Graduation Year

2005

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Geology

Major Professor

H. Leonard Vacher, Ph.D.

Keywords

Homogeneous, Heterogeneous, Variable-density, Equivalent freshwater heads, Asr cycles, Recovery efficiency

Abstract

Aquifer Storage Recovery (ASR) is a recently developed (circa 1970) method (in the U.S.A.) to reduce groundwater-pumping stresses by injecting treated wastewater or surface water during periods of low demand into an aquifer followed by its recovery during periods of high demand. This method has been successfully implemented in numerous locations across the U.S.A. and worldwide, mainly due to the cost savings provided by the use of an unlimited reservoir (aquifer) in which to store water compared to the costs to construct surface impoundments and the inherent problems with storing such water for extended periods of time under evaporative atmospheric conditions.

"This study describes the use of a highly discretized, three-dimensional, variable-density, numerical model (SEAWAT 2000) that incorporates the vertical variation of hydraulic conductivities, measured foot by foot, from a continuous core collected from the upper Floridan aquifer in southwest Florida, to evaluate the effects of small-scale heterogeneities on a hypothetical ASR system well. In order to compare these effects to the more general case in which average hydraulic parameters are used to characterize flow zones, a model is constructed with average parameters taken from the heterogeneous case. This study attempts to determine whether aquifer heterogeneities influence the performance of ASR systems, compared to assumed homogeneous conditions, by quantifying differences in recovery efficiency, horizontal and vertical flow due to advection and dispersion, plume dimensions, and storage periods.

The results of this study indicate that 1) the geometry of the injectate plume under homogeneous and heterogeneous conditions differ significantly; 2) background formation total dissolved solids (TDS) concentrations significantly control the quantity of potable water available for recovery; 3) dispersion exhibits a strong control on vertical mixing; 4) multiple injection cycles are required to generate a plume of potable water for long term storage; and 5) the percent recoveries under homogeneous and heterogeneous conditions are generally similar only in low-salinity background concentrations, due to the absence of the effects of buoyancy. Although the percent recoveries of the systems modeled are similar, the success of an ASR well is strongly controlled by the existence of heterogeneities, which essentially determine the degree of horizontal and vertical mixing of the injectate with formation waters.

Heterogeneities result in varying groundwater and mass transport paths during injection and recovery periods. Presumably these variations would need to be considered when evaluating potential variations in groundwater quality due to mixing between formation and injected water. Understanding potential variations in groundwater quality and treatment alternatives due to the presence of ASR-associated geochemical conditions, e.g., elevated arsenic concentrations, may also be improved with a detailed heterogeneous numerical model.

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