[Note: the following memo was shared for posting on the Davisite]
To: Clancy McConnell, David vanRijn
cc: Yolo County Planning Commission (via Jeff Anderson), Phil Pogledich
From: Alan Pryor, Chair, Sierra Club Yolano Group
Date: October 26, 2026
Re: A Subsequent EIR is Required for the Vulcan Mining Permit Extension instead of just an Addendum to the Previous EIR
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Executive Summary
Under the California Environmental Quality Act (CEQA), when a project changes or new information emerges after an Environmental Impact Report (EIR) is already certified, lead agencies must use a specific legal framework to determine if further environmental review is necessary. Yolo County has determined that just an Addendum to the 1998 SEIR for the Syar-Vulcan Madison Mine is required to extend the operating permit for that mine from 2029 to 2049.
We strongly disagree with this determination believing instead that a Subsequent EIR must be prepared. This is because new information has arisen regarding a plethora of more severe adverse environmental impacts that have occurred at the Vulcan and other Cache Creek off-channel mining sites since the 1998 certification of the SEIR for the Vulcan Madison mining project. This new information requires disclosure and analysis in a Subsequent EIR subject to public circulation.
These adverse environmental impacts for which additional information has arisen include widespread unresolved methyl mercury contamination of the deep wet impoundment pits at the Madison mine and other mining sites along Cache Creek site.
The adverse environmental impacts also include failures to fully reclaim ag lands to their former prime condition. New information is available describing the systemic ongoing difficulties to ever meet the ag reclamation requirements of the mining companies.
Further, adequate financial resources required to implement final mitigation to resolve these looming environmental problems has not been posted in a surety bond by the Vulcan mining company. This unfunded liability potentially leaves Yolo County as the funder of last resort of over $100 million in restoration and mitigation obligations.
Direct Comparison of Subsequent and/or Supplemental EIRs with an Addendum to an Existing EIR
Following are the primary differences between a Subsequent EIR, a Supplemental EIR, and an Addendum to an EIR which differences primarily depend on the severity of the new environmental impacts identified since the original certification of the prior EIR. This determines how much of the original analysis needs to be rewritten.
| Feature | Subsequent EIR (§ 15162) | Supplemental EIR (§ 15163) | Addendum to EIR (§ 15164) |
| Impact Severity | New or substantially more severe significant environmental impacts. | New or substantially more severe significant impacts. | No new or substantially more severe significant impacts. |
| Scope of Revision | Major revisions required; the original EIR can no longer adequately cover the project. | Only minor additions or changes are needed to make the previous EIR apply to the new situation. | Only minor technical changes or additions that do not alter previous conclusions. |
| Public Review? | Yes (Full public notice and recirculation required). | Yes (Only the supplement itself needs to be circulated, not the whole EIR). | No public review required; it is simply attached to the final EIR. |
1. Subsequent EIR (CEQA Guidelines § 15162)
A Subsequent EIR is triggered when substantial changes to the project, major changes in surrounding circumstances, or new information reveal brand new significant environmental effects or a substantial increase in the severity of previously identified impacts. It requires extensive new analysis and is treated like a full EIR, meaning it must undergo a full public comment and notification period.
2. Supplemental EIR (CEQA Guidelines § 15163)
A Lead Agency will choose a Supplemental EIR instead of a Subsequent EIR if the strict impact thresholds of § 15162 are met, but only minor additions or alterations are needed to fix the original document. Instead of rewriting the entire analysis, the agency crafts a focused “supplement” containing only the info necessary to make the previous EIR accurate for the project’s new scenario. It still requires formal public review and notices, but only the supplement itself is circulated.
3. Addendum to an EIR (CEQA Guidelines § 15164)
An Addendum is used when project modifications or updates are necessary, but none of the conditions triggering a Subsequent or Supplemental EIR have occurred. In other words, any modifications are minor, and the environmental impacts stay within the bounds of what was already analyzed and approved. Because it introduces no new risks, it bypasses requirements for public review entirely. The agency simply includes a brief technical explanation and attaches the addendum to the final EIR prior to project approval.
In the case of the Vulcan mine, neither the original 1996 EIR or the 1998 Supplemental EIR fully and properly addressed new information that has arisen regarding the substantially increased severity of two adverse environmental impacts surrounding i) the remediation of the ongoing methyl mercury contamination problems in the wet impoundment pits and ii) the difficulties in reclaiming agricultural land to the same “prime” quality as existed before mining.
The County claims there was sufficient disclosure about these potential adverse environmental impacts in the 1996 EIR and the 1998 SEIR to warrant their certification 28 years ago. They further claim that only technical changes are being proposed in the current mining permit extension thus requiring only the preparation of an Addendum to the 1998 SEIR to satisfy their disclosure obligations.
Unfortunately, however, this determination completely ignores substantial new information that has been made available since 1998 indicating the severity of these adverse impacts is greatly increased thus requiring the preparation of a Subsequent EIR to properly disclose, analyze, and evaluate them.
