The specific study questions the Commission requested technical opinions from Weiss to address are:
1. What were the effects of potential and actual changes in hydraulic gradient since January 2017, and what is the potential for these changes to affect potential seawater intrusion to, and capture of fresh water from, aquifers tapped by the well field?
2. What is the potential for the well field to adversely affect or capture previously unidentified volumes of fresh water? and
3. What are the possible project modifications to avoid or reduce the potential effects?
SUMMARY
OF FINDINGS
Weiss’s findings with respect to the Commission’s study questions are addressed in detail in this report and summarized as follows:
1. A
steepening of the hydraulic gradient seaward in the Dune Sand Aquifer in 2017 will
likely result in a limited to negligible effect on seawater intrusion, and
likely result in an increase in the fresh water percentage (FWP) of the well
field flow due to capture of fresh water from the aquifers tapped by the well
field. The gradient change appears to result from local and regional aquifer
recharge due to increased rainfall in the 2016-2017 and 2018-2019 rain years.
This is significant to the evaluation of the FWP percentages resulting from the
MRWSP since there are significant data gaps with respect to groundwater flow
paths in the Dune Sand Aquifer and the transfer of fresh water (total dissolved
solids [TDS] < 3,000 milligrams per liter [mg/L]) from the Sand Dune Aquifer to the 180-foot Aquifer. Therefore,
to be able to rely on Cal-Am’s model results to accurately predict FWP, Weiss
recommends additional data collection to address these data gaps, development
of a consensus conceptual site model (CSM) and modifications of the model assumptions
based on the CSM, and then calibration of the model to match the effects of
these recent rainfall events.
2. The well field capture analysis presented in the project’s Final EIR/EIS appears to be flawed as it does not account for potential freshwater capture beyond the identified capture zone of the well field due to seaward gradients. If such capture is greater than what is already accounted for, it will decrease the ocean water percentage (OWP) in water extracted by the well field. The uncertainty in the range of OWP depends on how the hydrogeology of the Dune Sand Aquifer and underlying Fort Ord Salinas Valley Aquitard (FO-SVA) is interpreted and modeled. It could be reduced through adjustments to the groundwater model and applying it in non-superposition mode to more accurately reflect the site hydrogeology and implications of the TSW pumping results.
3.
RECOMMENDATION
To
obtain a more accurate and definitive groundwater capture zone and OWP estimates
due to proposed pumping from the MRWSP well field, it is recommended that
additional hydrogeologic data be obtained from the 2 square-mile area east of
MW-7S so that a single CSM can be accepted to represent that area. In addition,
the area west of MW-7, between MW-4 and MW-7, should be investigated to
determine potential aquitards contiguous with those at MW-7, and vertical groundwater
gradients between the Dune Sand Aquifer and 180-Foot Aquifer. The new data
should be incorporated into NMGWM, 2016
which should be modified as follows:
•
Change the thickness of the FO-SVA (Layer 3) inland from MW-7, and configure so
that the top and bottom of Layer 3 approximates the configuration depicted in the
geologic cross-section, such that the top resembles a “stair-step” surface;
•
Potentially divide Layer 2 into two or more layers;
•
Increase the HK of the Dune Sand Aquifer (Layer 2) in parameter Zones 16 and 20,
currently modeled with HK of 2 and 4 ft/day, respectively, to values in the range
of 50 to 200 ft/day, more akin to the actual HK for dune sand, and in the middle
of the range in values from other sources (Figure 27); and
•
Modify HK and VK as appropriate in Layers 2, 3, and 4 of the model in the vicinity
of the well field, such that drawdowns in more distant wells, particularly MW-4
and MW-7, are in accord with those estimated from a conservative extrapolation
of the TSW drawdown data.
Along
with these changes, the model should be run in non-superposition mode in a
range of scenarios, and flow lines plotted to illustrate the revised capture pattern.
Mass balance information should be obtained for those portions of the model affected
by groundwater flow to the well field, and from the Dune Sand Aquifer to the
180-Foot Aquifer, and used to calculate new fresh water capture and OWP
estimates.
CLOSING
Weiss
Associates’ work at the California-American Water test slant well site and
vicinity was conducted
under my supervision. To the best of my knowledge, the data contained herein
are true and accurate, based on what can be reasonably understood as a result of
this project while satisfying the scope of work prescribed by the client for
this project. The data, findings, recommendations, specifications, and/or
professional opinions were prepared solely for the use of the California Marine Sanctuary Foundation and the California Coastal Commission in accordance with
generally accepted professional engineering and geologic practice. Weiss makes
no other warranty, either expressed or implied, and is not responsible for the
interpretation by others of the contents herein
EXHIBIT
7 document copy is embedded.
Weiss Associates
November 1, 2019
Addendum to Staff Report for CDP Application 9-19-0918 and Appeal A-3-MRA-19-0034 (California American Water Company
Addendum to Staff Report for CDP Application 9-19-0918 and Appeal A-3-MRA-19-0034 (California American Water Company
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