In addition to groundwater level monitoring, the agencies are committed to continuing to conduct monitoring of water quality, groundwater production, subsidence, and any other monitoring that may be required to satisfy sustainable management criteria.
Part 2: Additional Information for La Habra-Brea Management Area
The City of La Habra GSA incorporated additional information into the La Habra-Brea Management Area section, including additional explanation and quantification of the management approaches, sustainable management criteria, and clarifying the evidence for the absence of undesirable results for the La Habra-Brea Management Area. Additionally, data gaps within the monitoring network are discussed in the 2022 Update, including steps currently being taken to address and fill the gaps. DWR Recommended Action 4 . Staff recommend the Agencies explain the timeframe that the Basin can safely operate, without experiencing undesirable results, after exceeding 500,000 AF below full conditions (23 CCR Section 354.26(b)(2)) and clarify the wells used to calculate the change in groundwater in storage and the overall groundwater in storage.
Response:
It is important to reiterate that exceeding 500,000 acre-feet from full is a hypothetical situation and is not an intended action under normal basin management circumstances. Such a hypothetical situation is considered an extraordinary circumstance; for example, an extended drought that is exacerbated by a lengthy shutdown of imported water into southern California due to a catastrophic event such as a major earthquake. This hypothetical situation would not be expected to last more than one year; however, other worse hypothetical cases can be conceived. Because the cause(s) and magnitude of a hypothetical temporary storage exceedance beyond 500,000 acre-feet below full are unknown, it is difficult to quantify a precise timeframe that the Basin can operate without experiencing undesirable results. That said, a cumulative exceedance of no more than 200,000 acre-feet beyond 500,000 acre- feet below full conditions over five consecutive years is suggested as a general guideline. This means that the average exceedance would be no more than 40,000 acre-feet during a maximum five-year period. If the period of exceedance was two years, then the average exceedance would be 100,000 acre-feet. As a general guideline, limiting the magnitude and duration of the exceedance will lessen the potential for undesirable results, as described below: 1. The rate of inland movement of seawater intrusion with the basin at 500,000 acre-feet below full conditions and with the seawater barriers is expected to be on the order of 1-2 feet per day. Over a 5-year period, this could represent a brackish water encroachment of roughly ½ mile into some of the coastal gaps. While this encroachment could cause localized water quality degradation, it is unlikely to reach production wells which are farther inland. Once the storage exceedance ends, and the basin storage is within the established operating range, the seawater barriers’ performance would be expected to recover to their design conditions and subsequently halt and reverse the brackish groundwater inland encroachment. 2. Reduction of pore pressures in and drainage of water from thick, compressible confining layers is a slow process due to the low permeability of the confining layers. Therefore, a maximum five-year period of basin storage exceeding 500,000 acre-feet from full would not be expected to cause significant inelastic land subsidence.
Appendix F - 285
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