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APPENDIX A Reference Water Quality A-1 Definition of "Reference Water Quality" and "Project Period" °`Reference water quality" is defined as the dissolved zinc (Zn) concentration and flow at reference point A-72, the US Geological Survey's stream flow gage no. 09359020, Animas River below Silverton, for the time period of September 5, 1991 to May 22, 1996. Measurements that were taken on the same day or on consecutive da}~s were averaged to reduce the relative importance of those measurements when using the data in statistical analysis. This set of 17 flow/zinc pairs is termed the "baseline set". Tlris set of data defined reference water quality against which later water quality will be compared. Both the raw data and the baseline data set are presented below in Table A-] . "Project period" is defined as the time period that begins with the closure of the valve at the American Tunnel internal portal plug and the diversion of Cement Creelc into the American Tum~el water treatment plant. The project period ends when there is final closure of the number two bulkhead in the American Tunnel. "Post project condition" is defined as the condition of water quality that is expected after the A- list Projects have been completed; mine pool equilibrium has been reached; American Tunnel, Terry Tunnel, the Mogul Mine and the Koehler mine have been plugged; and diversion and treahnent of Cement Creek have ceased. A-2 Relationship between flow and dissolved Zinc in the baseline set. The high-mountain headwaters portion of a river such as the Animas is subject to seasonal fluctuations in flow due to hydrologic response of the basin to climatic factors. In this basin, stream flow and concentrations of dissolved metals are inversely related. At low flows concentrations of metals are high, and at high flows concentrations are low, exhibiting a dilution effect from snowmelt. Figure A-1 presents the baseline Zinc concentrations and streamflow at A-72. When compazing varying concentrations of dissolved metals in surface waters, standazd practice is to adjust for the variation due to streamflow. This is an analytical technique used when pollutant concentrations vary as a function of streamflow, as shown in Figure A-1. If the data set consists of sanrples taken at a variety of streamflows, some or all of the concentration differences could be due to differences in streamflow and not to changes in upstream inputs. By computing a regression equation between concentration and streamflow, the effect of streamflow can be "subtracted out." Appendix A 232565