Photo of mobile drilling rig enclosed by temporary sound walls
Monitoring well drilling in highly urbanized area required use of sound walls

Key Issues

  • Determine potential impacts of IPR on water levels, flow and water quality
  • Define nearest potable supply wells and retention times

Solutions

  • Develop refined flow model to reliably estimate water levels, flow and travel times
  • Determine purified water buffering necessary to prevent leaching of metals

Indirect Potable Reuse Groundwater Studies

Santa Clara County, California

Santa Clara Valley Water District actively manages groundwater, storm water and imported water supplies for high quality and sustainable supply in the face of increasing demands.

The Santa Clara Valley Water District ("District") has short- and long-term plans that include development of purified recycled water as a substantial source of additional water supply.

In 2015, the fourth successive year of an extreme drought, the District initiated an Indirect Potable Reuse (IPR) project to evaluate supplemental groundwater recharge with advanced treated recycled water (purified water). Ultimately three locations on the Santa Clara Plain were identified for potential recharge of purified water, including two existing surface spreading facilities and one new grouping of injection wells.

Todd Groundwater was retained by the District to conduct groundwater studies assessing the potential IPR projects. These studies include:

  • Re-evaluation and update of the hydrogeologic conceptual model
  • Refinement, calibration and application of the Districtís MODFLOW model to several IPR scenarios
  • Dissolution studies
  • Infiltration testing
  • Monitoring well siting, installation and testing
  • Groundwater monitoring plan development

Hydrogeologic Conceptual Model Reevaluation

The hydrogeologic reevaluation used the ArcHydro tool within a Geographic Information System (GIS) platform to generate multiple cross sections of the Study Area based on:

  • Lithology and geophysical logs
  • Well construction logs
  • Groundwater elevations
  • Recently completed stratigraphic studies
  • Distribution of hydrogeologic properties
  • Other existing model layering

Groundwater Flow Modeling

Building on the Districtís existing MODFLOW model, a refined numerical groundwater flow model was developed, calibrated and applied to evaluate proposed purified water spreading and injection. The modeling was used to evaluate project feasibility in terms of potential impacts to groundwater levels and flow, and California Division of Drinking Water (DDW) travel time separation to potable supply wells.

Dissolution Studies

Working with Mahoney Geochemical Consulting, dissolution testing was conducted to evaluate potential chemical reactions between purified water and vadose zone and aquifer materials. The purpose of the dissolution testing was to evaluate the potential for dissolution (leaching) of naturally-occurring minerals and metals that could impact groundwater quality. The testing included:

  • Purified water quality analysis and buffering
  • Soil leaching testing using a modified USEPA Synthetic Precipitation Leaching Procedure
  • Hydroxylamine-Hydrochloride extraction testing (to estimate the potential for metals to desorb into solution)
  • Geochemical modeling of leaching and extraction tests, to determine which inorganic solutes are likely to remain in solution with potential to affect water quality

Most samples showed low or non-detectable metals concentrations, providing strong evidence that interaction of buffered purified water and alluvial sediments will not result in significant leaching of metals to groundwater.

Percolation Testing

At one existing spreading basin site and an adjacent site, infiltration testing was conducted to assess recharge potential. The study included data collection and analysis of:

  • Historical recharge operations
  • Sonic borings and soil sampling for analysis of aquifer properties to assess the infiltration capacity
  • Cone penetrometer (CPT) soundings to characterize lithology and depth to groundwater
  • Two 10-day concurrent infiltration tests to further assess infiltration capacity

The study showed that the sites have good recharge potential.

IPR Monitoring Well Siting, Design, Installation and Testing

Based on groundwater flow modeling, the area suitable for IPR monitoring well installation was defined based on the travel time requirements specified in DDW regulations for IPR projects. Multi-completion monitoring wells (including five screened intervals) were installed in two borings located downgradient of one potential IPR site.

The installation and testing of the monitoring wells included sonic borehole drilling, geophysical logging, mud rotary borehole drilling, well construction, well development, and soil and groundwater quality sampling.

IPR Groundwater Monitoring Plan

Groundwater monitoring at recycled water recharge sites is required by DDW regulations and the District actively manages the groundwater with the objective that groundwater is protected from existing and potential contamination.

The plan included:

  • Monitoring well siting requirements
  • Identification of potential well locations based on modeled travel times
  • Nearby well owner requirements
  • Identification of constituents and frequency of monitoring including constituents of emerging concern
  • Sampling protocols
  • Quality assurance and control
  • Reporting requirements