Evaluation of the Validity of Groundwater Samples Obtained Using the Purge Water Management System at SRS

摘要

As part of the demonstration testing of the Purge Water Management System (PWMS) technology at the Savannah River Site (SRS), four wells were equipped with PWMS units in 1997 and a series of sampling events were conducted at each during 1997-1998. Three of the wells were located in A/M Area while the fourth was located at the Old Radioactive Waste Burial Ground in the General Separations Area. The PWMS is a 'closed-loop', non-contact, system used to collect and return purge water to the originating aquifer after a sampling event without having significantly altered the water quality. One of the primary concerns as to its applicability at SRS, and elsewhere, is whether the PWMS might resample groundwater that is returned to the aquifer during the previous sampling event. The purpose of the present investigation was to compare groundwater chemical analysis data collected at the four test wells using the PWMS vs. historical data collected using the standard monitoring program methodology to determine if the PWMS provides representative monitoring samples.The analysis of the groundwater chemical concentrations indicates that the PWMS sampling methodology acquired representative groundwater samples at monitoring wells ABP-1A, ABP-4, ARP-3 and BGO-33C. Representative groundwater samples are achieved if the PWMS does not resample groundwater that has been purged and returned during a previous sampling event. Initial screening calculations, conducted prior to the selection of these four wells, indicated that groundwater velocities were high enough under the ambient hydraulic gradients to preclude resampling from occurring at the time intervals that were used at each well. Corroborating evidence included a tracer test that was conducted at BGO-33C, the high degree of similarity between analyte concentrations derived from the PWMS samples and those obtained from historical protocol sampling, as well as the fact that PWMS data extend all previously existing concentration trends to the present time. The latter line of evidence is considered to be the most powerful in demonstrating that representative samples are being acquired by the PWMS because it is highly unlikely that previously existing concentration trends would continue if resampling had occurred. Standard procedure for obtaining protocol groundwater monitoring samples at the Savannah River Site (SRS) calls for extracting or 'purging' sufficient quantities of groundwater to allow removal of stagnant water and to allow certain key indicator parameters to stabilize prior to collection of samples. The water extracted from a well prior to sample collection is termed 'purge water' and must be managed in an approved fashion if it contains hazardous and/or radiological constituents that exceed specified health-based limits described in the Investigation Derived Waste Management Plan (WSRC, 1994). Typical management practices include containerization, transportation, treatment, and disposal via Clean Water Act-permitted facilities. A technology for handling purge water that eliminates the need to containerize and transport this water to a disposal facility has been developed. This technology, termed the Purge Water Management System (PWMS), is currently under pilot stage deployment at SRS.