GROUNDWATER RECHARGE IN THE CUATRO CIÉNEGAS BASIN, MEXICO: INSIGHTS FROM STRONTIUM ISOTOPE AND TRACE ELEMENT ANALYSES

The Cuatro Ciénegas Basin (CCB) of north-eastern Mexico hosts a unique groundwater-dependent ecosystem that consists of dozens of springs and pools with a long-term environmental stability that has lead to the evolution of over 70 endemic species. One goal of the larger research project is to identify and protect groundwater recharge areas that sustain these wetland habitats. To understand groundwater recharge mechanisms in the semi-arid 1,200 km² CCB, spring water samples were collected from 20 valley springs and pools surrounding the 3,000-m high Cretaceous limestone Sierra de San Marcos (SSM). Carbonate travertine samples were collected from a deposit of inferred Pleistocene age located at the western base of the SSM and a more recent deposit located on the eastern side of the SSM. Water samples were analyzed for trace elements and strontium isotopes to assess potential temporal changes in groundwater flow systems. Strontium isotopic analyses from the inferred Pleistocene travertine and down gradient spring water are within analytical error. These data suggest that the source of groundwater to the CCB has not changed substantially since the travertine deposits were formed. Strontium isotopic analyses of the more recent travertine and its associated spring water imply that the groundwater system has not changed significantly in modern times. The Sr isotope composition of 20 springs throughout CCB are all within analytical error and demonstrate that groundwater discharging in CCB springs has a homogeneous source for dissolved ions. Based on secular variations in marine Sr isotopes, this source is likely carbonate rocks of Barremian to Cenomanian age (127 to 94 ma) that form part of the SSM, although diagenesis and groundwater interaction with volcanic intrusions may affect the strontium isotopic signature of the spring waters. Trace element variations in nine springs adjacent to the SSM mountain front suggest that either locally derived recharge cannot account for trace element variability in groundwater, or we do not fully understand trace element constraints.