Increasing shallow groundwater CO sub(2) and limestone weathering, Konza Prairie, USA

• Published in: Geochimica et cosmochimica acta Vol. 72; no. 23; pp. 5581 - 5599
• Main Authors: Macpherson, G L, Roberts, JA, Blair, J M, Townsend, MA, Fowle, DA, Beisner, K R
• Format: Journal Article
• Language: English
• Published: 01-12-2008
• Subjects: USA, Kansas, Konza Prairie; USA
• ISSN: 0016-7037
• DOI: 10.1016/j.gca.2008.09.004

In a mid-continental North American grassland, solute concentrations in shallow, limestone-hosted groundwater and adjacent surface water cycle annually and have increased steadily over the 15-year study period, 1991- 2005, inclusive. Modeled groundwater CO sub(2), verified by measurements of recent samples, increased from 10 super(-2.05) atm to 10 super(-1.94) atm, about a 20% increase, from 1991 to 2005. The measured groundwater alkalinity and alkaline-earth element concentrations also increased over that time period. We propose that carbonate minerals dissolve in response to lowered pH that occurs during an annual carbonate-mineral saturation cycle. The cycle starts with low saturation during late summer and autumn when dissolved CO sub(2) is high. As dissolved CO sub(2) decreases in the spring and early summer, carbonates become oversaturated, but oversaturation does not exceed the threshold for precipitation. We propose that groundwater is a CO sub(2) sink through weathering of limestone: soil-generated CO sub(2) is transformed to alkalinity through dissolution of calcite or dolomite. The annual cycle and long-term increase in shallow groundwater CO sub(2) is similar to, but greater than, atmospheric CO sub(2).