CO2 OF SOIL AND GROUNDWATER IN RELATION TO THE STORM RECHARGE, KONZA PRAIRIE LTER SITE, NE KANSAS

The effect of large recharge events on CO₂ transport from the soil into an aquifer was studied at the N4D watershed in the Konza Prairie LTER site near Manhattan, KS during the summer of 2010. Our preliminary data consists of water from 2 wells completed in the Morrill Limestone aquifer, and from 2 soil water samplers, and soil gas from 3 gas samplers installed in the 150 cm-thick soil zone collected in 5 sampling series, 2 of which were performed a few hours after a recharge event.

The amount of CO₂ in the soil gas from the uppermost A horizon was about 3.3 - 3.6%. In the B and C horizons, CO₂ concentration was higher: 6.9 - 7.2 % and 6.4 - 6.7 % respectively.

Alkalinity of the soil water from the A horizon was typically just 1.3 - 1.75 mmol/L, decreasing to 0.6 mmol/L following the rain. Water from the B horizon had higher alkalinity (5 - 6 mmol/L) without significant variations related to the storm events.

Alkalinity of the groundwater from the upper part of the aquifer was increasing through the growing season from 4.26 mmol/L up to 6.93 mmol/L in late July representing a build-up stage of the annual cycle, reaching the value of 9 mmol/L in the lower part of the aquifer. However, water, collected several hours after the precipitation events, was not following this trend: alkalinity dropped by 0.8-1.8 mmol/L below the expected value.

Based on the preliminary results it is possible to identify basic trends in CO₂ distribution and shallow groundwater chemistry under alteration of intense storm events. The A horizon is in active contact with the atmosphere: low CO₂ concentration is the result of efflux of soil-generated CO₂ partially substituted by air; soil water chemistry shows a clear and fast response to precipitation events. In the B horizon, intensive soil respiration but lower CO₂ efflux results in highest CO₂ concentration. Part of this CO₂ dissolves in soil water, increasing the alkalinity in comparison with the A horizon. The relatively stable Cl/alkalinity ratio suggests a predominant role of dilution by the rainwater rather than change in the rate of CO₂ production.

An annual carbonate-mineral saturation cycle that causes weathering of limestone is the main source of groundwater DIC. The detailed snapshot of δ¹³C of CO₂ and DIC will allow estimation of DIC enrichment caused by soil CO₂ dissolved in storm recharge water flushing the soil zone.