Map of Vulnerability Study area

Key Issue

  • The District wished to develop a county-wide groundwater vulnerability characterization to support and prioritize groundwater protection activities


  • Develop a scientifically-rigorous and defensible method to assess groundwater sensitivity and vulnerability to contamination
  • Development of a web-based GIS tool to readily assess site vulnerability and potential impacts of existing land use and potential land use changes

Groundwater Vulnerability Study

Santa Clara County, California

Development and application of an innovative methodology to assess groundwater vulnerability to contamination

Santa Clara Valley Water District (District) manages groundwater in Santa Clara County, California through various protection and augmentation programs. The District's groundwater protection programs allow them to assess regional groundwater quality, identify and evaluate threats, and help prevent or mitigate contamination.

The District retained Todd Groundwater to conduct a Groundwater Vulnerability Study to predict groundwater vulnerability to potentially contaminating land use activities and allow the District to:

  • prioritize groundwater protection efforts for vulnerable areas
  • evaluate groundwater quality impacts from land use changes, and
  • provide information to decision makers at the District, land use agencies, and regulatory agencies to improve groundwater protection.

Todd Groundwater developed an innovative groundwater vulnerability assessment methodology including two key components:

  1. groundwater sensitivity, and
  2. potentially contaminating activities.

Groundwater sensitivity is 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 overlying unsaturated materials. Groundwater sensitivity is combined with the potentially contaminating activities (PCA) risk to characterize overall groundwater vulnerability.

Todd Groundwater developed a statistical method to quantify groundwater sensitivity based on hydrogeologic properties. Water quality data (e.g., nitrate 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 most important in characterizing groundwater sensitivity:

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

The PCA risk analysis included four main risk factors or categories:

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

The sensitivity assessment and PCA risk were combined to create the overall vulnerability map shown below. A web-based GIS tool was developed to readily assess individual site conditions relative to groundwater vulnerability.

Map showing vulnerability rating over Study area

Contact Todd Groundwater

We understand groundwater protection. Let us assist you in understanding and protecting your groundwater. For more information, contact Todd Groundwater