1995-05-16_REVISION - M1977378 (2)

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05/16/1995 13: 02 303-782-0390 CDH WQCD WQCC PAGE 03 Mr.William Goodhard May 12, 1995 Page No. 2 Definition of Ambient The Division has evaluated your comments coocectung the definition of ambient and we have thought about how to impose the 520 ug/1 in an efficient manner. The determination as to whether the water quality goal at A-72 is met should be based on using all of the paired streamflow and zinc concentrations data. At issue here is the need to determine the criteria to be used to evaluate whether the zinc concentration has increased,decreased,or not changed at A-72. The Division has established the 85th percentile Zinc concentration standard,based on a simple rank ordering of the data for stations A-72 and RPS-82,as 520 mg/l. The following is a concept on which we would like your thoughts. The existing concentration of zinc is a function of stream flow at A-72. The relationship between stream flow an the coaceatratian of dissolved zinc shows that 520 ug/l zinc concentration,on average,corresponds to a discharge of 44 efs which approximates the annual low flow. The attached graph illustrates the variability in the zinc concaatration, especially at the low flow end of the curve. The flow-concentration"model",rather than a single number,allows use of the entire data set to evaluate change/no change in zinc concentration at A-72. For example,reduced zinc concentration would result in most or all of the data points falling below the existing line. Concentrations at a given flow above the line would indicate a lowering of water quality. This"model"assumes that the percent change in zinc concentration owing to BMP's and mine closure is uniform throughout the flow range of interest. The loading analysis done by the Non-Point Source Program for four synoptic events indicates that the load percentage from the various watersheds is based on a relationship between stream flow and concentration. It was proposed by SGC that weighing samples to the low flow period on the theory that past sampling was biased toward high flows. The flow-concentration model makes use of what ever data is obtained. If unusually high now or low flaw conditions are encountered during the evaluation period,the flow-c000eetration model should be neutral Moreover,the streamtlow at which 85th percentile zinc concentration is expected to occur should not change, therefore,the traditional approach-to setting an ambient standard can still be used. We can discuss this further at our meeting. VobMry Mitigation PrRoects The list of mitigation projects is good.We would like to have additional information on the expected loadings to be removed by the projects. The Division needs to have a better understanding of the information that will be gathered prior to any remedmon efforts. The Division will want to ensure that there are adequate reviews of the plans and there is a reasonable amount of information which supports the activities at the sites and that the likelihood for success is high. Specifically,detailed information as to where and to what extent the Koehler&oagfellow,Gold Prince,Eureka and South Fork of Cameut Credo dumps and mill tailings will be removed Both the Water Quality Control Division and the Division of Minerals and Geology roust be involved in the process of design and in the implementation and final inspection of the A and B projects.