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Unit 8: Water Resources // Section 8: Water Pollution


Many different types of contaminants can pollute water and render it unusable. Pollutants regulated in the United States under national primary drinking water standards (legally enforceable limits for public water systems to protect public health) include:

These pollutants come from a wide range of sources. Microorganisms are typically found in human and animal waste. Some inorganic contaminants such as arsenic and radionuclides such as uranium occur naturally in geologic deposits, but many inorganic and most major organic pollutants are emitted from industrial facilities, mining, and agricultural activities such as fertilizer and pesticide application.

Sediments (soil particles) from erosion and activities such as excavation and construction also pollute rivers, lakes, and coastal waters. As discussed in Unit 3, "Oceans," availability of light is the primary constraint on photosynthesis in aquatic ecosystems, so adding sediments can severely affect productivity in these ecosystems by clouding the water. It also smothers fish and shellfish spawning grounds and degrades habitat by filling in rivers and streams (Fig. 13).

Sedimentation in Chattahoochee River, Atlanta, Georgia

Figure 13. Sedimentation in Chattahoochee River, Atlanta, Georgia
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Source: United States Geological Survey.

Water supplies often become polluted because contaminants are introduced into the vadose zone or are present there naturally and penetrate to the water table or to groundwater, where they move into wells, lakes, and streams. Many dissolved compounds can be toxic and carcinogenic, so keeping them out of water supplies is a central public-health goal. One critical question is how compounds of concern behave in water. Non-aqueous phased liquids (NAPLs) form a separate phase that does not mix with water and can reside as small blobs within the pore structure of aquifers and soils. Some, such as gasoline and diesel fuel, are lighter than water and will float on top. Others, including chlorinated hydrocarbons and carbon tetrachloride, are denser and will sink. Both types are difficult to remove and will slowly dissolve into groundwater, migrating downgradient as groundwater flows.

Other contaminants completely dissolve in water and, if they enter the aquifer at a single location (e.g., from a point source), are transported with flowing groundwater as plumes that gradually mix with native groundwater (Fig. 14). Over time, contaminated zones become larger but concentrations fall as the plume spreads. The paths that plumes follow can be extremely complex because of the complicated patterns of permeability within aquifers. Groundwater velocities are much higher through channels of high permeability, so these channels transport dissolved contaminants rapidly through the subsurface.

Contaminant flow in groundwater

Figure 14. Contaminant flow in groundwater
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Source: United States Geological Survey.

As a plume moves through groundwater, some contaminants in it may bind to soil particles, a process called sorption. High organic material and clay content in soils generally increases sorption because these particles are chemically reactive and have large surface areas. Sorption may prevent contaminants from migrating: for example, in some spills containing uranium, the uranium has moved only a few meters over decades. However, contaminants like uranium can also adsorp to very small suspended particles called colloids that migrate easily through aquifers. Even if a contaminated plume is pumped out, sorbed contaminants may remain on the solid matrix to desorb later back into the groundwater, so sorption makes full cleanup of the contamination more expensive and time-consuming.

Water pollution is relatively easier to control when it comes from a point source—a distinct, limited discharge source such as a factory, which can be required to clean up or reduce its effluent. Nonpoint source pollution consists of diffuse, nonbounded discharges from many contributors, such as runoff from city streets or agricultural fields, so it is more challenging to control.

Approaches for controlling nonpoint source pollution include improving urban stormwater management systems; regulating land uses; limiting broad application of pesticides, herbicides, and fertilizer; and restoring wetlands to help absorb and filter runoff (Box 1). U.S. regulations are increasingly emphasizing limits on total discharges to water bodies from all sources (for details, see the discussion of Total Maximum Daily Loads below in Section 10, "Major Laws and Treaties").

Types of wetlands

Types of wetlands
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Source: United States Environmental Protection Agency.

How wetlands improve water quality

In recent decades, appreciation has grown for the role that wetlands play in improving water quality. Specific functions vary with each site's vegetation, geology, and hydrologic patterns, but wetlands typically perform a number of purifying functions. For example:

"Water chemistry in basins that contain a large proportion of wetlands is usually different from that in basins with fewer wetlands. Basins with more wetlands tend to have . . . lower concentrations of chloride, lead, inorganic nitrogen, suspended solids, and total and dissolved phosphorus than basins with fewer wetlands."

Virginia Carter, U.S. Geological Survey
Wetland Hydrology, Water Quality, and Associated Functions (1997)


Along with freshwater bodies, many coastal areas and estuaries (areas where rivers meet the sea, mixing salt and fresh water) are severely impacted by water pollution and sedimentation. Ocean pollution kills fish, seabirds, and marine mammals; damages aquatic ecosystems; causes outbreaks of human illness; and causes economic damage through impacts on activities such as tourism and fishing.

A 2000 National Research Council report cited nutrient pollution (excess inputs of nitrogen and phosphorus) as one of the most important ocean pollution problems in the United States (footnote 14). As discussed in Unit 3, "Oceans," and Unit 4, "Ecosystems," nutrient-rich runoff into ocean waters stimulates plankton to increase photosynthesis and causes "blooms," or population explosions. When excess plankton die and sink, their decomposition consumes oxygen in the water.

Since the beginning of the industrial age, human activities, especially fertilizer use and fossil fuel combustion, have roughly doubled the amount of nitrogen circulating globally, increasing the frequency and size of plankton blooms. This process can create hypoxic areas ("dead zones"), where dissolved oxygen levels are too low to support marine life—typically less than two to three milligrams per liter. Seasonal dead zones regularly appear in many parts of the world. One of the largest, in the Gulf of Mexico, covers up to 18,000 square kilometers each summer, roughly the size of New Jersey (Fig. 15), where river and groundwater flow deliver excess nutrients from upstream agricultural sources to the coast.

Gulf of Mexico Dead Zone, July 2006

Figure 15. Gulf of Mexico Dead Zone, July 2006
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Source: NOAA Satellite and Information Service, National Environment Satellite, Data, and Information Services.

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