The concurrent use of karst aquifers as drinking water resources and receptors of combined sewer overflow lacking appropriate pre-treatment may cause conflicts between drinking water supply and storm water manage- ment. A storm water tank (SWT) for combined wastewater is identified as the source of sporadic contamination of a karst spring (Gallusquelle, ''Schwabische Alb'', SW Ger- many) used for public water supply. Spring water quality was examined by routine and event sampling and by evaluating physicochemical and microbiological parame- ters. The total number of microbial colonies growing at 20C and the number of Escherichia coli colonies rose to values up to four orders of magnitude higher than back- ground, 2-5 days after overflow of the SWT. High concentrations of chloride, sodium, and total organic car- bon (TOC) and high values of turbidity coincide with this increase. However, high bacterial contamination is also observed while turbidity and TOC are low. Several wastewater-related organic micro-pollutants such as chlo- rinated and non-chlorinated organophosphates were detected in the SWT and, depending on their Kow values and their biodegradability, in lower concentrations at the spring.