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Information to Support Denison BLM Environmental Assessment Remand <br />Denison Sunday Mines Group <br />and sampling is needed to verify the groundwater impact of continued <br />mining and oxidation that the current sampling appears to indicate at this <br />minesite. <br />Discussion CDM concurs that a cone of depression is present in the area of the <br />underground mines as a result of the following: (1) previous dewatering <br />prior to approximately 1990, (2) evaporation in the mine ventilation <br />circuit and discharge of this water vapor through the mine ventilation <br />exhaust, (4) negligible recharge to the Saltwash aquifer in the mine area <br />(because it is overlain by hundreds of feet of bentonitic shale), and (5) <br />very slow regional groundwater flow rates. These data suggest that the <br />water table in the underground mine has not fully recovered since <br />dewatering ceased in approximately 1990. These site conditions support <br />that it is unlikely that regional groundwater quality has changed <br />significantly since the existing Plans of Operations were approved by <br />BLM. <br />The referenced ventilation shafts referenced by BLM have not been <br />operating since approximately 1990 when mineral production ceased in <br />that portion of the mine. These ventilation shafts were connected to the <br />underground workings when they were in operation prior to <br />approximately 1990, but the water level data referenced by BLM shows <br />that these shafts are not currently connected to the underground <br />workings. If these shafts were connected to the underground workings at <br />this time, a pipe flow system would be present within the underground <br />workings and the water levels within the underground workings and all <br />ventilation shafts would be identical (Wolkersdorfer 2004). The <br />observation that the water levels in the ventilation shafts are different <br />indicates that these shafts are not connected to the underground <br />workings. The ventilation shafts are not lined, and it is thought that <br />portions of the shafts have collapsed since approximately 1990 resulting <br />in plugging at the base of the shafts. The specific location and cause of the <br />lack of hydraulic connection between the shafts and the underground <br />workings is not known, but the water level data provide conclusive <br />information that shows that the shafts are not interconnected with the <br />underground workings. BLMs interpretation that the water chemistry of <br />samples collected from standing water within the two ventilation shafts <br />represents a sequential maturation of mine water quality as inflowing <br />groundwater migrates through the mine is not supported by the water <br />level data (ventilation shafts are not hydraulically connected with the <br />mine pool). Rather, this apparent trend is coincidental and is likely <br />related to water/ rock interactions occurring within the ventilation shafts <br />and /or natural groundwater conditions. <br />Ventilation of underground workings is absolutely necessary to support <br />mining activities and it is an inherent part of the mineral development <br />Final Sunday Mines BLM Remand Letter.doc 8 <br />