The objectives of our study were to determine (i) removal of bacteriophage MS2 and phiX174, as surrogates for human pathogenic viruses, in an anoxic aquifer and (ii) the safe length of the microbial protection zone in anoxic aquifers. 3.5 Log units of MS2 were removed by adsorption and inactivation during 63 days residence time, which was 1.4 log units lower than removal of phiX174. These removal rates were considerably lower than previously reported for MS2 and phiX174 in oxic aquifers and consequently longer protection zones around anoxic aquifers might be needed. Therefore, the observed log removal of MS2 was used in a risk assessment approach to determine the required safe length of the microbial protection zone. In case of a leaking sewer in the vicinity of a well in an anoxic aquifer, the risk assessment demonstrated that a microbial protection zone of 110 m may be needed to meet a risk of infection of 10(-4) persons per year. This length can be two to three times larger than the length of the protection zone currently used in a number of countries.