Helium and Neon Groundwater Tracers to Measure Residual DNAPL: Laboratory Investigation

Abstract

A laboratory investigation was conducted to evaluate the applicability of dissolved He and Ne as partitioning tracers for detecting and quantifying nonaqueous phase liquid (NAPL) in the saturated zone. Based on the results of batch experiments, the equilibrium NAPL–water partition coefficients ( K N,W ) of these tracers for two common dense NAPLs (DNAPLs), tetrachloroethene (PCE) and trichloroethene (TCE), are: K PCE,W = 1.28 and K TCE,W = 2.42 for He, and K PCE,W = 1.84 and K TCE,W = 3.24 for Ne. Tracer partitioning is linear across the range of concentrations tested, and appears to be linear even near aqueous solubility limits of the gases. Multiple partitioning tracer tests (PTTs) were conducted in columns, and residual TCE saturations ( S TCE ) ranging from 4.7 to 10.5% were successfully measured by the tracers. Sensitivity analysis for the column experiments indicates that random tracer-measurement error of up to ±20% had little effect on results; however, accurate characterization of the tail region of the tracer curves is particularly important. Therefore, the low analytical detection limits possible with dissolved He and Ne (4 to 5 orders of magnitude below aqueous solubility) may permit better tracer curve characterization than commonly used alcohol partitioning tracers, and is a notable advantage for these tracers. Due to their high Henry9s Law constants, these gases will also partition into trapped air present in the tracer sweep zone. Equations are presented for estimating both trapped air and NAPL saturation for PTTs where three phases are present (water, trapped air, and residual NAPL). The results of this investigation provide a basis for field-scale application of dissolved He and Ne as groundwater partitioning tracers.