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-26- 2) The Division assumed that the water in the abandoned Caldirola Mine workings would be representative of water quality from mined areas of the Twin Pines No. 2 t4ine (the highest recorded TDS in the adjacent area) and data from the Dorchester No. 1 t4ine sump are assumed to be representative of water quality from the mined areas of the Dorchester No.l Mine. 3) The Division assumed that data from Twin Pines No. 2 monitoring well, TP-MW-1, and Dorchester's monitoring well, MW-10, are representative of dater quality in the undisturbed portions of the Vennejo Formation. s Total Dissooved Solid. Concentration Unmined portions of the Vernejo Formation - 829 mg/1 t+lined area within Twin Pines permit area - 1,019 mg/1 Mined area within Dorchester permit area - 1,660 mg/1 TDS in aquifer = .004 (1,019) mg/1) + .03 (1660)mg/1) + .966 (829 mg/1) = 909 mg/1 Net increase in TDS = 909 mg/1 - 829 mg/1 = 80 mg/1 Mixing of the ground water from the mined areas of the Twin Pines No. 2 Mine and the Dorchester No. 1 Mine with ground water in the Vermejo Formation aquifer throughout the remaining area of the ground water basin would result in an increase in dissolved solids of approximately 80 mg/1. (See calculations above). This water quality degradation may have localized effects to water users near the mine site. Over time, the concentration of total dissolved solids will be diluted as the pollution plume spreads. This impact to the total ground water quality in the basin is considered to be negligible. B. Impacts on Surface Waters The Division, based on review of the permit applications for the mining operations considered in this assessment, has identified several possible effects of mining on surface water quality and quantity in the Canon City Coal Field. Surface water quality could be affected by: 1) Erosion and subsequent increased sediment loads to streams; 2) Discharge of mine water to receiviny streams; 3) Seepage water through waste rock or coal refuse material; or 4) Depletion of stream flow via subsidence-induced infiltrations. The potential impact to surface waters is expected to be negligible as discussed below.