Modeling a Ground Water Circulation Well Alternative

Ground water circulation wells (GCWs) provide an appealing alternative to typical pump-and-treat ground water remediation systems because of the inherent resource-conservative nature of the GCW systems. GCW performance prediction is challenging because the consideration of extraction and recharge in a single well is unusual for most practitioners, the technology is relatively new, and a meaningful body of literature has not been published. A three-part evaluation process using state-of-the-practice numerical ground water flow and mass transport models was developed for application during GCW pilot studies at the former Nebraska Ordnance Plant site. A small-scale ground water flow model was developed during the pilot study planning process to predict the system performance and to locate performance-measuring monitoring wells. Key predictions included the capture zone predicted to develop upgradient of the GCW, the downgradierit GCW recharge zone, and the circulation zone centered on the GCW. The flow model was subsequently verified using ground water elevation data and contaminant concentration data collected during pilot study operation. Aquifer parameters were reestimated as a result of the verification process. Those parameter values were used as input to a larger scale model, which was used to develop a remedial alternative consisting of multiple GCW systems.     

Demonstration of a Microbiologically Enhanced Vertical Ground Water Circulation Well Technology at a Superfund Site

A full-scale ground water circulation well (GCW) system was installed and operated to demonstrate in situ remediation of soil and ground water impacted with a mixture of chlorinated and nonchlorinated organic compounds at a Superfund site in upstate New York. System performance and applicability under site-specific conditions were evaluated based on the system's ability to meet the New York State Department of Environmental Conservation (NYSDEC) cleanup goals for target compounds in ground water and soil. Contaminants from the unsaturated zone were mobilized (volatilized) by one-way vacuum extraction, and treated via enhanced biodegradation (bioventing). In the saturated zone, contaminants were mobilized by soil flushing (solubilized) and treated by a combination of air stripping and biodegradation. An in situ aqueous phase bioreactor, and an ex situ gas phase bioreactor, were integrated into the system to enhance treatment via bioremediation. After 15 months of operation, the mass of target contaminants in soil and ground water combined had been reduced by 75%. Removal by biological mechanisms ranged from 35% to 56% of the total observed mass reduction. The in situ and the ex situ bioreactors mineralized 79% and 76%, respectively, of their target biodegradable contaminant loads. Results indicate that some mass reduction in target contaminants may have been from aerobic and aerobic processes within the circulation cell. Nonchlorinated compounds were relatively easy to mobilize (volatilize, solubilize, and/or transport) and treat when compared to chlorinated compounds. The data collected during the 15-month study indicate that remediation could be accomplished at the Sweden-3 Chapman site using the technology tested.     

Submarine Ground Water Discharge Driven by Tidal Pumping in a Heterogeneous Aquifer

Submarine ground water discharge (SGD) is now recognized as an important water pathway between land and sea. It is difficult to quantitatively predict SGD owing to its significant spatial and temporal variability. This study focuses on quantitative estimation of SGD caused by tidally induced sea water recirculation and a terrestrial hydraulic gradient. A two-dimensional hydrogeological model was developed to simulate SGD from a coastal unconfined aquifer in the northeastern Gulf of Mexico, where previous SGD studies were performed. A density-variable numerical code, SEAWAT2000, was applied to simulate SGD. To accurately predict discharge, various influencing factors such as heterogeneity in conductivity, uncertain boundary conditions, and tidal pumping were systematically assessed. The tidally influenced sea water recirculation zone and the fresh water–salt water mixing zone under various tidal patterns, tidal ranges, and water table heights were also investigated. The model was calibrated and validated from long-term, intensive measurements at the study site. The percentage of fresh SGD relative to total SGD ranged from 4% to 50% under normal conditions. Based on simulations of two field measurements in summer and spring, respectively, the fresh water ratios were 9% and 15%, respectively. These results support the hypothesis that the SGD induced by tidally driven sea water recirculation is much larger than terrestrial fresh ground water discharge at this site. The estimates of total and fresh SGD are at the low and high ends, respectively, of the estimation ranges obtained from geochemical tracers (e.g., ²²²Rn).     

