NUTRIENT MANAGEMENT IN THE SAN JACINTO WATERSHED: TMDLS DRIVING STORMWATER MANAGEMENT DECISIONS

The Santa Ana Watershed Project Authority (SAWPA) has coordinated a watershed assessment and modeling study to support development of TMDLs for Lake Elsinore and Canyon Lake, as well as development of a nutrient management plan for the San Jacinto watershed. Both lakes have exhibited eutrophic conditions in recent years that have triggered periodic fish kills. The effort has involved development and application of a watershed and lake modeling system that evaluates nutrient sources and transport in the 780-mi2 watershed under a range of hydrologic regimes. Model simulation results have played a key role in identifying sources that critically impact water quality conditions in both lakes. The modeling system is also being used, through consultation with an active stakeholder group, to identify potential strategies that will ultimately meet lake water quality goals.The San Jacinto watershed is complex and has presented a challenge for successfully modeling hydrology, pollutant loading and transport, and lake processes. Shrublands dominate the headwaters, while the middle and lower regions of the watershed consist mostly of agriculture – namely cropland and dairy and animal feeding operations. The watershed is, however, experiencing a rapid increase in conversion to urban land with continued development. Although most streams in the watershed are generally dry, periods of excessive rainfall present several unique factors that impact flow and pollutant transport throughout the watershed. A number of impoundments in the watershed, most notably Mystic Lake, play a major role in retaining water during storm events and settling out nutrients. Large storms, however, also lead to septic system failures, urban and cropland stormwater runoff, and in extreme situations, inundation of dairy and animal feeding operation detention ponds.EPA's Loading Simulation Program in C++ (LSPC) was the modeling platform of choice for the San Jacinto Watershed Modeling System and was used to simulate watershed processes, including hydrology, pollutant accumulation and washoff, and flow and pollutant transport in the streams and headwater lakes. In addition to using many of the algorithms in EPA's Hydrological Simulation Program – FORTRAN (HSPF) recoded in the C++ programming language, LSPC provides useful GIS functionality and advanced database and model archival components. LSPC was fully calibrated and validated for hydrology and nutrient loading at multiple locations in the watershed and predicted flow and nutrient loads to Canyon Lake over an extended time period. In order to simulate nutrient loading impacts on Canyon Lake (from the watershed) and transport to the downstream lake, Lake Elsinore, a simplified one-dimensional Environmental Fluid Dynamics Code (EFDC) model was developed. EFDC is capable of simulating hydrodynamics, salinity, temperature, suspended sediment, water quality, and the fate of toxic materials. It was successfully calibrated to available hydrodynamic and water quality data for a multi-year period covering mean, dry, and wet years.Various alternative loading scenarios have been simulated with the LSPC and EFDC models, in order to test the sensitivity of different nutrient source loadings to lake water quality response. These scenarios are being used by the Santa Ana Regional Water Quality Control Board (SARWQCB) to arrive at source-based load allocations for development of the nutrient TMDLs. Many of these scenarios have and will involve implementation of Best Management Practices (BMPs), and these will be prescribed in a watershed nutrient management plan. The plan is intended to provide a roadmap for future load reduction and lake restoration efforts. Watershed stakeholders are playing a major role in evaluating results of the scenario modeling effort and proposing watershed nutrient management projects. The comprehensive modeling system and watershed nutrient management plan provide the basis for making future management decisions in the watershed.