Estimating subsurface water and contaminant discharge is fundamental to understanding the nature and extent of site contamination. In recent years, the use of contaminant flux and contaminant mass discharge as robust metrics to assess risks at contaminated sites and evaluate the performance of site remediation efforts has been gaining increased acceptance within the scientific, regulatory, and user communities. This project demonstrated and validated the passive flux meter (PFM) as a new tool for direct in situ measurement of both cumulative subsurface water and multiple contaminant fluxes.
The passive flux meter is a self-contained permeable unit that is inserted into a well or subsurface boring such that it intercepts groundwater flow but does not retain it. The interior composition is a matrix of hydrophobic and hydrophilic permeable sorbents that retain dissolved organic and inorganic contaminants. The sorbent matrix is also impregnated with known amounts of one or more fluid soluble "resident tracers." These tracers are leached from the sorbent at rates proportional to the fluid flux. Following exposure to groundwater flow for a period ranging from days to months, the meter is removed and the sorbent carefully extracted to quantify the mass of all contaminants intercepted by the sorbent matrix and the residual masses of all resident tracers. The contaminants' masses are used to calculate time-averaged contaminant mass fluxes, while residual resident tracer masses are used to calculate cumulative fluid flux.
The PFM was demonstrated and validated at five sites, including Canadian Forces Base Borden in Ontario, Canada; Hill AFB in Layton, Utah; NASA Launch Complex 34 in Cape Canaveral, Florida; the Naval Construction Base in Port Hueneme, California; and the Naval Surface Warfare Center at Indian Head, Maryland. At Borden, field tests demonstrated that the PFM could be deployed in the field for 45 days and yield accurate cumulated measures of water, tetrachloroethene (PCE), and trichloroethene (TCE) fluxes. It was also shown under controlled subsurface flow conditions that the PFM measured MTBE fluxes, which compared closely with independent estimates derived from multilevel samplers, and that the PFM could function as an instrument to measure local aquifer conductivities in addition to groundwater fluxes if local hydraulic gradients were also measured during deployments. Field tests conducted at Hill AFB validated the PFM as a tool for characterizing the performance of source zone remediation. At Port Hueneme, the PFM was evaluated in multiple wells of different construction, and it was demonstrated that the PFM could be deployed as a characterization tool to obtain high resolution vertical profiles of groundwater flux. Finally, at the Naval Surface Warfare Center site, the project demonstrated that the PFM could measure the mass fluxes of an inorganic contaminant, perchlorate, using surfactant-modified activated carbon as a sorbent. The focus of the NASA site was to demonstrate and validate the PFMs under an aggressive environment where NASA was demonstrating bioaugmentation to enhance the removal of TCE using an engineered microbial culture, KB-1™. Results showed that the PFM was capable of measuring fluxes of multiple contaminants (TCE, dichloroethene, and vinyl chloride) but had difficulty in measuring water fluxes.
PFMs provide simultaneous measurements of both water and contaminant fluxes that can be used for process control, for remedial action performance assessments, and for compliance purposes. The only alternative technology for comparison purposes is to measure groundwater contaminant concentrations through multilevel samplers (MLS) and then calculate contaminant fluxes using groundwater fluxes estimated from borehole dilution (BHD) tests. Cost estimates, per linear foot of well screen tested, for PFM deployments and BHD/MLS measurements indicate that the PFM method results in lower unit costs depending on cost variability and the number of wells monitored. Contaminant flux values derived from the BHD/MLS method further represent short-term (instantaneous) evaluations that reflect current conditions and not long-term trends. PFMs produce robust flux estimates that reflect long-term transport conditions and are less sensitive to day-to-day fluctuation in flow and contaminant concentration. (Project Completed - 2006)