Usefulness of the Cl/Br ratio to identify the effect of reverse osmosis treated waters on groundwater systems

The chloride-to-bromide molar ratio (R Cl/Br) is widely used in tracing groundwater salinity. Less experience exists on how some industrial processes such as the reverse osmosis (RO) water treatment modify the R value of desalinated water (P) and reject brine (T), and even less on how distinctive for hydrogeological applications the R changes are. This paper assesses the Cl/Br ratio usefulness to identify the effect of P and T on groundwater systems. First, the experimental R changes produced in P and T during standard RO operations and treatments of raw (I) seawater, brackish groundwater, and tertiary-treated domestic wastewater in nine RO plants (three in Gran Canaria Island and six in south-eastern continental Spain) were determined by means of the X-P = R-P/R-I (R change in P) and X-T = R-T/R-I (R change in T) ratios. X-P and X-T were respectively 0.90 and 1.07 for treatment 1 (weak pre-acidification) in one RO plant, 0.82 +/- 0.09 and 0.94 +/- 0.05 for treatment 2 (weak pre-acidification and weak pre-chlorination) in five RO plants, 0.63 and 0.97 for treatment 3 (moderate pre-acidification and strong pre-chlorination) in one RO plant, and 3.21 +/- 2.02 and 1.00 +/- 0.00 for treatment 4 (post-chlorination) in two RO plants. P was for irrigation (treatments 1 to 3) and for domestic use (treatment 4). Latter, the experimental X-P and X-T ratios were input data for six theoretical mixing scenarios aimed at showing how groundwater R changes in response to progressive contributions of P and T produced from different I water. The Cl/Br ratio enables to identify the effect of P from treatments 3 and 4, is scarcely effective for treatments 1 and 2, and is especially useful when P produced from seawater is used in other aquifer having different R. The Cl/Br ratio did not clearly identify T from any treatment.