Depth and Temporal Variations in Water Quality of the Snake River Plain Aquifer in Well USGS-59 near the Idaho Chemical Processing Plant at the Idaho National Engineering and Environmental Laboratory

In-situ measurements of the specific conductance and temperature of ground water in the Snake River Plain aquifer were collected in observation well USGS-59 near the Idaho Chemical Processing Plant at the Idaho National Engineering and Environmental Laboratory. These parameters were monitored at various depths in the aquifer from October 1994 to August 1995. The specific conductance of ground water in well USGS-59, as measured in the borehole, ranged from about 450 to 900 flS/cm at standard temperature (25� C). The pumping cycle of the production wells at the Idaho Chemical Processing Plant causes changes in borehole circulation patterns, and as a result the specific conductance of ground water at some depths in the well varies by up to 50% over a period of about 14 hours. However, these variations were not observed at all depths, or during each pumping cycle. The temperature of ground water in the well was typically between 12.8 and 13.8 C The results of this study indicate that temporal variations in specific conductance of the ground water at this location are caused by an external stress on the aquifer - pumping of a production well approximately 4000 feet away These variations are believed to result from vertical stratification of water quality in the aquifer and a subsequent change in intra well flow related to pumping. When sampling techniques that do not induce a stress on the aquifer (i e thief sampling) are used, knowledge of external stresses on the system at the time of sampling may aid in the interpretation of geochemical data