Laboratory validation study of new vapor-phase-based approach for groundwater monitoring

Recent improvements in field-portable analytical equipment allow accurate on-site measurement of VOCs present in air at concentrations of less than 0.1 parts per million volume (ppmv). The objective of this project is to determine if the use of these instruments for vapor-phase measurements of headspace in a monitoring well can serve as a reliable and accurate method for monitoring volatile organic compound (VOC) concentrations in groundwater under equilibrium conditions. As part of a comprehensive research project investigating the utility of this proposed monitoring method, the authors have completed a laboratory validation study to identify instruments and sample-collection methods that will provide accurate measurement of VOC concentrations in groundwater. This laboratory validation study identified two field-portable instruments (a gas chromatograph and a photoionization detector) with sufficient sensitivity to measure VOCs in groundwater at concentrations below typical monitoring standards (i.e., 1 to 5 μg/L). The accuracy and precision of these field instruments was sufficient to satisfy typical data-quality objectives for laboratory-based analysis. In addition, two sample-collection methods were identified that yield vapor-phase samples in equilibrium with water: direct headspace sampling and passive diffusion samplers. These sample-collection methods allow the field instruments (which measure VOC concentrations in vapor-phase samples) to be used to measure VOC concentrations in water. After further validation of these sample-collection methods in the field, this monitoring method will provide a simple way to obtain accurate real-time measurements of VOC concentrations in groundwater using inexpensive field-portable analytical instruments.