San Diego Regional Water Quality Control Board September 11, 2026

Emily Allen Area Operations Manager United States Section International Boundary and Water Commission 2995 Clearwater Way San Diego, CA 92154 emily.allen@ibwc.gov

Subject: Most Sensitive Species for Whole Effluent Toxicity Testing

Dear Emily Allen, The San Diego Water Board acknowledges the ongoing constraints in obtaining giant kelp (Macrocystis pyrifera) for use in required chronic whole effluent toxicity (WET) testing. In response to USIBWC’s request, the San Diego Water Board is granting a conditional variance allowing the use of the purple sea urchin for chronic WET testing of SBIWTP effluent, when giant kelp cannot be obtained despite reasonable efforts.

Summary of Giant Kelp Supply Constraints

In April 2025, USIBWC first reached out to the San Diego Water Board regarding challenges in obtaining a reliable supply of giant kelp for chronic WET testing of effluent from the SBIWTP. Nautilus Environmental and Bio-Analytical Laboratories, subcontractors for USIBWC’s contracted operator Veolia, described alleged restrictions imposed by the California Department of Fish and Wildlife (CDFW) on scientific collection permits (SCP) for giant kelp off the coast of San Diego. These alleged restrictions resulted in a significantly reduced take of giant kelp, making it difficult to consistently obtain enough giant kelp for weekly testing as required by USIBWC’s NPDES permit. Nautilus Environmental and Bio-Analytical Laboratories recommended that USIBWC request a variance from the most sensitive species requirement, allowing for the use of the second most sensitive species, the purple sea urchin (Strongylocentrotus purpuratus), to satisfy this weekly monitoring requirement.

mailto:emily.allen@ibwc.gov

Emily Allen, USIBWC - 2 - September 11, 2026

To corroborate this claim, San Diego Water Board staff met with the relevant staff from CDFW who explained that the SCP in question authorized a take of 50 sporophyll blades over the 3-year permit term, and the allowable take was routinely exceeded. A new SCP has since been issued, authorizing a take of 80 sporophyll blades per 3-year permit cycle. CDFW staff explained that the allowable take in the applicable SCP was not actually reduced, but it would be enforced moving forward. Additionally, CDFW staff noted that they may consider amending the new SCP to increase the take of sporophyll blades if the SCP holder identifies additional sites for sporophyll harvesting.

Per staff discussions with Nautilus Environmental, 10 to 20 sporophyll blades are required for a single weekly bioassay test. Accordingly, USIBWC’s weekly WET requirement would necessitate 520 to 1,040 sporophyll blades per year, or 1,560 to 3,120 sporophyll blades per 3-year SCP term.

Conditional Variance

To ensure uninterrupted weekly WET testing, the San Diego Water Board is granting a conditional, limited variance to use purple sea urchin in place of giant kelp only under the following conditions:

1. Applicability. This variance applies only when giant kelp is unavailable for a given week’s test after exhausting all reasonable procurement options. This variance does not waive weekly WET testing.

2. Exhaustion of procurement options (documentation required). Prior to first invoking the variance, USIBWC and its subcontractors must document diligent efforts to obtain giant kelp, including:

o Contact attempts and responses from permitted collectors (local and regional), including Point Loma, La Jolla, and other Southern California kelp beds; and o Consideration of logistics and quality risks for longer-haul sources (e.g., Channel Islands, Monterey), including cost constraints and test performance history; and o Any CDFW SCP-related constraints (permit numbers, quotas, and denial or deferral notices), and any coordination with CDFW staff, including, but not limited to, coordination to amend the current SCP.

3. Substitution with next most sensitive species. When giant kelp cannot be procured despite the above, USIBWC may test with the next most sensitive species identified by the most recent 2026 rescreening, the purple sea urchin.

The substitution shall be limited to that week’s test and shall revert to giant kelp as soon as it is available.

Emily Allen, USIBWC - 3 - September 11, 2026

4. Ongoing use of giant kelp on a monthly basis. USIBWC shall conduct WET testing using giant kelp no less than once per month.

5. Duration and review. This variance is effective retroactively to April 2025 and remains in place through the conclusion of the 2026 species sensitivity rescreening cycle, subject to revocation or modification if circumstances materially change (e.g., CDFW SCP adjustments that restore reliable giant kelp availability). The variance does not alter effluent limitations or the requirement for weekly chronic WET testing.

This conditional variance preserves weekly WET monitoring while acknowledging documented market, regulatory, and logistical constraints affecting giant kelp collection.

The approach reduces test cancellations and repeat runs due to substandard kelp samples and aligns with the Ocean Plan-based monitoring framework and WET program intent.

If you have questions, please contact Mr. Riley Nolan, Water Resource Control Engineer, at 619.521.3358 or riley.nolan@waterboards.ca.gov.

Sincerely, David W. Gibson Executive Officer, San Diego Water Board cc:

Emily Allen, emily.allen@ibwc.gov Gilbert Anaya, gilbert.anaya@ibwc.gov Wayne Belzer, wayne.belzer@ibwc.gov Melissa Corona, melissa.corona@waterboards.ca.gov Lauren Kim, lauren.kim@waterboards.ca.gov Riley Nolan, riley.nolan@waterboards.ca.gov Jacob Schmidt, jacob.schmidt@veolia.com mailto:riley.nolan@waterboards.ca.gov

San Diego Regional Water Quality Control Board