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Schwartzwalder Mine -Hydrologic Evaluation of Mine Closure and Reclamation 43 5.3.4 Tritium Dating of Mine Water Tritium occurs naturally in the atmosphere, but by far, the most important source of tritium in meteoric water is from nuclear testing between 1952 and 1969. Pre-bomb tritium levels in rainwater averaged about 10 TU (Dreyer, 1988). During the 1960s, tritium levels in the atmosphere spiked above 103 TU before decaying to the current level of about 10 to 15 TU. Tritium has a half life of 12.3 years and is not formed in the subsurface. Water that entered the groundwater system at any time during the last 50 years has approximately the same tritium concentration as meteoric water today because of equivalent rates of radioactive decay in the atmosphere and subsurface. Meteoric water that entered the groundwater system prior to 1952, had an initial tritium concentration of about 10 TU which will have decayed to less than 0.6 TU today. Groundwater that infiltrated after 1952 have tritium concentrations of about 10 to 15 TU. Groundwater with tritium concentrations between about 0.6 and 10 TU indicate mixtures of pre 1952 and post 1952 water, or result from subsurface ionic fractionation which is rare. Results of the tritium analysis returned a value of 12.4 for Ralston Creek. This value is typical of precipitation, and suggests that the water in Ralston Creek was recent precipitation and/or groundwater baseflow with a residence time of less than 52 years. Water samples from the 700 and 1,700 levels had tritium concentrations of 14.5 and 14.3 respectively. These values indicate that the water infiltrated from meteoric water less than 52 years ago. Tritium concentrations for water samples from the 1,900 level varied from 3.1 to 6.0 TU and indicate a mixture of old (pre 1952) and new (post 1952) water. Based on these results it is reasonable to conclude that the pre- mining groundwater at the 1,900 level was more than 52 years old and that vertical flow induced by pumping from the mine induced younger water to be drawn down to the 1,700 Level and below. During operation, the mine would have acted as a 2,000 foot deep well that increased the velocity of flow toward the mine and reduced the transit time of infiltrating meteoric water. Under the pre-mining condition, however, a much longer transit time would have been required. In short, the tritium data do not definitively indicate the presence or lack of a hydraulic connection between Ralston Creek and the mine. Under dewatered conditions, the transit of meteoric derived water to workings above the 1,700 Level is documented to be less than 52 years. Transit of water to the deeper portions of the mine is more lengthy with a substantial component of the water infiltrating at the surface before 1952. 5.3.5 Summary A strong direct hydraulic connection between Ralston Creek and the Schwartzwalder Mine is not indicated by the available evidence. However, a weak hydraulic connection may exist, so that while some water may have flowed from the creek into the mine during dewatering, flows from the creek to the mine were small and controlled by the inherent low permeability of the rock mass (2.8x10-7 cm/sec average bulk hydraulic conductivity). The low permeability of the bedrock limited flow from the creek into the mine, and dewatering the mine did not drain Ralston Creek. 6. SURFACE WATER QUALITY The water quality in Ralston Creek is affected by natural runoff and groundwater inflow, upgradient abandoned mines, the fill materials in the valley at the Schwartzwalder mine, and previously by discharge from the Schwartzwalder water treatment plant. The magnitude of loading from these sources was evaluated by studies performed during 1998 and 1999, and by previous investigations. 4109B.071116 Whetstone Associates •