Hydrologic and Land‐Use Factors Associated with Herbicides and Nitrate in Near‐Surface Aquifers

Abstract

Selected herbicides, atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐s‐triazine) metabolites, and NO − 3 were examined in near‐surface unconsolidated and bedrock aquifers in the midcontinental USA to study the hydrogeologic, spatial, and seasonal distribution of these contaminants. Groundwater samples were collected from 303 wells during the spring and late summer of 1991. At least one herbicide or atrazine metabolite was detected in 24% of the samples collected for herbicide analysis (reporting limit 0.05 µ g/L). No herbicide concentration exceeded the USEPA's maximum contaminant level (MCL) or health advisory level. The most frequently detected compound was the at razine metabolite deethylatrazine [2‐amino‐4‐chloro‐6‐(isopropylamino)‐s‐triazine] followed by atrazine, deisopropylatrazine [2‐amino‐4‐chloro‐6‐(ethylamino)‐s‐triazine], prometon (2,4‐bis(isopropylamino)‐6‐methyoxy‐s‐triazine), metolachlor [2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl)‐N‐(2‐methoxy‐1methylethyl)acetamide], alachlor [2‐chloro‐N‐(2,6‐diethylphenyl)‐N‐(methoxymethyl)acetamide], metribuzin [4‐amino‐6‐( tert ‐butyl)‐3‐methylthio‐ as ‐triazine‐5(4H)‐one], simazine [2‐chloro‐4,6‐bis(ethylamino)‐s‐triazine], and cyanazine [2‐[[4‐chloro‐6‐(ethylamino)‐1,3,5‐triazin‐2‐yl]amino]‐2‐methylpropionitrile]. Nitrite plus nitrate, as nitrogen (N), exceeding 3.0 mg/L (excess NO − 3 ), was found in 29% of the samples, and 6% had − 3 exceeding the MCL of 10 mg/L. Ammonium as N was detected in excess of 0.01 mg/L in 78% of the samples. A nonlinear increase in the frequency of atrazine detection occurred with decreases in reporting limit. The frequency of atrazine residue detection (atrazine + deethylatrazine + deisopropylatrazine) was 25% greater than for atrazine alone. Herbicide detections and excess NO − 3 were notably lacking in the eastern part of the study region where it was estimated that herbicide and fertilizer use were among the largest in the region. Prometon, the second most frequently detected herbicide, was associated with non‐agricultural land use. Herbicide and excess NO − 3 were more frequent in unconsolidated aquifers than in bedrock aquifers. Aquifer depth, as direct measurement of proximity to recharge sources, was inversely related to frequency of herbicide detection and excess NO − 3 .