SITE INFORMATION AND CORRESPONDENCE_CASE 2

Home Browse Search

Ms. Jeanine Townsend - 3 - *40 4 March 2009 effects of the discharge can occur very close to the outfall and the exposure periods are short (i.e. one hour for acute toxicity and four days for chronic toxicity). The same level of detail, however, is not necessary for human health criteria where the only exposure pathway (in this case, a drinking water intake) is far downstream from the discharge and exposure periods are very long. This is consistent with EPA guidance, which recommends dilution credits for human health criteria include reasonable assumptions regarding exposure pathways.2In this case, the closest drinking water intake is over 10 miles downstream at the Tracy Pumping Plant in the Delta Mendota Canal.3 While the SIP requires a "mixing zone study" for incompletely mixed discharges, it does not provide any guidance on what an appropriate mixing zone study must include for different types of criteria. Similarly, the SIP does not preclude the use of a study to justify dilution credits for long-term human health merely because that study does not adequately justify a mixing zone for constituents based on protection of aquatic life (e.g., because it does not confirm adequate fish passage). The SIP defines mixing zone studies, without limitation, as, "...tracer studies, dye studies, modeling studies, and monitoring upstream and downstream of the discharge that characterize the extent of actual dilution." (SIP, § 1.4.2.1.) The site-specific modeling performed by the Department of Water Resources to evaluate the Discharger's impact on salinity demonstrates that "actual dilution" (complete mixing) occurs within the mixing zone.° The modeling includes data that show the point in the receiving water where the effluent completely mixes with the receiving water. The modeling focused on salinity5, a conservative pollutant, so it provides an additional margin of safety for dibromochloromethane and dichlorobromomethane. These constituents are volatile organic compounds that degrade over time, in addition to mixing into the receiving water. Nothing in the SIP prohibits defining a mixing zone boundary based on a calculated location where monitoring, dye studies or— as in this case — modeling indicates where complete mixing occurs. Such calculations, in appropriate cases such as this one, are "modeling studies ... that characterize the extent of actual dilution." We believe that the record includes adequate information to support findings consistent with the draft Order. We are not requesting the State Water Board to consider these substantive conclusions at this time, because they are not included or explained in the findings of the Permit. However, we are concerned that the draft Order, in its current form, unintentionally precludes the Regional Water Board from relying on information that is already in the record. Therefore, we suggest the following changes to the draft Order: On page 10, in the last paragraph (beginning with "Discussion:"), delete the following sentence: "These are not in the record." On page 12, in the carryover paragraph, revise the second-to-last sentence to read, "The feserd-findings does not demonstrate that an independent mixing-zone Y Water Quality Standards Handbook: Second Edition, EPA-823-B94_005a, p. 5-7 3 The Delta Mendota Canal serves water to the lower San Joaquin Valley (including providing a portion of the drinking water supply for Tracy). The State Water Project intake is only a few miles further to the north. During reasonable worst-case flow conditions in the south Delta, water quality modeling has shown that very little of the Tracy effluent reaches these export pumps. "As noted above, we agree that this is not a completely-mixed discharge as defined by the SIP, and the Permit must specify the mixing zone boundary. b A"mixing zone study" need not be specific to any particular constituents in the effluent, since the SIP allows tracer and dye studies.