GEOCHEMICAL MODELING OF DUST AS THE SOURCE OF SOLUTES IN A MEROKARST AQUIFER, KONZA LTER SITE, NORTHEASTERN KANSAS

Although the geochemistry of groundwater in the Permian limestones of the Flint Hills of Kansas is similar to that derived from dissolution of limestone, the weathering rates are too fast for the length of time the landscape has been subaerially exposed. The rate of dust deposition (600 kg hc^-1 yr^-1) in this tallgrass prairie region is similar to that of the annual chemical weathering rates calculated from 1992-2015 (<200 to ~1800 kg hc^-1 yr^-1), suggesting that dissolution of dust could be a source of solutes. I performed inverse modeling using PHREEQC to show that the 2014-2015 monthly to bimonthly stream water geochemistry, with the contribution from evapo-concentrated precipitation subtracted, could be derived by reacting with several simple phases. Anhydrite was used to account for SO₄ in excess of that contributed by precipitation. Ion exchange among Ca (removed from the water), and Mg, Na, and K (added to the water) accounts for small amounts of the solutes, and supports the dominance of Ca on the ion exchange sites as well as the decreasing ratios of Ca to Mg, Na, and K with depth in the soil. A chemical composition for dust was calculated from previous work on the geochemistry of dust in the region. The Al was removed from that composition as kaolinite, since dissolved Al is not detectable in the water samples. The “dust” accounts for most of the solutes, and phytoliths, with a composition containing K similar to that measured in barley, remove some of the Si from the water contributed by the “dust”. A small amount of calcite, with a composition equal to the bedrock composition measured by laser ablation-ICP-MS (containing 1.5 mol% Mg and 0.8 mol% Sr) is dissolved to account for the water chemistry. The molar ratio of “dust” to bedrock calcite dissolved ranged from 1.4 to 4.6 over the time series. Although not necessarily a unique result, this modeling demonstrates that it is possible for fine-grained, high-surface-area dust, along with other minerals documented as being present in the soil and ion exchange processes, to explain the stream water chemistry with less reliance on dissolution of the limestone aquifers.