Water pollution, defined as the introduction of contaminants into water bodies, disrupts the Earth’s continuous global water cycle. This hydrologic cycle moves water through the atmosphere, oceans, land surface, and subsurface, sustaining all life. Pollutants from human activity interfere with the physical and chemical mechanisms governing this cycle. This fundamentally alters how water is stored, moves, and changes state, impacting the quality and distribution of global water resources.
The Baseline: Understanding the Water Cycle
The natural water cycle continuously transfers water between the land, the atmosphere, and the oceans. Movement begins with evaporation, where solar energy converts liquid water into vapor that rises into the atmosphere. This is supplemented by transpiration, the release of water vapor from plants.
As moist air ascends and cools, the water vapor undergoes condensation, forming clouds around tiny airborne particles. Water then falls back to the surface as precipitation, such as rain or snow. Once on the ground, water either flows as runoff or soaks into the soil via infiltration, replenishing groundwater reserves and feeding surface water bodies.
How Pollution Alters Surface Processes
Pollutants directly modify physical processes at the water’s surface, particularly evaporation and runoff. Thermal pollution, such as discharging heated water, increases the temperature of surface water bodies, raising the rate of evaporation. This localized increase in water vapor can alter regional humidity and precipitation patterns.
Conversely, chemical contaminants like oil slicks or industrial surfactants form thin chemical films on the water’s surface. These films act as a physical barrier, significantly suppressing the natural rate of evaporation. This reduced evaporation impacts the quantity of water vapor available for atmospheric processes.
Sedimentation and nutrient loading also alter the surface water balance. Excessive sediment increases turbidity, reducing sunlight penetration and altering the water’s thermal structure. Nutrient runoff causes eutrophication and dense algal blooms. These blooms change the water’s optical properties, indirectly influencing surface water temperature and evaporation rates.
How Pollution Impacts Atmospheric Processes
Pollution interferes with cloud and precipitation formation by introducing foreign matter into the atmosphere. Polluted water bodies and land surfaces release aerosols, or fine airborne particulate matter, through evaporation and industrial emissions. These particles travel high into the atmosphere and act as Cloud Condensation Nuclei (CCN) for water vapor to condense upon.
Increased concentrations of man-made CCN lead to clouds containing more numerous but smaller water droplets. This change makes clouds brighter, increasing their reflectivity and causing them to reflect more sunlight back into space. The smaller droplets also resist coalescing and falling as rain, which suppresses precipitation and increases the cloud’s lifetime.
Pollutants also engage in chemical transport, cycling volatile substances into the atmosphere that return to the surface in precipitation. For example, sulfur dioxide and nitrogen oxides dissolve in cloud water to form acid rain. This chemically altered precipitation contaminates surface water, soil, and vegetation upon return to the Earth’s surface.
How Pollution Affects Subsurface Processes
The final stage of the water cycle, where water moves beneath the surface, is severely compromised by contamination. Surface contaminants, including heavy metals and excessive salts, can drastically change soil permeability. Some chemicals clog soil pores, reducing the infiltration rate and increasing surface runoff and erosion.
Conversely, some contaminants can cause soil particles to aggregate or shrink, which may increase permeability. Regardless of the change, this alteration impacts the natural rate of groundwater recharge. When pollutants are carried downward into the saturated zone, they result in aquifer contamination, forming a harmful plume.
Once contaminants are in the groundwater, they are extremely slow to disperse and break down due to limited flow and oxygen availability. Aquifers act as long-term storage for these pollutants, often persisting for decades or centuries and compromising drinking water sources. Pollutants can also change the hydraulic conductivity of the underground rock and soil, disrupting the natural movement of subsurface water.