USGS Groundwater Information
Impact of Organic Contaminants on the Evolution of Aquifer Geochemistry
By Isabelle M. Cozzarelli
Numerous studies of aquifers contaminated with organic compounds have demonstrated that the introduction of organic material results in significant changes in the aqueous and solid-phase chemistry. These changes are largely driven by the degradation of the organic compounds by indigenous microorganisms. A range of electron acceptors available in subsurface environments can be utilized by bacteria to mediate the oxidation of labile organic compounds. Dissolved oxygen is the most readily used electron acceptor. In the absence of oxygen, the oxidized forms of other inorganic species are used by microorganisms as electron acceptors. A sequence of reactions, in order of decreasing energy yield, is typically observed during the decomposition of organic matter in aquifers, reflecting the ecological succession of progressively less efficient modes of metabolism. The order of utilization of electron acceptors is typically dissolved O2, NO3-, Mn (IV)-solids, Fe (III)-solids, and dissolved SO42-.
In most sand and gravel aquifers, Fe (III), as iron oxides, is abundant. Although less abundant, manganese oxides (Mn(IV)) are easily reducible by microorganisms. Organic matter oxidation coupled to Mn (IV) and Fe(III) reduction have resulted in the accumulation of high concentrations of dissolved Fe2+ and Mn2+ in ground water (Baedecker and Back, 1979; Chapelle and Lovley, 1992; Lyngkilde and Christensen, 1992; Baedecker et al., 1993). The absence of these oxides, or the depletion of microbially reducible Fe (III)-oxides in narrow zones in anoxic aquifers, has allowed sulfate reduction (e.g. at the Galloway site (Cozzarelli et al., 1999)) or methanogenesis (e.g. at the Bemidji site (Bekins et al., 2001 and Cozzarelli et al., 2001)) to occur at small spatial scales. In most aquifers, nitrate and sulfate are supplied during ground-water recharge by rainwater, by mixing with seawater, or by contamination of ground water with fertilizers. The reduction of nitrate and sulfate has resulted in the accumulation of reduced nitrogen species and sulfide in aquifers impacted by organic contaminants (e.g., Baedecker and Back, 1979; Baedecker and Cozzarelli, 1992).
Geochemical reactions may be complicated or obscured by the spatial or temporal scales at which they occur. Although organic degradation reactions may occur at a micro-scale, the products of these reactions become mixed in the groundwater, causing significant variations in the chemical character of the water and the aquifer material. Changes in the aquifer solids include formation of authigenic minerals, such as iron sulfides and iron carbonates (Baedecker et al., 1992; Cozzarelli et al, 1999; Tuccillo et el., 1999) further complicating the interpretation of water-rock interactions. This mixing of the end products of different biogeochemical reactions can be enhanced by fluctuations in the water table and the small spatial scale at which the reactions occurred.
A surficial aquifer contaminated with leaded gasoline was studied by the USGS to determine the effects of organic contaminant degradation on the ground-water chemistry of a local part of the New Jersey Coastal Plain (Cozzarelli et al., 1999). This study provides a good example of the impact of organic compounds on aquifer geochemistry. At the Galloway research site the microbial degradation of hydrocarbons and organic acids in ground water resulted in changes in the oxidation-reduction potential and overall geochemistry of ground water and alterations in the mineral composition of the aquifer. The geochemical reactions that gave rise to the observed composition of the contaminated ground water were a number of oxidation-reduction reactions mediated by bacteria. Inorganic carbon concentrations and pH increased in the contaminated water as organic carbon was oxidized to bicarbonate through microbial degradation reactions. Aerobic degradation of dissolved hydrocarbons resulted in the depletion of oxygen. In the absence of oxygen, anaerobic degradation reactions became important. Nitrate, iron, and sulfate reduction were important biogeochemical processes in the shallow ground water at this site. Organic acids, which are intermediates in the degradation of hydrocarbons, accumulated in the contaminated water.
Further detailed geochemical investigations shed light on what controlled the microbial processes in this system and how they varied in time and space. A combination of nitrate and oxygen concentrations and the availability of metabolizable organic carbon were determined to be important controls on nitrate depletion coupled to organic-carbon oxidation. In another vertically narrow zone of the aquifer, Fe (III) reduction and sulfate reduction occurred where nitrate and oxygen were depleted. Significant vertical heterogeneities of these constituents reflected the small-scale spatial and temporal heterogeneities in biogeochemical reactions. Temporal variations in geochemical reactions occurred at the Galloway site as well, reflecting the dynamic hydrologic system. Increased availability of electron acceptors was observed during periods of the increased recharge as evidenced by rises in the water table and lowered concentrations of degradable organic compounds in the ground water. High concentrations of hydrocarbon metabolites (organic acids) indicated that enhanced degradation of hydrocarbons occurred under these conditions. The geochemical character of the aquifer at the Galloway site was thus affected by numerous microbially mediated reactions that occured at a small spatial scale as well as by changes in these reactions over time due to mixing with infiltrating water and rising and falling water levels.
