Long-term monitoring of pesticides allows for the development of daily models of concentration and load, which can help guide best management practices and provide insights to consequences of climatic shifts, such as droughts. Drought or near drought conditions have prevailed in California since 2012. Although some parts of the state received near normal precipitation in water year 2016, other locations were still below average. Extended drought can impact aquatic organisms in a variety of ways because of decreased flows and elevated water temperature. However, lower precipitation and availability of irrigation water may limit subsequent runoff, resulting in reduced concentrations and loads of certain environmental toxicants, such as pesticides and ammonia, thereby limiting their toxic effects. In this study, funded by the U.S. Geological Survey National Water Quality Program, the occurrence of 227 pesticides and degradation products and nutrients was assessed before and during this current drought in the two largest rivers draining to the San Francisco Bay: the Sacramento and San Joaquin Rivers (two long-term trend sites for this program). The watersheds of both rivers include substantial agricultural and urban land uses. Herbicides, insecticides, fungicides, and ammonia were detected throughout the study (2010 to 2016) and models of daily concentration using the seasonal wave model (rloadest) were formulated to assess the amount of time that concentrations potentially exceeded benchmark levels known to be toxic to aquatic organisms. Frequently detected pesticides included the fungicide azoxystrobin, herbicides or their degradation products such as diuron, glyphosate, and metolachlor, and insecticides such as imidacloprid. Compounds that are transported primarily by surface runoff generally showed decreasing concentrations as the drought progressed, especially in the San Joaquin River. Compounds mainly transported by groundwater, as indicated by seasonal concentration profiles, had more stable concentrations in the rivers. Mass loads to the Bay all decreased, as expected, because of the lower river discharge. When compared to aquatic-life benchmarks, modeled concentrations indicated that individual compounds were not contributing to toxicity to zooplankton, non-vascular plants, or fish at these two locations where most of the freshwater inputs discharge to this estuary.

Daily models of agricultural chemical concentrations and loads in rivers draining the Central Valley, California to the San Francisco Bay-delta estuary: Before and during an extended drought

253rd National Meeting (2017)

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