Regional zonation of groundwater recharge mechanisms in alluvial basins of Arizona: Interpretation of isotope mapping

Abstract

Stable hydrogen and oxygen data for groundwater in alluvial basins of Arizona form four characteristic patterns corresponding to different recharge mechanisms: 1. Evaporation trends of slope near 5 and originating at (δ18O, δ2H) = (−11.2, −77‰); 2. Linear trends or elongate clusters with minimal evaporation, plotting beneath local meteoric water lines; 3. Data plotting on evaporation trends of major rivers; 4. Pattern 2 superimposed on to Pattern 1. The distribution of patterns forms a map in which Pattern 1 predominates in the northern and northwestern parts of the study area, Pattern 2 in the south and southeast, and Pattern 3 in basins with rivers. Pattern 4 is found in three basins of central-western Arizona, where ion chemistry suggests that Pattern 2 water is younger than Pattern 1 water. Recharge in Pattern 1 basins originates dominantly as winter precipitation, for which there appears to be no isotope altitude effect. In Pattern 2 basins, isotope altitude effects appear to apply in both winter and summer precipitation, and both contribute to recharge. Pattern 2 recharge occurs mainly in the wettest 30% of months, as indicated by δ18O and δ2H dependence on monthly rainfall intensity. Climate-related factors such as potential evaporation and presence/absence of isotope altitude effects are related to the distribution of patterns 1 and 2; however, current understandings of regional patterns of climate, geology and vegetation are insufficient to explain the differences between patterns 1 and 2. Similar patterns of δ18O and δ2H data occur in basins of the region around Arizona, but with too few examples to indicate any regular spatial arrangement.