Volatilization Losses of Organics During Ground Water Sampling from Low Permeability Materials

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AbstractVolatilization biases that can affect a aground water sample before its collection from a monitoring well were evaluated in this study. Previous studies showed that volatilization losses during sampling of high permeability materials do not introduce unacceptable bias, except; for the most volatile compounds. In low permeability materials, however, ground water must normally accumulate for hours to days after flushing before a volume sufficient for sampling is available. During this period, the ground water sample is open to the atmosphere and volatilization can lower the concentration of volatile compounds in solution.Laboratory simulations were conducted to evaluate this bias using four chlorinated, one‐ and two‐carbon compounds. Two distinct conditions of headspace exposure were investigated: (a) the water standing in the well casing, and (b) formation water entering the screen of a well that has been dewatered during purging.Water standing in the well was depleted in volatile organics by exponential decay with a half life of about four days. Volatilization losses will be less than 10 percent if the standing time is less than about six hours. In wells that have been purged dry, volatilization losses of 10 percent are likely in as little as five minutes as the recovering formation water trickles through the headspace in the dewatered sand filter pack. Losses may reach 70 percent for recovery periods of one hour. When the sand filter pack is drained by the purging procedure, the sample should not be analyzed for volatile constituents since volatilization biases are likely to be substantial.Conventional open system monitoring wells should be used to collect volatile organic samples only if fresh formation water can be drawn into the well with minimal turbulence and exposure to the atmosphere. One should therefore avoid drawing the water level down into the sand pack when the well is purged. Specialized sampling methods should be developed and evaluated for volatile organics where sample integrity is critical.