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establishing water quality standards may be more restrictive than EPA detection limits for <br /> effluent monitoring. <br /> 10. The dissolved oxygen standard is intended to apply to the epilimnion and metalimnion <br /> strata of lakes and reservoirs. Respiration by aerobic micro-organisms, as organic matter <br /> is consumed, is the primary cause of a natural decrease in dissolved oxygen and <br /> anaerobic conditions in the hypolimnion. Therefore, this stratum is exempt from the <br /> dissolved oxygen standard. <br /> 11. Where numeric standards are established based on historic instream water quality data at <br /> the level of x+ s, it is recognized by the Commission that measured instream parameter <br /> levels might exceed the standard approximately 15 percent of the time. <br /> 12. It is the Commission's intention that the Division implement and enforce all water quality <br /> standards consistent with the manner in which they have been established. <br /> 13. Hardness/Alkalinity <br /> Where hardness and alkalinity numbers differed, the Commission elected to use alkalinity <br /> as the controlling parameter, in order to be consistent with other river basins and because <br /> testimony from the Division staff indicated that in most cases alkalinity has a greater <br /> effect on toxic form of metals than does hardness. <br /> VI. Water Ouality Standards for Unionized Ammonia <br /> The Commission retains the use of unionized ammonia as a parameter rather than total ammonia <br /> because unionized ammonia is the toxic portion. Furthermore,the relationship of total ammonia <br /> as a function of temperature and pH is recognized. <br /> On some Class 2 Warm Water Aquatic Life streams containing similar aquatic life communities <br /> to those found in the plains streams of the South Platte & Arkansas Basins, .1 mg/I ammonia was <br /> selected as being appropriate to protect such aquatic life. <br /> The Commission has relaxed unionized ammonia standards to .1 mg/l or greater on several <br /> streams for the following reasons: <br /> 1. limited nature of the aquatic life present; <br /> 2. limited recreational value of species present; <br /> 3. habitat limitations, primarily flow and streambed characteristics,that impose significant <br /> limitations on the nature of aquatic life, even if ammonia reductions were attained; <br /> 4. rapid dissipation of ammonia in streams, reducing the impact of such discharges <br /> downstream; and <br /> 5. economic costs of ammonia removal, especially where such costs would fall primarily on <br /> publicly-owned treatment works, and while the availability of construction grant funds is <br /> questionable. <br /> 6. Biosurveys with support from a bioassay conducted on fathead minnows performed in the <br /> Cache la Poudre River show that a .1 mg/1 standard is appropriate to protect existing biota <br /> in that stream. The results of these studies may be reasonably extrapolated to similar <br /> streams; i.e.,those streams that demonstrate similar chemical, physical, and biological <br /> characteristics. <br /> Not all warmwater streams are comparable in terms of flow and habitat,and types and numbers <br /> of species of aquatic life. Therefore, some variations in an appropriate ammonia standard must <br /> be tolerated, with the objective of protecting existing aquatic life. The Commission found this <br /> approach preferable to totally removing the aquatic life classification from impacted or marginal <br /> aquatic life streams. <br /> VII. Water Ouality Standards for Uranium <br /> 13 <br />