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(001) Seasonality, Storm Runoff Events, and Critical Pollutant Loading Conditions for an Effluent-Dominated, Semi-Arid Urban/Agricultural Coastal California Watershed.
Duke, L.*,1,2, Erickson, Elizabeth2, Becker, Melinda2, 1 University of South Florida Department of Environmental Science and Policy, Tampa, FL, US2 California Regional Water Quality Control Board, Los Angeles, CA, US
ABSTRACT- Effective TMDL load allocation must accommodate critical conditions: watershed-specific loading and assimilative capacity conditions when impairment is at its maximum. Many watersheds and pollutants reach critical conditions at low flow (minimum assimilative capacity), and are not impaired during wet weather's increased flow. That is not true for Calleguas Creek, California, US, where chloride from urban and agricultural sources impairs an irrigation beneficial use. Watershed hydrology is typical of the semi-arid U.S. west: discrete storm events clustered into a wet season, each with runoff discharges enduring hours to days, separated by days or weeks of no rainfall; and dry-season stream flow, historically near zero, now sustained by baseline flow from municipal wastewater, elevated water tables, and urban activities. Seasonally-defined TMDL allocations are not effective because wet weather does not sustain high flows: mean daily discharge (mdd) statistically is low on 30% of days in wet seasons, compared to 80% of days in dry seasons. Wet season impairment cannot be neglected because irrigation occurs year-round as local farms yield 3 to 4 crops annually. However, lowest flow also is not the critical condition. Groundwater, impacted by decades of irrigation and leaching, contributes chloride load disproportionately to its flow, so periods of high groundwater discharge (high water table) are more impaired than low flow periods. The critical condition is maximum non-storm flow, defined statistically using cumulative frequency of mdd. Calleguas Creek storm flow exhibits log-normal distribution, typical of natural systems; non-storm flow is normally distributed, as in randomly combined decisions. Allocating load more stringently for wet seasons is ineffective because water table elevation does not vary consistently, affected by varying rainfall (volume, timing) and multiple dispersed decisions (irrigation runoff, groundwater pumpage, residential water use, wastewater treatment, etc.) USEPA's TMDL allocates loads conservatively, assuming critical conditions year-round. Relaxing allocations for non-critical periods would require water table elevation monitoring and forecasting maximum discharge with sufficient notice to implement more stringent allocations for critical flow.
Key words: total maximum daily load, critical loading flow conditions, wet weather season non-storm flow, chloride
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