The upper water table: a chemically reactive zone with respect to contaminant fate and mobility.

Armand Groffman, and Laura J. Crossey Presented at 8th Annual Goldschmidt Converence, 1998, Toulouse, France

INTRODUCTION        In shallow phreatic aquifers, the upper water table represents an interface between terrestrial and subsurface reservoirs. Because of the intimate contact and exchange between these two reservoirs, biogeochemical activity is more active than at depth resulting in greater fluctuation in the chemical structure. The redox regime exhibits transient behavior throughout the year and as a result influences the mobility and fate of many trace elements and contaminants. In the event of a contaminant release on the land surface, the upper phreatic surface will be impacted first as the pulse travel through the vadose zone to the aquifer. With this in mind, we are investigating the biogeochemical dynamics of this region with a focus on biologically mediated redox especially terminal electron acceptors (TEAs) and oxidation reaction products. Monitoring TEAs in ground water gives insight into microbially mediated processes in the aquifer especially redox regimes that control the mobility of many trace elements and contaminants in ground water. THE RIO CALAVERAS RESEARCH SITE Site description        We are characterizing biogeochemical responses by measuring redox-sensitive trace elements, nutrients, major cations and anions in ground water and sediments throughout the aquifer at the Rio Calaveras research site (Groffman et al., 1997a; 1997b; 1998a; 1998b). The Rio Calaveras site, located approximately three kilometers northwest of the Valles Caldera in the Jemez Mountains of northern New Mexico at an elevation of 2475 meters (8300 ft), is an ideal example of shallow phreatic systems because of its small scale and dramatic response to infiltration events. Instrumentation includes over 200 wells, 12 suction lysimeters, two flumes and a meteorological station. Methods        Using dialysis cells and a DMLS@ high resolution ground water sampling system (Margaritz et al., 1989) we have defined vertical concentration gradients of the biogeochemically sensitive TEAs oxygen, sulfate, nitrite, nitrate, iron, and manganese at multiple locations in the aquifer over seasonal cycles.        All water samples were analyzed according to APHA (1992). Cation (Ca, Mg, Na, K, Mn and Fe) analysis was performed by atomic absorption with a Perkin Elmer model 303 spectrometer. Inorganic anions (SO4, Cl, F, NO3, Br and PO4) and low molecular weight organic acids (acetate, formate, propionate, pyruvate and oxalate) were analyzed with a Dionex-500X ion chromatograph using a NaOH gradient method with an AS11 analytical column, an AG-11 guard column and an anion trap. Results        The biogeochemistry of the upper table is very sensitive to infiltration events that transport organic carbon from the vadose zone especially during spring snow melt and summer monsoon rain events. Microbially mediated redox shifts as bacteria respond to the bolas of labile organic carbon in the upper water table. By measuring a suite of terminal electron acceptors that decrease (SO4, DO) or build up (Fe, Mn) during a reducing event, a reactive zone can be defined. This is illustrated by Figure 1 which shows organic carbon and select TEA from July of 1997 in the upper aquifer at the Rio Calaveras research site.        Soluble iron is elevated in the upper 80 cm of the aquifer and decreases with depth while sulfate is low in the upper aquifer and increases with depth. Background concentrations of iron is below the analytical detection limit and sulfate is approximately 5 mg/L as measured from springs that feed the system. Dissolved oxygen, measured down hole with a probe (YSI-50B), is below 0.5 mg/L (the analytical detection limit of the instrument) most of the time indicating suboxic to microaerophillic conditions through the entire profile. Concentrations of low molecular weight organic acids (LMWOA) (acetate, formate and oxalate) are elevated in the upper 40 cm of the aquifer and decrease with depth. Acetate concentration is occasionally high (up to 15 mg/L (260 :M)), an unusual occurrence in both diffusion and advection dominated environments. The presence of LMWOA, especially acetate at the top of the aquifer imply either production from acetagenic processes at the vadose/saturated zone interface or transport from the vadose zone.        Iron and sulfate reduction appear to be robust in the upper aquifer during summer/autumn base flow conditions but decrease during late winter and spring as dissolved oxygen increases during spring infiltration of snow melt. These events shift redox to suboxic/oxic conditions and may stimulate aerobic microbial activity in addition to reacting with ferrous iron resulting in ferric iron phases (oxidation products) coating aquifer sediments. These coatings are highly reactive sorption surfaces and control trace element mobility in ground water. Both iron and sulfate reducing bacteria can use numerous carbon sources as substrates including hydrocarbons and natural organic complexes, an important consideration when addressing biodegradation of hydrocarbons in the subsurface. DISCUSSION        Many contaminated industrial sites are situated above shallow phreatic systems where the initial impact of contaminants affects the upper aquifer. Like the Rio Calaveras system, these shallow aquifers are intimately connection with the overlying vadose zone and the surficial reservoir of organic carbon. Fermentation products, from the microbial decay of liter and sediment bound organic matter in addition to root exudates are episodically transported from the overlying reservoirs to the upper aquifer. The same process would occur in the event of a hydrocarbon release, with the subsequent transfer of anthropogenic carbon to the upper water table. Figure 2 shows a conceptual model as organic carbon phases and solute are flushed out of the vadose zone during episodic infiltration events. The microbial community at the interface of the vadose/saturated zone reduces O2, SO4, Fe, Mn, NO3, NO2, during the terminal electron accepting processes. Both aerobic and anaerobic processes may occur at this interface depending upon the precipitation regime and nutrient/solute structure.        The mobility of many inorganic contaminants, including As, Se and U, are highly sensitive to redox conditions in ground water. Allochthonous imports of organic carbon to the top of the aquifer would initiate reducing conditions and modify the oxidation state of transition metals such as Fe, Mn, Zn, Cu, Cr, V, U in addition to metaloids such as As, Se and S. The shift in valence directly affects their speciation and mobility. For example, ferric iron (Fe3+) is many orders of magnitude less soluble that ferrous iron (Fe2+) and as a consequence is often not detected in ground water unless chelated by carrier complexes. Conversely, ferrous iron is relatively soluble and stable under moderately reducing environments and as a result is generally more mobile in ground water.

Research Projects | Faculty/Staff | Students | Facilities | Courses | Publications

Created: 10/7/98 Updated: 05/05/1999