Water Quality in Walnut Creek Watershed: Herbicides and Nitrate in Surface Waters
- D. B. Jaynes *,
- J. L. Hatfield and
- D. W. Meek
There is a lack of quantitative information describing the impact of farming 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, flow 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-(methylthio)-1,2,4-triazin-5(4N)-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 µg 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 µg 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 03 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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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