In August 2008 an intensive pumping experiment was carried out in order to assess the capacity of Brestovica – Klarici water supply in summer conditions, and its chemical quality. Groundwater was pumped for a period of 30 days, from three wells at the Klarici pumping station, and from well B-10/06, with a common capacity of 470 L/s. The fact that pumping caused a decrease in the water table in well B-10/06 by only 17 cm at a pumping rate of 265 L/s suggests that larger quantities of groundwater could be pumped from this well. The tracer experiment, performed at the B-10/06 temporary pipeline discharge area, showed that no pumped water was returned to the area of the exploited wells during the pumping test. Tracer uranine, which had been injected at the effluent from the pumped water from the well, was detected only at the Sardos Spring. The intensive pumping resulted in changes in the chemical and isotopic composition of oxygen in the water, due to a greater impact of the water from the intergranular SocaRiver aquifer on the karst aquifer.The chemical quality of groundwater from well B-10/06 is adequate for the purpose of drinking water supply. The presence of bacteria in groundwater is typical of karst aquifers and the water needs to be disinfected for further use.