STUDY OF PESTICIDE OCCURRENCE IN 2 CLOSELY SPACED LAKES IN NORTHEASTERN NEBRASKA

Dissolved pesticide concentrations in 168 water samples collected from two closely-spaced lakes between early May 1990 and mid-june 1991 indicated large differences in impacts from watershed nonpoint source inputs. Pesticide levels in Maskenthine Lake, a small impoundment of 34 ha, increased in response to spring and early summer runoff events. Twelve pesticides [atrazine, 2-chloro-4-ethylamino-6-isopropylamino-s-triazine; alachlor, 2-chloro-N-(2,6-ethylphenyl)-N-(methoxymethyl)acetamide; metolachlor, 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide; cyanazine, 2-[[4-chloro-6-(ethylamino)s-triazin-2-yl] amino]-2-methylpropionitrile; EPTC, S-ethyl dipropylthiocarbamate; butylate, S-ethyl diisobutylthiocarbamate; propachlor, 2-chloro-N-(1-methylethyl)N-phenylacetamide; trifluralin, alpha,alpha,alpha-trifluoro-2, 6-dinitro-N, N-dipropyl-p-toluidine; simazine, 2-chloro-4,6-bis(ethylamino)-s-triazine; propazine, 2-chloro-4,6-bis(isopropylamino)-s-triazine; fonofos, O-ethyl S-phenyl ethylpbosphonodithioate; metribuzin, 4-amino-6-(1,1-dimethylethyl) -3-(methylthio)-1,2,4-triazin-5(4H-one] and two atrazine metabolites, deethylatrazine, 2-amino-4-chloro-6-isopropylamino-s-triazine (DEA) and deisopropylatrazine, 2-amino-4-chloro-6-ethylaminostriazine (DIA) were detected. Atrazine, cyanazine, DEA and DIA levels were greater than 1 mug/L. Atrazine remained above the maximum contaminant level for potable water of 3 mug/L throughout the period of investigation. The pesticide response to spring and summer runoff events was much less pronounced at Willow Lake, a 284 ha impoundment. Ten of 12 pesticides and two atrazine metabolites in Maskenthine Lake were also detected in Willow Lake; however, concentrations did not exceed 1 mug/L. After the spring flush of pesticides in May and June, concentrations decreased until the following spring. Ratios of DEA to atrazine and DIA to atrazine remained relatively constant suggesting that appreciable amounts of biodegradates were not produced. Apparent half-lives were calculated from observed concentration decreases that appeared to follow first-order kinetics. Atrazine was the most persistent agrichemical (t1/2 = 193, t1/2 = 124) introduced in the spring runoff events. Extracted pesticide levels were higher in bottom cores from Maskenthine Lake than from Willow Lake. Differences in pesticide levels in the two lakes were related to watershed slope, soil drainage capacity, landuse, and rainfall.