Radon as a Complementary Well-Purging Indicator for Sampling Volatile Organic Compounds in a Petroleum-Contaminated Aquifer

摘要

Ground water sampling protocols generally require that a well be purged prior to sampling. At present, the stability of conventional field measurements such as electrical conductivity, water temperature, and pH is used as a criterion to determine whether a well has been purged sufficiently to yield "representative" water quality samples. The primary objectives of this study were (1) to evaluate the validity of using the stability of conventional field measurements as a well-purging criterion for sampling volatile organic compounds (VOCs) in an aquifer region contaminated with free-phase gasoline and (2) to investigate the possibility of using radon-222 as a complementary well-purging indicator. Monitoring wells in a refinery were sampled at locations both upstream and inside an area contaminated with free-phase gasoline. The variation of conventional field measurements, VOCs, and radon-222 was evaluated with time and the number of casing volumes flushed. The results indicated that the number of casing volumes required for purging prior to sampling for VOCs is significantly larger in the gasoline-impacted area than in the uncontaminated aquifer. In addition, the stability of conventional field measurements alone was not sufficient to determine if a well had been purged sufficiently to yield representative VOC water quality samples. Radon-222 concentrations appeared to follow the temporal variation of dissolved VOCs in the gasoline-contaminated region of the aquifer, suggesting that radon-222 might be a complementary well-purging indicator if a field method were available for rapid assessment of dissolved radon concentrations.