In order to achieve a more accurate input value for groundwater age determining models, δ13CVPDB values for soil gas were evaluated at 50 cm depths in locations throughout Utah in order to define correlations between δ13C and environmental parameters. 16 sites were chosen that exploited large changes in elevation and latitude which provided variations in climate, precipitation, plant community, etc. Gas samples were collected over 1-1½ years, and soil samples were collected at depth during installations. Field and laboratory studies were also used to evaluate CO2 and δ13C change with depth. It was discovered that in mountainous recharge areas, the mean δ13C value was −21.8 ± 2.8‰ (2σ). Use of this value reduced the uncertainty in groundwater age models by close to half relative to the current assumed range of -13‰ to -28‰. One arid and one semi-arid site were located at Pilot Valley (mean -11.0‰ ± 2.0, 2σ) and the mouth of Cedar Canyon (mean -17.5‰ ± 0.8, 2σ). The high values at these locations may be due to low root respiration and low microbial activity rates caused by high temperatures, low precipitation, and low vegetation density. Correlations among environmental parameters and δ13C values were discovered with ANOVA, but the differences were small enough that for practical purposes they are insignificant.