ITEM:		DISCUSSION ITEM
11.	DISCUSS SEASIDE SUBBASIN GROUNDWATER DIVIDE TECHNICAL MEMORANDUM
Meeting Date:			March 17, 2025	Budgeted:	N/A
From:			David J. Stoldt	Program/
			General Manager	Line Item No.:	N/A
Prepared By:			David J. Stoldt	Cost Estimate:	N/A
General Counsel Approval: N/A
Committee Recommendation: N/A
CEQA Compliance: This action does not constitute a project as defined by the California Environmental Quality Act Guidelines Section 15378.

SUMMARY: As part of ongoing District-funded work to understand the influence of groundwater levels in the Salinas Valley on conditions in the Seaside Subbasin, the District contracted with Montgomery & Associates (Consultant) to further investigate and summarize the dynamics of the groundwater flow divide that defines the northern boundary of the Seaside Subbasin. The Consultant’s technical memorandum is attached as Exhibit 11-A.

The District’s Water Supply Planning Committee reviewed and discussed the technical memorandum at its March 3, 2025 meeting and recommended informing the full Board of its findings.

Principal conclusions of the memorandum are as follows:

• The physical flow divide is dynamic and moves over time in response to changes in pumping and recharge, both seasonally and long-term;

• The flow divide does not align with the court-adjudicated boundary, nor the State Department of Water Resources boundary published in its Bulletin 118;

• The interpretation of inflow and outflow across the adjudicated basin boundary needs to be re-considered in light of this, as in theory there would be zero flow across an actual flow divide.

Previous estimates and discussions of inter-basin flows have been based solely on the position of the jurisdictional subbasin boundary rather than on the actual position of the flow divide. In fact, at its November 6, 2024 meeting the Seaside Groundwater Basin Watermaster adopted a “target” for annual replenishment water – if such water was available – based in part upon net outflows from the basin (see “Summary of Updated Replenishment Water Analyses”, October 10, 2022 and updated September 10, 2024, an Attachment to Watermaster agenda Item VIII.A, November 6, 2024. aka “Watermaster Summary”)

This new technical memorandum effectively calls into question the entire concept of “Net Flows from the Deep Aquifer to the Monterey Subbasin” as shown in Figure 11 of the Watermaster Summary (Exhibit 11-B.) Hence, several of the principal conclusions of the Watermaster Summary presented November 6, 2024 cannot be substantiated. Specifically, groundwater is not predictably “lost” to the Monterey Subbasin. Therefore, the replenishment “target” adopted by the Watermaster is based upon an unproven assumption of leakage or outflow and should be revisited with additional groundwater modeling analyses.

The Consultant proposes that a potential new analysis framework could be developed and used as part of Seaside Boundary Conditions Sensitivity Analysis work currently underway by the Watermaster.

EXHIBITS

11-A Seaside Subbasin Groundwater Divide Technical Memorandum

11-B Figure 11 of the Watermaster Summary 11/6/24

U:\staff\Boardpacket\2025\031725\Discussion Items\11\Item-11.docx