Characterization of ground-water flow and chemical transport beneath two irrigated fields in south-central Kansas, 1988

Abstract

Hydrologic and chemical data were collected at two irrigated fields in south-central Kansas during 1988. These data pertain to ground-water flow and chemical transport in the unsaturated and saturated zones with particular emphasis on the movement of the herbicide atrazine. One field was irrigated by using an overhead center-pivot sprinkler system (S field), and the second field was irrigated by using flood-irrigation techniques (F field). Soil-moisture profiles indicated that the available water capacity of the unsaturated zone was approximately 1 inch at the S field and 2 inches at the F field. A mathematical scheme that calculates water balance on a weekly basis was used to estimate that, between January 10, 1988 and January 31, 1989, 7.7 inches of water were available for recharge at the S field. On the basis of a chloride tracer test, the seepage velocity through a depth interval of about 6 to 10 feet at the S field was estimated to be about 4.6 feet per year, and the estimated maximum recharge during 1988 was 5 inches. Lithologic and water-level data indicate that water flow in the saturated zone beneath the S field acts as a two-unit ground-water system, with the irrigation well extracting more water out of the more-permeable lower sand unit, and that water flow beneath the F field acts as a one-unit ground-water system exhibiting the effects of partial penetration of the aquifer by the pumping irrigation well. Soil-water samples collected in 1988 from the unsaturated zone at depths from about 6 to 10 feet showed no definitive movement of atrazine. A possible explanation is that the atrazine detected was applied before 1988, and that the 1988 atrazine application had not reached the 6-foot depth during 1988 because of retardation. This interpretation is supported by analyses of soil samples that showed the largest concentrations of atrazine in the top 0.3 foot at a time when chloride (which had been applied at the same time as the atrazine) had already reached a depth of 6 feet. A mathematical model shows that, below the root zone, atrazine follows a nonsorbing tracer, such as chloride, by approximately 1.2 years. There is a greater decrease in chemical concentrations, from the land surface to the subsurface, for atrazine as compared to nitrite plus nitrate. On the basis of 1988 data, the fraction of chemical applied at land surface that remains in water that reaches the top of the saturated zone is 0.15 to 0.3 for nitrogen, but the fraction of atrazine remaining is less than 0.0002 for the S field or 0.004 for the F field. Additional avenues of atrazine loss are suggested. The chemical variables of chloride, nitrite plus nitrate, and atrazine show stratification in concentrations in the saturated zone; concentrations decrease with increasing depth. Concentrations of chloride and nitrite plus nitrate in the saturated zone show the effects of deep circulation at the S field and the predominance of shallow circulation at the F field; concentrations in water from deep test wells are larger at the S field than at the F field. INTRODUCTION The widespread use of agricultural chemicals poses a possible threat to groundwater quality under cropland in south-central Kansas. This threat may be of particular concern where the potential for downward movement of pesticides and fertilizers is increased by irrigation. The herbicide atrazine has been detected in ground water beneath irrigated cropland in south-central Kansas, but not at the frequency or in the concentrations that might be expected in view of the widespread use of this chemical. Among 13 ground-water samples collected in