The City of Modesto faces water supply challenges in the form of regional competition for limited groundwater, drought impacts that reduce treated surface water delivery, and loss of wells from aquifer contamination. As of 2018, 18 of the City of Modesto’s water supply wells, representing over 16 million gallons per day (MGD) of the system’s total groundwater production capacity (48 MGD), were unavailable for potable use due to contamination from arsenic, nitrate, or uranium. To help ensure the long-term resilience of its groundwater supplies, the City of Modesto was looking for water resources solutions.
Trihydro supported the City of Modesto in procuring Proposition 1 grant funding to address water supply challenges and promote aquifer resilience. In 2018, The City of Modesto entered into a grant agreement with the State Water Resources Control Board (SWRCB) to implement the City of Modesto Wellfield Remedial Investigation/Feasibility Study (RIFS), which was designed to evaluate approaches to mitigate impacts from arsenic, nitrate, and uranium in its wellfield.
The RIFS’s objectives were to characterize the aquifer system beneath the City of Modesto and identify, analyze, and compare six alternatives to address contaminants of concern (i.e., arsenic, nitrate, and uranium). Trihydro developed a solute transport model as part of the feasibility study (FS). The RIFS presented a model of the aquifers beneath the City of Modesto as shallow, intermediate, and deep groundwater supply zones. Trihydro then conducted an alternatives analysis to assess the effects of the six alternatives on each zone.
Ultimately, the FS recommended a hybrid approach focusing on the two alternatives that provided the highest benefit to address arsenic, nitrate, and uranium contamination. Based on the analysis, a combination of managed aquifer recharge with spatial and temporal pumping management was selected to comprise the Recommended Project Alternative, achieving a higher relative score in the FS than any individual alternative on its own. The hybrid approach also included minor components from other evaluated alternatives that scored highly (e.g., abandoning wells that act as conduits for cross-contamination between aquifers and swaging [blocking] aquifer intervals that contribute water containing undesirable levels of arsenic, nitrate, or uranium).
- Successful procurement of project funding through a grant
- A vetted project alternative designed to protect deeper groundwater quality, improve shallow groundwater quality, increase groundwater quantity, and reduce overdraft