Sierra Colina Village
P. O. Box 129
Lake Tahoe, NV 89448

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Yes. The anticipated average concentrations from the Sierra Colina project stormwater runoff for pollutants of concern to lake clarity (i.e. suspended sediment, phosphorus, and nitrogen) will be significantly less (estimated to be between 60% and 90% less) than the maximum allowable concentrations under the existing TRPA Code of Ordinances for discharge to surface water and to groundwater of pollutants of concern to lake clarity. See discussion below from nhc’s Supplemental Stormwater Analysis (Sierra Colina Final EIS (Appendix AA) for detailed analysis); see also Sierra Colina Final EIS, Master Response F at pages 2-251 through 2-253 .

In February 2009, in response to public comments received on the Project’s Environmental Impact Statement – Public Review Draft (EDAW, 2008), requesting additional water quality analysis of the proposed Project, nhc prepared Supplemental Water Quality Analysis (Sierra Colina Final EIS, Appendix AA).  The Supplemental Water Quality Analysis included 1) an evaluation of Project performance relative to current concentration-based regulations, and 2) a quantification of Project performance in terms of pollutant loading versus existing conditions (without the proposed project and its associated benefits). 

When nhc prepared the Project’s Storm Water Management Plan in 2006, it was common practice in the Tahoe Basin to forecast acceptable project performance based on runoff volume retention estimates, since storm water runoff concentrations are highly variable and difficult to accurately predict over a range of hydrologic and physical conditions.  Since 2006 when nhc prepared the Project’s Storm Water Management Plan, analysis performed for the Lake Tahoe TMDL has developed data and modeling approaches that make it more feasible to 1) evaluate project performance relative to current TRPA concentration-based regulations, and 2) quantify project performance in terms of pollutant loading.

Section 2 of the nhc Supplemental Water Quality Analysis (See Sierra Colina Final EIS at Appendix AA) develops the anticipated post-project runoff concentrations for the Project and compares them to existing TRPA standards for discharge to surface water and groundwater for pollutants of concern to lake clarity (i.e. suspended sediment, phosphorus, and nitrogen).  Anticipated runoff concentrations are estimated based on a comparison and linkage of the Stormwater Management Plan developed for the Project relative to the assumptions and data provided in the TMDL PRO Report (LRWQCB and NDEP, 2008).

Because the Sierra Colina Stormwater Management Plan will have significant capacity to store and treat Project runoff, the storm water treatment system will be the controlling factor influencing the quality of runoff leaving the Sierra Colina parcel.  As discussed above, the Sierra Colina Stormwater Management Plan is consistent with TMDL Treatment Tier 2, and anticipated average runoff concentrations for the proposed Project are estimated based on data provided in the TMDL PRO Report from Table 3-7 (LRWQCB and NDEP, 2008).  The storm water treatment system for the proposed Project is a two stage treatment, which includes a filtration vault followed by a detention basin connected in series.  Relative to Table 3-7 of the PRO Report, the filtration vaults are most similar to the definition of SWT-2B, while the detention basins are most similar to the definition of SWT-1A.  To estimate anticipated concentrations in outflows from the proposed Project, the average of effluent concentrations between a detention basin (SWT-1A) and a filtration vault (SWT-2B) in Table 3-7 was used (See Final EIS Appendix AA). 

Table 2 below (with respect to concentrations for discharge to surface water) and Table 3 below (with respect to concentrations for discharge to groundwater) show that anticipated average concentrations from the proposed Project, for pollutants of concern to lake clarity (i.e. suspended sediment, phosphorus, and nitrogen), are significantly lower than maximum allowable concentrations under existing TRPA Code for discharge to surface water and groundwater, respectively.  Because storm water concentrations are highly variable and “maximum” concentrations are difficult to apply in practice for assessing performance, the TMDL approach relies on pollutant load estimates.  There is no established relationship between average concentrations used for load estimates in the TMDL approach and the current regulatory approach using maximum concentrations.  However, the Sierra Colina Project design includes both filtration treatment and detention storage, and effluent quality is projected to be much more uniform than the highly variable storm water influent concentrations to the proposed storm water treatment system.  The results therefore indicate a high level of anticipated Project performance relative to existing TRPA maximum pollutant discharge standards.   

Table 2. Comparison of Concentrations for Discharge to Surface Water

Table 3. Comparison of Concentrations for Discharge to Groundwater

Current TRPA concentration-based requirements include additional pollutants not analyzed by the Lake Tahoe TMDL, or the majority of monitoring studies completed to date in the Lake Tahoe Basin.  These include iron, turbidity, and grease and oil.  Because these pollutants are not analyzed by the TMDL, an evaluation of Project performance relative to these concentration-based standards is not possible using the analysis process described above.  However, it is reasonable to assume that the advanced treatment practices included in the storm water treatment system for the proposed Project are sufficient to meet or exceed existing regulatory requirements for iron, turbidity, and grease and oil.  The storm water treatment system proposed for the Project will route runoff from all poor quality sources (road and driveways) through storm water filtration vaults (prior to any infiltration), and then to detention basins for further treatment, prior to any surface runoff.

Thus, the anticipated Sierra Colina project average concentrations from stormwater runoff for pollutants of concern to lake clarity (i.e. suspended sediment, phosphorus, and nitrogen) will be significantly less (estimated to be between 60% and 90% less) than the maximum allowable concentrations under existing TRPA Code of Ordinances for discharge to surface water and to groundwater of the same pollutants. See discussion below from nhc’s Supplemental Stormwater Analysis (Sierra Colina Final EIS (Appendix AA)) for detailed analysis.

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