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 ind icated large differences in impacts from watershed nonpoint source inp uts. Pesticide levels in Maskenthine Lake, a small impoundment of 34 h a, increased in response to spring and early summer runoff events. Twe lve pesticides [atrazine, 2-chloro-4-ethylamino-6-isopropylamino-s-tri azine; alachlor, ro-N-(2,6-ethylphenyl)-N-(methoxymethyl)acetamide; me tolachlor, -ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamid e; cyanazine, 2-[[4-chloro-6-(ethylamino)s-triazin-2-yl] amino]-2-meth ylpropionitrile; EPTC, S-ethyl dipropylthiocarbamate; butylate, S-ethy l diisobutylthiocarbamate; propachlor, 2-chloro-N-(1-methylethyl)N-phe nylacetamide; trifluralin, alpha,alpha,alpha-trifluoro-2, 6-dinitro-N, N-dipropyl-p-toluidine; simazine, 2-chloro-4,6-bis(ethylamino)-s-tria zine; 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 atrazin e metabolites, deethylatrazine, 2-amino-4-chloro-6-isopropylamino-s-tr iazine (DEA) and deisopropylatrazine, 2-amino-4-chloro-6-ethylaminostr iazine (DIA) were detected. Atrazine, cyanazine, DEA and DIA levels we re greater than 1 mug/L. Atrazine remained above the maximum contamina nt level for potable water of 3 mug/L throughout the period of investi gation. The pesticide response to spring and summer runoff events was much less pronounced at Willow Lake, a 284 ha impoundment. Ten of 12 p esticides and two atrazine metabolites in Maskenthine Lake were also d etected 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 D IA to atrazine remained relatively constant suggesting that appreciabl e amounts of biodegradates were not produced. Apparent half-lives were calculated from observed concentration decreases that appeared to fol low first-order kinetics. Atrazine was the most persistent agrichemica l (t1/2 = 193, t1/2 = 124) introduced in the spring runoff events. Ext racted pesticide levels were higher in bottom cores from Maskenthine L ake than from Willow Lake. Differences in pesticide levels in the two lakes were related to watershed slope, soil drainage capacity, landuse , and rainfall.