Page 43 - Yucaipa Sustainable Groundwater Management Agency - Yucaipa SGMA

Gateway/Wilson Creek Subbasins

Of the two existing storm detention/recharge basins in this area, the Oak Glen Creek Basins
appears to provide the best infiltration capacity on a per‐acre basis. The predicted long‐term
rate for the middle basin tested is 5.2 feet/day. This rate assumes historically accumulated
silt/clay in the upper 3 feet of the basin is removed, and a basin maintenance plan is
established. From a hydrogeologic perspective, the site is favorably located upgradient of
major production wells in the Gateway and Wilson Creek subbasins. Water levels indicate
that recharging at this location should benefit aquifers on both sides of the Chicken Hill
Fault. The site is underlain by coarse‐grained deposits that should allow for uninhibited
vertical migration of recharge water to the underlying water table, which was at 302 ft‐bgs
in 2014.

The Wilson Creek Basins provides a reasonable infiltration capacity (predicted long‐term
rate of 1.8 feet/day, assuming historically accumulated silt/clay in the upper 2‐3 feet of the
basin is removed). From a hydrogeologic perspective, the site is favorably located
upgradient of major production wells in the Gateway Subbasin. Recharging at this location
has historically benefited areas on both sides of the Chicken Hill Fault. The site is primarily
underlain by coarse‐grained deposits; however, shallow fine‐grained (clay) deposits create
perched conditions beneath the basins during recharge and limit surface infiltration rates.
Such conditions were observed both during pilot‐scale infiltration test by the USGS in 1970
and during field‐scale testing. Accordingly, for this site, the long‐term infiltration rate of 1.8
feet/day accounts not only for long‐term clogging, but also for predicted mounding (given
the results are similar to the 1970 pilot test, which identified the shallow perching
condition).

The Wilson Creek III site provides an exceptionally high infiltration capacity (predicted long‐
term rate of 13.0 feet/day). While the site is underlain by highly permeable sand and gravel
deposits that should allow for uninhibited vertical migration of recharge water to the
underlying water table, measured infiltration rates may be slightly overestimated due to the
presence of a gravel‐filled trench next to the site. The site is located in the Gateway
Subbasin upgradient of aquifers relied upon for groundwater production. Available vadose
zone storage is sufficient based on a 2014 measured depth to water of 202 feet‐bgs.
Recharging at this location would likely yield benefits both south and north of the Chicken
Hill Fault. Confirmation of the infiltration rate for a full‐scale basin requires further
investigation.

4.2. RECOMMENDATIONS

Key findings from this study are based on integration of infiltration testing results with
available site‐specific and basin‐wide hydrogeologic information. We recommend that
Valley District and its partners use the long‐term infiltration rates presented in this report to
quantify potential groundwater level/storage benefits from recharge and establish site‐
specific recharge goals that maximize basin yield while meeting subbasin and basin
sustainability criteria as part of the GSP development process. We envision these tasks will

Infiltration Testing at Eleven
Investigation Sites in the Yucaipa
Basin, Yucaipa Valley, CA 22 TODD GROUNDWATER

Yucaipa Sustainable Groundwater Management Agency - January 23, 2019 - Page 41 of 137