Hydrology of Mountain Blocks in Arizona and New Mexico as Revealed by Isotopes in Groundwater and Precipitation

Abstract

Mountain-block groundwater in the Southern Basin-and-Range Province shows a variety of patterns of delta O-18 and delta H-2 that indicate multiple recharge mechanisms. At 2420 m above sea level (masl) in Tucson Basin, seasonal amount-weighted means of delta O-18 and delta H-2 for summer are -8.3, -53 parts per thousand, and for winter, -10.8 and -70 parts per thousand, respectively. Elevation-effect coefficients for delta O-18 and delta H-2 are as follows: summer, -1.6 and -7.7 parts per thousand per km and winter, -1.1 and -8.9 parts per thousand per km. Little altitude effect exists in 25% of seasons studied. At 2420 masl, amount-weighted monthly averages of delta O-18 and delta H-2 decrease in summer but increase in winter as precipitation intensity increases. In snow-banks, delta O-18 and delta H-2 commonly plots close to the winter local meteoric water line (LMWL). Four principal patterns of (delta O-18, delta H-2) data have been identified: (1) data plotting along LMWLs for all precipitation at >1800 masl; (2) data plotting along modified LMWLs for the wettest 30% of months at <1700 masl; (3) evaporation trends at all elevations; (4) other patterns, including those affected by ancient groundwater. Young, tritiated groundwater predominates in studied mountain blocks. Ancient groundwater forms separate systems and mixes with young groundwater. Recharge mechanisms reflect a complex interplay of precipitation season, altitude, precipitation intensity, groundwater age and geology. Tucson Basin alluvium receives mountain-front recharge containing 50%-90% winter precipitation.