Mountain-block groundwater in the Southern Basin-and-Range Province shows a variety of patterns of δ
18O and δ
2H that indicate multiple recharge mechanisms. At 2420 m above sea level (masl) in Tucson Basin, seasonal amount-weighted means of δ
18O and
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Mountain-block groundwater in the Southern Basin-and-Range Province shows a variety of patterns of δ
18O and δ
2H that indicate multiple recharge mechanisms. At 2420 m above sea level (masl) in Tucson Basin, seasonal amount-weighted means of δ
18O and δ
2H for summer are −8.3, −53‰, and for winter, −10.8 and −70‰, respectively. Elevation-effect coefficients for δ
18O and δ
2H are as follows: summer, −1.6 and −7.7 ‰ per km and winter, −1.1 and −8.9 ‰ per km. Little altitude effect exists in 25% of seasons studied. At 2420 masl, amount-weighted monthly averages of δ
18O and δ
2H decrease in summer but increase in winter as precipitation intensity increases. In snow-banks, δ
18O and δ
2H commonly plots close to the winter local meteoric water line (LMWL). Four principal patterns of (δ
18O, δ
2H) 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.
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