Abstract

Isotope data and major ion chemistry were used to identify aquifer recharge mechanisms and geochemical evolution of groundwaters along the US–Mexico border. Local recharge originates as precipitation and occurs during winter through preferential infiltration pathways along the base of the Gila Range. This groundwater is dominated by Na–Cl of meteoric origin and is highly concentrated due to the dissolution of soluble salts accumulated in the near-surface. The hydrochemical evolution of waters in the irrigated floodplain is controlled by Ca–Mg–Cl/Na–Cl-type Colorado River water. However, salinity is increased through evapotranspiration, precipitation of calcite, dissolution of accumulated soil salts, de-dolomitization, and exchange of aqueous Ca2+ for adsorbed Na+. The Na–Cl-dominated local recharge flows southwest from the Gila Range and mixes with the Ca–Mg–Cl/Na–Cl-dominated floodplain waters beneath the Yuma and San Luis Mesas. Low 3H suggests that recharge within the Yuma and San Luis Mesas occurred at least before the 1950s, and 14C data are consistent with bulk residence times up to 11,500 uncorrected 14C years before present. Either the flow system is not actively recharged, or recharge occurs at a significantly lower rate than what is being withdrawn, leading to aquifer overdraft and deterioration.

Highlights

  • This study aims to establish sources of solutes, sources of aquifer recharge, groundwater residence time, and geographic variation of major ion chemistry in groundwater on the eastern flank of the Colorado River Delta

  • We focus on the upper ~300 m of the younger water-bearing sediments where most of the production wells in the study area are found

  • All δ13C values were expressed in delta notation relative to the Vienna Pee Dee Belemnite (VPDB) standard

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Summary

Introduction

In the Lower Colorado River Basin, south of the Utah–Arizona border, more than 27 million people depend on the river for sustenance. 1.2 million ha of farmland are irrigated with Colorado River water in the fertile and productive fields of the Mexicali and Imperial Valleys [1]. The lower Colorado River basin has seen extensive land-use changes in the last century. Pastures and crops have replaced native vegetation, and surface Colorado River water has been diverted for irrigation. These changes led to a massive loss of natural habitat in the Colorado River Delta (termed “Delta” below). The river no longer reaches the lower part of the Delta today, and riparian, wetland, and estuarine habitats occupy less than 5% of their original 780,000 ha extent [2]

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