Addendum No. 2 to Canyon Lake Hybrid Treatment Process – Phase 1

Addendum No. 2 to Canyon Lake Hybrid Treatment Process – Phase 1

In 2013, the City of Canyon Lake prepared an Initial Study to evaluate the environmental effects of a pilot alum application program in Canyon Lake. During the pilot program, 840 tons of alum was applied to Canyon Lake in five separate events (Sept., 2013, Feb., 2014, Sept., 2014, April, 2015 and Sept., 2015). The Initial Study states that after the five pilot applications were completed, the Lake Elsinore Canyon Lake Total Maximum Daily Load (TMDL) Task Force (Task Force) agencies would evaluate water quality data from the Lake to determine whether the alum was working as intended, to confirm that there were no adverse environmental effects, and to decide whether additional applications were necessary and appropriate. That evaluation was conducted and reported in the TMDL Progress Report for Evaluation of Compliance with the 2015 Interim Response Targets. In the interim period following the adoption of the 2013 IS/MND, Addendum No. 1 was prepared to in response to the stakeholders’ desire to extend the pilot alum application program in the lake for the next 10 years (2015-2025). Subsequent updates to the effectiveness of alum additions were incorporated into the 2020 TMDL Compliance Demonstration Report (Lake Elsinore and San Jacinto Watersheds Authority (LESJWA), 2021) and annually, in the Riverside County stormwater program report appendices. The current proposal is to continue the phosphorus applications to Canyon Lake for up to five additional years. If this extension is approved/authorized by LESJWA, the Canyon Lake phosphorus applications can be continued as necessary for the next 5 years, i.e., through 2030, subject to Santa Ana Water Quality Control Board review and approval of the existing offsets program as required by the Revised Total Maximum Daily Load for nutrients in Lake Elsinore and Canyon Lake (adopted by the Santa Ana Water Board on July 25, 2025). Thus, the proposed minor modification to the previously approved project is: 1) The extension of the pilot alum application program in Canyon Lake for the next 5 years. 2) Continue treating the transition area immediately above the north causeway (Vacation Drive) at the confluence between the San Jacinto River and Canyon Lake. 3) Provide additional flexibility to apply alum at times and under water quality conditions that will assure the highest level of effectiveness and the lowest potential for any unintended impacts. Greater flexibility in timing is a new operational measure that was developed based on knowledge and experience gained from the pilot program. 4) Provide additional flexibility to make alum applications in the Main Body and in the East Bay of Canyon Lake at different times of year. The decision as to when and how the alum will be applied will be made by the Task Force after consulting with the alum application contractor, their internal consulting team, and the Santa Ana Water Board. 5) Clarify that the program allows, but does not require, stakeholders to make up to two alum applications annually. And, if the alum applications for the Main Body and the East Bay occur at different times, this may result in up to a total of six application events on the Lake (e.g. 2 x Main Body, 2 x San Jacinto/lake interface, and 2 x East Bay). The documentation in this Addendum, combined with the adopted 2013 MND and Initial Study, will serve as the basis for this second tier environmental review of the proposed continued alum treatment program. The alum treatment will continue to be carried out in the Main Body, East Bay, and transition area from the San Jacinto River.

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