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Nonpoint source pollution of surface water by nitrate from agricultural activities is a national problem. An agricultural watershed in the Iowa Loess Hills with a 23year history of annual corn production with average N fertilization is studied. Headcut seepage is transported through a natural riparian zone and observed as weir baseflow; surface runoff is measured separately. Nitrate runoff graphs illustrate the importance of high-frequency sampling of each event to permit quantitative estimation of chemical loss. The concentration of nitrate carried from the field in basin drainage steadily increased from <1 mg L -1 in 1969 to >20 mg L -1 in 1991. The rate of cumulative increase in the amount of applied N is greater than the rate of removal by the crop. Over the 23-year record, 23% of the mean annual application of N remains stored and available for leaching or chemical conversion by soil microbes. Nitrate removal during early spring snowmelt surface runoff shows a diurnal pattern that corresponds to the daily freezing and thawing of the surface soil in early March. Contribution to the load of nitrate deposited on the soil surface by rainfall is very small in comparison to the amount applied by fertilizer application. Measurable changes in water quality within various hydrogeologic compartments are seldom observed in just a few years of monitoring. Therefore, these results emphasize the importance of long-term data sets incorporating temporal variability when evaluating the impact of agricultural practices on surface water resources.

Atrazine is a commonly used herbicide in corn (Zea mays L.) growing areas of the USA. Because of its heavy usage, moderate persistence, and mobility in soil, monitoring of atrazine movement under field conditions is essential to assess its potential to contaminate groundwater. Concentrations of atrazine, deisopropylatrazine (DIA), and deethylatrazine (DEA) were measured in subsurface drainage and shallow groundwater beneath continuous, no-till corn. Water samples were collected from the subsurface drain (tile) outlets and suction lysimeters in the growing seasons of 1990 and 1991, and analyzed for atrazine and two principle degradates using solid-phase extraction and HPLC. In 1990, atrazine concentration ranged from 1.3 to 5.1 µg L −1 in tiledrain water and from 0.5 to 20.5 µg L −1 in lysimeter water. In general, concentrations of parent and degradates in solution were atrazine > DEA > DIA. Lesser levels of atrazine were measured in 1991 from Plots 2 and 4; however, greater concentrations of atrazine (6.0-8.4 µg L −1) were measured from Plot 5. Throughout the two growing seasons, atrazine concentration in Plot 5 tile-drain water was greater than that of Plots 2 and 4, suggesting a preferential movement of atrazine. Concentrations of DIA and DEA ranged from 0.1 to 2.2 and 0.9 to 3.2 µg L −1 , respectively, indicating that the degradation products by themselves or in combination with parent atrazine can exceed the maximum contaminant level (mcl) of 3 µg L −1 even though atrazine by itself may be <3 >µg L −1. The deethylatrazineto-atrazine ratio (DAR) is an indicator of residence time in soil during transport of atrazine to groundwater. In Plots 2 and 4, DAR values for tile-drain water ranged from 0.43 to 2.70 and 0.50 to 2.66, respectively. By comparison, a DAR of 0.38 to 0.60 was observed in Plot 5, suggesting less residence time in the soil.

A 40-ha field is under study in the loess hills of southwestern Iowa to determine the impact of corn production in ridge-tilled soils on the nitrate-nitrogen loading in groundwater. Within the vadose zone, nitrate concentration between June 1989 and December 1991 ranged from <10 to >80 mg/L. Well water concentrations increased from <5 mg/L in 1972 to >60 mg/L in 1994. In both hydrogeologic compart ments, time of sampling and landscape position are important factors influencing concentrations. The unsaturated zone groundwater system has a high potential for storage of unutilized nitrogen as nitrate. Leaching resulted in the drinking water MCL being exceeded for several wells screened within the saturated loess, which is characterized by relatively high hydraulic conductivity. Concentrations within and below the loess−glacial till interface did not exceed the standard. A conservative solute transport model was used to predict the concentration of nitrate exiting the field in basin drainage. Denitrification in which nitrate is reduced to nitrite by autotrophic bacteria and then further reduced geochemically to nitric oxide, nitrous oxide, or nitrogen may be an important mechanism for reducing the nitrate concentration within selected landscape positions, especially those in near proximity to the water table. Due to its relatively rapid conductance of both water and applied agchemicals, the loess hills represent a vulnerable agricultural landscape on which nitrogen fertilization impacts groundwater quality.

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