Water Pollutants Emitted by Transport Systems

Author: Dr. Jean-Paul Rodrigue

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1. Modal Impacts

Transportation contributes significantly to the pollution of the hydrosphere in various ways ranging from air pollution fallouts to the construction and maintenance of infrastructure such as roads, railways and ports. The first type of impacts are related to the transport modes.

Air Pollution Fallouts

• Fallouts occur when a pollutant goes from an airborne state (gas, solid or liquid) towards a solute or colloidal state. Water is a very good solvent for several pollutants, notably acid depositions. Fallouts are accelerated and concentrated in an area by rainy conditions.
• As an important source of air pollution, transportation accounts on a similar scale for fallouts. In some areas transportation may account for up to 25% of nitrogen fallouts in water. It is estimated that acid rains may account for more than 75% of the growth of acidity of lakes.
• Since fallouts are a continuous accumulation and occur over a longer period than most water pollution sources, they have a higher impact on still-water (lentic) environments than running-water (lotic). The most notable and destructive fallouts are sulfuric and nitric acids that may alter the pH of water if they are present in sufficient concentrations. Several northeastern United States and eastern Canadian lakes have seen their entire fish population destroyed as a result of increased acidity levels. It also includes damage to forests like reduced photosynthesis (sparse foliage) and acidified soils (limited nutrients). Nitrous oxides may affect the ecological balance of marine life by favoring algae blooms.
• Other fallouts such as HC/VOC and lead are poisonous and may disrupt marine life if they accumulate in the aquatic food chain. Particulate fallouts, when in sufficient quantities, may increase the turbidity of water and thus reduce the photosynthesis capacity of aquatic plants. A long term accumulation of air pollution fallouts of various nature will contaminate and disrupt whole aquatic ecosystems.

Marine Vessels Discharges and Spills

• After unloading their bulk loads like oil, coal, nitrates and mineral products, marine vessels require cleaning. Since this practice is restricted in several port and coastal areas, operators wait until they are in international waters to proceed. Oil products residuals carried by tankers are the major source for discharges.
• It is estimated that for every million tons of oil carried, one ton is spilled through washouts. Once a spill has occurred, it is extremely difficult to contain it. From 1989 to 1992, 105 accidental oil spills by tankers were accounted worldwide, totaling 991,000 tons of oil being spilled. Annually, an average of 1.1 million tons of oil comes from discharges and 400,000 tons are spilled. They depend on the nature of the residue discharged.
• Petroleum products are the most harmful and include environmental effects like the destruction/disruption of aquatic plant/animal life and of shore ecosystems. Since most marine life is in neritic (continental shelf) and epipelagic (less then 100 meters) zones, it is particularly vulnerable to marine vessels discharges. The Exxon Valdez oil spill is a well known example of such an occurrence.

2. Infrastructure Impacts

The second type of impact involves transport infrastructures.

De-Icing of Infrastructure and Runoffs

• Salt (NaCl) has the characteristic of lowering the melting point of water and thus presents an useful compound for keeping safe road conditions in sub-zero climates. Other elements like sand and gravel are also added to provide adherence.
• Runoffs occur when substances accumulated by a surface (notably a road) are dissolved / carried by water and evacuated elsewhere. It is often the convergence of a surface to a point. De-icing of transportation infrastructure (roads, parking lots, airfields etc.) is almost the only artificial source of salt release in the environment. Salt mostly comes from mining (halite) or in fewer proportions from sea water evaporation. Other compounds like calcium and magnesium can be used, but they work more slowly and cost ten times as much.
• Lubricants (from car leakages - engine, brakes, and transmission), heavy metals (Zn, Cd, Cu, Ni, Cr and Fe from abrasion of tires and brake linings) and dry fallouts (HC/VOC, particulates) account for harmful sources of runoffs.
• Since road infrastructure (parking lots, roads, drainage systems) occupy a significant land surface in developed countries, it is the major source of runoffs. For instance, while highways occupy 5-8% of the urban catchment area, it contributes for as much as 50% of the total suspended solids, 16% of the total HC and 75% of the total metal inputs to a receiving stream.
• High concentrations of salt, notably chlorine ions, in fresh water environments disrupt life cycles and may be fatal to some organisms like larvae. Runoffs from infrastructure will alter the turbidity and the oxygen level of water (warm water holds less oxygen), and contaminate the food chain. It may increase the eutrophication process of several lakes, particularly in recreational areas where dirt roads are dense. De-icing salt has the tendency to accumulate in snow and soils beside roadways. During early springtime, nearly all the salt accumulated will be released in the hydrographic system where it will contaminate ground water and interfere with the growth of plants and the reproduction cycle of aquatic life, particularly vulnerable at this time of year.
• Infrastructure runoffs collected by the sewage system of urban areas often converge at evacuation points and contaminate whole hydrographic systems at high concentrations. It is worth noting that most cities have 30 to 70% of their surface occupied by roads and parking space. They thus represent important sources of runoffs.

Construction and Maintenance of Infrastructure

• Several transportation infrastructures have important territorial handholds. When a transportation infrastructure is built over a hydrological environment like a river, wetland or a coastal area, disruption occurs.
• The maintenance of transportation infrastructure, particularly harbor and waterways (dredging), have also a significant impact. Each mode needs a specific set of infrastructure that interfere with hydric systems.
• Road infrastructure accounts for most of the territorial handhold of transportation with structures like bridges and parking facilities. Railways have also an important handhold over continental hydric systems. Maritime transportation, by its intrinsic link with hydric systems have several disruptive infrastructure like piers, canals, harbors and terminals. Airports have similar effects when constructed over wetland. Dredging accounts alone for 80% of the waste released in aquatic environments.
• The most widespread effect of transportation infrastructure on hydric systems is the removal of natural habitats along shorelines. The aquatic / land interface to which several animal and vegetal species depend is considerably reduced. Further, a modification of the aquatic environment occurs, particularly during dredging in port harbors and along waterways. This notably influences the turbidity of water and destroys habitats. Roads and rails, when running through wetland, reduce the water regeneration / purification capacity by splitting available areas and disrupting water flows. Large ports occupy extensive areas along the shorelines of waterways and coasts. The construction and maintenance of those infrastructure have thus extensive impacts over aquatic environments. The construction of canals changes whole hydrographic systems by altering water flows (quantity and speed) at regional and often at continental levels.