Use of Atmospherically Derived Dissolved Gases and Tritium to Characterize Ground-Water Flow In the Arbuckle- Simpson Aquifer

A geochemical reconnaissance of the Arbuckle-Simpson aquifer in south-central Oklahoma was initiated in 2004 to investigate the ground-water quality at an aquifer scale, describe the chemical evolution of ground water as it flows from recharge areas to streams and springs, and determine the residence time of ground water in the aquifer.  Dissolved gas samples were collected to characterize model parameters (e.g. recharge temperatures and amount of excess gas) associated with the application of the tritium-helium (³H-³He) and chlorofluorocarbons (CFC) dating techniques.  Noble gas recharge temperatures (NGRTs) and concentrations of excess air (fractionated and un-fractionated) varied greatly within the study area.  The mean annual air temperature associated with the aquifer is 16.1°C; whereas the average NGRT is 14.3 +/- 2.2°C.  The NGRTs are consistently too low to be a record of the mean annual temperature, as is typically assumed.  A review of temperature and water level data obtained from stream gaging stations and monitoring wells in the aquifer suggests that the NGRTs appear to record seasonal and focused recharge events.  Measured concentrations of age indicators from ³H-³He and CFC dating techniques are consistent with tracer input curves, which supports the concept that ground-water flow within the modern-age zone of the aquifer is dominated by piston flow conditions.  Exceptions were observed in a few samples of older water taken from wells and springs near the periphery of the aquifer.  These distal samples contain measurable amounts of terrigenic helium-4 and are interpreted as either sub-modern (³H dead) or mixed modern and sub-modern waters.  These older waters have considerably colder NGRTs and possibly record either cooler climatic conditions or represent mixing between modern waters and ancient oil field brines.