Determining critical flow conditions for chloride impairment in an effluent-dominated, storm-peaking, Western U.S....

Extensive agricultural land use and intensive urban residential growth of the Calleguas Creek, California, watershed has increased chloride load and impaired beneficial uses. The hydrology of the watershed is typical of the semiarid U.S. West in that nearly all rainfall occurs in a small number of discrete storm events that each produces peak discharges of a duration of several days or less; conversely, during the dry weather season, discharge has historically been near zero. Currently, a year-round flow is sustained by two factors: base line flow sustained by discharges from publicly owned treatment works (POTWs) and increased groundwater discharge from a shallow water table elevated by intensive agricultural irrigation (deep groundwater basins used for water supply have declined into overdraft). Water quality impairment of Calleguas Creek increases during low-flow days, but cannot be defined seasonally because days not influenced by storm discharge occur at substantial proportions during all months. Impairment is greatest not during lowest flows, which are dominated by POTW effluent, but when groundwater and other nonpoint sources are highest, thereby contributing chloride load disproportionately to their flow. The highest nonstorm days are identified through cumulative frequency of mean daily discharge (MDD) as the transition from nonstorm conditions (described by normal distribution) to storm conditions (described by log-normal distribution). Transition occurs at approximately the 80th to 85th percentile MDD at three Calleguas Creek locations. Critical conditions for chloride impairment are defined as volumetric flow at those percentiles of cumulative MDD distribution.