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Milos M. Novotny and William E. Sanford estimates of the ambient temperature at the time of recharge. The apparent residence times were determined using 14C, and recharge temperatures were estimated using the concentrations of atmospheric noble gases (Ne, Ar, and Kr). Background The Denver Basin is centered in northeast Colorado, east of the Front Range of the Rocky Mountains. It is a foreland basin containing Upper Pennsylvanian to mid -Ter- tiary sedimentary rocks, underlain by Precambrian igneous and metamorphic rocks. The structural basin is asymmetric with steeply dipping to overturned and faulted beds on the western margin where it crops out against the Front Range. The names of the formations comprising the bedrock aquifers have been adapted to represent distinct aquifers within the Denver Basin (see Fig. 2 in Raynolds, 2004, this issue). The formations of the principal bedrock aquifer sys- tem of the Denver Basin are the Fox Hills Sandstone, the Laramie, Arapahoe and Denver formations, and the Daw- son Arkose. These translate into the Laramie -Fox Hills aquifer, the Arapahoe aquifer, the Denver aquifer and the Dawson aquifer. The bedrock aquifers of the Denver Basin are considered to be regionally continuous and were mapped using electric log signatures and outcrop locations around the Basin (Robson, 1987). Aquifer properties and model parameters were determined from formation sam- ples and aquifer tests. Aquifers may cut across geologic boundaries to include parts of other formations (e.g. the Laramie -Fox Hills aquifer; cf Crifasi, 1992). The Front Range urban corridor is found in the Great Plains region of northeastern Colorado, just east of the Front Range of the Rocky Mountains. Land surface eleva- tions range from about 1400 to 2100 m above MSL. During the last two million years, the climate of the area has varied dramatically, including periods of glaciation. The most recent glacial period peaked about 25 ka and ended about 12 ka. Since that time, the regional climate has been gener- ally drier and /or warmer (Stute et al., 1992; Muhs et al., 1999). Today, the area has a semi -arid climate with highly variable temperatures at any given location. Modern mean annual temperature near the southern outcrop of the bedrock aquifers is 9SC (National Oceanic and Atmos- pheric Administration, 2002) and modern mean annual pre- cipitation ranges from 30 -45 cm /yr (Hansen et al., 1978). Groundwater Flow and Environmental Tracers For most regional -scale groundwater models, such as those of the Denver Basin, groundwater flux through the aquifers is not measured but is calculated in a water bud- The Rocky Mountain Association of Geologists 162 get. Uncertainty in model parameters (e.g., hydraulic con- ductivity, porosity, specific yield) (Woodard et al., 2002) can greatly affect the calculated flux. However, the flux can be estimated independently of the model if groundwater resi- dence times are known. Residence time is estimated from groundwater samples using environmental tracers, naturally - occurring substances in groundwater that vary in concentra- tion with time or process. Residence times may be applied to constrain parameters that affect the groundwater flux, or included as an additional model parameter. Other environ- mental tracers are used to characterize groundwater flux in a broad, qualitative sense by defining recharge areas or indi- cating the approximate period of recharge. Carbon -14 Residence Time Carbon -14 is the most routinely applied dating tool for pre- modern (pre -1950) groundwater and has been used in numerous studies to characterize flow in regional aquifers (e.g., Phillips et al., 1989; Clark et al., 1998). Because the subsurface residence time is a function of both the aquifer properties and recharge rates, residence times have been used variously to constrain these values. Modern ground- water systems have been strongly influenced by temporal changes in recharge (Phillips, 1995). Where the distribution of aquifer properties is well constrained, groundwater models calibrated using residence times have demon- strated significantly higher past recharge rates in some southwestern U.S. basins (e.g., Zhu et al., 1998; Sanford, 2002). If evidence suggests that a change in recharge rate does not significantly affect the distribution of residence times, these may be used to constrain aquifer properties. Phillips et al. (1989) used hydraulic heads and residence times estimated from i4C activity to estimate the distribu- tion of transmissivity in the lower Tertiary aquifers of the San Juan Basin, New Mexico. Residence times are calculated from the percent modern carbon (pmQ of groundwater samples using the standard isotopic decay equation for 14C and, usually, an adjustment for the dilution of 14C activity. The dilution occurs as a result of interaction of groundwater with 14C -free carbon sources, such as limestone, and causes the groundwater to appear `old.' Several methods been have proposed to quantify the magnitude of the dilution. However, theoreti- cal uncertainty exists because of parameter assumptions, especially in cases where parameters cannot be easily mea- sured, such as the environmental conditions during groundwater recharge (Clark and Fritz, 1997). Because of the uncertainty in 14C dilution, these residence times are generally viewed as maximum, and another environmental tracer can be used in tandem to constrain the uncertainty in 14C residence time.