Newly Available Information on Increased Severity of Adverse Environmental Impacts
1) Substantially Increased Methyl Mercury Contamination in Wet Impoundment Pits and the Potential Failure to Remediate the Pits Requiring that they be Filled – The two Vulcan wet impoundment pits have indisputably elevated concentrations of methyl mercury since 2015 as shown in the following information excerpted from the 2025 annual methyl mercury monitoring report prepared for the County by Dr. Daryl Slotten.
Fish Mercury Monitoring Summary – Vulcan Sites, 2015-2025
In recognition of this ongoing severity of contamination, Dr. Slotten recommended in 2021 that the County immediately begin to implement Lake Management Plans at Vulcan to remediate the contamination. To date, however, these Lake Management Plans have not even been approved by the County much less implemented at the Vulcan mine.
Additionally, the County’s consultant and designer for the Lake Management Plans, Dr. Stephen McCord, acknowledged at a previous Yolo County Planning Commission meeting that there are no guarantees these plans will ever be successful in reducing the excessive concentrations. Dr. McCord also indicated that, even if successful, the Lake Management Plans may be required to be operated at substantial cost in perpetuity to keep the impoundment pits’ concentrations of methyl mercury within acceptable levels.
Failing that, the pits are required to be completely filled in by the mining companies at enormous costs. Neither of these eventualities are provided for in the amount of the surety bond posted by Vulcan to ensure they meet their restoration and reclamation obligations.
The matter of ongoing excessive concentrations of methyl mercury in the wet impoundment pits at Vulcan and the extensive and costly mitigation strategies to remediate the problems were neither analyzed nor explicitly discussed in the 1996 EIR or the 1998 SEIR.
This site-specific information on the severity and complexity of methyl mercury contamination of the Vulcan wet impoundment pits indisputably represents definitive new information of more severe environmental impacts of the project. As such, CEQA regulations explicitly indicate that a Subsequent EIR must be employed in the current permit extension process instead of simply issuing an Addendum to the earlier 1998 SEIR which is now approaching 30 years old.
2) Potential Failure to Reclaim Farmland to its former “Prime” Productive Capacity – Substantial new evidence has arisen since the certification of Vulcan’s SEIR and mining entitlements issued in 1998 that indicates that ag reclamation to former “prime” condition, as required in the Surface Mining Reclamation Ordinance, may not be physically or economically attainable. This clearly represents a more severe environmental impact that previously, discussed, and/or analyzed.
For instance, to date, of the approximate 2,300 acres currently permitted for mining, only a single 30-acre site (Muller-30) has been reclaimed for agriculture in 2007 following 2 years reported winter wheat production purportedly meeting industry standards. However, there was no other information provided at the time otherwise indicating the soil tilth and productivity met the criteria for “Prime” farm land. Indeed, the site has not been farmed since and, today after almost 20-years, the site remains an unproductive weed-filled, rocky, fallow field that has certainly NOT been restored to its former productive prime capacity as is intended in the Off-Channel Mining Plan.
Indeed, the existing condition of this particular field is indicative of the general impediments to returning former prime farm lands to pre-existing productive capacity as further described and explained in the “House Report” in 2021 (from p.7 of the Executive Summary)
“1.1.1 Significant findings
Not all reclaimed land is suitable for agriculture, and not all prior agricultural uses may remain economic after reclamation.
– The regulatory standard for success in terms of evaluating productivity of land reclaimed to agriculture is established in the California Surface Mining and Reclamation Act of 1975, used statewide. This is when the productive capability of the land in reclamation is demonstrated to be equivalent to or exceeds, for two consecutive crop years, that of the pre-mining condition or similar crop production in the area. This productivity standard is not rigorous and presents some ambiguities for implementation.
– Successful farming of reclaimed mined lands is possible, but it requires extra inputs, especially organic-matter additions to the reclaimed soil, and it takes years, even decades in some cases, to establish a biologically healthy and productive soil ecology.”
Other than spreading stored overfill back onto the ag lands followed by leveling and ripping, there are no provisions in the Vulcan mining permit for additional costs incurred in bringing the ag lands back to the productive standards that existed prior to mining (e.g. prime, Class 1 or Class 2). If such ag lands cannot be brought back to their former prime productivity status, Vulcan will be required to purchase conservation easements on other equivalent land in the County at a likely 3:1 mitigation.
This clearly represents a new, more severe, environmental impact that previously disclosed in either the 1996 EIR or the later 1998 SEIR thus requiring the preparation of a Subsequent EIR instead of a mere Addendum for the current planned Vulcan mine permit extension.
Additionally, there are currently no provisions in the surety bond posted by Vulcan to provide funds to purchase any such such agricultural easements in the event ag reclamation of the designated lands at the Vulcan mining site are unsuccessful being reclaimed to “prime” productive status.
Conclusion
In summary, in the last 28 years since the certification of the 1998 SEIR it has become clear that these are 2 very important potential adverse environmental impacts which have not been sufficiently disclosed, analyzed, and addressed in the prior 1996 Vulcan Mine EIR and the 1998 SEIR. As such, they indisputably represent new or more severe environmental impacts thus requiring the preparation and circulation of a Subsequent EIR rather than preparing an Addendum to the 1998 SEIR.
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