Utilization of Environmental Isotopes, Water Geochemistry, and Hydrogeologic Properties to Assess the Interconnectedness of Shallow Aquifers at a Copper Mining Site

 

Chandler E. Noyes, Jihyun Kim, and Jennifer McIntosh

Department of Hydrology and Atmospheric Sciences

The University of Arizona, Tucson, Arizona

Assessing the interconnectedness of aquifers is an important water resource management practice to mitigate potential migration of contaminants from a mined aquifer (e.g. in-situ copper mining) to adjacent aquifers.  This study is focused in the metal-rich Lisbon Valley of the Paradox Basin in southeastern Utah where numerous faults may act as conduits or barriers to cross-formational flow.  Tritium, 14C, and 81Kr lay a foundational groundwork to assess the relative ‘ages’ of two distinct aquifers separated by an aquitard of varying thickness.  Analysis of isotopes such as δ18O/δD, δ34S-SO4, and δ18O-SO4 provide insight into the climate during recharge, regions of source-water recharge, and groundwater flowpaths.  Assessment of geochemistry data extending back decades indicates that these two aquifers have distinct geochemical signatures; the upper Burro Canyon aquifer has a calcium-sulfate signature with high TDS and basic pH, while the lower Navajo aquifer has a sodium-bicarbonate water with low-to-moderate TDS and near-neutral pH.  Uncorrected radiocarbon ‘ages’ from 16,000-20,000 BP in the Burro Canyon aquifer and 32,000-42,000 BP in the Navajo aquifer, coupled with low δ18O/δD values relative to modern precipitation, suggests both aquifers contain fossil groundwater recharged during the Pleistocene.  Future work may include noble gas age tracers and hydrologic modelling.