Kenneth G. Stollenwerk

Large-scale natural gradient tracer test in sand and gravel, Cape Cod, Massachusetts: 1. Experimental design and observed tracer movement

A large-scale natural gradient tracer experiment was conducted on Cape Cod, Massachusetts, to examine the transport and dispersion of solutes in a sand and gravel aquifer. The nonreactive tracer, bromide, and the reactive tracers, lithium and molybdate, were injected as a pulse in July 1985 and monitored in three dimensions as they moved as far as 280 m down-gradient through an array of multilevel samplers. The bromide cloud moved horizontally at a rate of 0.42 m per day. It also moved downward about 4 m because of density-induced sinking early in the test and accretion of areal recharge from precipitation. After 200 m of transport, the bromide cloud had spread more than 80 m in the direction of flow, but was only 14 m wide and 4-6 m thick. The lithium and molybdate clouds followed the same path as the bromide cloud, but their rates of movement were retarded about 50% relative to bromide movement because of sorption onto the sediments.

Geochemical Processes Controlling Transport of Arsenic in Groundwater: A Review of Adsorption

Adsorption is the predominate mechanism controlling transport of arsenic in many ground water systems. Hydrous oxides of iron, aluminum, and manganese, and clay minerals are commonly associated with aquifer solids and have been shown to be significant adsorbents of arsenic. The extent of arsenic adsorption is influenced by the chemistry of the aqueous phase including pH, arsenic speciation, and the presence and concentration of competing ions. Under moderately reducing conditions, trivalent arsenite is stable and adsorption increases with increasing pH. In an oxidizing environment, arsenate is stable and adsorption decreases with increasing pH. The presence of phosphate, sulfate, carbonate, silica, and other anions have been shown to decrease adsorption of arsenic to varying degrees. The effects of complex aqueous and solid phase chemistry on arsenic adsorption are best simulated using surface complexation models. Coupling of such models with hydrologic solute transport codes provide a powerful method for predicting the spatial and temporal distribution of arsenic in ground water.

Geochemical interactions between constituents in acidic groundwater and alluvium in an aquifer near Globe, Arizona

Abstract Acidic water from a copper-mining area has contaminated an alluvial aquifer and stream near Globe, Arizona. The most contaminated groundwater has a pH of 3.3, and contains about 100 mmol/1 SO4, 50 mmol/1 Fe, 11 mmol/1 Al and 3 mmol/1 Cu. Reactions between alluvium and acidic groundwater were first evaluated in laboratory column experiments. A geochemical model was developed and used in the equilibrium speciation program, MINTEQA2, to simulate breakthrough curves for different constituents from the column. The geochemical model was then used to simulate the measured changes in concentration of aqueous constituents along a flow path in the aquifer. The pH was predominantly controlled by reaction with carbonate minerals. Where carbonates had been dissolved, adsorption of H+ by iron oxides was used to simulate pH. Acidic groundwater contained little or no dissolved oxygen, and most aqueous Fe was present as Fe(II). In the anoxic core of the plume, Fe(II) was oxidized by MnO2 to Fe(III), which then precipitated as Fe(OH)3. Attenuation of aqueous Cu, Co, Mn, Ni and Zn was a function of pH and could be quantitatively modeled with the diffuse-layer, surface complexation model in MINTEQA2. Aluminum precipitated as amorphous Al(OH)3 at pH After the alluvium in the column had reached equilibrium with acidic groundwater, uncontaminated groundwater was eluted through the column to evaluate the effect of reactants on groundwater remediation. The concentration of Fe, Mn, Cu, Co, Ni and Zn rapidly decreased to the detection limits within a few pore volumes. All of the gypsum that had precipitated initially redissolved, resulting in elevated Ca and SO4 concentrations for about 5 pore volumes. Aluminum and pH exhibited the most potential for continued adverse effects on groundwater quality. As H+ desorbed from Fe(OH)3, pH remained below 4.5 for more than 20 pore volumes, resulting in dissolution of AlOHSO4 and elevated aqueous Al.

Modeling the Effects of Variable Groundwater Chemistry on Adsorption of Molybdate

Laboratory experiments were used to identify and quantify processes having a significant effect on molybdate (MoO42−) adsorption in a shallow alluvial aquifer on Cape Cod, assachusetts. Aqueous chemistry in the aquifer changes as a result of treated sewage effluent mixing with groundwater. Molybdate adsorption decreased as pH, ionic strength, and the concentration of competing anions increased. A diffuse-layer surface complexation model was used to simulate adsorption of MoO42−, phosphate (PO43−), and sulfate (SO42−) on aquifer sediment. Equilibrium constants for the model were calculated by calibration to data from batch experiments. The model was then used in a one-dimensional solute transport program to successfully simulate initial breakthrough of MoO42− from column experiments. A shortcoming of the solute transport program was the inability to account for kinetics of physical and chemical processes. This resulted in a failure of the model to predict the slow rate of desorption of MoO42− from the columns. The mobility of MoO42− ncreased with ionic strength and with the formation of aqueous complexes with calcium, magnesium, and sodium. Failure to account for MoO42− speciation and ionic strength in the model resulted in overpredicting MoO42− adsorption. Qualitatively, the laboratory data predicted the observed behavior of MoO42− in the aquifer, where retardation of MoO42− was greatest in uncontaminated roundwater having low pH, low ionic strength, and low concentrations of PO43− and SO42−.

