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RATIONALE - Page 4 <br /> Permit No: COG-070000 <br /> Total Suspended Solids <br /> In the absence of promulgated Federal BAT, BCT or BPT limitations, the State is <br /> required to place State Effluent Limitations in this permit. For TSS these <br /> limitations are 30 mg/1 for a 30 day average and. 45 mg/1 for the 7 day average <br /> limitation. It should be pointed out that operations covered under this permit <br /> primarily control runoff in a manner similar to sand and gravel, and coal <br /> mining operations. Settling of suspended material would be rapid and much like <br /> disturbed ground water associated with a sand and gravel operation. The <br /> Division has found general compliance with the 30/45 TSS limitations for sand <br /> and gravel mining facilities which are properly operated. <br /> Data are available for two construction dewatering operations. These data show <br /> the following: <br /> 30-Day Average Daily Maximum <br /> Arithmetic Mean 5.98 36.18 <br /> Thus, it appears that TSS limits of 30/45 are appropriate. With the limited <br /> data available, the application of more stringent limits is inappropriate. <br /> For the purpose of facilitating compliance monitoring activities, a daily <br /> maximum limitation of 60 mg/l has been set. Monitoring shall be specified as <br /> twice monthly. This should be adequate to protect water quality. <br /> Oil and Grease <br /> Oil and Grease may be present in the discharge as the result of pump lubricant <br /> contamination or contaminated extraneous runoff entering the discharge. A <br /> limitation of 10 mg/l for Oil and Grease is recommended based on th-e Colorado <br /> Regulations for Effluent Limitations. This level can generally be attained by <br /> conventional oil skimming methods or a submerged overflow. Further, at 10 mg/l <br /> of Oil and Grease a visible sheen normally becomes evident. Monitoring shall <br /> be weekly for Oil and Grease. <br /> pH <br /> The pH range of 6.5-9.0 for state water quality standards (WQS) is the effluent <br /> limitation appropriate for this permit. Though some stream segments in the <br /> state have a WQS of 6.0-9.0, a sufficient number of streams have the more <br /> stringent limitation of 6.5-9.0 thus warranting imposition of this limitation. <br /> Waters normally encountered are at a pH of 7.0-8.0. Natural surface and <br /> groundwaters in a few locations in the state will not be within these limits <br /> and a facility at such a location will very likely be considered an abnormal <br /> operation and therefore required to obtain an individual permit. <br /> Flow <br /> Flow monitoring will be required under this permit. • Flow volumes will be <br />