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Document Type

Thesis - University Access Only

Award Date

1995

Degree Name

Master of Science (MS)

Department

Civil and Environmental Engineering

First Advisor

Darrell W. DeBoer

Abstract

High concentrations of naturally occurring selenium are found in soils of the Lake Andes Wagner project area. Increasing public concern about environmental issues, including water quality, could jeopardize the project because of potential selenium concentrations in project drain age waters. Hence it would be wise to use drainage system geometries and water management strategies that limit drainage discharge and thereby reduce selenium with it. The water management model DRAINMOD was used to investigate the influence of drainage system geometries and irrigation management practices on drainage discharge. Nine years of climatic data (1985- 1993) from the automatic weather station at Oacoma, SD, were available for simulation purposes. Soil information was obtained from various sources. Over 80 simulations were carried out for hydraulic conductivities ranging from 0.25 to 6.25 cm/hr, drain spacings varying from 30 to 210 m and two irrigation management practices of 2.5 cm/4 day and 5.0 cm/8 day. The study was limited to soil conditions east of Choteau Creek since data west of Choteau Creek were not available. Results of the study indicate that drain spacings, hydraulic conductivities and irrigation management practices all have profound effects on drain outflows. As spacing increased for a given hydraulic conductivity, the annual drain outflows decreased. Average yearly drain outflows for a hydraulic conductivity of 0.25 cm/hr decreased from 4.24 cm for a 30 m drain spacing to 0.33 cm for a 210 m spacing. Higher hydraulic conductivities produced higher drain outflows than the lower hydraulic conductivities. Average yearly drain outflows for the 30 m drain spacing increased from 4.24 cm for a hydraulic conductivity of 0.25 cm/ hr to 6.23 cm for a hydraulic conductivity of 6.25 cm/ hr, a 31 percent increase. It was observed that the drain outflows were small or even zero during dry years indicating that drainage may be necessary only during wet years. Drain outflow was less for the 2.5 cm practice than for the 5.0 cm irrigation practice. Soils with a hydraulic conductivity of 1.25 cm/hr and a spacing of 52 m produced an average yearly drain outflow of 2.85 cm for the 2.5 cm practice and 3.46 cm for the 5.0 cm practice. Both irrigation management practices did not meet the Bureau of Reclamation recommended leaching ratio of 7 percent except for the 0.25 cm/hr hydraulic conductivity scenario. An average leaching ratio of 4.5 percent was found. A statistical analysis was performed on the soil selenium concentration data sets for soils of east of Choteau Creek. Soil samples were collected at four depths (0.0-0.5, 0.5-1 .0, 1 .0 -2.0, 2.0-3.0 m) every 200 m depth along a 16 .7-km north and south leg and a 10-km east and west leg of the transect for a total of 133 locations. For readily available selenium, a maximum concentration of 7,338 ppb was found in 2.0-3.0 m layer. Mean percentage of readily and conditionally available selenium increased with depth whereas unavailable selenium decreased with depth. It was found that lognormal distribution fit the selenium concentration data sets. No trends were detected for the horizontal spatial distribution of the selenium fractions.

Library of Congress Subject Headings

Soils -- South Dakota -- Selenium content
Irrigation -- South Dakota -- Management
Drainage -- South Dakota -- Computer simulation
DRAINMOD

Format

application/pdf

Publisher

South Dakota State University

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