The paper presents research on filtration characteristics of the alluvial sediments of the Stip aquifer in order to determine the degree of protection from ground water pollution. Previous studies determined that alluvial sediments of the Stip aquifer are built of two layers of different granulometric and filtration characteristics. The water-bearing layer is a coarse-grained porous media built of sand and gravel with hydraulic conductivity ranging from 48 to 280 m/day. This layer is overlaid with fine-grained sediments having low filtration characteristics. Overburden consists of dusty and clayey dusty sands with hydraulic conductivity ranging from 2×100 - 1.5×10-3 m/day. This research showed that overlay sediments don’t provide sufficient natural protection for ground water of the Stip aquifer and that they pertain to susceptible geological environment.