Chloride/bromide ratios as a natural tracer of urban runoff in the Tucson basin, Arizona

Abstract

The weight ratio of naturally occurring chloride and bromide (Cl/Br) can be used to identify water origin, and as a tracer. In the Tucson basin, where geologic inputs of these constituents are limited, the mean Cl/Br ratio of uncontaminated ground water is 130. This ratio may be altered by artificial recharge, sewage effluent disposal, and agricultural practices. In this study, Cl/Br ratios of 30 Tucson urban runoff samples averaged 34, one quarter that of the ground water. Cl/Br ratios may be useful in studying the fate of urban runoff recharged via dry wells. Urban runoff contains bromide from ethylene dibromide in leaded gasoline. Both species behaved conservatively in sorption and volatilization experiments; thus, these processes should not significantly alter Cl/Br ratios in surface or ground water. Bromide analytical methods were compared and methods best suited for analysis of dilute natural waters are discussed.