Diazinon Concentrations in Surface Water of the Lower San Joaquin Basin, California

Sandra Y. Panshin, Charles R. Kratzer, JoAnn M. Gronberg, Neil M. Dubrovsky, and Joseph L. Domagalski

In:
Society of Environmental Toxicology and Chemistry abstract book, Second SETAC World Congress (16th annual meeting), Global Environmental Protection: Science, Politics, and Common Sense, Vancouver, British Columbia, Canada, p. 157

Abstract:
More than 61,000 pounds of diazinon are used in the lower San Joaquin Basin annually. About 34,000 pounds are applied to almonds, primarily during two periods: the dormant orchard period of January to February (26,000 pounds) and the pre-harvest period of June to August (4,700 pounds). The spatial and temporal distribution of diazinon in the San Joaquin River and its tributaries was assessed by collecting water samples at six sites during 1993. The frequency of diazinon detection at the six sites ranged from 60 to 96% (detection limit = 0.01 ug/L). The highest concentrations at five sites occurred during storms in January and February. Maximum concentrations ranged from 1.4 to 7.0 ug/L; concentrations corresponded to the amount applied in the western watersheds. Concentrations at sites on the small, ephemeral western tributaries generally were low ([less than or equal to]0.09 ug/L) from March through December, with a few high values (0.18 to 0.48 ug/L (in May, July, August, and September. In contrast, concentrations in a large, perennial eastern tributary (Merced River) remained low (0.01 ug/L) from March through December. Differences in the spatial distribution of concentrations in the summer months could be due to differences in crop distribution, physiographic characteristics, and water management. The east side of the valley has coarse-grained, well-drained soils. West-side soils are finer grained and less permeable. Western tributaries consist exclusively of irrigation return flows in the summer. Conversely, the Merced River consists of both reservoir releases of water originating in the forested Sierra Nevada and irrigation return flows. Load calculations estimate that on the order of 1% of the total diazinon applied annually is transported out of the basin in surface-water runoff.