Successful remediation requires an accurate conceptual site model (CSM) that includes geology, hydrogeology and contaminant distribution.
However, subsurface geologic heterogeneity has been a continuing challenge to successful characterization and remediation of dense non-aqueous phase liquid (DNAPL) sources and plumes. The understanding of DNAPL transport over time has recently been advanced due to the availability of high density, real-time tools and strategies to cost effectively collect data for all three domains.The approach described below determines mass flux, the impact of back diffusion and improves the understanding of risk.
Measurement of DNAPL flux in both the advective and impermeable zones at a site helps determine the stage of the plume and the potential effects of back diffusion can be accurately assessed. There are now tools available for collecting high density data in real-time to accurately delineate subsurface stratigraphy and hydrogeology. The Hydraulic Profiling Tool (HPT) and Waterloo Profiler provide quantitative measurement of hydraulic conductivity. Our direct sampling ion trap mass spectrometer (DSITMS) measures contaminant mass distribution in high density. Using these tools in combination allows flux and mass transport to be assessed directly without the need for monitoring wells.
TriadES assists clients in the direct measurement of DNAPLflux in groundwater using the hydraulic profile and quantitative measurements of discrete groundwater samples. Based on the CSM developed during high resolution site characterization using the DSITMS and the HPT, a limited number of plane view locations and depths are selected for groundwater sampling and analysis based on varying hydraulic conductivity and contaminant concentration. The measured groundwater concentration at a specific location and depth is multiplied by the hydraulic conductivity to generate a quantitative index of flux for the advectively transported groundwater. The index of flux allows quantitative comparison of mass transport zones at the site.
A similar approach is used to determine stored vs. mobile dissolved phase contamination and address the issue of back diffusion of contaminant from immobile dissolved phase contamination. This approach is applicable down-gradient from DNAPL source areas that have been subject to diffusive contaminant loading into low permeability zones during years of advective transport of contaminant in adjacent permeable zones. At locations where contamination has been detected in the saturated soils in low permeability zones, soil samples are collected and the bulk contamination is measured. The assumption is made that the bulk phase contamination is present only as dissolved phase in the soil pore water. The bulk phase soil concentration is converted to an equivalent groundwater concentration based on pore volume and soil density. The (calculated) groundwater concentration is multiplied by the hydraulic conductivity for this plan view and depth to generate a quantitative index of flux.
By measuring the hydraulic conductivity and contaminant concentration in both the advective transport zones of aquifers and the low permeability zones where dissolved phase contamination is stored, site managers can determine the flux of contamination from these different transport processes. This knowledge will allow better site remediation and management decisions.