Photo of drilling rig at Seaside Basin site
Drilling proceeded 24x7 for four days

Key Issues

  • Characterization of the hydraulic properties and contaminant leaching potential of the vadose zone to support well design, predict well injection yield, and assess water quality impacts from recycled water recharge through vadose zone injection wells
  • Geochemical compatibility of native groundwater, aquifer materials, and recycled water in shallow and deep formations
  • Specifications for shallow vadose zone and deep injection wells to optimize individual well and overall well field performance


  • Design and implementation of a comprehensive field investigation program to characterize potential groundwater quality impacts from recycled water recharge and support design of shallow vadose zone and deep injection wells
  • Siting, design, and operations of shallow and deep injection wells to accommodate projected recycled water flows to increase the operational yield of the Seaside Basin

Recycled Water Injection

Seaside Basin, Monterey County, California

Recycled water injection is being planned to replenish the Seaside Groundwater Basin to enhance critically-short water supplies and mitigate potential seawater intrusion (link to recycled water project description). As part of that work, Todd Groundwater conducted a field program to fill data gaps and support design of vadose and saturated zone injection wells.

The Monterey Regional Water Pollution Control Agency and Monterey Peninsula Water Management District are developing a groundwater replenishment (GWR) project, which includes plans for injection of highly purified recycled water into both the vadose zone and deeper semi-confined aquifer in the Seaside Basin.

Diagram showing soil and water attributes with depth
Well Profile and Lithology

Todd Groundwater conducted a field investigation program to address knowledge gaps for GWR vadose zone injection wells. The program included:

  • continuous coring through the 400-foot deep vadose zone and shallow aquifer using the sonic drilling method,
  • core sampling and analysis for mineralogical and hydraulic properties, and contaminant leaching potential
  • installation of a dedicated groundwater monitoring well
  • a comprehensive groundwater quality sampling program, including age dating and isotopic analyses in six nearby wells including an ASR monitoring well

Groundwater quality samples were analyzed for a suite of 300+ constituents/parameters including constituents of emerging concern (CECs) to establish baseline conditions for compliance with future regulatory requirements. Collected data were used to refine the conceptual hydrogeologic model and estimate travel time through the relatively thick vadose zone. Geochemical modeling was used to assess leaching of naturally-occurring constituents (such as arsenic, chromium and lead) and the potential for injection well clogging.

Todd Groundwater is also assisting with basis-of-design reports for shallow vadose zone and deep injection wells. The reports detail the recommended drilling and construction methods; materials and dimensions of casing, screen, and borehole annulus; and associated downhole control valve and wellhead equipment. Construction management and operational considerations associated with specific design components (including scheduling for back-flushing and well re-development) as well as construction cost estimates are also considered.

Cross-sectional diagram of the deep injection well field
Deep Injection Well Field