PRELIMINARY EVALUATION OF δ13C AND CO2 CONCENTRATION IN SOIL GAS IN UTAH BASED ON CONSTRAINING ENVIRONMENTAL VARIABLES FOR A MORE ACCURATE AND PRECISE INPUT PARAMETER FOR GROUNDWATER MODEL AGES

Groundwater model ages typically use an assumed value for δ¹³C_PDB of soil gas and may range from about -15 to -25‰. A sensitivity study using the Pearson model (Pearson and Hanshaw, 1970) shows that apparent ages can range over 4500 years using assumed activities of 13.6 ppm for the sample, 24.52 ppm for the initial activity, and a δ¹³C and activity of dissolving carbonate minerals of -18.2‰ and 0 ppm, respectively.

In order to obtain more accurate and precise input for groundwater model ages, δ¹³C values in soil gas are being measured in 17 locations throughout Utah, which will permit testing a large variety of environmental variables, including plant community, elevation, latitude, lithology on which the soil has developed, season, and timing of associated recharge events. We are producing a database of associated δ¹³C values and CO₂ soil atmosphere concentrations. CO₂ concentrations and δ¹³C values are also being measured as a function of depth in uniform soil in both the field and laboratory to evaluate whether current isotopic and concentration gradient assumptions are valid.

Preliminary results show that, with two exceptions, δ¹³C values range throughout the state between -19 and -24‰ with an overall average of -21.5±0.1‰ and a mean standard deviation of 1.3‰.

CO₂ soil concentrations are at least an order of magnitude greater than atmosphere, but no seasonal correlations have been observed. As exceptions, Cedar Canyon has δ¹³C values between -16.9 and -17.9‰ reflective of succulent plants found in the area. δ¹³C values in the west desert vary from -9 to -12‰, with low soil CO₂ concentrations between 400 and 1260 ppm, suggesting low biological activity and strong atmospheric exchange. Preliminary laboratory results for the CO₂ concentration gradient with depth diverge from predicted diffusive profiles, exhibiting a distinct spike at ~35cm depth.

Except for plant community and low biological activity in extremely arid settings, there are no strong correlations between δ¹³C values and environmental parameters other than the indirect relationship of plant community to elevation, latitude, etc. Rather, it appears that as long as the plant community represents a single metabolic type (C3, C4, or CAMS), and single value can be used over a broad range of elevation, latitude, and season as long as typical values are characterized.