Most water pollutants are eventually carried by the rivers into the oceans. In some areas of the world the influence can be traced hundred miles from the mouth. Indicator filter feeding species such as copepods are used to study pollutant fates in the New York Bight , for example. The highest toxin loads are not directly at the mouth of the Hudson River , but 100 kilometers south, since several days are required for incorporation into planktonic tissue. The Hudson discharge flows south along the coast due to the coriolis force . Further south then are areas of oxygen depletion , caused by chemicals using up oxygen and by algae blooms , caused by excess nutrients from algal cell death and decomposotion. Fish and shellfish kills have been reported, because toxins climb the foodchain afer small fish consume copepods , then large fish eat smaller fish, etc. Each step up the food chain concentrates certain toxins like heavy metals and DDT by approximately a factor of ten.

Many chemicals undergo reactive decay or change especially over long periods of time in groundwater reservoirs. A noteworthy class of such chemicals are the chlorinated hydrocarbons such as trichloroethylene (used in industrial metal degreasing) and tetrachloroethylene used in the dry cleaning industry. Both of these chemicals, which are carcinogens themselves, undergo partial decomposition reactions leading to new hazardous chemicals.

Groundwater pollution is much more difficult to abate than surface pollution because groundwater can move great distances through unseen aquifers . Non-porous aquifers such as clays partially purify water of bacteria by simple filtration (adsorption and absorption), dilution, and, in some cases, chemical reactions and biological activity: however, in some cases, the pollutants merely transform to soil contaminants . Groundwater that moves through cracks and caverns is not filtered and can be transported as easily as surface water. In fact this can be aggravated by the human tendency to use natural sinkholes in seldomly occuring areas of Karst topography.

There are a variety of secondary effects stemming not from the original pollutant, but a derivative condition. Some of these secondary impacts are:

• Silt bearing runoff from can inhibit the penetration of sunlight through the water column, hampering Photosynthesis in aquatic plants.
• Thermal pollution can induce fish kills and invasion by new thermophyllic species