J.K. Böhlke S. C. Christenson P.B. McMahon 2004 <p><span>Water samples from short-screen monitoring wells installed along a 90-km transect in southwestern Kansas were analyzed for major ions, trace elements, isotopes (H, B, C, N, O, S, Sr), and dissolved gases (He, Ne, N</span>₂<span>, Ar, O</span>₂<span>, CH</span>₄<span>) to evaluate the geochemistry, radiocarbon ages, and paleorecharge conditions in the unconfined central High Plains aquifer. The primary reactions controlling water chemistry were dedolomitization, cation exchange, feldspar weathering, and O</span>₂<span> reduction and denitrification. Radiocarbon ages adjusted for C mass transfers ranged from &lt;2.6 ka (</span>¹⁴<span>C) B.P. near the water table to 12.8&nbsp;±&nbsp;0.9 ka (</span>¹⁴<span>C) B.P. at the base of the aquifer, indicating the unconfined central High Plains aquifer contained a stratified sequence of ground water spanning Holocene time. A cross-sectional model of steady-state ground-water flow, calibrated using radiocarbon ages, is consistent with recharge rates ranging from 0.8 mm/a in areas overlain by loess to 8 mm/a in areas overlain by dune sand. Paleorecharge temperatures ranged from an average of 15.2&nbsp;±&nbsp;0.7 °C for the most recently recharged waters to 11.6&nbsp;±&nbsp;0.4 °C for the oldest waters. The temperature difference between Early and Late Holocene recharge was estimated to be 2.4&nbsp;±&nbsp;0.7 °C, after taking into account variable recharge elevations. Nitrogen isotope data indicate NO</span>₃<span> in paleorecharge (average concentration=193 μM) was derived from a relatively uniform source such as soil N, whereas NO</span>₃<span> in recent recharge (average concentration=885 μM) contained N from varying proportions of fertilizer, manure, and soil N. Deep water samples contained components of N</span>₂<span> derived from atmospheric, denitrification, and deep natural gas sources. Denitrification rates in the aquifer were slow (5&nbsp;±&nbsp;2×&nbsp;10</span>⁻³<span> μmol N&nbsp;L</span>⁻¹<span>&nbsp;a</span>⁻¹<span>), indicating this process would require &gt;10 ka to reduce the average NO</span>₃<span> concentration in recent recharge to the Holocene background concentration.</span></p> application/pdf 10.1016/j.apgeochem.2004.05.003 en Elsevier Geochemistry, radiocarbon ages, and paleorecharge conditions along a transect in the central High Plains aquifer, southwestern Kansas, USA article