Geochemistry, radiocarbon ages, and paleorecharge conditions along a transect in the central High Plains aquifer, southwestern Kansas, USA

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(2), Ar, O(2), CH(4)) 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(2) reduction and denitrification. Radiocarbon ages adjusted for C mass transfers ranged from <2.6 ka ((14)C) B.P. near the water table to 12.8 +/- 0.9 ka ((14)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 +/- 0.7 degreesC for the most recently recharged waters to 11.6 +/- 0.4 degreesC for the oldest waters. The temperature difference between Early and Late Holocene recharge was estimated to be 2.4 +/- 0.7 degreesC, after taking into account variable recharge elevations. Nitrogen isotope data indicate NO(3) in paleorecharge (average concentration = 193 muM) was derived from a relatively uniform source such as soil N, whereas NO(3) in recent recharge (average concentration = 885 muM) contained N from varying proportions of fertilizer, manure, and soil N. Deep water samples contained components of N(2) derived from atmospheric, denitrification, and deep natural gas sources. Denitrification rates in the aquifer were slow (5 +/- 2 x 10(-3) mumol N L(-1) a(-1)), indicating this process would require >10 ka to reduce the average NO(3) concentration in recent recharge to the Holocene background concentration. Published by Elsevier Ltd.