EFFECTS OF SAMPLE ISOLATION AND HANDLING ON THE RECOVERY OF PURGEABLE ORGANIC-COMPOUNDS

This report compares the recovery of purgeable organic compounds (POCs) obtained by using a downhole isobaric sampler developed by the U.S. Geological Survey, a helical-rotor submersible pump, and a point source bailer to collect and isolate samples of ground water from three wells in New York and New Jersey, the samples contained a total of 13 POCs detectable at concentrations ranging from 0.5 mug/L to about 400 mug/L. This report also compares the effects of sample handling, specifically the differences in POC concentration recovery when an isobaric sample container is filled at land surface vs. when it is filled downhole, and when samples are taken using a bailer with and without a bottom-emptying device. These case studies are used to quantify the possible effects of different sample-isolation and sample-handling techniques on POC recovery. The relative effectiveness of the three devices varied by site and by compound. Overall, the POC recoveries achieved by using the helical-rotor submersible pump and the downhole isobaric sampler were not significantly different at the 95 percent confidence level. POC recovery obtained by using the point source bailer was 11 percent lower overall. The downhole isobaric sampler results exhibited smaller coefficients of variation than did the helical-rotor submersible pump or the point-source bailer results. However, the differences between the coefficients of variation of the downhole isobaric sampler and those of the helical-rotor submersible pump were not significant the 95 percent confidence level. The nonsignificant smaller coefficient of variation of the downhole isobaric sampler apparently resulted from two fewer sample handling steps that exposed the sample water to ambient air. An independent experiment performed with a different downhole sampler at one of the wells used in this investigation produced a similar statistical result. Also, the POC recovery obtained by pouring sample water out the top Of a point source bailer into 40-mL vials was 8 percent lower than that obtained by filling vials from a bailer with a bottom-emptying device.