ARSENIC SOURCES TO GROUND WATER AND SIMULATION OF GEOCHEMCIAL EXPERIMENTS ON AQUIFER CORES AT A LANDFILL, SACO, MAINE: IMPLICATIONS FOR NATURAL REMEDIATION
John A. Colman 1, Kenneth G. Stollenwerk 2 and Forest Lyford 1
U.S. Geological Survey, Northborough MA 01532-1528 1 and Denver CO 80225-0046 2.

Concentrations of arsenic (As) in leachate-plume ground water at the municipal landfill, Saco, Maine, varied with the character of geologic materials that underlie separate landfill piles. Comparisons were made between leachate plumes that pass through (1) glaciomarine fine-sand deposits not in contact with bedrock, and (2) sand and gravel upper- and till lower-layer deposits, over a fine-grained hornfels bedrock. A low average As concentration (0.033 mg/L) and ratio of As-to-iron (As:Fe = 0.031 percent, by weight) were measured in plume water as-sociated with the fine-sand deposits. A high average concen-tration (0.33 mg/L) and ratio (0.64 percent) were associated with the sand and gravel deposits. These differences corresponded with differences in average As concentrations and As:Fe values measured between the solid materials associated with the plumes, using both whole-rock digestions and hydroxylamine-hydrochloride leaches of aquifer solid materials. Speciation analysis of As in the leachate plumes indicated that high concentrations were primarily inorganic As (III), associated with anaerobic conditions and high concentrations of organic carbon and dissolved Fe and manganese. A core from the uncontaminated portion of the sand and gravel aquifer was eluted with uncontaminated ground water to which sucrose had been added to simulate the effect of reducing conditions in ground water from organic carbon. After reducing conditions were established in this core, As and Fe concentrations increased in the core effluent (after 50 pore volumes, average As = 0.2 mg/L, As:Fe = 0.5 percent). The results are compatible with a hypothesized source of As in the Fe-hydroxide minerals coating the aquifer solid materials. The coatings could be dissolved under reducing conditions in anaerobic leachate plumes releasing As and Fe to the ground water. An ad-ditional bedrock source is required to explain the highest As concentrations and As:Fe values, mea-sured in the ground-water samples from deeper parts of the sand and gravel aquifer, and from bedrock wells. Cores from contaminated portions of the aquifer were eluted with uncontaminated ground water to simulate natural remediation conditions. Results indicated that substantial amounts of organic carbon have accumulated on the aquifer solids, causing continued biological oxygen demand. In laboratory leaching experiments of the most contaminated core, this pool of organic carbon caused complete consumption of the influent oxygen for 230 pore volumes. A geochemical model was developed to simulate the concentration changes of selected constituents in the natural remediation experiments. Concentrations of dissolved oxygen, As, and Fe in effluent from one core were used to calibrate the model. The model was then used successfully to simulate constituent concentrations in leachate from two other cores. The modeling indicates that reductive dissolution and sorption were the processes controlling As and Fe concentrations in the experiment effluents. Precipitation of As solid phases was not important. The data show that elimination of the source of landfill leachate and flushing with uncontaminated ground water may not return some constituents, including As, to pre landfill concentrations for decades. (Funded by the U.S. Environmental Protection Agency, Region I and the U.S. Geological Survey, National Research Program)