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recharge basin. This factor is recommended based on discussions with OCWD (Adam
Hutchinson, Recharge Planning Manager, personal communication, November 7, 2018) and
is applied to the average infiltration rate calculated from the constant‐head test to provide
an “initial” full‐scale infiltration rate for each site (second‐to‐last column in Table 2). It is
important to recognize that this scale‐up factor does not account for the potential impacts
to surface infiltration rates from possible low‐permeability hydraulic boundary conditions
(e.g., clay layers in the deeper vadose zone or clay gauge associated with nearby geologic
faulting) that can inhibit radial flow of recharge water and increase recharge mounding
beneath the recharge basin. Additional field investigations, such as pilot‐scale infiltration
testing, is needed to address the influence of hydraulic boundary conditions, including
faults.

Accurate prediction of the effects of clogging on long‐term infiltration rates in a full‐scale
recharge basin relies on a clear understanding of the site‐specific recharge goal, anticipated
active spreading period, and approach and frequency of basin maintenance. These factors
are effectively unknown at this time. Recent research has improved the understanding of
clogging dynamics to allow operators to reliably predict infiltration rate declines and
optimize basin maintenance when site‐specific information for active recharge basins are
available. Key governing factors include the intrinsic permeability of sediments comprising
the basin bottom and the suspended solids load in source water. A study by OCWD (Phipps,
D.W., Lyon, S., and Hutchinson, A., 2007, see Appendix A) indicated that approximately 90
percent of the infiltration rate decline over time in selected OCWD basins were attributable
to the accumulation of suspended solids mass on the basin floor and its interface with
sediment on the basin floor.

The physical clogging potential of SWP water was recently characterized by Valley District by
analyzing the total suspended solids (TSS) concentration of a sample collected from the
Yucaipa Turnout of the SWP East Branch Extension. The TSS concentration of the SWP water
was 1.4 mg/L, which is well below TSS concentrations of Santa Ana River water in the OCWD
study. An additional clogging factor ‐ algal growth ‐ has also been observed in local SWP
water recharge basins by the San Bernardino County Water Conservation District (SBCWCD)
(personal communication, Richard Corneille, November 21, 2018). Basin design and
operations will need to address long‐term basin clogging through a combination of pre‐
treatment design solutions and a basin maintenance program.

For planning purposes, a “long‐term” full‐scale infiltration rate is predicted for all tests by
reducing the initial full‐scale infiltration rate by 50 percent. The 50 percent assumption
accounts for currently unknown site‐specific variables including the site recharge goal,
active spreading period, and the frequency of basin maintenance over time.

3.1. INVESTIGATION SITE SUMMARIES

Conclusions integrating infiltration test results and local hydrogeologic conditions for each
investigation site are presented along with recommendations for additional work below.

Infiltration Testing at Eleven
Investigation Sites in the Yucaipa
Basin, Yucaipa Valley, CA 12 TODD GROUNDWATER

Yucaipa Sustainable Groundwater Management Agency - January 23, 2019 - Page 31 of 137