Oxygen-18 and deuterium distribution in rainfall, runoff and groundwater in a small semi-arid basin: The Aravaipa Valley in the Sonora Desert, Arizona

The stable isotope distribution in a confined alluvial aquifer may indicate the altitude as well as the climatological conditions which prevail in the recharge area. However, study of stable isotope distribution across ten kilometres of a confined alluvial aquifer in the Aravaipa Valley, Sonora Desert, Arizona, revealed an extreme difference in isotopic ratios in both sides of the aquifer, which cannot be explained by simple climatological factors. The stable isotope distribution in precipitations across the valley revealed that the heaviest average oxygen-18 values in the valley are -4.9 per mille and -8.9 per mille in summer and winter storms, respectively. However, although the western sampler and one of the samplers in the east were located almost at the same elevation, 14 kilometres apart, the rainfall appeared to be significantly heavier in the west. The weighted averages of oxygen-18 were found to be -7.9 per mille in the west and -8.7 per mille in the west. The fact that water in the east is depleted in oxygen-18 compared with the water in the west at a given altitude can be attributed to the relation between the path of the storm and the topography. Thus, the western rain sampler is located on the lee side of the Galivro Mountains, and the eastern sampler is located on the foreside of the Santa Teresa Mountains. On the lee side, the vertical distance travelled by the raindrops is greater than on the foreside, causing the lee side to be relatively enriched in oxygen-18. The only mechanism to explain further enrichment in the groundwater is evaporation from the top soil and from sheet-flow over the alluvial pediments. The large extent of coarse alluvial pediments in the western portion of the Aravaipa Valley compared with limited outcrops along the eastern pediments explains the heavy isotopic ratio in the western portion of the groundwater system. 12 refs, 8 figs, 2 tabs