A simulation is presented of the operation of a brackish karst spring with the MODKARST deterministic mathematical model under not enough data conditions. MODKARST is a deterministic mathematical model for karst springs, able to accommodate brackish water. It incorporates mass and energy balances in a system of reservoirs properly connected that approximates the karst system and the sea. A complete fitting for this model needs field measurements about spring discharge and spring water chloride concentration at least for a hydrological year. The particular case analysed is the “Potami” brackish karst spring at Himara in Albania. MODKARST simulated the hydrograph along with the variation with time of the chloride concentration. The fitting of the model was based only on chloride concentrations measurements of the spring water that have taken place from 1/5/2007 till 27/12/2007, since there were not spring discharge measurements except for a single one, which simply gave the order of magnitude of this parameter. From the simulation of the model, information was drawn about the recharge area, the effective infiltration coefficient and the dominant seawater intrusion mechanism of the spring. Taking the above information into account, a set of proposals for the sustainable development of the spring was sketched.