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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.

A study site underlain by a claypan soil was instrumented to examine the transport of fertilizer nitrogen (N) under corn (Zea mays L.) cultivation. The study was designed to examine N transport within the unsaturated zone and in intedlow (the saturated flow of water on top of the claypan). A 15N‐labeled fertilizer (labeled N), bromide (Br), and chloride (Cl) were used as field tracers. Rapid or prolonged infiltration events allowed water and dissolved solutes to perch on the claypan for brief periods. However, a well‐developed network of preferential flow paths quickly diverted water and solutes through the claypan and into the underlying glacial till aquifer. Excess fertilizer N in the unsaturated zone supplied a continuous, but declining input of N to ground water for a period of 15 mo after a single fertilizer application. Calculated solute velocities through the claypan matrix (6.4 × 10−6 cm s−1) were similar to horizontal transport rates along the claypan (3.5 to 7.3 × 10−6 cm s−1) but much slower than infiltration rates determined for preferential flow paths (1.67 × 10−3 cm s−1). These flow paths accounted for 35% of the transport. A seasonally variable, dual mode of transport (matrix and preferential flow) prevented the daypan from being an effective barrier to vertical transport. Simulations of selected field observations, conducted using the variably saturated two‐dimensional flow and transport model, VS2DT, confirmed the presence of a dual flow regime in the claypan.

In July 1991, the U. S. Geological Survey and the Missouri Department of Health collected water samples from 130 rural domestic wells in Caldwell, Clinton, Daviess, Gentry, and Nodaway Counties in northwestern Missouri. Triazine or Cl (chlorinated)-acetamide herbicide concentrations exceeded 0.05 microgram per liter in 32 percent of all samples analyzed with enzyme-linked immunosorbent assays. Samples from 79 wells were analyzed using gas chromatograph/mass spectrometry methods. One or more of the herbicides alachlor, atrazine, cyanazine, metolachlor, metribuzin, and trifluralin were detected at concentrations greater than or equal to 0.05 microgram per liter in water from 19 of the 79 wells. Atrazine was detected in samples from 16 of the wells in concentrations that ranged from 0.05 to 9.6 micrograms per liter. Atrazine concentrations exceeded 3 micrograms per liter in only one sample. Nitrite plus nitrate as nitrogen concentrations in water samples from 129 wells sampled in northwestern Missouri during 1991 ranged from less than 0.05 to 63 milligrams per liter. Nitrite plus nitrate concentrations were greater than or equal to 10 milligrams per liter in water samples from 31 wells. Mean well depth for wells with nitrite plus nitrate concentrations less than 0.05 milligram per liter was 72.7 feet and mean well diameter was 20.2 inches. Wells with nitrite plus nitrate concentrations greater than 0.05 milligram per liter had a mean well depth of 44.2 feet and a mean well diameter of 32.6 inches. Arsenic was detected in two samples and the concentrations in all samples ranged from less than 5 to 9 micrograms per liter. Total recoverable iron concentrations ranged from less than 50 to 6,600 micrograms per liter and were greater than 500 micrograms per liter in 23 wells. Manganese concentrations in water samples ranged from less than 20 to 2,600 micrograms per liter and were greater than 50 micrograms per liter in 38 samples. In July 1992, water samples were collected from 147 wells in Audrain, Clark, Lewis, Monroe, Scotland, and Shelby Counties in northeastern Missouri and analyzed for herbicides. Alachlor, atrazine, cyanazine, metribuzin, and metolachlor were detected at concentrations greater than 0.10 microgram per liter in water samples from 19 of the 147 wells. Atrazine was detected in water from 18 of the 19 wells with detectable herbicide concentrations. Atrazine concentrations exceeded 3.0 micrograms per liter in two of the samples.

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