TRENDS IN WATER CHEMISTRY OF SHALLOW LIMESTONE AQUIFERS: KONZA PRAIRIE LTER SITE, NORTHEASTERN KANSAS USA

Fifteen years of monthly groundwater chemistry data from a tallgrass prairie in central North America shows that shallow carbonate aquifers are responding to recent climate change. In northeastern Kansas, the Konza Prairie Long-Term Ecological Research Site comprises 35 km² of unplowed original tallgrass prairie that encompasses the entire watershed of Kings Creek, an ephemeral stream. On the south branch of Kings Creek, 35 wells are used to monitor water level and chemistry in the lower part of a 1.2 km² watershed. Alternating nearly-horizontal Permian-aged thin limestones and thicker shales create a stepped topography and a sandwich-type system of aquifers. Over the monitoring period, dissolved Mg in groundwater has increased by 20-30% in the shallow and deeper parts, respectively, of a relatively high permeability aquifer, and decreased by about 10% in an intermediate-depth lower permeability aquifer. These changes in water chemistry are attributed to changes in chemical weathering phases and/or rates. The stream receiving discharge from these aquifers has similar chemistry to the higher-permeability aquifer, implying that the rapid response of chemical weathering to climate is manifested in stream water chemistry that in turn could alter residence times of dissolved species in oceans.