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Groundwater professionals require methods to estimate the potential time required to achieve remedial goals, including locations within and downgradient of zones containing nonaqueous phase liquids (NAPLs). NAPLs have long been recognized as persistent contaminant sources to groundwater. Dissolution of multicomponent NAPLs is particularly complex, and numerical models that explicitly simulate it are not widely available. This study introduces an equilibrium partitioning approximation to simulate the dissolution of the most soluble chemical components from multicomponent NAPL containing a significant fraction of relatively insoluble mass. The effective distribution coefficient that describes depletion of a specific compound from NAPL is estimated based on the properties of the NAPL and the porous medium. This study also presents numerical modeling results that support the utility of the method, with verification using published empirical data collected during dissolution of residual coal tar in a controlled laboratory sand tank experiment. The numerical modeling method uses equilibrium partitioning as an approximation and matched the concentrations of the two most soluble NAPL components in and downgradient of the NAPL zone with reasonable accuracy. The results suggest that the method should be useful for screening‐level assessments and can be adapted to compare relative groundwater restoration timeframes of select NAPL components for various remedial alternatives. [ABSTRACT FROM AUTHOR]

Matrix diffusion can attenuate the rate of plume migration in fractured bedrock relative to the rate of ground water flow for both conservative and nonconservative solutes of interest. In a system of parallel, equally spaced constant aperture fractures subject to steady-state ground water flow and an infinite source width, the degree of plume attenuation increases with time and travel distance, eventually reaching an asymptotic level. The asymptotic degree of plume attenuation in the absence of degradation can be predicted by a plume attenuation factor, β, which is readily estimated as R'([φ.sub.m]/[φ.sub.f]), where R' is the retardation factor in the matrix, [φ.sub.m] is the matrix porosity, and [φ.sub.f] is the fracture porosity. This dual-porosity relationship can also be thought of as the ratio of primary to secondary porosity. β represents the rate of ground water flow in fractures relative to the rate of plume advance. For the conditions examined in this study, β increases with greater matrix porosity, greater matrix fraction organic carbon, larger fracture spacing, and smaller fracture aperture. These concepts are illustrated using a case study where dense nonaqueous phase liquid in fractured sandstone produced a dissolved-phase trichloroethylene (TCE) plume ~300 m in length. Transport parameters such as matrix porosity, fracture porosity, hydraulic gradient, and the matrix retardation factor were characterized at the site through field investigations. In the fractured sandstone bedrock examined in this study, the asymptotic plume attenuation factors (β values) for conservative and nonconservative solutes (i.e., chloride and TCE) were predicted to be ~800 and 12,210, respectively. Quantitative analyses demonstrate that a porous media (single-porosity) solute transport model is not appropriate for simulating contaminant transport in fractured sandstone where matrix diffusion occurs. Rather, simulations need to be conducted with either a discrete fracture model that explicitly incorporates matrix diffusion, or a dual-continuum model that accounts for mass transfer between mobile and immobile zones. Simulations also demonstrate that back diffusion from the matrix to fractures will likely be the time-limiting factor in reaching ground water cleanup goals in some fractured bedrock environments., Introduction Characterizing solute plume transport in fractured bedrock is often an essential component of environmental investigations at sites contaminated by dense nonaqueous phase liquids (DNAPLs). Examples of DNAPLs frequently encountered [...]

Groundwater professionals require tools to evaluate a variety of technical issues related to per‐ and polyfluoroalkyl substances (PFAS). These include the potential impact of PFAS precursors on groundwater plumes of perfluoroalkyl acids (PFAAs). Numerical modeling results show that, by adjusting the mass loading rate, source zones with or without a precursor can produce similar PFAA plumes. However, if a precursor is present, it can impact PFAA plume concentrations and extend PFAA plume durations by decades. Additional research regarding in situ precursor transformation rates—and improvements in source area characterization—will further advance the predictive value of modeling. [ABSTRACT FROM AUTHOR]

The maximum steady state flow rate into a well can be predicted by observing the water potential water table drawdown. The prediction was first carried out by Kozeny in 1953. Controlling factors of water table drawdown such as the initial aquifer thickness, its radius of influence during steady state pumping at maximum stage, the pumping well effective radius and the seepage face area along the inside of the well screen were estimated.

This paper describes a practical field method of using applied tracers to determine how much purging is required to collect representative groundwater samples after the introduction of drilling water during borehole advancement. In general, the approach involves adding a tracer of known concentration to the drilling water and then measuring the tracer in the purge water until the tracer concentration declines to a defined target level. If necessary, the dilution effects of residual drilling water can be quantified, and the measured contaminant concentration can be corrected based on the measured tracer concentration. A project example is presented to demonstrate that this method is straightforward and reliable and that applied tracers can be used to quantify the influence of residual drilling water on formation water quality while also ensuring that purge times and volumes are not unnecessarily large. [ABSTRACT FROM AUTHOR]