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Field Investigation of Vapor-Phase-Based Groundwater Monitoring



The use of in-field analysis of vapor-phase samples to provide real-time volatile organic compound (VOC) concentrations in groundwater has the potential to streamline monitoring by simplifying the sample collection and analysis process. A field validation program was completed to (1) evaluate methods for collection of vapor samples from monitoring wells and (2) evaluate the accuracy and precision of field-portable instruments for the analysis of vapor-phase samples. The field program evaluated three vapor-phase sample collection methods: (1) headspace samples from two locations within the well, (2) passive vapor diffusion (PVD) samplers placed at the screened interval of the well, and (3) field vapor headspace analysis of groundwater samples. Two types of instruments were tested: a field-portable gas chromatograph (GC) and a photoionization detector (PID). Field GC analysis of PVD samples showed no bias and good correlation to laboratory analysis of groundwater collected by low-flow sampling (slope = 0.96, R2 = 0.85) and laboratory analysis of passive water diffusion bag samples from the well screen (slope = 1.03; R2 = 0.96). Field GC analysis of well headspace samples, either from the upper portion of the well or at the water-vapor interface, resulted in higher variability and much poorer correlation (consistently biased low) relative to laboratory analysis of groundwater samples collected by low-flow sample or passive diffusion bags (PDBs) (slope = 0.69 to 0.76; R2 = 0.60 to 0.64). These results indicate that field analysis of vapor-phase samples can be used to obtain accurate measurements of VOC concentrations in groundwater. However, vapor samples collected from the well headspace were not in equilibrium with water collected from the well screen. Instead, PVD samplers placed in the screened interval represent the most promising approach for field-based measurement of groundwater concentrations using vapor monitoring techniques and will be the focus of further field testing.