RSS FeedsSurface Water Quality
For the surface water quality chapter of the BDR, GIS-based soil erosion, sediment loading, and nonpoint source water quality loading tools are coupled to the MIKE-SHE integrated model developed for surface hydrology and groundwater to determine annual loadings and transport to (and within) the stream network. Specific constituents examined include sediment, temperature, nitrate, total phosphorus, and coliforms (e.g., E. coli). The dynamic coupling between the loading model and water quality modeling provide a tool for:
- analyzing current water quality conditions,
- assessing cumulative impacts of water quality following future land use changes in the watershed.
 In the higher elevations, geologic structures that surround the structural troughs/basins of the County create source areas for surface water and groundwater. |
Where possible, the water quality analysis is making use of existing and available water quality information from on-going studies, including the TMDL process, Napa nutrient analyses, and other studies. Loadings are assessed using a GIS-based non-point calculation tool. Different types of land use have different run-off concentrations of nutrients, organic matter, and bacteria. Predominant land uses include: residential, commercial, industrial, mixed (variety of land uses), transportation, open space, forest, wetlands, and agriculture. These run-off concentrations are often refereed to as the land use estimated mean concentration (EMC). The pollution load from agricultural activity (primarily the use of fertilizer and domestic animals) is included in the non-point pollution load estimation. The GIS tool also links point sources and provides a first screening-level of assessment of the combined pollutant loadings. The GIS tool is applied to estimate the total load from the housing in the rural areas, taking the local treatment system efficiencies into account.
Potential sediment loading arising from soil erosion is analyzed based on available information about topography, soils, vegetation, land use, and rainfall. The analysis is a combination of GIS-based soil erosion modeling and evaluation of existing data. The soil source erosion assessment is applied to describe delivery index and sediment yield (ton sediment /acre/year) for all drainages. These soil erosion rates are applied as loadings to an in-stream sediment transport model that is fully integrated with the MIKE-SHE model developed. The river sediment transport model (including the main stem Napa River and major tributaries in the Napa River watershed only) will transport sediment as either suspended sediment or bedload. The model predicts areas prone to sedimentation and erosion and predicts total loadings from the watershed to the San Pablo Bay.
The coupling of GIS-based tools and MIKE-SHE ISGW allows the County to examine loading of water quality constituents and sediment throughout Napa County.
Results for the Surface Water Quality analysis are documented in a supporting technical report (Napa BDR Surface Water Hydrology Modeling Report), which includes a more complete description of the models’ data requirements, computational algorithms, and outputs. Resulting maps include:
- Surface water pollutant-level maps for temperature, nutrients, organic material, and pathogens
- Sediment source map
