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Deeply-derived fluids influence river and groundwater hydrochemistry at the Socorro "constriction" in the central New Mexico Rio Grande rift. New hydrochemical transects in the Rio Grande rift provide three-dimensional perspectives on sources contributing to surface and groundwaters. In addition to abrupt salinity increases in the Rio Grande, major and trace ion chemistry, delta O-18, delta D, delta C-13, Sr-87/Sr-86, H-3 and He-3/He-4 variations in springs and wells across the rift are best explained by mixing of deeply-derived endogenic fluids, containing both mantle and crustal inputs, with shallow groundwater. Endogenic fluids ascend along rift-bounding and intra-rift faults, and can cause increased salinity accompanied by an increase in metals and overall degradation of water quality. Our data reinforce models that attribute a downstream increase in salinity to geological, not agricultural, influences. Hydrochemical mixing of endogenic waters along faults in shallow aquifers has a major influence on groundwater and surface water quality and variability in arid tectonically active regions, with important ramifications for water management. (c) 2013 Elsevier B.V. All rights reserved.