USING NATURAL BROMINE TO TRACE THE ORIGIN AND MIXING OF GROUNDWATERS: LARGE AND SMALL SCALE

Saline waters are found through out a wide range of aquifer scales: from smaller “km” scales to basinal “100's of km” scales; and from metre to km's depths. The origin of salinity in these aquifers can be caused by a number of mechanisms, with the two dominant ones being dissolution of evaporites and evaporative-concentration. However, differentiating the different origins of saline waters can be problematic using common water analyses because typically both types of brines are concentrated in the common major ions.

Bromine concentrations and Cl/Br ratios have been used to differentiate, and hence trace, the origin of saline waters. Although not universally applicable, bromine is a useful tracer for differentiating waters formed by the dissolution of halite from those formed by the evaporative concentration of seawater. Recent advancements in the analytical determination of bromine in water using Neutron Activation Analysis (NAA) have enabled the routine and accurate determination of low levels (e.g., <1 mg/L) of bromine in groundwater.

Two examples of using NAA to determine bromine concentrations and Cl/Br ratios demonstrate the utility of this approach. In the first example, deep (up to 4 km) Paleozoic brines from across the Williston Basin (Canada-USA) were analyzed for their bromine concentrations and these data were used to distinguish waters formed by halite-dissolution from those with an evaporated-seawater component. In the second example, a groundwater flow system in a shallow buried-valley aquifer in eastern Alberta with a saline brine plume was investigated to determine the origin of the salinity in the aquifer. Br/Cl ratios demonstrated that the salinity was not linked to deeper oilfield brines.

Bromine and Cl/Br ratios were successful in determining the source of the salinity in shallow and deep aquifers, and thus good tools for fingerprinting and tracing saline water.