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from spoils springs at several discharge points where the spring water converges before flowing into the creek. The original permit documents the expectation that concentrations of total dissolved solids (TDS) would rise in Trout Creek during and immediately after mining. As is the case for virtually all Colorado coal mines, the rock comprising the stratagraphic sequence containingthe coal deposits contain a lazge amount of highly soluble salts. The process of mining breaks this spoils rock into small fragments, exposing enormous amounts of surface azea from which these soluble salts aze easily leached. In such a situation, the most readily- available salts aze dissolved by infiltrating groundwater. As the most available salts are dissolved early in the life of the spoils aquifer, leaving less soluble material available to groundwater as the years pass, the highest concentrationsof dissolved salts aze produced in the first several yeazs, and the concentrationof TDS in groundwater diminishes as time passes. The data for the Edna mine from the period directly before mining activity to the present show this to be the case. Baseline data are available from several yeazs before mining was initiated. Trout Creek upstream of the mine contained a range of total dissolved solids (TDS) of 62 to 260 mg/1, varying seasonally;downstreamofthe mine, TDS concentrationsranged from 126 to 565 mg/I depending on the season. The increase is attributable to inflow affected by pre-SMCRA mining activity on the site. In the period since SMCRA- governedmining was initiated, the measured concentration of TDS upstream of the mine has ranged from 14 to 250 mg/I; downstream measurements have ranged from100 to 1440 mg/l. That is, low concentrations aze similar to pre-mining levels, but higher concentrations occur than were measured before the recent mining activity. Analysis of the data shows that the highest salt concentrations occur eazly in the spring and later in the summer, when inflow from snowmelt and precipitationaze at their lowest. A seasonal variation is the function of several factors, but the primary factor is dilution -the proportion of water in Trout Creek that comes from the mine area. Mass balance has been calculated to analyze stream flow in Trout Creek above the mine and compaze it to the flow below the mine. The analysis shows that during spring runoff, 15% of the flow downstream of the mine has entered Trout Creek below the upstream monitoring site. In the summer, when irrigation withdrawals reduce the flow on upper Trout Creek even more, 28% of the flow at the downstream station originates from the stretch along the mine. Further calculations show that most of that inflow is from groundwater, not surface water. All the known influent surface water on the mine side of the creek is measured at the existing CDPS monitoring points. In the spring, this monitored flow is approximately 2.6% of the flow measured in Trout Creek downstream of the mine, leaving 12.4% to Edna Mine ? 1 Prnnit Rmevnl No. 3