Pitfalls in groundwater recharge evaluation using chloride mass balance in the Canadian Prairies

ABSTRACT: The chloride mass balance (CMB) method is commonly used for recharge evaluation in semi-arid settings. However, its results can be biased by incorrect assumptions about chloride input rates and local hydrogeological settings. We have investigated this effect at prairie sites in Alberta by combining the CMB with other tracers (stable isotopes of water and tritium). The study revealed an existence of the chloride plume in the shallow subsurface with concentration surpassing background values by as much as an order of magnitude. The association between elevated chloride concentrations and tritium presence suggests increased chloride inputs in the last 60 years, likely, associated with agricultural practices. An existence of an anthropogenic chloride plume in the vadose zone and groundwater breaks the link between high chloride concentrations and low recharge, which underpins CMB.