Groundwater is water that lies below the soil surface and fills the pore spaces in and around rock, sand, gravel, and other materials. Contrary to popular belief, groundwater does not move through vast underground rivers and lakes, but through water-saturated zones called aquifers. The upper level of the aquifer is called the water table and fluctuates throughout the year depending on use and recharge. Contamination of groundwater occurs when unwanted substances move through fractures or the soil profile to the saturated zone.
Groundwater moves both vertically and horizontally in response to gravity and hydraulic pressure. Generally, the groundwater "flow" rate is only several feet per year except in permeable sand and gravel aquifers where the movement may be one or two feet per day. The withdrawal of water through wells would directly influence that movement in the vicinity of the well intake, but generally it may take several years for groundwater and substances dissolved in it to move a mile. Because of the slow rate of degradation of substances in groundwater, contamination at one location may show up years later in a different location.
Four major factors determine whether a pesticide is likely to reach groundwater: