Showing papers by "Fernando Miralles-Wilhelm published in 1996"

Stochastic analysis of sorption macrokinetics in heterogeneous aquifers

Abstract: A stochastic analysis of the transient characteristics of sorption at field scales shows that sorption macrokinetics arise as a result of aquifer physical and chemical heterogeneities. Analytical expressions for the time evolution of the field scale retardation factor and longitudinal macrodispersivity for a reactive solute are obtained and compared with previous theoretical findings. Sorption macrokinetics are characterized by a timescale related to the average solute velocity, mean retardation factor, and variability correlation scale. A comparison of this timescale with the intragrain diffusion characteristic timescale provides a quantitative criterion to determine the relative importance of intragrain processes and aquifer physical and chemical heterogeneities on the characteristic field scale features of reactive solute plumes: retardation and dispersion. Calculations performed for the Borden field site in Ontario, Canada, show that the macrokinetics timescale is potentially much larger than the intragrain diffusion characteristic time, rendering natural heterogeneities as the controlling mechanism in the transient development of reactive plumes in the field. Analytically derived expressions for spatial moments of a reactive plume are evaluated to obtain results for the mass in solution, center of mass displacement, and longitudinal second moment. These results compare favorably with numerical simulations, but a comparison with field measurements is limited at this time owing to a lack of reliable data.

Stochastic Analysis of Transport and Decay of a Solute in Heterogeneous Aquifers

Abstract: A stochastic analysis of transport and first-order decay for a solute plume in a three-dimensionally heterogenous aquifer shows that a spatially variable decay rate produces effects both in the transient and steady state characteristics of the effective decay rate, effective solute velocity, and longitudinal macrodispersivity. The effective decay rate is found to be less than the mean, which implies that decay predictions that employ spatial averages of observations will tend to overestimate the rates of mass transformation due to decay. The effective solute velocity is found to be the only field scale coefficient dependent on the correlation between decay and log hydraulic conductivity. The longitudinal macrodispersivity is found to be reduced relative to that of a conservative/nondecaying solute. The characteristic timescale of the transient development of all field-scale coefficients is reduced by the presence of a heterogeneous decay rate. All of these trends are accentuated with increasing decay rate variability. Increased peak concentrations, earlier arrival times, and decreased plume spreading are practical consequences of the derived results. The stochastic analysis presented here provides a vehicle to link laboratory-scale measurements with these field-scale predictions.