Toxicity Problems at the IWTP

Excerpts from Executive Summary of the IWTP, available at:
http://www.epa.gov/region09/water/iwtp/seis1.pdf
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Areas of Significant Impact that cannot be mitigated
Impacts that cannot be mitigated to below significance are marine water quality, marine biological resources, land use, traffic and transportation, public health and safety, and air quality. (The location of the detailed discussion of each resource area in this SEIS is noted in parentheses.) The inclusion of a resource area in this section does not indicate that impacts are associated with all alternatives.

Marine Water Quality (Section 3.1.3)
Compliance with California Ocean Plan Standards
Marine water quality parameters were analyzed in this SEIS using an ocean modeling evaluation to determine compliance with the California Ocean Plan standards (see Appendix C of this SEIS). All of the secondary treatment alternatives were found to be in compliance with the California Ocean Plan standards for Table B toxic compounds. For the Advanced Primary Only alternative, the ocean model identified copper and DDT as exceeding the standards. DDT was also identified as exceeding the standard by the Partial Secondary Treatment alternative.

As a result of these exceedances, the toxicity standard could also be exceeded. Note that in contrast to these results from the ocean model, effluent data collected from the SBIWTP to date, albeit limited, does not identify an exceedance of these compounds. Effluent data will continue to be collected from the SBIWTP and will be used to assess the ocean modeling evaluation.

Toxic Spikes
Although Tijuana is currently developing a pretreatment program, there exists a potential for unusually high concentrations of toxic compounds (toxic spikes) to enter the SBIWTP from time to time. It is possible that a toxic spike could cause an exceedance of a Table B limit, as well as the toxicity standard, because of the high concentration of a compound passing through the treatment plant without receiving adequate treatment. This occurrence is more likely to occur from the Advanced Primary Only and Partial Secondary Treatment alternatives because of the lower levels of treatment.

Toxic spikes can upset the secondary processes as well, reducing treatment for a period and exceeding discharge limits as a result. Of the secondary alternatives, the pond treatment systems would provide the best management of toxic spikes because of the anaerobic digester pits and the large water volume that dilutes the spike and minimizes an upset.

The proposed mitigation to address the impacts to the marine environment from Table B compounds and toxic spikes, as well as to ensure the overall performance of the SBIWTP, is the successful implementation of a pretreatment program in Tijuana . The pretreatment program should target those pollutants most likely to exceed Ocean Plan standards, as well as provide an overall reduction in pollutants from entering the SBIWTP. A more detailed discussion regarding the status of Mexico’s pretreatment program is provided in Appendix A2.

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