THE EFFECT OF SOIL CO2 ON DISSOLVED INORGANIC CARBON IN SHALLOW GROUNDWATER, KONZA PRAIRIE, NE KANSAS

Sources and seasonal trends of dissolved inorganic carbon (DIC) were studied in a shallow limestone aquifer in northeastern Kansas. From summer 2010 to spring 2011, the concentration and stable carbon isotope composition of soil CO₂ and groundwater DIC were measured on a monthly basis at the N4D watershed in the Konza Prairie LTER site.

Soil CO₂ concentrations reached 7% in July to early August, when temperature was highest and moisture did not limit soil respiration, and dropped to less than 0.5% in March. δ¹³С of soil CO₂ varied from -12.3 to -17.2 ‰ PDB with a mean of -14.8‰, which is typical for CO₂ respired by warm-season grasses following the C₄ photosynthetic cycle. Groundwater DIC demonstrated a seasonal trend from 7.0 to 9.0 mmol/L with the highest values late in the growing season. Groundwater δ¹³С_DIC varied between -6.4 and -4.7 ‰ and was out-of-phase with DIC.

Carbon isotope enrichment of groundwater DIC relative to soil CO₂ is explained by isotopic fractionation between CO₂ and carbonate species in infiltrating water, and water-rock interaction. Calcite (average δ¹³С of 0 ‰) dissolution released additional HCO₃^- to the solution, contributing heavier carbon to DIC. Seasonal ranges of Ca²⁺ and δ¹³C in groundwater remained at nearly equilibrium conditions with calcite in a system open to CO₂, based on C soil-horizon conditions over the study period. The highest soil pCO₂ in summer months resulted in groundwater with the lightest isotopic composition; isotopic enrichment was less even though Ca²⁺ was highest from more active calcite dissolution. A lag time of 2-3 months between soil and groundwater CO₂ maximas reflect a travel time of recharge water through the low-permeable unsaturated zone. Several samples, collected in early July with DIC up to 2 mmol/L lower than expected from the seasonal trend, were related to extreme storm events during which high streamwater rapidly recharged the aquifer and diluted groundwater.

One year of monitoring showed that soil CO₂ and carbonate minerals are two main sources of shallow groundwater DIC at the Konza Prairie. The annual cycle of soil respiration controls seasonal variations of limestone weathering rates and the amount and isotopic composition of DIC. Long-term monitoring may show that increased respiration rates could result in higher carbon flux to groundwater.