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. ANPRM Comments December 8, 1998 Page 5 EP~ also states in this sediori its current interpretation "that, where em effluent discharge creates an essentially pererinial flow for.i"vhat naturally' would be ephemeral or intermittent waters;.the. resulting aquatic communitY is. to be prot.ected." What does EPAundefstandto be the legal mechanism to "achieve this r~sUlt?; DoesEPA believe thatau~hoi"ityexists to'!E3.gur~EF.? dischargerto continue a3lischargethat has created a downstream use? 4. III.BA.c. Use Attainability Analysis: . We do not believe that EPA should adopt new regulatory provisions specifying the minimum requirements of a use attainability analysis (UAA) , beyond ttie general description contained in 40 CFR 131.3(g). What constitutes an appropriate "structured scientific assessment" will vary greatly with individual circumstances. For example, the attainability of primary contact recreational uses such as swimming may sometimes be determined based upon simple observations and descriptions of physical realities. In other circumstances, some aquatic life use attainability analyses may need to be much more extensive. Establishing a specific set of minimum requirements that apply to all use attainability analyses is likely to establish a "floor" that is either too high or too low for many applications. !!.mi=iY;J)~e usefumorEPA";to'addfes"sstheTheea'f6"r.~timel' "'deter'fiiinaticiffs":oethE!7~r-r6'-riate -...-, ,~---~._- _~""""~_'r-"..b.,;,.-....,""",,,,,,,,,,,,.~_^,,u.;:;;...,_...y',,._~_ ,_.............._.._.~'" ......,p.J:2...'""P............." .~C?f.[@.@!cJ.~I1!~atti!ill!!QJ!jIy;a'i:i'.i!!y.~eS':at'tlie!ffOnt'ef1d~6frthe-'p19Sess~tQ avo'Cl;;art'jQ~~!lrQ:SiD5fIresourceSl . . .. .. 5. III.BA.d. Alternatives to "Downgrade" of the Designated Use: Colorado has for many years utilized the adoption of ambient quality-based water quality standards and temporary modifications of water quality standards as tools in its water quality standards system. These tools have been applied most often in highly mineralized areas of the State which have been subjected to extensive past mining activity. . a. Ambient quality-based standards are adopted where natural or irreversible human-induced ambient water quality levels are higher than levels identified in applicable water quality criteria, but are determined adequate to protect designated uses. Because EPA section 304(a) criteria are not attainable' in such circumstances, the primary alternative tci-arlit?~hXc:fi.Jali!y&~sEfQ]lanC!iM2~ would be to downgrade the use designation of such waters. Colorado believes that in these circumstances it is preferable to recognize the, perhaps limited, use that is present or attainable, and to adopt ambient quality-based standards, which then essentially constitute a nondegradation requirement for those parameters for those waters.