Causes of Water Pollution [Part 1]
Pesticides
Pesticides that get applied to farm fields and roadsides—and homeowners' lawns—run off into local streams and rivers or drain down into groundwater, contaminating the fresh water that fish swim in and the water we humans drink. It's tempting to think this is mostly a farming problem, but on a square-foot basis, homeowners apply even more chemicals to their lawns than farmers do to their fields! Still, farming is a big contributor to this problem. In the midwestern United States, a region that is highly dependent on groundwater, water utilities spend $400 million each year to treat water for just one chemical—the pesticide Atrazine.
Fertilizers / Nutrient Pollution
Many causes of pollution, including sewage, manure, and chemical fertilizers, contain "nutrients" such as nitrates and phosphates. Deposition of atmospheric nitrogen (from nitrogen oxides) also causes nutrient-type water pollution.
In excess levels, nutrients over-stimulate the growth of aquatic plants and algae. Excessive growth of these types of organisms clogs our waterways and blocks light to deeper waters while the organisms are alive; when the organisms die, they use up dissolved oxygen as they decompose, causing oxygen-poor waters that support only diminished amounts of marine life. Such areas are commonly called dead zones.
Nutrient pollution is a particular problem in estuaries and deltas, where the runoff that was aggregated by watersheds is finally dumped at the mouths of major rivers.
Oil, Gasoline and Additives
Oil spills like the Exxon Valdez spill off the coast of Alaska or the more recent Prestige spill off the coast of Spain get lots of news coverage, and indeed they do cause major water pollution and problems for local wildlife, fishermen, and coastal businesses. But the problem of oil polluting water goes far beyond catastrophic oil spills. Land-based petroleum pollution is carried into waterways by rainwater runoff. This includes drips of oil, fuel, and fluid from cars and trucks; dribbles of gasoline spilled onto the ground at the filling station; and drips from industrial machinery. These sources and more combine to provide a continual feed of petroleum pollution to all of the world's waters, imparting an amount of oil to the oceans every year that is more than 5 times greater than the Valdez spill.
Shipping is one of these non-spill sources of oil pollution in water: Discharge of oily wastes and oil-contaminated ballast water and wash water are all significant sources of marine pollution, and drips from ship and boat motors add their share. Drilling and extraction operations for oil and gas can also contaminate coastal waters and groundwater.
As for gasoline and gas additives, leaking storage tanks are a big problem. The US Environmental Protection Agency (EPA) estimates that about 100,000 gasoline storage tanks are leaking chemicals into groundwater. In Santa Monica, California, wells supplying half the city's water have been closed because of dangerously high levels of the gasoline additive MTBE.
Mining
Mining causes water pollution in a number of ways:
- The mining process exposes heavy metals and sulfur compounds that were previously locked away in the earth. Rainwater leaches these compounds out of the exposed earth, resulting in "acid mine drainage" and heavy metal pollution that can continue long after the mining operations have ceased.
- Similarly, the action of rainwater on piles of mining waste (tailings) transfers pollution to freshwater supplies
- In the case of gold mining, cyanide is intentionally poured on piles of mined rock (a leach heap) to chemically extract the gold from the ore. Some of the cyanide ultimately finds its way into nearby water.
- Huge pools of mining waste "slurry" are often stored behind containment dams. If a dam leaks or bursts, water pollution is guaranteed.
Perhaps the worst offense in the category of mining vs. water pollution causes: Mining companies in developing countries sometimes dump mining waste directly into rivers or other bodies of water as a method of disposal. Developed countries are not immune from such insanity: The US government in 2003 reclassified mining waste from mountaintop removal (a type of coal mining) so it could be dumped directly into valleys, burying streams altogether.
Sediment
When forests are "clear cut," the root systems that previously held soil in place die and sediment is free to run off into nearby streams, rivers, and lakes. Thus, not only does clearcutting have serious effects on plant and animal biodiversity in the forest, the increased amount of sediment running off the land into nearby bodies of water seriously affects fish and other aquatic life. Poor farming practices that leave soil exposed to the elements also contribute to sediment pollution in water.
Chemical and Industrial Processes
Almost all bodies of water in the world have some level of pollution from chemicals and industrial waste.
In the United States, 34 billion liters per year (60%) of the most hazardous liquid waste—solvents, heavy metals, and radioactive materials—is injected directly into deep groundwater via thousands of "injection wells." Although the EPA requires that these effluents be injected below the deepest source of drinking water, some pollutants have already entered underground water supplies in Florida, Texas, Ohio, and Oklahoma.
The US is not alone in careless treatment of its groundwater. In the late 1990s, India's Central Pollution Control Board found that groundwater was unfit for drinking in all 22 major industrial zones it surveyed.
Plastic
Plastics and other plastic-like substances (such as nylon from fishing nets and lines) can entangle fish, sea turtles, and marine mammals, causing pain, injury, and even death. Plastic that has broken down into micro-particles is now being ingested by tiny marine organisms and is moving up the marine food chain.
Sea creatures that are killed by plastic readily decompose. The plastic does not—it remains in the ecosystem to kill again and again.