2018-03-01_HYDROLOGY - C1981010 (2)

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�` COLORADO 2- SWater Quality Assessment ,;-ti•' Department of Public <8 Health b Environment For mercury, which is a bioaccumulative toxic pollutant, the bioaccumulative significance test can now be completed with some minor additional calculations for the baseline water quality load (BWQload), the threshold load (TL), the new load based on the WQBELs, and the threshold load concentration (TL conc). These terms are defined by the following equations: BWQload = BWQ* AD low flow (chronic) * 8.34 Threshold Load (TL) = 0.1 * BWQload Threshold Load Concentration (TL Conc) = TL : new design flow : 8.34 WQBEL Load = new WQBEL (concentration) * new design flow * 8.34 The antidegradation low flow used in the BWQload equation was determined based on the same approach and using data from the same location as discussed in Section IV, but the period of record used was October 1, 1990 through September 30, 2000. The resulting chronic flow is 75.6 cfs. The discharge is considered to be insignificant if the new load (WQBEL load) is less than the threshold load (TL), or if the new WQBEL (concentration) is less than the TL Conc. The results of the calculations and the comparisons are shown in Table A-16. Note the bioaccumulative significance test is performed using the combined design flow; therefore, the result is the same for all outfalls. Table A-16 Bioaccumulative Significance Test for Total Mercury Threshold Load Threshold Load (TL) WQBEL Conc WQBEL Load Concentration L Conc) 3.477E-4 .0580 0.067 11.1756 For mercury, the TL Conc is less than the WQBEL Conc. The antidegradation review for this parameter will continue with the calculation of the SCT and ADBACs, in the same manner as the other non-bioaccumulative toxic pollutants. Significant Concentration Threshold The SCT is defined as the BWQ plus 15% of the baseline available increment (BAI), and is calculated by the following equation: SCT = (0.15 x BAI) + BWQ The BAI is the concentration increment between the baseline water quality and the water quality standard, expressed by the term (WQS - BWQ). Substituting this into the SCT equation results in: Where, SCT = 0.15 x (WQS-BWQ) + BWQ WQS = Chronic standard or, in the absence of a chronic standard, the acute standard BWQ = Value from Table A-15 When the BWQ concentration is equal to zero, the following equation results: SCT = 0.15 x WQS PagQe 35 of 40 4300 Cherry Creek Drive S., Denver, CO 80246-1530 p 303-692-2000 www.coLorado.gov/cdphe/wqcd I