Understanding Groundwater Resources in Cascabel, Arizona

Pima County Regional Flood Control District Monthly Brown Bag Series

Understanding Groundwater Resources in Cascabel, Arizona: An Exercise in Citizen Science

Date: Wednesday, October 12, 2016
Time: 12:00 – 1:00
Location: 9th Floor Public Works Building, 201 N. Stone, Tucson AZ, 85701
Speaker: Chris Eastoe, Retired, Department of Geosciences, University of Arizona

An aquifer recharged from Hot Springs Canyon, the source of domestic and irrigation water for many Cascabel residents, has been delineated using stable isotope and electrical conductivity measurements in a study with citizen participation. Eighteen years of drought have led to the depletion of the upgradient parts of the aquifer. The entire aquifer would be vulnerable to droughts lasting 50–100 years. Many residents participated in a study of local groundwater resources by collecting of water samples for measurement of stable O and H isotopes and electrical conductivity (EC), and by attending two public presentations on the results. Stable isotope and EC data indicate four principal sources of water recharging the basin-fill alluvium along the San Pedro River:

  • The southern Galiuro Mountains, via Hot Springs Canyon
  • The Rincon Mountains, via Paige Canyon
  • The margins of the middle San Pedro basin, via other tributary washes
  • Summer runoff in the San Pedro River

Groundwater originating in Hot Springs Canyon (HSC) is distinctive because of low-δ18O, low-δD contributions from Hooker Hot Springs. It extends 7 km downstream from the HSC confluence along the east side of the San Pedro River, some water being resident in the aquifer for 60-70 years on the basis of tritium data. The small volume of base flow from the hard-rock reach of HSC to basin sediments and declining static water levels in domestic wells in HSC alluvium indicate an aquifer vulnerable to prolonged drought. The present, continuing drought of about 18 years’ duration is short compared with prehistoric droughts, evidenced in tree rings from the Rio Grande basin, where droughts have characteristically lasted 50 to 100 years over the last millennium. Such droughts would render the entire HSC aquifer vulnerable, indicating the need for prudent use of groundwater, and suggesting the advisability of supplementary roof-water collection.

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