2017 Lake Oswego TMDL Benchmarks: Summary
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2017 Lake Oswego TMDL Benchmarks: Summary In accordance with the City of Lake Oswego’s (City’s) 2012 National Pollutant Discharge Elimination System (NPDES) municipal separate storm sewer (MS4) permit, Schedule D.3.d, the City must develop total maximum daily load (TMDL) pollutant load reduction benchmarks. Benchmarks must be developed for each TMDL parameter where existing best management practice (BMP) implementation is not estimated to be achieving the wasteload allocation (WLA). The TMDL benchmarks must be submitted to the Oregon Department of Environmental Quality (DEQ) with the City’s NPDES MS4 permit renewal application, due March 1, 2017. The City is a designated management agency (DMA) in both the Willamette Basin and Tualatin Basin TMDLs due to urban stormwater discharges. The City’s MS4 discharges runoff either directly or via tributaries to the Tualatin River and the Lower Willamette River. The applicable TMDL parameters are summarized in Table 1. The City conducted a pollutant load reduction evaluation (PLRE) in 2015. Based on results of the PLRE, the City is estimated to be meeting TMDL WLAs for bacteria and TSS in the Lower Tualatin subbasin and for bacteria in the Fanno Creek subbasin. The City is required to establish TMDL pollutant load reduction benchmarks for the remaining parameters and TMDL waterbodies for the next permit term as shown in Table 1. Table 1. Lake Oswego Applicable WLAs TMDL TMDL TMDL Anticipated to meet WLAs WLA document water body parameter (based on the PLRE 2015) 5,000 counts/100 mL (winter storm event concentration) Bacteria (E. coli) Yes 12,000 counts/100 mL (summer storm event concentration) Lower Tualatin Total phosphorus 0.14 mg/L (summer seasonal concentration) No. Benchmarks are needed. DO (TSS as a 20% reduction (summer seasonal) Yes surrogate) Tualatin 5,000 counts/100 mL (winter storm event concentration) Bacteria (E. coli) Yes Subbasin 12,000 counts/100 mL (summer storm event concentration) TMDL Fanno Creek Total phosphorus 0.13 mg/L (summer seasonal concentration) No. Benchmarks are needed. DO (TSS as a 50% reduction (summer seasonal) No. Benchmarks are needed. surrogate) 0.09 mg/L (summer seasonal concentration) Oswego Lakea Total phosphorus No. Benchmarks are needed. 0.15 mg/L (fall/winter/spring seasonal concentration) Lower Willametteb Bacteria (E. coli) 78% reduction (annual) No. Benchmarks are needed. Willamette Springbrook Bacteria (E. coli) 80% reduction (annual) No. Benchmarks are needed. Basin TMDL Creekc Tryon Creek Bacteria (E. coli) 78% reduction (annual) No. Benchmarks are needed. a. For the Tualatin Subbasin TMDL, WLAs for total phosphorus are specified for the Oswego Lake subbasin (which includes both Oswego Lake Direct and Springbrook Creek subbasins). b. For the Willamette Basin TMDL, WLAs for bacteria associated with the Oswego Lake subbasin area are specified under two other TMDL waterdbodies. The Oswego Lake Direct subbasin is accounted for in the Lower Willamette subbasin and the Springbrook Creek subbasin is accounted for individually. 1 2017 Lake Oswego TMDL Benchmarks This summary document outlines the City’s plans for implementation of additional BMPs that will result in further reduction of TMDL parameters over the next permit term and presents the associated TMDL benchmarks. Detailed information with regards to modeling methods, assumptions, and results are provided in the City’s TMDL Pollutant Load Reduction Evaluation and Benchmark Report (January 2017), which is included as an appendix to this NPDES MS4 permit renewal application. BMP Identification By definition, TMDL benchmarks are estimates of pollutant load reductions in the future. They reflect current BMP implementation and projected BMP implementation over the next permit term (i.e., through 2022). City staff identified anticipated stormwater facility installations associated with upcoming public works and transportation related projects. They also identified pending and constructed private stormwater facility installations associated with recent or in-progress development activities since the PLRE analysis was completed in March 2016. In total, new public structural stormwater BMPs are anticipated to be installed as described in the City’s 2015 Retrofit Strategy result in treatment of approximately 74.5 acres (ac). Table 2 lists the projected stormwater facility installations by TMDL watershed, facility type, and drainage areas. Table 2. Projected Stormwater Facilities and Drainage Areas for 2022 Benchmarks Approximate projected TMDL Subbasin Projected BMP categoriesa new drainage area (acres) Fanno Creek Lined planter - filtration raingardens 7.8 Swales 2.6 Lower Willamette Lined planter - filtration raingardens 3.3 Oswego Lake direct Detention pond (dry) 1.3 Infiltration raingarden 5.0 Lined planter - filtration raingardens 9.9 Retention pond (wet) 1.6 Sedimentation manholes 0.9 Swale 0.5 UIC 0.5 Springbrook Creek Lined planter - filtration raingardens 25.7 Sedimentation manhole 0.6 UIC 0.6 Tryon Detention pond (dry) 0.7 Infiltration raingardens 10.0 Lined Planter - filtration raingardens 3.5 Total 74.5 a. The projected BMP categories and associated drainage areas in this table are subject to change. They are current best estimates of what is projected through 2022 for the purposes of developing benchmarks. 