Todd Engineers

Key Issues
Identification and implementation of scientifically-defensible methods to assess groundwater vulnerability to contamination Development of a GIS tool to assess vulnerability and potential impacts of land use changes

Todd Engineers developed and implemented an innovative groundwater vulnerability assessment methodology for all groundwater subbasins in Santa Clara County (County) for the Santa Clara Valley Water District (District), the primary water resources agency in the County. This Groundwater Vulnerability Study was conducted to predict the vulnerability of groundwater to potentially contaminating land use activities and aid the District in its management and protection activities.

Groundwater vulnerability is comprised of two key components: 1) groundwater sensitivity, and 2) potentially contaminating activities. Groundwater sensitivity is generally defined as the relative ease with which a contaminant on or near the land surface can migrate to the aquifer of interest based on the intrinsic characteristics of the aquifer and the overlying unsaturated materials. Groundwater sensitivity is combined with the potentially contaminating activities (PCA) risk to characterize overall groundwater vulnerability.

The Study Area is comprised of three groundwater subbasins: the Santa Clara, Coyote, and Llagas. Each has unique hydrogeologic characteristics as well as specific current and historic land uses.

Working with Practical Stats, Todd Engineers developed a statistical method to quantify groundwater sensitivity based on hydrogeologic properties. Water quality data (i.e., nitrate

Services Provided

County-wide vulnerability assessment Assessment of intrinsic hydrogeologic properties Survey and ranking of potentially contaminating activities Development of a hybrid assessment methodology through statistical analysis of water quality data Compilation and assessment of data on hydrogeology, water quality, and potentially contaminating activities Use of GIS to assess spatial relationships and development of an interactive tool to assess vulnerability

concentrations and distribution) were used for calibration and verification purposes to identify and rank groundwater sensitivity factors using the logistic regression statistical analysis. Four factors were found to be the most important in characterizing groundwater sensitivity:

soil media characteristics in the vadose zone, groundwater recharge, depth to top of the aquifer, and annual groundwater production.

Kennedy-Jenks Consultants provided assistance in assessment of PCA risk and development of a GIS tool implementing the study results. The PCA risk ranking was based on subjective observation and experience (e.g., California Drinking Water Source Assessment and Protection Program) with emphasis placed on characterizing the maximum risks. The potentially contaminating activities risk analysis included four main risk factors or categories:

general plan land use, potentially contaminating business activities, known contaminated sites, and supplemental data.

The sensitivity assessment and PCA risk were combined to create the overall vulnerability map shown at left. Following completion of the vulnerability assessment, a web-based geographical information system (GIS) tool was developed, to enable District staff to evaluate potential impacts of new developments, prioritize basin management activities, and prioritize review of known contamination sites.