Abstract
Mountain-block groundwater in the Southern Basin-and-Range Province shows a variety of patterns of δ18O and δ2H that indicate multiple recharge mechanisms. At 2420 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 2420 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, 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.
Highlights
In the Basin and Range Province [1] of Southwestern North America, mountain blocks play an important role in regional hydrology
Four principal patterns of (δ18 O, δ2 H) data have been identified: (1) data plotting along local meteoric water lines (LMWL) for all precipitation at >1800 masl; (2) data plotting along modified LMWLs for the wettest 30% of months at
Recharge mechanisms reflect a complex interplay of precipitation season, altitude, precipitation intensity, groundwater age and geology
Summary
In the Basin and Range Province [1] of Southwestern North America, mountain blocks play an important role in regional hydrology. Water from the highest mountain ranges is conveyed to adjacent basin-fill aquifers either as surface flow or as sub-surface mountain-block recharge [2]. Groundwater hydrology studies in mountain ranges are potentially difficult because of the challenges of data collection in remote and inaccessible terrain with few sampling points [2]. Studies of mountain-system hydrology have been conducted at a variety of spatial scales. Isotope and noble gas ratio data have identified groundwater moving from a mountain block into basin alluvium in Salt Lake Valley, Utah, USA [9,10]. Stable isotope studies have addressed the residence groundwater of Table Mountain, South Africa, and the Cascade Range, Oregon, 2 of 22
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