An Evaluation of Some Systems for Sampling Gas‐Charged Ground Water for Volatile Organic Analysis

Abstract

AbstractLoss of volatile organics during sampling is a well‐recognized source of bias in ground water monitoring; sampling protocols attempt to minimize such loss. Such bias could be enhanced for ground water highly charged with dissolved gases such as methane. Such ground water was the object of this study. A positive‐displacement bladder pump, a momentum‐lift pump and a suction‐lift, peristaltic pump were employed in sampling both methane‐charged ground water for volatile aromatic hydrocarbons and a CO2‐charged reservoir water for volatile chlorinated hydrocarbons. In both cases, the suction‐lift pump produced samples with a significant negative bias (9 to 33 percent) relative to the other methods. Little difference between samples produced by the other pump Systems was noted at the field site, but in sampling the reservoir, the bladder pump produced samples that were 13 to 19 percent lower in halocarbon concentration than were samples from the momentum‐lift pump.These negative biases are tentatively interpreted as losses due to volatilization during sampling. Slightly greater negative biases occur for compounds of higher volatility as estimated from their Henry's law constants. Additional studies appear to be warranted in order to adequately establish the scientific basis for recommending protocols for sampling ground water in which degassing could enhance the loss of volatile organics during sampling.