ISOTOPIC EVALUATION OF SOIL GAS IN UTAH FOR A MORE ACCURATE INPUT VARIABLE IN GROUNDWATER AGE DETERMINING MODELS

¹⁴C groundwater model ages often assume an input value for d¹³C_PDB of soil gas that ranges from about -15 to -25‰.  A sensitivity analysis using a mixing model (Pearson and Hanshaw, 1970) shows that calculated ages for a hypothetical water with 25 percent modern carbon (pmc) can vary by >4000 years when other inputs are held constant at reasonable values. Thus, to improve inputs of soil gas d¹³C values for groundwater model ages, d¹³C values and CO₂ concentrations in soil gas were measured synoptically at 17 sites in Utah to permit testing the influence of a large variety of environmental variables on this parameter.

For 15 of 17 sites, d¹³C values range between -19 and -24‰ with a narrow overall mean of -21.7 ± 1.4‰.  Near the mouth of Cedar Canyon (Cedar City), d¹³C values vary from -17 and -18.5‰, reflective of the partial influence from CAMS or C4 plants, atmospheric exchange, or both.  d¹³C values at low elevations in the west desert vary from -9 to -12‰, with low soil CO₂ concentrations between 220 and 1260 ppm, suggesting low biological activity and strong atmospheric exchange.   Thus, except for low biological activity in arid settings, plant community is the only statistically significant parameter controlling soil atmospheric d¹³C values. 

In this study, three groupings of associated plant species were found in the common, mountainous recharge areas of Utah.  Although statistically distinct, the absolute differences among the groups are small and introduce relatively little uncertainty into the calculation of groundwater age.  No strong correlations exist between d¹³C values and environmental parameters other than the linked relationship of plant community to elevation, latitude, etc.  Rather, it appears that as long as the plant community in a given watershed represents a single metabolic type (C3, C4, or CAMS), a single d¹³C value can be used to represent soil gas over a broad range of elevation, latitude, and season.