Degree Type

Dissertation

Date of Award

2017

Degree Name

Doctor of Philosophy

Department

Civil, Construction, and Environmental Engineering

Major

Civil Engineering (Environmental Engineering)

First Advisor

Chris Rehmann

Abstract

Flow, stream depletion rate (SDR), and transport are evaluated with three analytical models for wells installed in a semi-infinite, homogeneous, anisotropic, unconfined aquifer near a fully penetrating stream with a streambed with reduced conductivity. The first model, presented in Maroney and Rehmann (2017), involves re-evaluating the calculations of Huang et al. (2012) for the SDR of a radial collector well. The present solution for SDR has a single integral that requires numerical evaluation, while the previous model has five. Analytical results show that at steady state the flow through the streambed equals the pumping rate of the well. That is, the steady SDR is one, and it does not depend on streambed or aquifer properties or the well design. Before the SDR reaches steady state, streambed conductance, aquifer anisotropy, and the position of the well relative to the stream affect SDR much more than the orientation, length, and depth of the lateral well screens.

The second model builds on the method developed for SDR for a radial collector well. It provides flow and SDR for a partially penetrating vertical well installed in a semi-infinite, homogeneous, unconfined aquifer adjacent to a stream with a reduced conductivity streambed. Like the model for SDR of a radial collector well, the SDR for this case has only one improper integral that must be evaluated numerically. Steady drawdown is symmetric across the horizontal plane at the center of the aquifer for a well centered in the aquifer. The supply of water from the stream decreases hydraulic gradients at the top and bottom of the aquifer. The model for a partially penetrating well provides quantitative guidance for practical applications. For example, for removing contaminants, a well with small degree of penetration should be placed near the level of the contamination, and for dewatering—which aims for maximum drawdown and minimum SDR, the well should be placed as high in the aquifer as possible.

Streamlines from a river to a nearby well are used to compute the concentration of a contaminant in a well. Concentration at the stream is constant and transport to the well is through advection, retardation, and decay. Both the well and steam fully penetrate the aquifer, which has horizontal anisotropy. The stream has a streambed with reduced conductivity. As shown by dimensional analysis and supporting arguments, the concentration at the well depends on four dimensionless parameters: dimensionless time, a streambed conductance coefficient χ, the ratio κy of the horizontal hydraulic conductivities, and the Damköhler number, which accounts for advection, retardation, and decay by combining the properties of the aquifer, well, and contaminant. For fixed χ and κy, the timing and magnitude of the steady state concentration at the well depend only on the Damköhler number. The special case of no streambed (→∞) yields conservative estimates of the time of first arrival, steady state concentration, and (except for low Damköhler number) the time to steady state.

Copyright Owner

Cynthia Louise Maroney

Language

en

File Format

application/pdf

Share

COinS