When pollutants enter the ground, they spreads out in the groundwater. If there is no movement of the groundwater, they disperse evenly in all directions. Click to see views of pollution dispursion as seen from above. These represent dispursion in a uniform material, such as soil. The above image was created using an Excel Spreadsheet.
Gravity is the prime factor in the movement of groundwater from higher locations to lower.
However other factors contribute. The rock material can retard and divert flow. Cracks can produce rapid flows. Exit points such as springs, lakes, streams or pumping locations can draw water towards those locations. As the groundwater flows, the pollutants follow the flow and spread out.