Water pollution has become a major global problem and requires an immediate action. It was generally considered that pollutants were diffused gradually from surface to underwater region, where plants and animals should have less affected. However, diffusion of contamination is very slow in comparison with penetration. Here we show saline water has a highly selective penetration, and the penetration rate is much higher than that of diffusion. An experiment was carried out using a drop of red ink as a point pollution source to check both diffusion and penetration processes in various salt solutions with different concentrations, the salt concentration greatly affects the penetration process and there is a threshold value beyond which no penetration occurs. An imaged capillary is adopted in this paper to explain the selective penetration on a molecular scale, revealing that the salt concentration and temperature will greatly affect the capillary pressure and the penetration depth. Our results demonstrate that the penetration is the main pollution dispersion, and we anticipate our theory would shed light upon the major pollution problem from theoretical analysis to pollution control.