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Document Type

Thesis - University Access Only

Award Date

2000

Degree Name

Master of Science (MS)

Department

Civil and Environmental Engineering

Abstract

The Big Sioux River Basin covers approximately 9,600-square-miles, of which about 70 percent is located in eastern South Dakota. The Big Sioux River Basin is a major source of ground water for municipal, domestic, and irrigation use. Studies have found that dissolved organic carbon (DOC) levels in Big Sioux River Basin ground water are generally low, averaging about 7.7 mg/L. However, in Brookings Grant and Minnehaha Counties, DOC levels are as much as 30 times higher than elsewhere in the basin. These elevated DOC levels could be an early indicator of ground water contamination due to land surface activities. To help interpret the significance of the elevated DOC levels, a better understanding of the relationship of the relationship between surface water and ground water is needed. A hydrologic investigation was conducted on a prairie pothole (wetland) in Brookings County South Dakota, located within the Big Sioux River Basin. The study focuses on ground-water and surface-water interaction. Material underlying the prairie pothole area is glacial till, which has been subdivided into weathered and unweathered till. The upper weathered till average 25 feet in thickness and is underlain by unweathered till. The Hydraulic conductivity of weathered glacial till in South Dakota generally from 1x10-4 to 1v10-7 feet/second with an average of about 2x10-5 feet/second. The hydraulic conductivity values of the unweathered till range up to 3 orders of magnitude lower than conductivity values of unweathered till. Due to the low hydraulics conductivity of unweathered, ground-water movement through the unweathered till is insignificant or non-existent. Therefore, the unweathered till is considered the lower limit is considered the lower limit of ground-water movement at the prairie pothole investigation site. As an aid in better understanding the ground water and surface water interaction at the prairie pothole, a digital ground-water flow model of the weathered till was developed. The model was to used to test various combinations hydraulic properties, recharged to, and discharge from the weathered till. The model was calibrated by comparison of observed ground water levels to model-simulated water levels. During calibration, the prairie pothole was represented as a constant head boundary. Once the model was calibrated the boundary representing the prairie pothole was modified as a river boundary (fluctuating head). The model verified conclusions observed by with the data collection. First, the general ground water flow direction with in the area of the prairie pothole is form west to east. Second, during a normal year the prairie pothole surface-water recharges the ground-water, with the exception of the northeast corner, where ground water is recharging the prairie pothole. The model further suggests that under normal conditions, wet years, and when the pond stage is high, the surface-water of the pothole recharges the ground-water. Therefore, under these conditions the surface-water in the prairie pothole has an influence on the ground-water around it. In terms of water quality high DOC levels in the prairie pothole could affect the DOC levels in the ground-water east and south of the prairie pothole.

Library of Congress Subject Headings

Hydrologic cycle -- Big Sioux River Watershed (S.D. and Iowa)

Water quality

Format

application/pdf

Number of Pages

199

Publisher

South Dakota State University

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