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4. CONCLUSIONS AND RECOMMENDATIONS

4.1. CONCLUSIONS

Based on the results of infiltration testing completed at eleven investigation sites in the
Yucaipa Basin, the following general conclusions can be made:

 Short‐term infiltration testing using temporary field‐scale basins provides a
reliable and cost‐effective means for confirming vadose zone infiltration
capacity, a critical component to assessing site recharge feasibility.

 Infiltration test results provide a technical basis for predicting “initial”
infiltration rates for a full‐scale recharge basin, the size for which is known
at existing storm detention/recharge facilities (e.g., Wilson Creek Basins,
Oak Glen Creek Basins, and Wildwood Creek Basins) and unknown for other
sites.

 Initial full‐scale infiltration rates presented herein do not account for
potential hydraulic boundary conditions that can inhibit radial flow of
recharge water and increase recharge mounding beneath a full‐scale
recharge basin. The primary boundary condition concern identified for some
investigation sites is the presence of a nearby geologic fault or barrier.
Additional focused field investigations, including pilot‐scale infiltration
testing with groundwater monitoring, are needed to address these effects.

 Infiltration test results also provide a technical basis for predicting “long‐
term” infiltration rates for a full‐scale basin, needed to support future
evaluation and design of recharge facilities to optimize groundwater water
level, storage, safe yield, and water quality benefits in the Yucaipa Basin.
Long‐term full‐scale infiltration rates presented herein consider the effects
of basin clogging which is dependent on currently unknown variables,
including the site‐specific recharge goals, anticipated active spreading
period, and approach and frequency of basin maintenance.

Combined with existing available hydrogeologic information, infiltration test results indicate
the following regarding site recharge feasibility, organized below by subbasin:

Western Heights Subbasin
The Tennessee Street Basins and Chapman Heights Basins appear to provide adequate
infiltration capacity (predicted long‐term rates of 2.7 and 2.5 feet/day, respectively). Both
sites are favorably located upgradient of aquifers relied upon for groundwater production
and beyond the extent of regionally extensive clay layers in the Western Heights Subbasin.
Whether the migration of recharge water across the Western Heights Fault into the Western
Heights Subbasin will be inhibited represents a critical knowledge gap that requires further
investigation and/or evaluation.

Infiltration Testing at Eleven
Investigation Sites in the Yucaipa
Basin, Yucaipa Valley, CA 20 TODD GROUNDWATER

Yucaipa Sustainable Groundwater Management Agency - January 23, 2019 - Page 39 of 137