City of San Diego La Jolla Area of Special Biological ...

City of San Diego La Jolla Area of Special Biological Significance Site Specific Dilution and Dispersion Model

June 2013

City of San Diego

Prepared by: AMEC Environment and Infrastructure, Inc. Project No. 5025121039

LA JOLLA AREA OF SPECIAL BIOLOGICAL SIGNIFICANCE SITE SPECIFIC DILUTION AND DISPERSION MODEL

Prepared for: City of San Diego Transportation and Storm Water Department 9370 Chesapeake Drive, Suite 100 San Diego, California 92123

Submitted by: AMEC Environment & Infrastructure, Inc.

9210 Sky Park Court, Suite 200 San Diego, California 92123

May 2013 AMEC Project No. 5025121039

City of San Diego La Jolla Area of Special Biological Significance Draft Dilution and Dispersion Model AMEC Project No. 5025121039 June 2013

This special study report is prepared for the City of San Diego, Transportation and Storm Water Department (City of San Diego) to complement multiple, ongoing storm water monitoring programs within the La Jolla Area of Special Biological Significance, No 29, located in La Jolla, California. In particular, this study is designed to provide a quantitative, site specific dilution and dispersion model to aid in determination of appropriate dilution factors per guidance provided in the California Ocean Plan (2009).

1.0 REGULATORY BACKGROUND

On October 18, 2004, the State Water Resources Control Board (State Water Board) notified the City of San Diego, as a responsible party, to cease storm water and non point source waste discharges into Areas of Biological Significance (ASBS) or to request an exception from the California Ocean Plan waste discharge prohibition. On December 15, 2004, the City of San Diego requested an exception for ASBS No. 29. On March 20, 2012, the State Water Board adopted Resolution No 2012-0012, approving an exception to the California Ocean Plan, General Exception to the California Ocean Plan for Areas of Special Biological Significance Waste Discharge Prohibition for Storm Water and Nonpoint Source Discharges, with Special Protections (herein referred General Protections). These general protections are in accordance with the Porter-Cologne Water Quality Control Act, California Water Code ?13000 et seq., and implementing regulations, including the current Ocean Plan (2009).

Under the California Ocean Plan (2009), Section III, C.4.a, Effluent limitations for water quality objectives listed in Table B shall be determined through the use of the following equation:

Ce = Co + Dm (Co - Cs)

where (in ug/l):

Ce = the effluent concentration limit, Co = the concentration (water quality objective) to be met at the completion of

initial dilution, Cs = background seawater concentration (references in Table C of Ocean Plan) Dm = minimum probable initial dilution expressed as parts seawater per part

wastewater.

Furthermore, the Ocean Plan provides guidance for the identification of dilution models for use in determining the initial dilution, Dm. Specifically, per Section III, C.4.a;

The Executive Director of the SWRCB shall identify standard dilution models for use in determining Dm, and shall assist the Regional Board in evaluating Dm for specific waste discharges. Dischargers may propose alternative methods of calculating Dm, and the Regional Board may accept such methods upon verification of its accuracy and applicability.

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City of San Diego La Jolla Area of Special Biological Significance Draft Dilution and Dispersion Model AMEC Project No. 5025121039 June 2013

2.0 HYDRODYNAMIC MODELING DESIGN

This study is designed to provide site specific dilution and dispersion model results for ASBS No. 29 to the San Diego Regional Water Quality Control Board (SDRWQCB). The effluents from three permitted outfalls within the La Jolla ASBS were studied using the SEDXPORT hydrodynamic modeling system. The model is designed to numerically simulate dry weather and wet weather case scenarios. The dilution study incorporated historical site specific outfall data on water mass boundary properties (bathymetry, salinity, temperature, ocean level/tides) and forcing functions (waves, currents and winds).

The SEDXPORT modeling system was developed at Scripps Institution of Oceanography (SIO) for the US Navy's Coastal Water Clarity System and Littoral Remote Sensing Simulator. The model has been reviewed and vetted by multiple regulatory agencies and has been calibrated for six previous water quality projects in the Southern California Bight. Due to the large number of oceanographic measurements that have been made in ASBS No 29 and the adjacent ASBS No 31, the model for this site is considered particularly robust. These include long term water quality, wind, wave, water height, tidal and temperature measures at the SIO pier as well as multiple special studies of currents and storm water inputs into La Jolla Bay and environs.

This model was selected as the most relevant model available for the following reasons;

1) The State Water Board required SIO to perform a similar study to determine the initial dilution and dispersion of the discharge during storm and non-storm periods at the adjacent ASBS No. 31 (San Diego-Scripps) as a requirement of SIO's NPDES Permit Exception, Order No. R9-2005-0008. SIO used the SEDXPORT modeling system to fulfill this requirement and the study was provided on February 9 2007 to the San Diego Regional Board and the California SWRCB for approval.

2) The model design and results of the SIO study were evaluated by the Natural Water Quality Committee (NWQC), and deemed appropriate (Summation of Findings, Natural Water Quality Committee, 2006-2009, SCCWRP Technical Report 625, September 2010). The NWQC was a scientific oversight committee established as a requirement of the SIO permit to evaluate monitoring results and special studies. With technical review and input from the NWQC, the SDRWQCB revised the initial 2:1 dilution factor in the initial waste discharge requirements (WDR) to a 7:1 dilution factor (representative of the minimum dilution ratio) and this 7:1 dilution factor was incorporated into the November 2008 revision of SIO's permit.

3) This study utilizes the same modeling program (SEDXPORT) and modeling assumptions and integrating the most recent long term historical trend data. In addition, the model calibration and outputs were generated by the same SIO scientists (Scott Jenkins, Ph.D. and Joseph Wasyl) that published the SDRWQCB approved 2007 SIO study.

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City of San Diego La Jolla Area of Special Biological Significance Draft Dilution and Dispersion Model AMEC Project No. 5025121039 June 2013

4) The mass flow model inputs are based on actual discharge data (flow and mass) measured from permitted outfalls; SDL-157, SDL-062 and SDL-186. To model the beach discharges, flow and chemistry results collected in November 2011 provided data representative of the worse case proxy scenario. These data were merged with the long term probability assessment in order to bracket possible dilution outcomes. The study provide the same data endpoints (dilution ranges of the outer far field [>-10m] of ASBS No. 29 and the near field [ ................
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