Chemical and Isotopic Tracers for Estimating Ground Water Recharge, Flow Paths, and Residence Times in the Middle San Pedro Basin, Southeast Arizona

Monday, April 20, 2009: 1:50 p.m.
Canyon Suites I/II (Hilton Tucson El Conquistador Golf & Tennis Resort )
Candice B. Adkins , USGS, Tucson, AZ
Christopher J. Eastoe, Ph.D. , SAHRA, Department of Geosciences, University of Arizona, Tucson, AZ
Jennifer McIntosh, Ph.D. , Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ
Jesse E. Dickinson , Arizona Water Science Center, USGS, Tucson, AZ
This study analyzes variations of major ion (Ca, Na, K, Mg, Sr, Fe, Zn, F, Cl, Br, NO3, SO4) and isotope (18O, 2H, 3H, 34S, 13C, 14C) chemistry of ground water, surface waters and precipitation with in conjunction with hydrogeologic data (e.g. hydraulic head and stratigraphy) to infer recharge areas, mixing of water sources, and residence times of ground water within the middle San Pedro watershed in southeastern Arizona. 

Ground-water chemistry in the middle San Pedro basin suggests compartmentalization of waters into an upper and lower alluvial aquifer system comprised of permeable sands and gravels ranging in depth from ten to over one thousand feet in the basin center; the units are separated by confining units of silt and clay in the basin center.  Variations include higher fluoride (up to 8 ppm) near the Dragoon Mountains, higher chloride (up to 54 ppm) near the Whetstone Mountains, and higher sulfate (up to 750 ppm) in both upper and lower sands and gravels owing to interaction with thick Permian or Neocene evaporites.  Chloride is generally lower (less than 8 ppm) in the lower unit of the aquifer due to limited evaporation.  The presence of high nitrates (up to 32 ppm) in the upper sands and gravels indicate modern recharge to this unit.  

 Oxygen isotope values ranging from -6.8‰ to -8.9‰ suggests recharge to the upper unit originates mostly from summer monsoon precipitation especially along certain reaches of the San Pedro River. Oxygen isotope values between -7.2‰ and -11.8‰ indicates recharge to lower units originates from a mixture of summer and winter precipitation and high elevation recharge.  Low percent modern carbon values (8.0 to 37.8 PMC) within lower units indicate recharge within the past ~14,000 years.  Detectable tritium (1.0-6.8 TU) near mountain blocks and shallow units indicate recharge within the past sixty years.