Agricultural Pesticides in Shallow Ground Water
of the Quincy and Pasco Basins, Washington
Joseph L. Jones and Lonna M. Roberts
Eighty-nine shallow wells were sampled for pesticides during 1993 to 1995 to investigate the effects of irrigated agriculture on the quality of shallow ground water in the Quincy and Pasco Basins. The effects of two different types of agriculture were investigated: irrigated row crops that include potatoes sometime during a 3- to 5-year rotation (row crops), and orchards. Forty-nine wells were located in immediate proximity to row crop land use areas, and 40 were similarly located near orchards.
Samples were analyzed for 144 pesticides and related compounds; these included herbicides, insecticides, and fumigants, along with nitrate concentration and other general water-quality indicators. Analytical detection levels were very low--for most pesticides, concentrations were reported in parts per trillion (nanograms per liter)--allowing an unprecedented study of the presence of pesticides in shallow ground water beneath the area. Detection levels were higher for most fumigants, and for common measures of water quality.
Two types of wells were used for each study: domestic wells and monitoring wells installed specifically for this study. The domestic wells selected were shallow (average depth 140 feet) and near the land use being investigated (usually within 20 to 50 feet). The monitoring wells averaging about 40 feet deep were installed typically within a few feet of the land use areas.
Eighteen domestic wells and 22 monitoring wells were sampled. The only compound detected above the maximum contaminant level (MCL) was the discontinued fumigant DCP (1,2-dichloropropane), which exceeded the MCL in one well (about 2% of the 40 wells sampled). Sixty-eight percent of wells had one or more pesticide detection. Twenty compounds were detected, six of which were detected at 10% or more of the wells sampled: desethylatrazine (a degradation product, 52%), atrazine (40%), simazine (28%), DCPA (15%), DCP (10%), and azinphos-methyl (10%). On average, 2.0 compounds were detected per well, with 4 wells (10 %) having 5 or more detections per well. Health criteria are available for about 80% of detected compounds and concentrations were usually less than one percent of the health standard.
Herbicides were detected the most frequently: twelve herbicides and two break-down products comprised 75% of all detections. Fumigants and associated volatile compounds were detected next most frequently (16% of detections). Insecticides (dinoseb, methyl paration, or azinphos-methyl) comprised 9% of detections.
Thirty domestic wells and 19 monitoring wells were sampled. The compounds detected above the maximum contaminant level (MCL) were the discontinued fumigant EDB, which exceeded the MCL in two wells (5% of the 41 wells sampled for EDB), and the discontinued insecticide dieldrin (4% of 49 wells) which exceeded the drinking water guideline in both wells in which it was detected (dieldrin has a very low guideline of 2 parts per trillion). Sixty-nine percent of wells had one or more pesticide detection. Twenty-four compounds were detected, five of which were detected at 10% or more of the wells sampled: atrazine (45%), desethylatrazine (a degradation product, 35%), DCP (1,2-dichloropropane, 12%), metribuzin (10%), and metolachlor (10%). On average, 1.9 compounds were detected per well, with 5 wells having 5 or more detections per well. Drinking water guidelines or drinking water standards are available for about 70% of detected compounds and concentrations were usually less than one percent of the health standard.
Herbicides were detected the most frequently: fourteen compounds and two break-down products comprised 79% of all detections. Fumigants and associated volatile compounds were detected next most frequently (19%). Insecticide compounds (dieldrin, or DDE, a break-down product of DDT) comprised 3% of detections.
No large differences in detection rates or numbers of compounds detected were found between the two agricultural land uses. In both land use areas, herbicides were most commonly detected; of these the triazine class of compound (including atrazine, simazine, and metribuzin) were most common. Fumigants were also found in both areas, but were slightly more common in the row crop areas where they are typically used. Similarly, insecticides were found slightly more often near orchards where they are more commonly used.
Oral presentation at Conference on Agriculture and Water Quality in the Pacific Northwest, 1st, Yakima, Washington, Oct. 22-23, 1996.