Eastoe and Wright reported in 2019 that mountain groundwater in the Southern Basin-and-Range Province shows a variety of patterns of δ18O and δ2H that indicate multiple recharge mechanisms. At 2,420 m above sea level (masl) in Tucson Basin, seasonal amount-weighted means of δ18O and δ2H for summer are -8.3, -53%, and for winter, -10.8 and -70%, respectively. Elevation-effect coefficients for δ18O and δ2H 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 2,420 masl, amount-weighted monthly averages of δ18O and δ2H decrease in summer but increase in winter as precipitation intensity increases. In snow-banks, δ18O and δ2H commonly plots close to the winter local meteoric water line (LMWL).
Four principal patterns of (δ18O, δ2H) 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, tritated groundwater predominates in the studied mountain areas. 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.
The authors concluded that:
1. In 1995–2004, precipitation at 2,420 masl at PRS in the Santa Catalina Mountains had seasonal amount-weighted averages of (δ18O, δ2H) and LMWLs as follows: in summer (-8.3, -53%), and δ2H = 7.0 δ18O + 4.2; in winter, (-10.8, -70%) and δ2H = 8.3 δ18O + 20.1. Wetter months have lower (δ18O, δ2H) in summer, but higher (δ18O, δ2H) in winter.
2. In 2008–2013, snow-bank samples defined a line δ2H = 9.0 δ18O + 26.3; in 2007, isotope metamorphism gave rise to (δ18O, δ2H) values deviating strongly from this line.
3. Long-term coeffcients for isotope altitude effect (δ18O, δ2H) between Tucson and PRS are (-1.6, -7.7 % per 1000 m) in summer, and (-1.1, -8.9 % per 1000 m) in winter. Altitude effect was absent in 25% of seasons examined.
4. Groundwater containing finite tritium predominates in all ranges studied except the Tucson Mountains., and is interpreted as young groundwater, or mixtures containing young groundwater. Ancient groundwater, indicated by 14°C and distinctive (δ18O, δ2H) data, is present in the Galiuro, Tucson, Rincon and Santa Rita Mountains.
5. Multiple recharge mechanisms of young groundwater are indicated by patterns of (δ18O, δ2H) data. The patterns are influenced by altitude, geology and mixing with ancient groundwater.
6. Regional, zoned recharge mechanisms of neighboring alluvial basins are present in the silicate mountain blocks. Winter-only recharge with evaporation occurs in the Mogollon Highlands and the Galiuro Mountains. Infiltration of winter and summer infiltration from the wettest months, in various proportions, occurs below 1,700 masl in the Santa Rita and Tucson Mountains, and below 1,400 masl in the Santa Catalina Mountains.
7. Additional recharge mechanisms include: 1) infiltration of both winter and summer precipitation in various proportions at high elevations in the Rincon and Santa Rita Mountains., yielding (δ18O, δ2H) data plotting along high-altitude LMWLs; 2) Infiltration of high-elevation summer-dominant precipitation, with strong evaporation, in the Rincon Mountains; 3) recharge of water with (δ18O, δ2H) values below those of long-term mean winter precipitation above 1,400 masl in the Santa Catalina Mountains.; and 4) alternation of winter recharge with evaporation (dry years) and recharge of mixed winter and summer precipitation without evaporation (wet years) in the carbonate rocks of the Sacramento Mountains.
8. A complex interplay of isotope effects related to altitude, precipitation seasonality, precipitation intensity and groundwater age is responsible for the multiple patterns of isotope data.
9. Assuming that mountain runoff from any altitude has an equal likelihood of reaching the basin floor, mountain-front recharge to alluvium in Tucson Basin consists of 50%–90% winter runoff from surrounding mountain areas.
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