Metals Site Specific Criteria Development at Twelve Mile Creek

Union County, North Carolina received a NPDES permit for the Twelve Mile Creek Wastewater Treatment Plant (12 MC WWTP) which had limits for total recoverable copper of 3.8 ug/L and for total recoverable zinc of 37 ug/L. Although the discharge is in North Carolina, these limits are based on the South Carolina standards as the diluted effluent is conveyed by Twelve Mile Creek into South Carolina. Twelve Mile Creek merges with the much larger Catawba River about nine miles from North Carolina/South Carolina border. Because both North Carolina and South Carolina regulatory agencies have input into the site specific alternative criteria and each has its own approach, the development of site specific criterion was particularly challenging. (refer to Figure ES-1). The permit also provided for the development of site specific alternative criteria for metals which typically allows for higher permit limits while fully protecting the environment.

The procedures for developing site-specific alternative are presented in the Environmental Protection Agency document “Interim Guidance on the Determination and Use of Water-Effect Ratio for Metals” (EPA-823-B-94-001). There are several methods available for developing site-specific alternative criteria which have a range of additional data requirements and hence cost including (1) recalculation procedure; (2) translator procedure; and (3) water effects ratio development. The County elected to pursue the most economical first and if this did not improve the permit limits, progressing to the more complex approach, if needed.

The recalculation procedure was selected as the most appropriate approach for an initial site-specific alternative. The recalculation procedure, as presented in the EPA guidance document consists of tabulating the resident species determined for this receiving water. The list of species used to develop the original  nationwide water quality is then modified by the deletion of non-resident species. The resident species list is then used to determine the most sensitive species and the toxicity characteristics of the most sensitive resident species.

To develop the resident species list, the receiving water must be surveyed for the habitat present. Based on the habitat, a resident list may be developed from similar habitat studies. Also the background copper, zinc and hardness and the effluent copper, zinc and hardness must be determined. To assure accurate representation data the samples were collected using near clean techniques. To have a statistical significant amount of reliable data, twenty samples were collected and analyzed.

This article evaluates the indigenous species toxicity information to develop site-specific alternative criteria for copper and zinc. Using the Environmental Protection Agency stipulated methodology the copper site-specific value (at a hardness of 50 ug/L) was determined to be 9.23 ug/L and for zinc the sitespecific alternative criterion is 234.5 ug/L again at a hardness of 50 mg/L. Using the ambient hardness of 59 mg/L, the copper site-specific alternative criterion is 13.22 ug/L and the zinc site-specific alternative criterion is 258.9 ug/L.

Using these site-specific alternative criterions and the reliable 20 sample data for copper and zinc, a reasonable potential evaluation was conducted. The previous effluent monitoring data is suspect as the collection and analysis did not use near clean or clean methods and tended to have high detection limits. A reasonable potential evaluation is a statistical evaluation of the available reliable data to project the maximum effluent concentration for the parameter of interest. Following Environmental Protection Agency methodology the coefficient of variation, which is the standard deviation divided by the mean, is determined. Using the coefficient of variation and the number of samples, a multiplier is taken from the Environmental Protection Agency table for 99th probability with 99th confidence level. This multiplier is then multiplied times the highest determinate datum to calculate the maximum predicted concentration. If the  aximum predicted concentration is below the maximum allowable concentration then there is no need for another permit monitoring or a numerical limit.

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