Pitfalls in application of the conventional chloride mass balance (CMB) in karst aquifers and use of the generalised CMB method

There is unanimity in the literature that chloride mass flux crossing the piezometric surface requires a steady-state for the saturated or unsaturated version of the conventional chloride mass balance (CMB) method to apply. Data indicate that chloride concentration in point recharge fluxes crossing the piezometric surface can remain at or near surface runoff chloride concentrations, rather than finding equilibrium with groundwater chloride. Preferential groundwater flows were observed through an interconnected network of highly permeable zones with groundwater mixing along flow paths. Measurements of salinity and chloride indicated that fresher water pockets exist at point recharge locations. A stable and measurable fresh water plume develops only when a large quantity of surface water enters the aquifer as a point recharge. In such circumstances, assumptions and boundary conditions of the conventional CMB method are not met, and the method requires modification to include both point and diffuse recharge mechanisms. This paper describes a generalised CMB that is applicable to groundwater basins with point recharge. In three case studies, point recharge flux is estimated to contribute 63, 85, and 98 % of total recharge. However, long-term average annual point recharge volumes are much smaller than the aquifer storage, at 1.5, 1.95, and 0.75 % and distributed across the basins at discrete locations. In the study basins, conventional CMB-estimated recharges are 46, 20, and 11 % of the recharge estimated using the generalised CMB, indicating the importance of accommodating point recharge into the CMB method. The generalised CMB method provides an alternative long-term net recharge estimation method for groundwater basins characterised by both point and diffuse recharge.