Geology topics

Kenneth G. Stollenwerk

Publications and source records attributed to Kenneth G. Stollenwerk.

24 records · Page 2Linked to original sources

USGS1998

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 p H (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 p H 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.

Massachusetts↗

USGS1996

Geochemical and hydrologic controls on phosphorus transport in a sewage-contaminated sand and gravel aquifer near Ashumet Pond, Cape Cod, Massachusetts

Currently (1993), about 170 kg/yr of phosphorus discharges into Ashumet Pond on Cape Cod from a plume of sewage-contaminated ground water. Phosphorus in the plume is mobile in two distinct geochemical environments--an anoxic zone containing dissolved iron and a suboxic zone containing dissolved oxygen. Phosphorus mobility in the suboxic zone is due to saturation of available sorption sites. Phosphorus loading to Ashumet Pond may increase significantly after sewage disposal is stopped due to phosphorus desorption from sediment surfaces.

Massachusetts↗

USGS1994

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

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 SO 4 , 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 MnO 2 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 < 4.7 and as AlOHSO 4 at pH < 4.7. Aqueous Ca and SO 4 were close to equilibrium with gypsum. 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 SO 4 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 AlOHSO 4 and elevated aqueous Al.

Applied Geochemistry↗

USGS1985

Adsorption and desorption of hexavalent chromium in an alluvial aquifer near Telluride, Colorado

A laboratory investigation of reactions between hexavalent chromium [Cr(VI)] and alluvium was conducted to evaluate reactions of Cr(VI) contaminating an alluvial aquifer near Telluride, CO and to determine the mechanisms responsible for these reactions. Uncontaminated alluvium and groundwater (spiked with CrO 4 2− ) from the study site were used in batch and column experiments. Results of these experiments show that Cr(VI) was adsorbed by the alluvium. Distribution coefficients from batch experiments ranged from 52 L/kg at an equilibrium CrO 4 2− concentration of 0.4 µ mol/L to 1.7 L/kg at an equilibrium concentration of 1400 µ mol/L. The zero point of charge for the alluvium was approximately 8.3, and the alluvium had a positive net charge at the groundwater pH of 6.8. Visual and chemical evidence indicated that Fe oxide and hydroxide coatings on the alluvial particles principally were responsible for the adsorption of Cr(VI). During column experiments, Cr(VI) initially was desorbed easily from the alluvium by Cr‐free groundwater; however, the rate of desorption decreased rapidly, and > 60 pore volumes of groundwater were required to decrease the effluent concentration of Cr(VI) to 3 µ mol/L [drinking water standard for Cr(VI) = 1 µ mol/L]. The quantity of Cr(VI) adsorbed varied with the type and concentration of other anions in solution.

Colorado↗

USGS1985

Reduction of hexavalent chromium in water samples acidified for preservation

Reduction of hexavalent chromium, Cr(VI), in water samples, preserved by standard techniques, was investigated. The standard preservation technique for water samples that are to be analyzed for Cr(VI) consists of filtration through a 0.45‐ µ m membrane, acidification to a pH < 2, and storage in plastic bottles. Batch experiments were conducted to evaluate the effect of H + concentration, NO 2 , temperature, and dissolved organic carbon (DOC) on the reduction of Cr(VI) to Cr(III). The rate of reduction of Cr(VI) to Cr(III) increased with increasing NO 2 , DOC, H + , and temperature. Reduction of Cr(VI) by organic matter occurred in some samples even though the samples were unacidified. Reduction of Cr(VI) is inhibited to an extent by storing the sample at 4°C. Stability of Cr(VI) in water is variable and depends on the other constituents present in the sample. Water samples collected for the determination of Cr(VI) should be filtered (0.45‐ µ m membrane), refrigerated, and analyzed as quickly as possible. Water samples should not be acidified. Measurement of total Cr in addition to Cr(VI) can serve as a check for Cr(VI) reduction. If total Cr is greater than Cr(VI), the possibility that Cr(VI) reduction has occurred needs to be considered.

Journal of Environmental Quality↗

USGS1985

NEUTRALIZATION OF ACIDIC GROUND WATER NEAR GLOBE, ARIZONA.

Highly acidic contaminated water is moving through a shallow aquifer and interacting with streams near Globe, Arizona. Dissolved concentrations reach 3,000 mg/L iron, 150 mg/L copper, and 16,400 mg/L total dissloved solids; pH is as low as 3. 6. Samples from 16 PVC-cased observation wells include uncontaminated, contaminated, transition, and neutralized waters. Chemical reaction with sediments and mixing with uncontaminated water neutralizes the acidic water. The reactions form a transition zone where gypsum replaces calcite and most metals precipitate. Ferric hydroxide also precipitates if sufficient oxygen is available. Abundant gypsum crystals and ferric hydroxide coatings have been recovered from well cuttings. Large sulfate concentrations produce sulfate complexes with many metals that inhibit removal of metals from solution.

Conference Paper↗