A reduction in pH can occur in streams with limited buffering capacity (alkalinity). One of the main causes of anthropogenic acidity is acid precipitation. Nitric and sulfuric acids from coal and other fossil fuel combustion such as automobiles' exhaust form within clouds and are deposited onto the watershed with rain and snow. Although streams located in regions with significant industrial, urban, or mining influences are at a higher risk, acidic deposition can be transported long distances in the atmosphere and deposited in pristine watersheds that are otherwise unaffected by humans. pH lowering acids also often enter streams through industrial waste water discharges.
pH regulates many biogeochemical processes within a stream. For example, it regulates the proportion of NH3 to NH4, the solubility of potentially toxic metals such as aluminum, and microbial decomposition rates. Increased acidity can reduce the diversity of every biological component from microbes to fish, and especially harm pH sensitive species such as invertebrates with shells composed of calcium carbonate as well as salmonid fishes.
Salinity pollution in streams can be due to the leach ing of salts from soils, or caused by runoff of road salt used in cold, snowy areas. Dryland salinity occurs when a reduction of natural vegetation allows more rainfall to penetrate deeper into the soil and bring up excess salts to surface waters. Irrigation salinity occurs through the same process but the role of rainfall is replaced by irrigation water. Excess salinity in streams causes increased channel erosion due to a breakdown of the soil structure, as well as an increase in salt tolerant species.
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