DENITRIFICATION AND PALEORECHARGE CONDITIONS IN A REGIONAL UNCONFINED AQUIFER SYSTEM

The High Plains aquifer underlies an area of about 450,000 km² in parts of eight western States and has saturated thicknesses as large as 350 m.  Dissolved gases (N₂, N₂O, O₂, He, Ne, Ar,), nutrients, and isotopes were measured in water samples collected from nested, short-screen monitoring wells installed along regional flow paths in Texas, Kansas, and Nebraska to evaluate denitrification and paleorecharge conditions in this regional, unconfined aquifer system.  Concentration and isotope (N and O) measurements of N₂ and NO₃ indicate that denitrification occurred along downgradient parts of most flow paths, with reaction rates increasing in order of Kansas<Texas<Nebraska.  Maximum denitrification rates were about 0.1 µmol N L^-1 yr^-1.  Water samples from deep flow-path wells in Kansas contained components of N₂ from a deep natural gas source in addition to atmospheric and denitrification sources.

Tritium and adjusted ¹⁴C ages ranged from modern near the water table to about 12,500 yr B.P. at the base of the aquifer, indicating the aquifer contained a stratified sequence of ground water spanning Holocene time.  Noble-gas recharge temperatures of water at the bottom of the aquifer were 13.7±1.0, 11.6±0.4, and 5.9±0.1^oC at the Texas, Kansas, and Nebraska flow paths, respectively, which are 1.5 ^oC (Texas) to 4.2 ^oC (Nebraska) colder than the present average annual air temperatures.  Paleorecharge contained substantial NO₃ concentrations (25 to 300 μM) derived from a relatively uniform source such as soil nitrogen, whereas recent (< 50 yr) recharge contained generally larger NO₃ concentrations (100 to 4,400 μM) derived from varying proportions of fertilizer, manure, and soil nitrogen.  Slow denitrification rates in the aquifer indicate this process would require > 10,000 yr to reduce the average NO₃ concentration in recent recharge to the Holocene background concentration.  Records of paleorecharge conditions derived from dissolved-gas and other data provide a useful baseline against which the effects of changes in climate and land use can be measured.