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Summary: GEOLOGY, December 2007 1063
ABSTRACT
Semi-arid and arid rivers typically exhibit increasing salinity levels
downstream, a trend often attributed to irrigated agriculture, primarily
due to evapotranspiration. In contrast, the results of our investigations
in one salinized river suggest that geological sources of salt added by
groundwater discharge are more important than agricultural effects.
We performed detailed synoptic sampling of the Upper Rio Grande
Rio Bravo, an arid-climate river with significant irrigated agricul-
ture, and identified a series of salinity increases localized at the distal
ends of sedimentary basins. Using Cl/Br, Ca/Sr, 87
Sr/86
Sr, and 36
Cl/Cl
ratios and 234
U values as environmental tracers, we show that these
increases result from localized discharge of high-salinity groundwater
of a sedimentary brine source. These groundwater fluxes, while very
small (<1 m3
s1
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