GAMA: Ground-Water Ambient Monitoring and Assessment
Figure 1. Study area map and diazinon use on almonds,
1992, (California Department of Pesticide Regulation).
Figure 2. Diazinon concentrations and discharge,
Creek, February 7, 8, 9, 1993.
Nearly two inches of rain fell during the storm of February 7-9, 1993. The first sample, collected at 1000 hours on February 8, had a concentration of 0.54 micrograms per liter at a discharge of 170 cubic feet per second (fig. 2). As discharge increased, the diazinon concentration also increased, reaching 3.8 micrograms per liter within 5 hours, at a discharge of 249 cubic feet per second. By 1900 hours, the diazinon concentration dropped to 0.7 micrograms per liter at a discharge of 418 cubic feet per second. The discharge then increased rapidly and peaked at 1,280 cubic feet per second, shortly after midnight. Although no samples were collected during the most rapid rise in discharge, a sample collected at 1315 hours on February 9, as the discharge was declining, had a diazinon concentration of 0.14 micrograms per liter. It is assumed that the diazinon concentration was less than 0.7 micrograms per liter at peak discharge because most of the flow at that point was uncontaminated water from the Coast Ranges. More limited data from Del Puerto Creek and the Spanish Grant Drain indicated similar patterns of decrease in diazinon concentration. The diazinon concentration was 5.3 micrograms per liter at Del Puerto Creek on February 8, which decreased to 0.3 micrograms per liter on February 9. The concentration at the Spanish Grant Drain was 1.5 micrograms per liter on February 8, which decreased to 0.3 micrograms per liter on February 9.
Figure 3. Diazinon concentrations and discharge at
Creek, February 18, 19, 20, 1993.
The next significant rainfall was February 18 to 20, when almost an inch of rain was recorded. The discharge was higher following this storm (figs. 2 and 3) because of near-saturated soil conditions. The highest diazinon concentration detected was only 0.2 micrograms per liter at Orestimba Creek on February 18 (fig. 3). All other concentrations were lower, indicating that most of the available diazinon had been transported during the previous storm.
Runoff in the drainage basins of the westside tributaries comes from two very different areas: the intensely farmed San Joaquin Valley and the steeply sloped, naturally vegetated Coast Ranges. An explanation consistent with the data is that agricultural runoff from orchards in close proximity to Orestimba Creek transports diazinon, and it is that runoff that causes the initial rise is discharge and concentration. The rapid decrease in diazinon concentration is attributed to two factors: (1) decrease of diazinon concentration in agricultural runoff, with time, and (2) dilution of agricultural runoff with pesticide-free water from the non-agricultural Coast Ranges. No quantitative evaluation of the relative importance of these two factors is possible from available data; however, streamflow data from the gage on Orestimba Creek at the valley margin, upstream from the sampling site, indicate that little runoff from the Coast Ranges was reaching the water-quality monitoring station at Orestimba Creek while the diazinon concentration was high. This information, along with the extremely rapid decrease in diazinon concentration with only a modest increase in discharge, indicates that decreasing diazinon concentration in the agricultural runoff may be the predominant factor.
Information on technical reports and hydrologic data related to the NAWQA program can be obtained from:
Project Chief--San Joaquin-Tulare Joseph Domagalski E-mail: firstname.lastname@example.org
Basin NAWQA Study
U.S. Geological Survey
Placer Hall, 6000 J Street Sacramento, California 95819-6129
Neil M. Dubrovsky
Charles R. Kratzer
Joseph Domagalski E-mail: email@example.com
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