EVALUATING SOURCES OF WATER CONTRIBUTING TO PERENNIAL WETLANDS USING RADIOGENIC SR AND U ISOTOPES, POTRERO CREEK, NOGALES, AZ, USA

Perennial wetlands occupy 0.14–0.26 km² near the confluence of Potrero Creek and Nogales Wash and represent one of the last desert marshlands in the Santa Cruz River valley. They provide important habitats supporting plant and resident/migratory species within an urban corridor that has substantially affected topography and land use in both drainages. The amount of water present in the wetlands fluctuates depending on amounts of evaporation, transpiration, and precipitation; however, surface flow in Potrero Creek is insufficient to maintain perennial saturation. Depths to the regional water table, hosted in Miocene to Holocene basin-fill sediments, are too great to supply groundwater directly to the wetlands, although leakage/runoff from cultural activities may contribute. Transient-electromagnetic data indicating a shallow, high-conductivity zone beneath the wetlands have been interpreted to reflect a saturated, clay-rich horizon that could limit downward drainage of perched water, although the source of that water remains unclear.

Water from wells (8) and surface locations along Nogales Wash (5) and throughout the wetlands (13) were analyzed for ⁸⁷Sr/⁸⁶Sr and ²³⁴U/²³⁸U. Those isotopes are well suited as tracers of hydrologic mixing in arid wetlands as they are not significantly fractionated by near-surface physical or chemical processes. Results indicate that each group has distinct isotopic signatures with lower ⁸⁷Sr/⁸⁶Sr and higher ²³⁴U/²³⁸U in groundwater from wells pumping the regional aquifer relative wetlands samples. Surface flow in Nogales Wash has intermediate compositions precluding it as a source for the wetland endmember. Nor can the wetland endmember be derived from the regional groundwater as sampled from upgradient wells, although some mixing between the wetlands and regional-groundwater endmembers is evident. A recently completed geologic framework model indicates that the newly mapped Grand Avenue Fault zone intersects Potrero Creek at the wetlands and likely plays an important hydrogeologic role. Probable geologic factors contributing to wetland hydrology include transmissive structures that focus upwelling groundwater from a distal, as-of-yet unidentified source, and formation of a flow barrier that causes mounding of the isotopically distinct water.