Agricultural Chemical Movement through a Field-Size Watershed in Iowa:  Subsurface Hydrology and Distribution of Nitrate in Groundwater

Abstract

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.

Herbicide and Nitrate in Surface and Ground Water: Results from the Iowa Management Systems Evaluation Area

“…In this topography, water that infiltrates through the hill-top and side-slope soils emerges later as stream baseflow at the foot of the watershed. Nitrate was often present in baseflow concentrations exceeding 10 mg L' 1 in the watershed managed with ridge-tillage which was monitored extensively (32,33,34). This was consistent with the measurement of high nitrate concentrations in both the vadose zone and the saturated zone, with some concentrations exceeding 50 mg Ν L" 1 in the same ridge-tilled watershed (33,34).…”

“…Nitrate was often present in baseflow concentrations exceeding 10 mg L' 1 in the watershed managed with ridge-tillage which was monitored extensively (32,33,34). This was consistent with the measurement of high nitrate concentrations in both the vadose zone and the saturated zone, with some concentrations exceeding 50 mg Ν L" 1 in the same ridge-tilled watershed (33,34). In the ridge-till watershed, NO3-N leaching loss accounted for 16% of the cumulative fertilizer Ν input from 1968 to 1991, with NQ5-N and NH4-N loss in surface runoff accounting for only 1% of the fertilizer Ν input (33).…”

Terrestrial denitrification: challenges and opportunities

“…Imbalances in detailed studies at smaller scales, where plant and soil processes are carefully accounted for, are more difficult to explain than in large scale balances (Addiscott 1995;Lowrance 1992;Steinheimer et al 1998). For example, analysis of the fates of fertilizer N added over 22 years in an Iowa watershed with continuous corn show large amounts of "unaccounted for (lost or stored) N" (Figure 4).…”

Effect on nitrate concentration in stream water of agricultural practices in small catchments in Brittany: I. Annual nitrogen budgets

“…Another possible assumption is that nitrate may have a long residence time in a catchment due to hydrological processes. Evidence from isotopic studies (Bölke and Denver, 1995), from groundwater monitoring (Steinheimer et al, 1998;Molénat et al, 2002) or from fractal analysis of stream chloride and sodium Neal and Kirchner, 2000) suggests that even shallow groundwater may constitute an important reservoir for solutes. Thus, nitrate losses would be essentially transport-limited and stream concentration would be determined by a source little influenced by annual variations in the soil nitrate supply (Trudgill et al, 1991) so that the annual mean concentration should be almost constant.…”