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Commission has determined that they have uniform applicability here,their inclusion as <br /> basic standards for the region accomplishes this purpose. <br /> 5. In many cases,the numeric water quality standards are taken from the"Tables"appended <br /> to the Basic Regulations. These table values are used where actual ambient water quality <br /> data in a segment indicates that the existing quality is substantially equivalent to, or better <br /> than, the corresponding table values. This has been done because the table values are <br /> adequate to protect the classified uses. <br /> Consistent with the Basic Regulations,the Commission has not assumed that the table <br /> values have presumptive validity or applicability. This accounts for the extensive data in <br /> the record on ambient water quality. However, the Commission has found that the table <br /> values are generally sufficient to protect the use classifications. Therefore, they have <br /> been applied in the situations outlined in the preceeding paragraph as well as in those <br /> cases where there is insufficient data in the record to justify the establishment of different <br /> standards. The documentary evidence forming the basis for the table values is included <br /> in the record. <br /> 6. In many cases, instream ambient water quality provides the basis for the water quality <br /> standards (See 7 below). In those cases where the classified uses presently exist or have <br /> a reasonable potential to exist despite the fact that instream data reflects ambient <br /> conditions of lower water quality than the table values, instream values have been used. <br /> In these cases, the evidence indicates that instream values are adequate to protect the <br /> uses. In those cases where temporary modifications are appropriate, instream values are <br /> generally reflected in the temporary modification and table values are reflected in the <br /> corresponding water quality standard. (Goals are established for the appropriate <br /> classification affected by the parameter). <br /> Cases in which water quality standards reflect these instream values usually involve the <br /> metal parameters. On many stream segments elevated levels of metals are present due to <br /> natural or unknown causes, as well as mine seepage from inactive or abandoned mines. <br /> These sources are difficult to identify and impractical or impossible to control. The <br /> classified aquatic life uses may be impacted and/or may have adjusted to the condition. <br /> In either case, the water quality standards are deemed sufficient to protect the uses that <br /> are present. <br /> 7. In those cases where there was no data for a particular segment, or where the data <br /> consists of only a few samples for a limited range of parameters, "table values"were <br /> generally recommended. Data at the nearest downstream point was used to support this <br /> conclusion. In some cases, where the limited data indicated a problem existed, additional <br /> data was collected to expand the data base. Additionally, where there may not be <br /> existing data on present stream quality, the Commission anticipates that if necessary, <br /> additional data will be collected prior to an economic reasonableness hearing required by <br /> C.R.S. 1973, 25-8-204(3), as amended. <br /> 8. In most cases in establishing standards based on instream ambient water quality, a <br /> calculation is made based upon the mean(average) plus one standard deviation (x+ s) for <br /> all sampling points on a particular stream segment. Since a standard deviation is not <br /> added to the water quality standard for purposes of determining the compliance with the <br /> standard,this is a fair method as applied to discharges. <br /> Levels that were determined to be below the detectable limits of the sampling <br /> methodology employed were averaged in as zero rather that at the detectable limit. This <br /> 11 <br />