The presence and distribution of nitrate and selected pesticides in ground water in the surficial aquifer along the Straight River in north-central Minnesota were studied. Local residents and public officials are concerned that these substances may pose a health hazard to humans and livestock. Nitrate and pesticides may move downward from cultivated croplands, livestock feedlots and manured fields, waste-water lagoons, and residential development to the ground water.

Ground water near the water table ranged in nitrate-nitrogen concentration (based on median values determined for sampled monitoring wells) from less than 5 to a little greater than 20 mg/L (milligrams per liter) except for one concentration of 50 mg/L downgradient from a feedlot and manured field. Increased nitrate-nitrogen concentrations generally were coincident with cultivated croplands. Decreased nitrate-nitrogen concentrations generally were coincident with forests. Trace amounts of atrazine were detected in 4 of 8 ground-water samples collected from 8 monitoring wells screened near the water table. Detections were more frequent in cultivated croplands (detections at 3 of 5 monitoring wells) than in forests (detections at 1 of 3 monitoring wells). Atrazine concentrations ranged from 0.01 to 0.11 (μg/L (micrograms per liter), which are well below the MCL (Maximum Contaminant Level) of 3 μg/L established by the USEPA (U.S. Environmental Protection Agency). Trace amounts of metolachlor and alachlor were detected in one of the samples in which atrazine was detected. MCLs for metolachlor and alachlor have not been established by the USEPA.

The median nitrate-nitrogen concentration determined from sampled monitoring wells along the direction of ground-water flow through five land-use settings: (1) increased from slightly greater than 0 to 50 mg/L near the water table at a feedlot and an adjacent manured field; animal waste was a nitrate source; (2) increased from 8.0 to 16 mg/L near the water table at cultivated croplands irrigated with municipal treated waste water; fertilizer probably was a nitrate source; (3) increased from slightly greater than 0 to 14 mg/L near the bottom of the aquifer at cultivated croplands irrigated with ground water; fertilizer probably was a nitrate source; (4) increased from 1.6 to 3.7 mg/L near the water table and from slightly greater than 0 to 2.8 mg/L near the bottom of the aquifer at a residential development; septic-system leachates, and possibly lawn fertilizer, may have been nitrate sources; and (5) decreased from 14 to 2.6 mg/L near the water table at three waste-water treatment lagoons; waste water was not a nitrate source.

Upgradient to downgradient mean or individual nitrogen isotope δ¹⁵N values in %_o (delta units in parts per thousand) determined for sampled monitoring wells along the direction of ground-water flow through the five land-use settings were: (1) 5.1 %_o and 4.0 %_o for the feedlot and adjacent manured field; (2) 1.1 %_o and 0.9 %_o for the cultivated croplands irrigated with waste water; (3) 3.8 %_o and 2.7 %_o for the cultivated croplands irrigated with ground water; (4) 3.4 %_o and 4.9 %_o for the residential development; and (5) 1.7 %_o and 3.0 %_o for the three waste-water lagoons. Nitrate from fertilizer appeared to have been present in ground water at the waste-water lagoons, cultivated croplands irrigated with waste water, and cultivated croplands irrigated with ground water. Nitrate from soil organic matter rather than from animal waste appeared to have been present in ground water at the feedlot and adjacent manured field.