent, has a high specific conductance and DOM, and is anoxic. This geochemical gradient can be used to experimental advantage because each zone has a distinct set of biogeochemical processes.

The Cape Cod site has an extensive three-dimensional sampling array consisting of multilevel sampling devices (MLS) spaced at 1-10 meter intervals aligned perpendicular to and along the regional groundwater flow path (LeBlanc and others, 1991). This array allows in-situ natural gradient experiments to be conducted within the various geochemical zones. Results from a series of pulsed and continuous injection tracer tests using LAS and sodium bromide indicate that: (1) biodegradation rates within the aquifer are low, spatially variable, and controlled primarily by dissolved oxygen content, (2) biodegradation can be self limiting as the LAS and oxygen are consumed, resulting in formation of stable metabolites, (3) under oxygen limited conditions, biodegradation selectively removes the LAS isomers having the longest linear side chains, (4) sorption to the aquifer sediments results in chromatographic separation of the various homologs and isomers that is inversely proportional to the water solubility, even at very low sediment organic carbon contents, (5) sorption to the sediments results in variable transport rates, thus breakthrough of contaminants in the recovery wells will be dynamic.

The impact of the organic component held in sediment storage on long-term water quality considerations is illustrated by observations on the time required for dissolved organic carbon (DOC) concentrations to decrease in monitoring wells near the infiltration beds after the wastewater disposal practices ceased (Barber and Keefe, 1999). After nearly 60 years of continuous disposal, the wastewater source was shut off in December 1995. Monitoring of an extensive network of MLS immediately down gradient of the disposal beds indicated that the soluble components of the plume, such as chloride and boron, were rapidly flushed from the system (concentrations returned to baseline conditions in less than 6 months). In contrast, after 6 years the dissolved oxygen and DOC levels have still not returned to baseline conditions.

In summary, depending on the operational controls of aquifer recharge activities, the nature of the recharge water, and the hydrogeology of the receiving aquifer, a variety of trace organic contaminants can be introduced into the groundwater, with little additional attenuation occurring with subsequent transport and recovery. Rates of in situ removal can be difficult to determine, and work at the Cape Cod site has developed methods and protocols to quantitatively determine the mechanisms, rates, and geochemical constraints of subsurface removal processes.

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In George R. Aiken and Eve L. Kuniansky, editors, 2002, U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4, 2002: USGS Open-File Report 02-89

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