Water quality in Walnut Creek watershed: Herbicides and nitrate in surface waters

There is a lack of quantitative information describing the impact of fanning on water quality at the watershed scale. This study documents the surface water quality of Walnut Creek-a 5130-ha watershed with about 86% of the land used for crop production. Starting in 1990, now and concentrations of NO3-N and four herbicides-atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methyl-thio)-1,2,4-triazin-5(4H)-one], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N- (2 -methoxy-1-methylethyl)acetamide]-were measured at eight locations. Nitrate-N concentrations often exceeded 10 mg L-1 during May, June, and July. Total losses from the watershed ranged from 4 to 66 kg ha(-1) yr(-1) and represented 6 to 115% of the N applied as fertilizer in any year. Atrazine and metolachlor were detected at concentrations >0.2 mu g L-1 in about half of all water samples, while alachlor and metribuzin were seldom detected. Median concentrations for atrazine and metolachlor were below 1 mu g L-1 for all locations within the watershed. During runoff events, herbicide concentrations in the stream increased while NO3-N concentrations decreased. Yearly losses from the watershed ranged from 0.2 to 7.5 g ha(-1) for atrazine and from 0.3 to 6.7 g ha(-1) for metolachlor. These losses represent 0.18 to 5.6% of the atrazine and 0.047 to 1.6% of the metolachlor applied in any year.