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, beta, which is readily estimated as R' (phi(m)/phi(f)), where R' is the retardation factor in the matrix, phi(m) is the matrix porosity, and phi(f) is the fracture porosity. This dual-porosity relationship can also be thought of as the ratio of primary to secondary porosity. Beta represents the rate of ground water flow in fractures relative to the rate of plume advance. For the conditions examined in this study, beta 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 approximately 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 (beta values) for conservative and nonconservative solutes (i.e., chloride and TCE) were predicted to be approximately 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.

No abstract

No abstract

The magnitude of water‐table drawdown achievable by a fully penetrating pumping well in an unconfined aquifer commonly is limited to a small fraction of the initial aquifer thickness. The maximum potential water‐table drawdown can be estimated based on the aquifer thickness, the pumping well effective radius, and the estimated radius of influence in the aquifer during maximum steady‐state pumping (Kozeny, 1953). The maximum steady‐state flow rate into a well can be predicted if the aquifer hydraulic conductivity is known based on observed water‐table drawdown during a pumping period. To achieve the maximum potential pumping rate from a fully penetrating well in an unconfined aquifer, the water level inside the well must be maintained at the bottom (Kozeny, 1953; this paper). This condition creates a seepage face along the inside of the well screen. The hydraulics of the seepage face control the removal of water from the aquifer and the size of the resulting cone of depression. The area of the seepage face, which is a direct function of the effective radius of the pumping well, strongly influences the maximum potential water‐table drawdown and the maximum steady‐state pumping rate. The vertical component of the hydraulic gradient in the formation is downward and increases toward the pumping well. The hydraulic potential, therefore, decreases with increasing depth below the water table. Only shallow observation wells that are screened across the uppermost fraction of the saturated zone are adequate for delineating the actual water‐table position during pumping. Observation wells screened across deeper portions of the aquifer, including fully penetrating, fully screened observation wells, exhibit potentiometric drawdown in excess of true water‐table drawdown.