Although water from 20 to 25% of shallow farmstead wells in northern Missouri has concentrations of nitrate (NO−3) exceeding 10 mg L−1 as nitrogen (N), many potential sources for this NO−3 are usually present. A field experiment was designed to trace and isolate the amount of a single application of N fertilizer lost to a glacial‐till aquifer and runoffrom a 400 m2 corn (Zea mays L.) plot with bromide (Br−) and isotopically labeled (15N) fertilizer. Soil at the plot is a Albaquic Hapludalf of the Adco Series containing a 61 cm claypan beneath 41 to 43 cm of topsoil. Groundwater levels ranged from 0.38 to 2.40 m below the land surface. Transport of water and NO−3 to the saturated zone was not substantially retarded by the claypan. Labeled‐N fertilizer accounted for as much as 8.6 mg L−1 of the NO−3 (as N) in groundwater, but only in the top 1 to 2 m of the saturated zone. After two growing seasons (16 mo), <2% of the labeled‐N fertilizer was lost to runoff, about 30% was in the saturated zone, 27.3% was removed with the grain, and about 5% remained in the unsaturated zone. A large part of the remaining labeled N may have been lost in gaseous N forms. The presence of labeled NO−3 only in the top 2 m of the aquifer, slow horizontal transport, and winter recharge indicate grass crops such as wheat (Triticum aestivum L.) or rye (Secale cereale L.) might be used to extract near‐surface N during the winter recharge period. Also, fall fertilizations can be expected to readily leach. Because groundwater concentrations of labeled NO−3 were still increasing after two growing seasons, rotation of crops requiring small N inputs could be expected to limit the cumulative effect of large annual fertilizer applications on groundwater.

The in situ measurement of water flow and chemical transport through clay pan soils is crucial to understanding potential water contamination from agricultural sources. It is important due to the large areal extent of these soils in agricultural regions of the midwestern United States and because of preferential flow paths caused by desiccation cracks, worms burrowing, and root development. A study plot at the Missouri Management Systems Evaluation Area near Centralia, Missouri, was instrumented to determine the rate of preferential flow of water and transport of NO3−1 fertilizer in the unsaturated zone through a claypan soil using 15N‐NO3−1 and Br‐1 tracers. The areal distribution of preferential flow paths was between 2 and 20% in the topsoil. Gravity lysimeter flow caused by preferential flow through the claypan was as much as 150 times greater than the estimated average rate of vertical recharge. As much as 2.4% of the volume of the soil below the clay pan may be occupied by preferential flow paths. The 15N‐NO3−1 concentrations in ground water indicate that substantial quantities of fertilizer‐derived NO3−1 were transported to ground water through the claypan during the first recharge event following fertilizer application even though that event occurred six months after application. Hydraulic conductivity, measured at three scales, ranged from 6.2 × 10−8 to 7.5 × 10−3 cm/s. The observed increase of calculated hydraulic conductivity with each increase in scale was attributed to the inclusion of more and larger preferential flow paths within the volume over which the measurement was made, indicating hydraulic conductivity measured at one scale may not describe flow and transport at another scale.

The Missouri River alluvial aquifer in the Kansas City metropolitan area is the only aquifer in the area capable of supplying large quantities of ground water. Hydrogeologic data for the Missouri River alluvial aquifer at Kansas City, Missouri and Kansas, were compiled from existing data into a regional geographic information system. The vertical hydraulic conductivity of soil was used to identify areas of the aquifer susceptible to surface contamination. The vertical hydraulic conductivity ranges from 0.002 to 0.152 meter per hour. Areas of vertical hydraulic conductivity between 0.023 and 0.043 meter per hour are greatest in extent. Large areas with vertical hydraulic conductivity greater than 0.043 meter per hour occur in conjunction with industrial land use within the Kansas City metropolitan area. Two potentiometric-surface maps of the aquifer were developed from synoptic water-level measurements collected from 187 wells during August 17 to 22, 1992, and from 155 wells during January 18 to 22, 1993. During these periods the Missouri River gained water from the aquifer and ground water flowed away from the valley walls, toward the river, and down the river valley. Under conditions of nearly equal pumping rates at active well fields, induced recharge from the river into the aquifer caused cones of depression at well fields near the river to be smaller than cones of depression around well fields located some distance from the river.

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The Missouri River alluvial aquifer near Ft. Leavenworth, Kansas, supplies all or part of the drinking water for Ft. Leavenworth; Leavenworth, Kansas; Weston, Missouri; and cooling water for the Kansas City Power and Light, Iatan Power Plant. Ground water at three sites within the alluvial aquifer near the Ft. Leavenworth well field is contaminated with trace metals well field for all well-pumping/river-stage scenarios. Groundwater travel times to the Ft. Leavenworth well field for average well-pumping/river-stage scenario ranged from about 33 years for the closest contamination site to about 71 years for the farthest contamination site. Groundwater flow was induced below the Missouri River by the Ft. Leavenworth and Leavenworth well fields for all well-pumping/river-stage scenarios.