Saline Cooling Tower Water for Irrigation in Minnesota: II. Drainage Water Composition, Leaching Fraction, and Effect on Aquifer Quality

Saline cooling tower water (CTW) from the Sherburne County electrical power plant in Minnesota is being considered as an irrigation water source on surrounding croplands. Since the soils are coarse-textured and this region receives an average 66 cm rainfall annually, salt will leach into the underlying aquifer. This study was undertaken to determine composition of drainage waters, leaching fractions (LF), and quality of the underlying aquifer (10–12 m) as affected by irrigation with CTW at two rates under three cropping systems. In a 3-yr field study (1979–1981), alfalfa (Medicago sativa L.), corn (Zea mays L.), and fallow plots were irrigated at rates that replaced net water deficits (I₁) and twice the net water deficits (I₂) in the cropped areas. The soil at this site was a Hubbard loamy sand (Udorthentic Haploboroll). Lysimeters, 57 cm in diam, were buried 150 cm below the plots to monitor the composition of the drainage waters and to determine the LF. Wells were installed in all plots to monitor deep movement of the CTW salts into the aquifer. Application of CTW increased the concentration of Ca and SO₄ in the drainage waters. These ions together comprised 70–75% of the total ion equivalents measured. Salt concentrations were generally higher under alfalfa than corn and fallow, reflecting its higher water requirement. Drainage waters were undersaturated with respect to gypsum; thus, SO₄, added from the irrigation water stayed in solution. The LF under the I₂ rate averaged 0.58. The LF under the alfalfa and corn (I₁ rate) averaged 0.34 and 0.38, respectively. The LFs under the I₁ rate were higher than anticipated. Apparently, even when irrigation was scheduled at a rate consistent with satisfactory crop growth, a considerable amount of water—and in turn salts—were lost from the soil profile due to heavy, erratic precipitation. Salt concentrations in the aquifer increased greatly over the 3 yr. Sulfate concentrations peaked at 6.4 mmol L⁻¹ in 1981, over 20 times greater than background levels and higher than U.S. Environmental Protection Agency drinking water standards.