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Milos M. Novotny and William E. Sanford Table 1. Location and physical characteristics of wells sampled; data in meters above mean sea level (MSL). Table 2. Land surface elevation of southern recharge zones (meters above MSL) used in model of atmospheric noble gases. Standard Coefficient Aquifer Mean Maximum Minimum Deviation of Variation Denver 2,084 2,250 1,861 112 0.05 Arapahoe 1,994 2,100 1,814 81 0.04 Model of Recharge Temperature Aeschbach -Hertig et al. (1999) have created a model that takes into account the effects of temperature, eleva- tion, salinity, and excess -air fractionation on atmospheric noble gas concentrations. Measured values of Ne, Ar, Kr, and Xe are used in the model to minimize an objective function, essentially producing a statistical measure of the "goodness -of -fit" of the data to the model. Novotny (2004) provides a more detailed discussion of the application of this model in the Denver Basin. Salinity of recharging water often cannot be measured and is assumed equal to zero; other researchers using simi- lar methodology have assumed that this parameter is negli- gible in waters with low salinity (e.g., Clark et al., 1998). The assumption appears valid in this study area where modern recharging water is closely tied to precipitation and no significant sources of salinity exist. Concentrations of two of the measured inert gases (helium and nitrogen) were not used because it is suspected that there are signifi- cant sources or modifications of these in the subsurface. The mean elevations of the Arapahoe and Denver aquifer recharge areas were estimated from digital elevation maps to be approximately 2,000 and 2,075 in above MSL, respec- tively (Table 2). With three known noble gas concentra- tions (Ne, Ar, Kr) and two unknown parameters (recharge The Rocky Mountain Association of Geologists 164 temperature and excess air), the model equations are over - determined and may be inverted. A statistical fit of the data to the model is given by the objective function, x2. For the equation with one degree of freedom, and having a x2 value of 3.84, the probability that the unknown parameters deviate from their true values because of errors in gas con- centration analysis is less than 5 %. RESULTS AND DISCUSSION The results of the noble gas and 14C analyses are pre- sented in Table 3• As expected, the majority of the gas content is dissolved nitrogen. The 14C activity is highest in the well located nearest the recharge areas and is lowest in samples from the middle of the Basin. Residence Times Applying the assumptions and techniques outlined above, the activity of 14C was used to determine the apparent residence time of the waters at each location (Table 3). The shortest residence time of 8,000 years was at Donala (DON -04) in the Arapahoe aquifer, the location nearest the outcrops and the highest potentiometric level. Screen Interval Surface Top Bottom CO -DWR PLSS Location Datum Depth Depth Sample Aquifer Permit T.R.S.Q.Q [m] [m] [m] DON -04 Arapahoe 16141 -F 11S.66W.31.NW.SW 2,103 243 359 PIN -02 Arapahoe 51782 -F 07S.65W.18.SE.NW 1,948 538 657 PARK -01 Arapahoe 50563 -F 06S.66W.15.NE.SW 1,787 396 518 AUR -08 Arapahoe 30326 -F 05S.65W.21.SE.NE 1,829 378 512 PARK -03 Denver 50562 -F 06S.66W.15.NE.NW 1,787 235 372 AUR -07 Denver 30323 -F 05S.65W.21.SW.NE 1,829 121 351 Table 2. Land surface elevation of southern recharge zones (meters above MSL) used in model of atmospheric noble gases. Standard Coefficient Aquifer Mean Maximum Minimum Deviation of Variation Denver 2,084 2,250 1,861 112 0.05 Arapahoe 1,994 2,100 1,814 81 0.04 Model of Recharge Temperature Aeschbach -Hertig et al. (1999) have created a model that takes into account the effects of temperature, eleva- tion, salinity, and excess -air fractionation on atmospheric noble gas concentrations. Measured values of Ne, Ar, Kr, and Xe are used in the model to minimize an objective function, essentially producing a statistical measure of the "goodness -of -fit" of the data to the model. Novotny (2004) provides a more detailed discussion of the application of this model in the Denver Basin. Salinity of recharging water often cannot be measured and is assumed equal to zero; other researchers using simi- lar methodology have assumed that this parameter is negli- gible in waters with low salinity (e.g., Clark et al., 1998). The assumption appears valid in this study area where modern recharging water is closely tied to precipitation and no significant sources of salinity exist. Concentrations of two of the measured inert gases (helium and nitrogen) were not used because it is suspected that there are signifi- cant sources or modifications of these in the subsurface. The mean elevations of the Arapahoe and Denver aquifer recharge areas were estimated from digital elevation maps to be approximately 2,000 and 2,075 in above MSL, respec- tively (Table 2). With three known noble gas concentra- tions (Ne, Ar, Kr) and two unknown parameters (recharge The Rocky Mountain Association of Geologists 164 temperature and excess air), the model equations are over - determined and may be inverted. A statistical fit of the data to the model is given by the objective function, x2. For the equation with one degree of freedom, and having a x2 value of 3.84, the probability that the unknown parameters deviate from their true values because of errors in gas con- centration analysis is less than 5 %. RESULTS AND DISCUSSION The results of the noble gas and 14C analyses are pre- sented in Table 3• As expected, the majority of the gas content is dissolved nitrogen. The 14C activity is highest in the well located nearest the recharge areas and is lowest in samples from the middle of the Basin. Residence Times Applying the assumptions and techniques outlined above, the activity of 14C was used to determine the apparent residence time of the waters at each location (Table 3). The shortest residence time of 8,000 years was at Donala (DON -04) in the Arapahoe aquifer, the location nearest the outcrops and the highest potentiometric level.