Getting to Better Than Natural – Multi-Agency Collaboration on a One Water Inspired Revision to the Canyon Lake and Lake Elsinore Nutrient TMDLs
In 2004, TMDLs were adopted for Canyon Lake (CL) and Lake Elsinore (LE) that set numeric targets and allocations to be protective of WARM aquatic life uses. In 2015, the Regional Board reopened these nutrient TMDLs armed with long term hydrology and water quality data, a portfolio of ongoing water quality controls, and a collaborative group of stakeholders referred to as the TMDL Task Force. For the current TMDL revision, a ‘reference watershed’ approach is being employed to set total allowable watershed loads to levels representative of a reference watershed. Dynamic multi-dimensional models served as the basis for a linkage analysis to estimate temporally and spatially varying lake water quality conditions that may be expected to occur naturally. Results were expressed as a cumulative distribution functions, which are proposed as numeric targets in the revised TMDL. This is in reverse to traditional stressor-response based TMDL methods. The reference watershed approach is necessary to facilitate allocations and targets that would support, rather than preclude, a ‘One Water’ approach to managing the watershed and downstream lakes. When the TMDL was reopened in 2015, significant prior investment had been made to construct and implement watershed and in-lake water quality controls, including urban and agricultural watershed BMPs, alum additions in Canyon Lake, the Lake Elsinore Management Project (LEMP), supplement water addition in Lake Elsinore, operation of the Lake Elsinore Aeration and Mixing System (LEAMS), and routine fishery management. The proposed TMDL revision takes a one water perspective by allowing for greater volumes of watershed runoff to reach Canyon Lake and be treated within the lake, thereby increasing local municipal water supply and reducing reliance on imported water. Also, greater volumes of reclaimed water are allowed to be discharged to Lake Elsinore to support recreational and aquatic life uses. By allowing for a portion of excess external nutrient loads to be offset within the lakes, compliance can be achieved sooner and at a fraction of the cost. The linkage analysis model shows that the collective benefit of ongoing operation of existing controls may be able to achieve close to natural water quality conditions. This finding surprised the Task Force considering the recent harmful algae blooms. For Lake Elsinore, the current period of extended drought amounts to the greatest levels of natural stress on water quality from reduced nutrient export, evaporative loss, concentration of TDS and nutrients, and aquatic food web instability on the lake when compared with a 100-year hydrologic period (1916-2016) of analysis. Complete lakebed desiccation occurs in ~10 percent of years with gnat infestations and fish kills in the surrounding years, most recently occurring in the mid-1950s to early-1960s. As of 2015, the lakebed would be completely desiccated without the one water forethought and action taken by stakeholders over past three decades. For Canyon Lake, nutrient loads have been substantially reduced through a combination of routine in-lake alum additions, sewer service area expansion, and active MS4 program implementation in the watershed. MS4 program implementation has included street sweeping, catch basin cleaning, and deployment of low impact development BMPs downstream of over 7000 acres of urbanized drainage area, many of which were formerly agricultural lands. Reduced levels of bioavailable phosphorus in the water column are limiting algae growth with concentrations approaching natural levels in recent years. Despite these successes, the TMDL revision identifies future actions to ensure conditions can be sustained at better than natural into the future, such as monitoring to validate estimated nutrient concentrations downstream of reference areas in the SJR watershed, evaluation of potential supplemental projects, and other adaptions that may be required as a result of climate change.