Simulation of phosphate transport in sewage-contaminated groundwater, Cape Cod, Massachusetts

Abstract Sewage-contaminated groundwater currently discharges to Ashumet Pond, located on Cape Cod, Massachusetts. Phosphate concentrations as high as 60 μmol l−1 have been measured in groundwater entering Ashumet Pond, and there is concern that the rate of eutrophication could increase. Phosphate in the sewage plume is sorbed by aquifer sediment; the amount is a function of phosphate concentration and pH. A nonelectrostatic surface-complexation model coupled with a one-dimensional solute-transport code was used to simulate sorption and desorption of phosphate in laboratory column experiments. The model simulated sorption of phosphate reasonably well, although the slow rate of approach to complete breakthrough indicated a nonequilibrium process that was not accounted for in the solute-transport model. The rate of phosphate desorption in the column experiments was relatively slow. Phosphate could still be measured in effluent after 160 pore volumes of uncontaminated groundwater had been flushed through the columns. Desorption was partly a function of the slowly decreasing pH in the columns and could be modeled quantitatively. Disposal of sewage at this site is scheduled to stop in 1995; however, a large reservoir of sorbed phosphate exists on aquifer sediment upgradient from Ashumet Pond. Computer simulations predict that desorption of phosphate could result in contamination of Ashumet Pond for decades.

Molybdate transport in a chemically complex aquifer: Field measurements compared with solute‐transport model predictions

A natural-gradient tracer test was conducted in an unconfined sand and gravel aquifer on Cape Cod, Massachusetts. Molybdate was included in the injectate to study the effects of variable groundwater chemistry on its aqueous distribution and to evaluate the reliability of laboratory experiments for identifying and quantifying reactions that control the transport of reactive solutes in groundwater. Transport of molybdate in this aquifer was controlled by adsorption. The amount adsorbed varied with aqueous chemistry that changed with depth as freshwater recharge mixed with a plume of sewage-contaminated groundwater. Molybdate adsorption was strongest near the water table where pH (5.7) and the concentration of the competing solutes phosphate (2.3 micromolar) and sulfate (86 micromolar) were low. Adsorption of molybdate decreased with depth as pH increased to 6.5, phosphate increased to 40 micromolar, and sulfate increased to 340 micromolar. A one-site diffuse-layer surface-complexation model and a two-site diffuse-layer surface-complexation model were used to simulate adsorption. Reactions and equilibrium constants for both models were determined in laboratory experiments and used in the reactive-transport model PHAST to simulate the two-dimensional transport of molybdate during the tracer test. No geochemical parameters were adjusted in the simulation to improve the fit between model and field data. Both models simulated the travel distance of the molybdate cloud to within 10% during the 2-year tracer test; however, the two-site diffuse-layer model more accurately simulated the molybdate concentration distribution within the cloud.

Natural Remediation Potential of Arsenic-Contaminated Ground Water

Migration of leachate from a municipal landfill in Saco, Maine has resulted in arsenic concentrations in ground water as high as 647 μg/L. Laboratory experimental data indicate the primary source of arsenic to be reductive dissolution of arsenic-enriched iron oxyhydroxides in the aquifer by organic carbon in landfill leachate. A core from an uncontaminated part of the aquifer yielded no dissolved iron or arsenic when leached with oxic ground water. Eluent ground water spiked with organic carbon in order to create reducing conditions mobilized both ferrous iron and arsenite from this core. The landfill was capped in early 1998 to eliminate the source of leachate. Cores from the contaminated portion of the aquifer were collected and leached with uncontaminated ground water in the laboratory to simulate natural remediation conditions. Data from these experiments show that significant concentrations of labile organic carbon have accumulated on aquifer solids, causing significant biological oxygen demand. In laboratory leaching experiments of the most contaminated core, the organic carbon caused complete consumption of the influent dissolved oxygen (6 mg/L) for 220 pore volumes. Arsenic leaching from contaminated cores rapidly decreased in concentration initially in response to flushing with uncontaminated ground water. Subsequent leaching produced more gradual decreases in dissolved arsenic concentrations, controlled by a combination of reductive dissolution of arsenic-enriched iron oxyhydroxides and adsorption/desorption. In leachate from the most contaminatedcore, arsenic concentrations exceeded the new United States Environmental Protection Agency drinking-water standard of 10 μg/L for more than 200 pore volumes. A geochemical model simulated the concentration of selected constituents as uncontaminated ground water eluted through contaminated aquifer solids. Concentrations of dissolved oxygen, arsenic, and iron, in leachate from one core were used to calibrate the model. This model was validated by successfully simulating constituent concentrations in leachate from cores collected from other contaminated areas of this aquifer.

In situ Arsenic Remediation in a fractured, alkaline aquifer

In situ removal of arsenic from ground water used for water supply has been accomplished in circum-neutral ground water containing high dissolved iron concentrations. In contrast, the ground water at our study site is alkaline, contains measurable dissolved oxygen and little dissolvediron. Because the dissolved iron concentration is low in the basalt aquifer, the iron oxide content of the aquifer would not increase with successive pumping cycles unless iron is added to the injected water. Additionally, the high pH limits adsorption onto iron oxide present in the aquifer. Having the ability to lower arsenic concentrations in high-pH, oxic ground water could have wide application because similar high arsenic ground water is present in many parts of the world.