Direct Push Chemical Sensors for DNAPL

The Department of Defense (DoD) critically requires faster, cheaper, and more accurate procedures to characterize and monitor volatile organic compounds (VOCs) in the subsurface. Chlorinated solvents in the form of dense non-aqueous phase liquids (DNAPLs) pose the most serious challenge. Failure to adequately define DNAPLs source terms plagues many remediation efforts, wasting millions of dollars and possibly exacerbating the problem by redistributing the contaminant over a larger area. The inadequacy of current characterization techniques makes for highly uncertain clean-up time and cost estimates. With present methods it is likely that decades and tens of billions of dollars will be required to cleanup DoD sites. Chlorinated solvents form DNAPLs because they are immiscible with and denser than water. Unlike petroleum hydrocarbons, DNAPLs sink as they travel through the vadose zone and into the ground water, leaving behind a trail of micro-globules in the soil matrix [1], [2]). The heterogeneously distributed free-product phase can continue to contaminate large volumes of groundwater for decades to centuries [3]. Tetrachloroethylene (PCE) has been detected in more than 10% of the wells tested in California [4]; the 5 ppb maximum allowable contaminant level (MCL) was exceeded in more than a quarter of the wells testing positive. Defining the 3-dimensional subsurface distribution of VOCs traditionally relies on drilling, discrete sampling, and laboratory analysis. This strategy is messy, yields an incomplete picture, and the data are often suspect. Samples are extracted from the soil cores at widely separated intervals (typically several feet), the choice of where to sample is made arbitrarily, and volatiles are easily lost during the process. Unconsolidated sands and silty soils tend to flow in the saturated zone, resulting in poor retention of samples collected via split spoon below the water table.