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.     

Screening of Ground Water Samples for Volatile Organic Compounds Using a Portable Gas Chromatograph

A portable gas chromatograph was used to screen 32 ground water samples for volatile organic compounds. Seven screened samples were positive; four of the seven samples had volatile organic substances identified by second-column confirmation. Four of the seven positive, screened samples also tested positive in laboratory analyses of duplicate samples. No volatile organic compounds were detected in laboratory analyses of samples that headspace screening indicated to be negative. Samples that contained volatile organic compounds, as identified by laboratory analysis, and that contained a volatile organic compound present in a standard of selected compounds were correctly identified by using the portable gas chromatograph. Comparisons of screened-sample data with laboratory data indicate the ability to detect selected volatile organic compounds at concentrations of about 1 microgram per liter in the headspace of water samples by use of a portable gas chromatograph.     

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.     

Suggested Methods to Mitigate Bias from Nondissolved Petroleum in Ground Water Samples Collected from the Smear Zone

This article provides actual site data that confirm that turbid ground water samples collected from within the smear zone at petroleum release sites can be significantly biased high by the inclusion of a nondissolved component that is an artifact of the sampling process. Side-by-side comparisons show that reducing sample turbidity can result in significant reductions of reported concentrations for the ground water samples and that the lower turbidity results are more representative of the petroleum actually dissolved in the ground water. Depending on site-specific factors, ground water sample turbidity can be reduced by four field-based and two laboratory-based methods. These methods should be used routinely at sites where turbid samples with a nondissolved component are being collected.     

Computer Modeling of Scale Formation During Treatment of Ground Water in Air Strippers

During treatment to remove volatile organic compounds from contaminated ground water, inorganic scale sometimes precipitates in an air stripper tower. This leads to increased costs and down-time associated with tower media replacement. In order to determine the kind, quantity, and rate of scale formation, the ground water from five locations in Florida was simulated using an aqueous equilibrium chemistry and flow process model. In all cases the pH of the outflow water is higher than that of the inflow water because of degassing of CO₂. This often results in the precipitation of calcium carbonate. The addition of air to reduced ground water results in the oxidation of iron and the precipitation of ferric hydroxides. Model estimates of scale formation are about a factor of two to five too high. This indicates that the precipitation reactions do not reach equilibrium within the air stripper. Future work will require the inclusion of biological fouling and a kinetic expression to account for the observed non-equilibrium.     

Effects of Selected Sampling Equipment and Procedures on the Concentrations of Trichloroethylene and Related Compounds in Ground Water Samples

Variations in concentrations of trichloroethylene and related compounds in ground water obtained from seven ground water samplers were used to compare the performance of three submersible pumps, a centrifugal pump, two peristaltic pumps, and a bailer. Two- and 4-inch diameter submersible pumps and a centrifugal pump produced samples whose trichloroethylene concentrations, on the average, did not differ significantly from each other. Ground water samples collected by using a peristaltic pump and silicone tubing had significantly lower trichloroethylene concentrations than samples from the submersible pumps. Concentrations of 1,2-dichloroethylene and trichloroethylene in ground water samples collected by using a bailer were indistinguishable from those in samples taken by a submersible pump when the concentrations were as much as 96 and 76 micrograms per liter, respectively, but were 15 and 12 percent lower when concentrations were as low as 29 and 23 micrograms per liter, respectively. Tests of different configurations of sampler placement in observation wells indicate that pump placement, rate of pumping, duration of pumping, and the uniformity of the vertical and lateral distribution of trichloroethylene in ground water near the well screen have a potentially significant influence on trichloroethylene concentrations in ground water samples and that these factors can have a greater effect than the type of sampler used.     

Hydrogeology of the Brunswick (Passaic) Formation and Implications for Ground Water Monitoring Practice

Fractured shales of the Brunswick Formation provide a major aquifer in the most industrialized region of New Jersey. Numerous cases of ground water contamination have been documented in this formation. However, effectiveness of monitoring and remediation efforts is often hampered by the use of inappropriate concepts regarding ground water flow controls in this complex aquifer system. One such concept presumes that near-vertical fractures parallel to the strike of beds provide principal passages for the flow and produce an anisotropic response to pumping stress. Field evidence presented in this paper confirms that the Brunswick Formation hosts a gently dipping, multiunit, leaky aquifer system that consists of thin water-bearing units and thick intervening aquitards. The water-bearing units are associated with major bedding partings and/or intensely fractured seams. Layered heterogeneity of such a dipping multiunit aquifer system produces an anisotropic flow pattern with preferential flow along the strike of beds. Within the weathered zone, the permeability of the water-bearing units can be greatly reduced. The commonly used hydrogeologic model of the Brunswick as a one-aquifer system, sometimes with vaguely defined "shallow" and "deep" zones, often leads to the development of inadvertent cross-flows within monitoring wells. If undetected, cross-flows may promote contaminant spread into deeper units and impair the quality of hydrogeologic data. Hydrogeologic characterization of the Brunswick shales at any given site should be aimed primarily at identification of the major water-bearing and aquitard units. Recommended techniques for this characterization include fluid logging and other in-well tests.