Field Investigation of Vapor‐Phase‐Based Groundwater Monitoring

Abstract

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.

On‐Site Vapor‐Phase Analysis as a Novel Approach for Monitoring Groundwater Wells

“…The results of the first phase of field testing supported the second of these conditions—that vertical stratification within individual monitoring wells was an important contributor to variability (Adamson et al ). In general, concentrations within the well water column did not appear to be uniform with depth within the majority of locations included in this program, as evidenced by the differences between vapor samples collected from the well screen and the water‐vapor interface, as well as differences between passive water samples at the screen vs. passive water samples at the interface.…”

“…On‐site vapor‐phase‐based monitoring represents a promising alternative method that can help accomplish these goals (e.g., Kampbell and Vandergrift 1998; USGS ; Adamson et al ). Vapor‐phase‐based groundwater monitoring relies on the equilibrium partitioning process for dissolved volatile organic compounds (VOCs) in contact with a gas phase (e.g., Sanford et al ; Vroblesky et al ; Vroblesky and Campbell ; USGS ; Divine and McCray ; Spalding and Watson , ; McLeish et al ).…”

“…As a result, the multistep process for obtaining concentration data from a well can be greatly simplified. A preliminary field study showed that collecting vapor samples from the screened interval of the well was most effective in obtaining strong correlations with low‐flow groundwater data (Adamson et al ).…”

“…On the basis of these findings, further field testing for validation purposes have eliminated headspace sampling from sealed monitoring wells and instead focused on two other vapor‐based approaches. The first approach—field equilibration of groundwater samples collected using low‐flow techniques, followed by on‐site analysis of the equilibrated vapor—has proven highly effective for the purpose of correlating vapor samples with low‐flow groundwater concentrations during initial studies (Adamson et al ). This strong performance was consistent with expectations as both methods analyze constituents that originate from the same sample, thus minimizing bias.…”

“…This method for collecting vapors in equilibrium with water has been described previously for quantifying dissolved gases (Sanford et al ; Spalding and Watson , ; McLeish et al ; Gardner and Solomon ) and volatile contaminants in soil gas (Kerfoot and Mayer ), sediment pore water (Vroblesky et al ; Vroblesky and Campbell ; USGS ), and lab‐scale systems (Divine and McCray ). Prior to the successful initial demonstration described by Adamson et al (), they were largely untested in groundwater monitoring wells. These samplers are wrapped in layers of gas‐permeable membrane and then submerged in the well.…”