Title

Effects of variability in surficial recharge on dispersion in groundwater

Date of Award

1997

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

David A. Chin, Committee Chair

Abstract

The predominant cause of dispersion in ground water is advective mixing due to variability in seepage rates. There has been extensive research investigating hydraulic conductivity variations as a cause of this seepage variability. This dissertation examines the impact of surficial recharge on this dispersion phenomena, as it induces varying seepage rates. Specifically, the effect of variations in surface recharge to a shallow surficial aquifer is investigated and related to the effects of hydraulic conductivity variations. An analytic formulation is developed which relates aquifer parameters and the statistics of recharge variability to increases in the dispersion coefficient. The analytic formulation is developed by solving Fourier transforms of the small perturbation forms of the ground-water flow equations. Two field studies are used to characterize the statistics of recharge variability for input to the analytic formulation. A time-series of water levels at a continuous ground-water recorder is used to investigate the temporal statistics of hydraulic head caused by recharge, and a series of infiltrometer measurements are used to define the spatial variability in the recharge. These field statistics, representing head fluctuations due to recharge, allow the analytic formulation to compute the dispersion coefficient without an explicit representation of the recharge boundary. In order to compare with, and define the limits of, this analytic formulation, a series of numerical experiments were developed. A sophisticated model is developed that uses a particle tracking algorithm, modified to account for temporal variations, to estimate dispersion. Rainfall timeseries statistics and an unsaturated flow model are used to develop the stochastic numerical model which indicates the effects of recharge on dispersion for various aquifer configurations. Significant increases in the dispersion coefficient, on the order of 10 percent, are indicated by the analytic formulation for the shallow surficial aquifer at the field site. Dimensionless analysis of the analytic scheme indicates that with a aquifer head fluctuation timescale on the order of days or less, with head fluctuation standard deviation greater than a tenth of a meter, in an aquifer several tens of meters or less, that recharge variability must be accounted for in the determination of dispersion. The analysis also indicates that if the timescale of the aquifer head response to recharge is on the order of several days or less and the aquifer is greater than 5 meters deep, it is probably not necessary to determine the infiltrometer statistics and the recharge-induced dispersion can be determined from the water level well statistics.

Keywords

Hydrology; Engineering, Civil

Link to Full Text

http://access.library.miami.edu/login?url=http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:9824533