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Microbiological Quality of Subsurface Drainage Water from Irrigated Agricultural Land¹

ABSTRACT

Irrigation and subsurface drainage waters sampled from an 82,150-hectare (203,000-acre) irrigation district in southern Idaho were evaluated for bacteriological quality. The soils in the district are wind deposited over fractured basalt, calcareous, and have a pH near 7.8. Drainage, where needed, is provided by horizontally mined tunnels or by tile drains connecting shallow relief wells that flow the year around. For the 12 month sending September 30, 1969, a 2-meter (6.5-foot) depth of water for the entire irrigation tract was diverted, and 50% of the water passed through the soil becoming subsurface drainage. The irrigation water and seven subsurface drains were sampled at 2-week intervals during the summer of 1969. Coliform, fecal streptococci, starch hydrolyzers, and bacteria able to grow at temperatures from 0 to 55C were counted. The diverted irrigation water contained from 140 to 3,300 coliform per 100 ml, but 86% of the subsurface drainage samples contained 5 or fewer coliforms per 100 ml. Numbers of other microorganisms were also low in the drainage waters. The outflow samples were oxygen saturated and the temperatures were 13.0 ± 1.1C for all samples. Percolation through the soil improved the water quality almost to domestic water standards.

Key Words: pollution control • water purification • ground water

¹ Contribution from the Northwest Branch, Soil & Water Cons. Res. Div., Agr. Res. Serv., USDA, Idaho Agr. Exp. Sta. cooperating. Paper presented Aug. 26, 1970, in Tucson, Arizona, at the annual meeting of the ASA as part of the symposium "Animal Wastes, Pesticides, and Water Quality."

² Soil Scientists and Research Agricultural Engineer, respectively, Snake River Cons. Res. Center, Kimberly, Idaho, 83341.

Received for publication August 2, 1971.