Survey for Herbicides in Well Water in Tulare County

The Environmental Hazards Assessment Program of the California Department of Food and Agriculture conducted a well survey in Tulare County to determine the presence of the herbicides simazine, atrazine, prometon, bromacil and diuron in well water. These herbicides were chosen because of their use on crops grown in Tulare County and on rights-of-way throughout the state. Twelve sampling areas were distributed from the Fresno to Kern County borders of Tulare County. Each area, referred to as a cell, consisted of four one-square mile sections. Ten wells were sampled in each cell. Prometon was not detected. Most concentration values were below 1 part per billion (ppb). The highest values were 1.7, 2.8, 8.5 and 6.7 ppb for simazine, diuron, atrazine and bromacil, respectively. Screens were conducted for organochlorine, organophosphate and chlorinated hydrocarbon pesticides in one well water sample from each cell. All screen results were negative. The frequency of detection and the herbicide found differed in roughly the northern and southern areas sampled in the county. In the northern cells labeled A-G, the frequency of detection was high with 40-100% of wells sampled in a cell containing a residue for at least one herbicide. In contrast, the frequency of detection in the southern cells labeled H-L ranged from lo-50% of wells sampled in a cell. Another indication of the difference between the areas was the number of different herbicide residues detected in one well water sample. In the northern area, residues for 2 or more herbicides were found in 33 of the 72 wells sampled whereas in the southern area only 1 of the 50 wells contained residues for 2 or more herbicides. Lastly, residues for simazine, atrazine, bromacil and diuron were detected in the northern cells whereas simazine and atrazine were predominant in the southern area. Data on the use of these herbicides for 1980-1984 were collected from the County Agricultural Commissioner. The data indicated that during this period atrazine was used only for rights-of-way, simazine and diuron were used on both crops and rights-of-way and bromacilwas mainly used on crops. Thus, uses of herbicides on both crops and rights-of-way appeared to be sources for the presence of pesticide residues in well water. Some factors that may have accounted for the spatial separation of the occurrence of herbicides between the northern and southern areas were pesticide-soil interactions or cropping patterns. Further investigation is needed to identify the most likely sources …