Wastes injected under pressure into a deeper aquifer are flowing with and against the groundwater flow direction. Again, as with all aspects of groundwater, the fate and transport of pollutants through both the saturated and unsaturated zones is highly site-specific, with contaminant movement depending on the local hydrogeology as well as the characteristics of the contaminants in question. For example, confining layers overlying a critical aquifer or significant depth to groundwater can slow and, in some cases, prevent contaminant entry. High recharge rates in humid regions of the country, on the other hand, or heavy pumping of wells may make contamination problems more pressing. In addition, different contaminants will interact with the natural groundwater environment in different ways.
Contaminants also interact and react with other contaminants in groundwater.
The movement of some contaminants can be attenuated or retarded by various physical, chemical and biological interactions with the local environment. Presume, for example, that a tanker truck spills a large volume of water with trace amounts of salt, iron and lead. The wastewater enters a sandy aquifer and a year later, the water itself has moved a horizontal distance of about 200 feet. The salt, moving along with the water, has also traveled about 200 feet. In contrast, the iron has moved only 100 feet in a year and the lead contamination reaches only 30 feet from the spill site.