Changes in Volatile Organic Chemical Concentrations After Purging Slowly Recovering Wells

摘要

AbstractMost guidelines for sampling ground water require the evacuation of multiple bore volumes from the well before a sample is collected. Such recommendations are impractical for wells that are finished in fine‐grained deposits with such slow recharge that they cannot recover rapidly enough for the requisite number of well volumes to be removed. In such situations, the sample is usually collected either 24 hours after evacuation or sometime during well recovery. Neither of these strategies has been supported by field evidence.The purpose of this study was to determine the changes in volatile organic chemical concentration during well recovery. Three hundred samples were analyzed for up to 19 different volatile organic compounds from 11 wells that were finished in fine‐grained glacial tills. Each well was sampled before purging and at several time intervals up to 48 hours after purging. The experiment was conducted three to five times on each well. Samples were collected with dedicated point‐source PTFE bailers equipped with bottom‐emptying devices that were designed for collecting samples for volatile organic chemical analysis. All wells were evacuated with a bailer because they were finished in slowly recharging materials having hydraulic conductivities of between 1 × 10−6and 7 × 10−5cm/sec.Results of the chemical analyses were examined statistically using a general linear model and the Tukey honestly significant difference test to ascertain whether the changes in water quality with time after purging were statistically significant. At the 95 percent confidence level, there was no significant difference in chemical compositions at any time interval after purging, although samples collected at four hours had slightly higher concentrations than did earlier or later samples. However, concentrations of volatile organics were significantly lower before purging than after purging. These results clearly show that wells finished in fine‐grained sediments should be purged before samples are collected for volatile organic ch