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CDPHE Groundwater Standard Classification <br />The groundwater in the Revenue Mine area cannot be classified for domestic use. The major reasoning for this <br />conclusion is due to the lack of current and potential future domestic use. It is extremely unlikely that this water <br />will be used for domestic use in the future as the City of Ouray has water rights that greatly exceed their projected <br />future needs. Additionally, the quality of the groundwater on the Revenue Mine site does not meet the CDPHE <br />Human Health Standards for domestic use for dissolved antimony and manganese. <br />The groundwater in the Revenue Mine area can be classified for agricultural use. The major reasoning for this <br />conclusion is due to the current and potential future agricultural use. It is likely that this water could be used for <br />agricultural use in the future as surface water in the area already being used for this purpose. Additionally, the <br />quality of the groundwater on the Revenue Mine site meets the CDPHE standards for agricultural use. Because <br />the groundwater can be classified as agricultural use, an examination of whether the water can be classified as <br />"potentially usable quality" is unnecessary. <br />The groundwater standards for agricultural use are set - numeric for all parameters except for total dissolved solids <br />(TDS). The standard for this parameter varies based on the background levels of TDS present in the water. Table <br />4, taken from in CDPHE Regulation Number 41 (shown below) outlines the different standards to be set dependent <br />on background levels. <br />Table 4: TDS Water Quality Standards [2] <br />Background TDS Value (m g/1) <br />Maximum Allowable TDS Concentrations <br />0 -500 <br />400 mg /I or 1.25 times the background level, <br />whichever is least restrictive <br />501 -10,000 <br />1.25 times the background value <br />10,001 or greater <br />No limit <br />Explanation of Methodology <br />To determine the DRMS standard, a list of parameters of concern was compiled based on the dissolved parameters <br />included in the CDPHE in- stream standards, the parameters included in the CDPHE discharge permit, as well as <br />the standards for ground water. To determine the proposed DRMS standard, the representative baseline <br />groundwater value (shown as Max of Wells + 25 %) was first compared to the In- Stream Water Quality Standard <br />for each parameter. For parameters in which the representative groundwater concentration exceeded the In- <br />Stream Water Quality Standard, the representative baseline groundwater value from the well samples was set as <br />the DRMS standard. For parameters in which the representative groundwater concentration was less than the In- <br />Stream Water Quality Standard, the In- Stream Water Quality Standard was set as the DRMS standard. This was <br />done to insure no increased impact occurred on Sneffels Creek and also the ground water due to mining activity. <br />In some cases, there was no value for the In- Stream Water Quality Standard for a parameter. In this case, the <br />representative baseline groundwater value was compared with the CDPHE agricultural standard for groundwater. <br />The previous section describes how this water is best classified as agricultural use. This was the most conservative <br />estimate of a standard for these parameters to insure no increased impact on the potential current and /or future <br />use of this water. If any parameter was measured in the wells but did not have a CDPHE standard for In- Stream or <br />agricultural ground - water, the DRMS standard for that specific parameter was set as "Report". This is to comply <br />with the agreement to monitor the groundwater wells for all of the parameters included in the underground mine <br />water samples. <br />14 <br />