2 2017 Lake Oswego TMDL Benchmarks TMDL Benchmark Results and Discussion The spreadsheet loads model used for the PLRE in 2015 was used to simulate predicted future BMP implementation and calculate future pollutant load reduction estimates (i.e., TMDL benchmarks). TMDL benchmarks are calculated as the difference between the modeled loads associated with the no- BMP scenario and the (future) with-BMP scenario. This load reduction is presented as a range to reflect the wide variability in stormwater data. Table 3 provides TMDL benchmarks as a load reduction and as a percentage load reduction. Calculation of the TMDL benchmarks as a percentage load reduction allows for direct comparison with the WLAs established for bacteria. Table 3. TMDL Benchmarks for 2017–2022 TMDL benchmarks TMDL Model time TMDL benchmarks Pollutant(s) WLA (in units matching the WLA)d, subbasin frame (load reduction)a, range range Lower Tualatin Summer season Total phosphorus 0.14 mg/L NA NA Total phosphorus 0.13 mg/L 10.1 – 23.0 lbs 0.16 mg/L to 0.21 mg/L Fanno Creek Summer season TSS 50% reduction 4,612 – 13,245 lbs 16.2% - 22.9% reduction Summer season Total phosphorus 0.09 mg/L 22.6 – 47.7 lbs 0.16 mg/L – 0.23 mg/L Oswego Lakeb Fall/Winter/Spring Total phosphorus 0.15 mg/L 110.0 – 232.0 lbs 0.16 mg/L – 0.23 mg/L season Lower Willamettec Annual Bacteria 78% reduction 2.48x1012 - 1.16x1013 counts 7% - 11% reduction Tryon Creek Annual Bacteria 78% reduction 9.83x1011 – 3.99x1012 counts 9% - 13% reduction Springbrook Creek Annual Bacteria 80% reduction 7.59x1011 - 3.4x1012 counts 7% - 10% reduction a. The TMDL benchmarks are a load reduction, calculated as the difference between the current no-BMP scenario load and the future with-BMP scenario load. b. The Oswego Lake subbasin includes both the Oswego Lake Direct and Springbrook Creek subbasins for the purposes of developing the total phosphorus benchmark. c. The Lower Willamette subbasin includes the Oswego Lake Direct subbasin for the purposes of developing the bacteria benchmark. d. The benchmarks have also been calculated as a percent reduction or as a concentration for direct comparison with the WLA. For this upcoming permit term, while the City anticipates more than 60 new facilities, no new BMPs are projected for the Lower Tualatin subbasin. This subbasin represents only 4% of the City’s MS4 area that is subject to TMDLs. In addition, it already has approximately 33% BMP coverage, which is relatively high for the City. As a result, over the next permit term, higher priority was placed on implementing BMPs in other subbasins where the BMP coverage is not as extensive. However, several planters and swales are projected to be installed in the Fanno Creek subbasin, which will reduce pollutants in runoff from approximately 10 acres that eventually drains to the Tualatin River and will collectively help reduce pollutants in the Tualatin River. The TMDL benchmarks presented in Table 3 are conservative estimates of the pollutant load reduction anticipated during the upcoming MS4 permit term with the installation of anticipated public and private structural BMPs. The City’s overall stormwater program also comprises non-structural BMPs and programmatic activities, so the TMDL benchmarks do not reflect the full range of pollutant load reduction anticipated through implementation of the stormwater program. Additional load reduction is expected through the following: • Non-structural and source control BMPs (e.g., erosion control, illicit discharge detection and elimination, street sweeping, public education, pet waste management activities, operation and maintenance) per the City’s Stormwater Management Plan. 3 2017 Lake Oswego TMDL Benchmarks • Infiltration that is likely occurring in unlined planter facilities given that they were modeled as lined planters as a conservative assumption to account for poor infiltration conditions across much of the City. • Additional private structural BMPs that are required to occur with redevelopment projects. The City prepared a WLA attainment assessment for DEQ in March 2016, which indicated that achievement of some of the WLAs would require construction and maintenance costs that far exceed the City’s definition of “maximum extent practicable.” Consequently, progress toward the WLA, and not achievement of the WLA, is the City’s goal for the TMDL benchmarks. 4 TMDL Pollutant Load Reduction Evaluation and Benchmarks Prepared for The City of Lake Oswego, Oregon March 2016 Amended January 2017, to include Benchmarks This is a draft and is not intended to be a final representation of the work done or recommendations made by Brown and Caldwell. It should not be relied upon; consult the final report. 6500 SW Macadam Avenue, Suite 200 Portland, OR 97239 Phone: 503.244.7005 Fax: 503.244.9095 Table of Contents 1.