Enhancement of In Situ Bioremediation of BTEX-Contaminated Ground Water by Oxygen Diffusion from Silicone Tubing

A field study of oxygen-enhanced biodegradation was carried out in a sandy iron-rich ground water system contaminated with gasoline hydrocarbons. Prior to the oxygen study, intrinsic microbial biodegradation in the contaminant plume had depleted dissolved oxygen and created anaerobic conditions. An oxygen diffusion system made of silicone polymer tubing was installed in an injection well within an oxygen delivery zone containing coarse highly permeable sand. During the study, this system delivered high dissolved oxygen (DO) levels (39 mg/L) to the ground water within a part of the plume. The ground water was sampled at a series of monitors in the test zone downgradient of the delivery well to determine the effect of oxygen on dissolved BTEX, ground water geochemistry, and microbially mediated biodegradation processes. The DO levels and Eh increased markedly at distances up to 2.3 m (7.5 feet) downgradient. Potential biofouling and iron precipitation effects did not clog the well screens or porous medium. The increased dissolved oxygen enhanced the population of aerobes while the activity of anaerobic sulfate-reducing bacteria and methanogens decreased. Based on concentration changes, the estimated total rate of BTEX biodegradation rose from 872 mg/day before enhancement to 2530 mg/day after 60 days of oxygen delivery. Increased oxygen flux to the test area could account for aerobic biodegradation of 1835 mg/day of the BTEX. The estimated rates of anaerobic biodegradation processes decreased based on the flux of sulfate, iron (II), and methane. Two contaminants in the plume, benzene and ethylbenzene, are not biodegraded as readily as toluene or xylenes under anaerobic conditions. Following oxygen enhancement, however, the benzene and ethylbenzene concentrations decreased about 98%, as did toluene and total xylenes.     

Effect of Well Disinfection on Arsenic in Ground Water

Domestic water wells are routinely subjected to in situ chemical disinfection treatments to control nuisance or pathogenic bacteria. Most treatments are chlorine based and presumably cause strongly oxidizing conditions in the wellbore. Water resource managers in Wisconsin were concerned that such treatments might facilitate release of arsenic from sulfide minerals disseminated within a confined sandstone aquifer. To test this hypothesis, a well was subjected to four disinfection treatments over 9 months time. The first treatment consisted of routine pumping of the well without chemical disinfection; three subsequent treatments included chlorine disinfection and pumping. Pretreatment arsenic concentrations in well water ranged from 7.4 to 18 μg/L. Elevated arsenic concentrations up to 57 μg/L in the chemical treatment solutions purged from the well are attributed to the disintegration or dissolution of biofilms or scale. Following each of the four treatments, arsenic concentrations decreased to less than 10 μg/L during a period of pumping. Arsenic concentrations generally returned to pretreatment levels under stagnant, nonpumping conditions imposed following each treatment. Populations of iron-oxidizing, heterotrophic, and sulfate-reducing bacteria decreased following chemical treatments but were never fully eradicated from the well. Strongly oxidizing conditions were induced by the chlorine-based disinfections, but the treatments did not result in sustained increases in well water arsenic. Results suggest that disruption of biofilm and mineral deposits in the well and the water distribution system in tandem with chlorine disinfection can improve water quality in this setting.     

Simulation of a Ground Water Recirculation Well with a Dual-Column Laboratory Setup

This paper describes a dual-column laboratory setup consisting of a glass column and a stainless-steel column filled with aquifer material. The setup was used to replicate a ground water recirculation well that serves as an in situ reactor and a combined injection/withdrawal well. The treatment solution consisted of a buffered titanium (III) citrate/vitamin B₁₂ mixture. The first column, representing the well, was made of glass, allowing for visual inspection of the mixing. The stainless-steel column was instrumented with redox (Eh) probes to monitor the changes in redox conditions. The redox measurement showed that, although the sand contained large quantities of iron oxides, the oxidation rate was relatively slow and the titanium solution would remain reduced for some time in the aquifer, continuing to react with the contaminants. This laboratory setup was used to optimize the reagent concentrations and rate of delivery for field implementation. It was found that 4 mM titanium citrate and 3 mg/L vitamin B₁₂ were sufficient to degrade 1,1,2,2-tetrachloroethane and carbon tetrachloride within one day, but not trichloroethylene, which required five days with 10 mM titanium citrate and 5 mg/L vitamin B₁₂.