Investigations of the aquifer solids were undertaken at the Galloway Site in addition to the aqueous geochemical investigations. Changes in the aquifer solids were a direct consequence of the degradation of the introduced organic compounds. Authigenic iron-containing minerals, siderite, ferroan calcite, and magnetite were identified in shallow aquifers associated with hydrocarbon degradation (Baedecker et al. , 1992). The authigenic minerals were reported in the anoxic zones of shallow aquifers containing high concentrations of dissolved hydrocarbons, ferrous iron, and bicarbonate. The formation of these authigenic minerals was clearly related to the interaction of the reduced end products of the microbial degradation of hydrocarbons, such as ferrous iron and hydrogen sulfide.
The availability of organic carbon did not limit either iron reduction or sulfate reduction; the hydrocarbons present in the anoxic, perched water have been shown to support growth of both iron and sulfate reducers in sediment from this site (Cozzarelli et al., 1999). It was the depletion of microbially reducible solid iron-oxyhydroxides, and the coating of iron oxide surfaces with reduced iron phases, in narrow zones in the anoxic aquifer that limited iron reduction and allowed sulfate reduction to occur. The stability of the iron oxides remaining in the aquifer allowed sulfate reduction to become energetically favorable at the expense of iron reduction, as described by Postma and Jakobsen (1996). The mixing of the end products of iron reduction and sulfate reduction in groundwater at the Galloway site, similar to the process observed by Baedecker et al. (1992) resulting in the siderite and magnetite formation, was enhanced by the fluctuations in the water table over time and the small spatial scale at which the reactions appear to be occurring.
The results of the Galloway study underscore the need to determine small-scale geochemical changes so that the biogeochemical processes occurring in organic-rich subsurface environments can be evaluated. The reactions that controlled the distribution of hydrocarbons and organic acids in the shallow aquifer were microbially mediated oxidation-reduction reactions. The concentration gradients observed in contaminated groundwater were complicated by the occurrence of these reactions at a micro-scale. The impacts of hydrologic processes were different in shallow groundwater and the regional aquifer due to the different hydrodynamics of these two regimes. Recharge water entering the perched-water zone is depleted of O2 and NO3- as it mixes with contaminated water in the shallow, higher permeability zone. These electron acceptors did not reach the deeper lower-permeability zone. The chemical effects resulting from the microbial degradation of the hydrocarbons lead to discrete zones where secondary minerals, such as iron sulfide precipitated. Thus, the biogeochemical processes and resulting changes in groundwater and aquifer composition had to be examined differently depending on the local aquifer physical and chemical properties. The heterogeneous distribution of both the hydrocarbon source material and the aqueous and solid phase electron acceptors were important controls on the progress of degradation reactions.
Similarly, the presence of biodegradable organic matter in any potentially recharged ground water or in the aquifer matrix of an Aquifer Storage and Recovery (ASR ) system will result in the consumption of oxygen and other electron acceptors. The resulting changes in oxidation-reduction potential and pH could have a significant impact on ASR systems. Redox changes can affect solubility and precipitation reactions, which impact both physical aspects of operating ASR systems, such as well clogging, and other aspects such as produced water quality (Pyne, 1995). The mineralization of the organic matter or dissolved organic compounds will also produce CO2. An important product of microbial nitrate reduction is ammonium, which can exchange for other cations on the aquifer solids. If solid phase iron oxides are present in the aquifer iron reduction will occur, resulting in high dissolved iron concentrations in the aquifer. Likewise if sulfate reduction occurs, due to the presence of dissolved sulfate, hydrogen sulfide will be produced and, due to its' low solubility, will likely precipitate as iron sulfide minerals. For other trace metals the solubility in the ASR system may increase with increasing reduction.
When considering ASR, it is important to keep in mind that many of these processes are reversible and that subsequent withdrawal of the recharged ground water may result in reoxidation of many of the reduced species. Taking into account the availability and stability of the aquifer solids is very important. Much of the oxidizing or reducing potential of the system will be stored in the aquifer solids and measurement of the dissolved concentrations alone will not give an adequate indication of the potential occurrence of geochemical alterations.
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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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