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.

During 1990, pesticides were detected in water from 29 of 92 wells and the 5 surface-water sites sampled in Bates, Cass, St. Clair, and Vernon Counties in west-central Missouri. Atrazine concentrations exceeded 3 micrograms per liter in samples from 5 wells and 2 surface-water sites. During 1991, pesticides were detected in water from 27 of 100 wells sampled in Bates, Cass, and Vernon Counties. During 1991, the mean tritium concentration was 28.3 picocuries per liter in water samples from 20 wells. Water samples from three wells had tritium concentrations less than 9.0 picocuries per liter. During 1990, nitrate, as nitrogen, concentrations exceeded 10 milligrams per liter in water samples from 21 of 89 wells. During 1991, nitrate concentrations exceeded 10 milligrams per liter in water samples from 23 of 99 wells and 1 of 3 springs. During 1991, the 815N (ratio of 15nitrogen to 14nitrogen in nitrate, relative to atmospheric nitrogen, expressed in per mil) in water samples from 22 wells ranged from +4.2 to +29.2 per mil. All but two of the samples had 815N of nitrate values equal to or greater than +10 per mil. During 1990, alpha radiation concentrations ranged from less than 1 to 33 picocuries per liter. Beta radiation concentrations ranged from less than 1 to 110 picocuries per liter. During 1991, arsenic concentrations ranged from less than 5 to 10 micrograms per liter. Iron concentrations ranged from less than 50 to 8,600 micrograms per liter. Manganese concentrations ranged from less than 20 to 7,800 micrograms per liter.

During 1988, pesticides were detected in 4 of 59 wells sampled in the northwestern reach of the Missouri River alluvium between the Iowa border and St. Joseph, Missouri. During 1989, pesticides were detected in 7 of 50 wells sampled in the northwestern reach and in 3 of 31 wells sampled in the west-central reach between Kansas City and Miami, Missouri. During June 1989, pesticides were detected in water from all 14 surface-water sites sampled in the west-central reach. Atrazine concentrations equalled or exceeded the U.S. Environmental Protection Agency's maximum contaminant level of 3 micrograms per liter in 8 of 14 surface-water samples. During 1988, nitrite plus nitrate concentrations as nitrogen equalled or exceeded the Missouri drinking-water-supply criteria of 10 milligrams per liter in samples from 9 of 60 wells in the northwestern reach. During 1989, samples from 7 of 51 wells in the northwestern reach and 3 of 31 wells in the west-central reach equalled or exceeded the criteria. Arsenic concentrations equalled or exceeded the Missouri drinking-water-supply criteria of 50 micrograms per liter in samples from five wells during 1988 and from two wells during 1989 in the northwestern reach of the Missouri River alluvium. Arsenic concentrations were less than 15 micrograms per liter in all wells sampled in the west-central reach. During 1989, the mean detected iron concentration was 8,000 micrograms per liter in samples from wells in the northwestern reach, and 6,300 micrograms per liter from wells in the west-central reach.

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