Grant Funding, Groundwater Quality, and Supply Resilience
Trihydro supported the City of Modesto in procuring grant funding to conduct a study to address impacts from arsenic, nitrate, and uranium in its wellfield and promote aquifer resilience.
The City of Modesto (City) faces water supply challenges from 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’s water supply wells, representing over 16 million of the system’s total 48 million gallons per day groundwater production capacity, were unavailable for potable use due to contamination from arsenic, nitrate, or uranium. The City of Modesto was looking for water resources solutions to support the long-term resilience of its groundwater supplies.
Remedial Investigation and Feasibility Study
Trihydro supported the City in procuring Proposition 1 grant funding to study how to best address water supply challenges and promote aquifer resilience. In 2018, the City entered into a grant agreement with the State Water Resources Control Board 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 and identify, analyze, and compare six alternatives to address contaminants of concern (i.e., arsenic, nitrate, and uranium). Trihydro applied groundwater flow and contaminant transport models as part of the feasibility study (FS) in addition to applying geochemical models to assess the long-term aquifer/recharge water interaction. Our team then used these models to assess the effects of six alternatives on the City’s water supply.
Selecting a Remedial Solution
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, we selected a combination of managed aquifer recharge with spatial and temporal pumping management—a solution that achieved 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 had high scores, such as abandoning wells that act as conduits for cross-contamination between aquifers and swaging (blocking) aquifer intervals with water containing undesirable levels of arsenic, nitrate, or uranium.
Following project completion, Trihydro helped the City secure additional Proposition 1 funding for the implementation phase of the Recommended Project Alternative.
