MOUNTAIN-BLOCK RECHARGE TO THE WESTERN DENVER BASIN, COLORADO

Recharge to the Mesozoic and Cenozoic clastic rock aquifers of the Denver Basin is poorly understood. According to existing studies, recharge occurs entirely by infiltration of precipitation near the margins of the basin where the aquifers crop out. However, considerable volumes of water from discrete geologic structures have been encountered during the drilling of water diversion tunnels in the Precambrian rocks of the Rocky Mountain Front Range immediately west of the Denver Basin. Little is known about the permeability structure of this mountain block. Although it is composed of crystalline rocks that generally have low permeability and storage capacity, discrete fault and/or fracture zones do occur and may be able to store and transmit sizeable volumes of ground water. The distribution of such zones and their ability to transmit ground water over considerable distances is unknown. Their presence in diversion tunnels, however, presents the possibility that ground water circulates deeply in the crystalline mountain block, and that subsurface flow from the mountain block to the Denver Basin aquifers (mountain-block recharge, or MBR) could be appreciable in some locations. Wells in a mountain-front drainage and in the western Denver Basin were sampled for noble gases, tritium, and major and trace element chemistry. Noble gas concentrations were measured in order to calculate recharge temperatures. Recharge temperatures from wells in the mountain-front drainage help define the elevation dependence of recharge temperature in the Front Range. Employing this elevation dependence, recharge temperatures from wells in the basin are being used to constrain the relative magnitude of MBR from the Front Range, and to determine whether it is from highly discrete or diffuse sources. Tritium data will be used to derive 3H/3He ages, which could confirm the presence of modern recharge in the aquifers. Major and trace element chemistry will be measured because the composition of the mountain block, with sites of significant metal-rich mineralization, is distinctly different from that of the basin aquifers. Therefore, MBR waters may have chemical signatures that are unique from water recharged in the basin.