New Pollutant Load, Prioritization, and Reduction Model for California
The 2013 California Phase II General MS4 Permit (MS4 Permit) specifically requires a Program Effectiveness Assessment and Improvement Plan (PEAIP) to include "quantification of pollutant loads and pollutant load reductions achieved by the program as a whole" (Section E.14.a.ii.a.6) and to "quantitatively assess BMP performance at reducing pollutant loads wherever feasible, using … science-based estimates of pollutant load removal for BMPs where direct measurement of pollutant removal is overly challenging" (Sections E.14.a.ii.d.2 and E.14.a.ii.b.4). Additionally, the PEAIP requirements include using "receiving water monitoring to identify priority areas for program improvement" (Sections E.14.b.i)
To address MS4 Permit requirements and information requests from the Central Coast Regional Water Quality Control Board (Regional Board), the County of Santa Barbara, and the Cities of Buellton, Solvang, Goleta, and Carpinteria, along with Geosyntec Consultants, developed a pollutant load quantification, catchment prioritization, and BMP load reduction model (LPR Model). The LPR Model is accessible from an easy to use non-proprietary Excel interface and consists of five main components:
- calculating baseline (i.e., existing conditions) annual average wet weather pollutant loads and runoff volumes by catchment, MS4 Permit Area, and watershed;
- prioritizing MS4 catchments to support program improvement planning;
- tracking structural BMP implementation details (e.g., BMP type, catchment location, tributary drainage area and land uses, etc.);
- quantifying (or taking credit for) non-structural and structural BMP pollutant load and runoff volume reductions (e.g., post-construction requirement projects); and
- summarizing, formatting, and graphically presenting all results for easy reporting.
The LPR Model utilizes local jurisdictional GIS data and scientifically defensible modeling assumption to calculate baseline loads using land use areas, runoff coefficients (based on land use, estimated imperviousness, and hydrologic soil groups), average annual precipitation data, and average event mean concentrations (EMCs). The EMCs for land use-pollutant combinations can be based on published datasets with summary statistics taken from recent Regional Board-approved modeling efforts used by other coastal California regions, or can be customizable with equivalent local data, as available.
The LPR Model leverages a catchment prioritization approach based on a published and ASCE-award winning structural retrofit BMP planning methodology (http://ladpw.org/wmd/bmpmethod/), which is coded into a public domain, open-source GIS-based, Regional Board-approved Strategic BMP Prioritization and Analysis Tool (SBPAT, available at www.sbpat.net), and has been successfully demonstrated in over fifteen Southern California TMDL implementation plans and watershed management plans for MS4 BMP siting and prioritization purposes. The LPR Model incorporates key components of SBPAT into a more simplified and streamlined spreadsheet model to appropriately support Central Coast Municipal Permittees in prioritizing catchments that reflect specific water body-pollutant combinations, as well as meet the Central Coast Regional Board expectations for quantifying pollutant load and pollutant load reductions.
The LPR Model's key features include:
- an easy to use non-proprietary Excel interface;
- ability to model 12 water quality parameters for baseline loading, catchment prioritization, and BMP performance;
- ability to reflect jurisdiction-specific water quality priorities (and guidance on this water body-pollutant prioritization process);
- ability to integrate modeling results for multiple water quality parameters into a single multi-pollutant catchment prioritization score;
- ability to adjust runoff volumes based on calibration results (e.g., based on comparison between predicted watershed volumes and measured values at a streamflow gauge);
- ability to quantify reductions from planned or current BMP implementation;
- advanced logic to accurately estimate BMP reductions when multiple BMPs treat the same area (e.g., source control BMPs and structural BMPs treating the same catchments);
- capacity for streamlined updates to the watershed GIS data (i.e., simple GIS file import/export); and
- capacity to customize inputs, e.g., by adding new BMP types, adjusting land use pollutant concentrations, or adding new pollutants as additional data become available.
Approach Memo: discusses the modeling approach and the default model values.
Guidance Document: describes the model organization, how users can add new BMPs and extract model results for future annual reports, how to modify model defaults, and how model calculations are performed.
2016 CASQA Conference Presentation: The County of Santa Barbara and Geosyntec presented the methodology and function of the LPR Model at the 2016 CASQA conference, Phase II Permit Track.