WA10 TMDL and Monitoring Programs for the San Joaquin River Basin (USA)
D135-136
8:00 AM - 12:00 PM, Wednesday
() Monitoring diazinon and chlorpyrifos for TMDL development.
Kratzer, C1, Zamora, C1, Domagalski, J1, 1 U.S. Geological Survey, Sacramento, California, USA
ABSTRACT- During the study period of 2000 and 2001, diazinon and chlorpyrifos were used in both agricultural and urban areas in the San Joaquin Basin. However, because of the dominance of agricultural land use, urban sources were a relatively small part of the load measured at the basin outlet. About 53 percent of the agricultural application of diazinon occurred during winter (primarily January and February) to dormant orchards, with the rest applied during irrigation season (primarily March through August) to orchards and field crops. Only about 6 percent of the agricultural application of chlorpyrifos occurred during winter to dormant orchards, with the rest applied during irrigation season to orchards and alfalfa. Thus, January through August was the primary period of concern for transport of diazinon and chlorpyrifos. Within this time period, two monitoring approaches were used: winter storm sampling plus fixed-interval sampling in January and February, and fixed-interval sampling during March through August. This monitoring approach was based on meeting three data interpretation objectives: describing concentrations over time, identifying peak concentrations, and calculating loads. Factors considered in selecting monitoring sites included application rates and times; locations of streamflow and rainfall gages and availability of real-time data; and travel times in the hydrologic system. The daily yield of diazinon from the agricultural area was 0.021 grams per day per square kilometer for the dormant season and 0.008 for irrigation season, whereas the dormant season and irrigation season yields of chlorpyrifos were both 0.004. The amount of applied diazinon that made its way to the basin outlet during the dormant season averaged about 0.22 percent, compared with the irrigation season amount of 0.17 percent. For chlorpyrifos, the percentages were 0.09 and 0.007, respectively. Thus, transport rates to surface water were higher during winter storms than during irrigation season.
Key words: chlorpyrifos, diazinon, monitoring