Nickel and Chromium in Ground Water Samples as Influenced by Well Construction and Sampling Methods

An investigation of elevated concentrations of nickel and chromium in certain ground water samples collected at Williams Air Force Base (AFB) indicated that type 304 stainless steel well materials are the source. Chloride in the ground water has apparently caused crevice corrosion of the stainless steel well screens installed during site characterization. An evaluation of site geochemistry suggested that chromium released from the well screen would precipitate, while nickel would remain dissolved. Thus, low-flow purging and sampling significantly reduces the chromium found in the ground water samples because such sampling minimizes the collection of artificially entrained particulates. In contrast to chromium, nickel concentrations did not decrease during low-flow purging and sampling, indicating that it is dissolved. Nickel and chromium concentrations are both low following high-volume purging when turbidity levels are stabilized below 10 nephelometric turbidity units prior to sampling. In the latter case, chromium concentration is low because particulate collection is minimized, and nickel concentration is low because of increased dilution. Based on these results, it is recommended that elevated levels of nickel and chromium in ground water samples collected from stainless steel monitoring wells be carefully evaluated, because well materials may be the source. In addition, although low-volume purging is increasingly becoming the sampling method of choice, high-volume purging may be a useful means of determining whether the well materials influence nickel and chromium concentrations.     

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.     

Acquisition of Representative Ground Water Quality Samples for Metals

R.S. Kerr Environmental Research Laboratory (RSKERL) personnel have evaluated sampling procedures for the collection of representative, accurate, and reproducible ground water quality samples for metals for the past four years. Intensive sampling research at three different field sites has shown that the method by which samples are collected has a greater impact on sample quality, accuracy, and reproducibility than whether the samples are filtered or not. In particular, sample collection practices that induce artificially high levels of turbidity have been shown to have the greatest negative impacts on sample quality. Results indicated the ineffectiveness of bailers for collection of representative metal samples. Inconsistent operator usage together with excessive purging generally resulted in excessive turbidity (>100 NTUs) and large differences in filtered and unfiltered metal samples. The use of low flow rate purging and sampling consistently produced filtered and unfiltered samples that showed no significant differences in concentrations. Turbidity levels were generally less than 5 NTUs, even in fine-textured glacial till. We recommend the use of low flow rates, during both purging and sampling, placement of the sampling intake at the desired sampling point, minimal disturbance of the stagnant water column above the screened interval, monitoring of water quality indicators during purging, minimization of atmospheric contact with samples, and collection of unfiltered samples for metal analyses to estimate total contaminant loading in the system. While additional time is spent due to use of low flow rates, this is compensated for by eliminating the need for filtration, decreased volume of contaminated purge water, and less resampling to address inconsistent data results.     

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.     

In-House Well Drilling Operations for a Ground Water Monitoring Program, Suffolk County, New York

All public agencies involved in ground water monitoring and protection face the requirement of obtaining well drilling services consistent with their needs. Most will rely on contracted drilling services, but a limited number of agencies having responsibilities for long-term aquifer monitoring and the promulgation of monitoring standards may find that implementing an in-house well drilling program is an effective approach to achieving their ground water protection goals.
Suffolk County, located on Long Island, New York, has been operating a well drilling program for 10 years, with three levels of drilling capability to meet the requirements of characterizing and monitoring an unconsolidated regional aquifer system. The program's cost and labor requirements are substantial, but this approach has provided uniformly high-quality data and the hands-on experience needed to test drilling innovations and produce monitoring regulations that are effective and feasible. If soundly organized and run, an agency-directed drilling operation will be readily adaptable to changing needs and priorities. In Suffolk County, this has included emergency response and fuel-spill monitoring. A successful program must also include a demonstrated geologic identification and well logging capability, close laboratory support, and an experienced hydrogeological or ground water engineering staff.
This approach may be suitable for other regional agencies based on their ground water conditions and water-supply protection needs.     

An Analytical Method to Determine Ground Water Supply Well Network Designs

An analytical method is provided where the ground water practitioner can quickly determine the size (number of wells) and spacing of a well network capable of meeting a known ground water demand. In order to apply the method, two new parameters are derived that relate theoretical drawdown to the maximum drawdown that is achievable without mining the aquifer. The size of a well network is shown to be proportional to the ground water demand and inversely proportional to the transmissivity and available head. The spacing between wells in a supply well network is shown to be most sensitive to a derived parameter r _{HA/ 3}, which is related to the available head and the propagation of drawdown away from a theoretical well if the total ground water demand was applied to that well. The method can be used to quickly determine the required spacing between wells in well networks of various sizes that are completed in confined aquifers with no leakance.     

Case History of a Large-Scale Air Sparging/Soil Vapor Extraction System for Remediation of Chlorinated Volatile Organic Compounds in Ground Water

A large-scale air sparging/soil vapor extraction (AS/SVE) project constructed within coastal plain sediments in New Jersey has demonstrated substantial progress toward remediating ground water through removal of volatile organic compounds (VOCs). Potential concerns identified prior to project implementation regarding hydraulic mounding, reduction in hydraulic conductivity, development of air channels, and the absence of hydraulic containment were assessed and addressed through testing and operational features incorporated into the project. At the project site, AS/SVE has successfully reduced the presence of many VOCs to undetectable levels, while reducing the concentrations of the remaining VOCs by factors of two to 500. The physical agitation caused by air sparging, and incomplete transformation from sorbed and nonaqueous phases to the vapor phase, appears to temporarily increase VOC concentrations and/or mobility of dense nonaqueous phase liquids (DN APLs) within source areas at the project site, but this is addressed in terms of subsequent removal of VOCs by properly placed downgradient treatment lines and VOCs by properly placed downgradient treatment lines and DNAPL recovery wells. This case study identifies and evaluates project-specific features and provides empirical data for potential comparison to other candidates AS/SVE sites.     

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.