James G. Speight - Encyclopedia of Renewable Energy

The water cycle describes the means by which water evaporates from the surface of the Earth, rises into the atmosphere, cools, condenses to form clouds, and falls again to the surface as precipitation. This system of the cycling of water is intimately linked with energy exchanges among the atmosphere, the ocean, and land that determine the climate of the Earth and cause much of natural climate variability. For example, approximately 75% of the energy (or heat) in the global atmosphere is transferred through the evaporation of water from the surface of the Earth. On land, water evaporates from the ground, mainly from soils, plants (through transpiration), lakes, and streams. In fact, approximately 15% v/v of the water entering the atmosphere is from evaporation from the land surfaces and evapotranspiration from plants which (i) cools the surface of the Earth, (ii) cools the lower atmosphere, and (iii) provides water to the atmosphere to form clouds.

The major physical components of the global water cycle include (i) the evaporation from the ocean and land surfaces, (ii) the transport of water vapor by the atmosphere and precipitation onto the ocean and land surfaces, (iii) the net atmospheric transport of water from land areas to ocean, and (iv) the return flow of fresh water from the land back into the ocean. The additional components of oceanic water transport are few, including the mixing of fresh water through the oceanic boundary layer, transport by ocean currents, and sea ice processes.

Water pollution has become a widespread phenomenon and has been known for centuries, particularly the pollution of rivers and groundwater. By way of example, in ancient time up to the early part of the 20 thcentury, many cities deposited waste into the nearby river or even into the ocean. It is only very recently (because of serious concerns for the condition of the environment) that an understanding of the behavior and fate of chemicals, which are discharged to the aquatic environment as a result of these activities, is essential to the control of water pollution. In rivers, the basic physical movement of pollutant molecules is the result of advection, but superimposed upon this are the effects of dispersion and mixing with tributaries and other discharges. Some of the chemicals discharged are relatively inert, so their concentration changes only due to advection, dispersion, and mixing. However, many substances are not conservative in their behavior and undergo changes due to chemical or biochemical processes, such as oxidation.

In addition, there are many indications that the chemical materials in the aquasphere (also called, when referring to the sea, the marine aquasphere) are subject to intense chemical transformations and physical recycling processes imply that a total carbon approach is not sufficient to resolve the numerous processes occurring. The transport of anthropogenically produced or distributed compounds such as crude oil hydrocarbon derivatives and halogenated hydrocarbon derivatives, including the polychlorobiphenyl derivatives the DDT family, and the Freon derivatives and the chemistry of these chemicals in water is not fully understood.

The effects of a chemical released into the marine environment (or any part of the aquasphere) depends on several factors such as (i) the toxicity of the chemical, (ii) the quantity of the chemical, (iii) the resulting concentration of the chemical in the water column, (iv) the length of time that floral and faunal organisms are exposed to that concentration, and (v) the level of tolerance of the organisms, which varies greatly among different species and during the life cycle of the organism. Even if the concentration of the chemical is below what would be considered as the lethal concentration, a sub-lethal concentration of an chemical can still lead to a long-term impact within the aqueous marine environment. For example, chemically-induced stress can reduce the overall ability of an organism to reproduce, grow, feed, or otherwise function normally within a few generations. In addition, the characteristics of some chemicals can result in an accumulation of the chemical within an organism ( bio-accumulation ) and the organism may be particularly vulnerable to this problem. Furthermore, subsequent bio-magnification may also occur if the chemical (or a toxic product produced by one or more transformation reactions) can be passed on, following the food chain up to higher flora or fauna.

In terms of a chemical spill into the environment, the complex processes of transformation start developing almost as soon as the chemical contacts the land (or the water) although the progress, duration, and result of the transformations depend on the properties and composition of the chemical, parameters of the spill, and environmental conditions. The major operative processes are (i) physical transport, (ii) dissolution, (iii) emulsification, (iv) oxidation, (v) sedimentation, (vi) microbial degradation, (vii) aggregation, and (viii) self-purification.

In terms of physical transport , the distribution of oil spilled on the sea surface occurs under the influence of gravitational forces and is controlled by the viscosity of the (liquid) chemical as well as the surface tension of the water. In addition, during the first several days after a spill of a liquid chemical or a mixture of liquid chemicals, a part of spilled chemical may be lost through evaporation and any water-soluble constituents disappear into the water. The portion of the chemical mixture that remains is the more viscous fraction. Further changes take place under the combined impact of meteorological and hydrological factors.

Many organic chemicals are not soluble in water, although some constituents may be water-soluble to a certain degree, especially low-molecular-weight aliphatic and aromatic hydrocarbon derivatives. Polar compounds formed as a result of oxidation of some oil fractions in the marine environment also dissolve in seawater. Compared to evaporation process, the dissolution of organic chemicals in water is a slow process. However, the emulsification of a chemical (or chemicals) in the aquasphere does occur but depends predominantly on the presence of organic functional groups in the spilled material which can increase with time due to oxidation. The rate of emulsification process can be decreased by use of emulsifiers – surface-active chemicals with strong hydrophilic properties used to eliminate oil spills – which help to stabilize oil emulsions and promote dispersing oil to form microscopic (invisible) droplets that accelerates the decomposition of the chemicals in the water.

Oxidation is a complex process that can ultimately results in the destruction of the crude boil constituents. The final products of oxidation (such as hydroperoxide derivatives, phenol derivatives, carboxylic acid derivatives, ketone derivatives, and aldehyde derivatives) usually have increased water solubility. This can result in the apparent disappearance of the chemicals from the surface of the water. This is due to the incorporation of oxygen-containing functional groups into the chemicals which results in a change in density with an increase in the ability of the transformed chemicals to become miscible (or emulsify) and sink to various depths of the water system as these changes intensify. These chemical changes also result in an increases in the viscosity of the chemicals which promotes the formation of solid oil aggregates. The reactions of photo-oxidation, photolysis in particular, also initiates transformation of the more complex (polar) chemicals.

As these processes occur, some of the chemicals are adsorbed on any to suspended material and deposited on the floor of the water system (sedimentation), the rate of which is dependent upon the ocean depth – in deeper areas remote from the shore, sedimentation of oil (except for the heavy fractions) is a slow process. Simultaneously, the process of biosedimentation occurs – in this process, plankton and other organisms absorb the emulsified chemical – and the transformed chemical is sent to the bottom of the water system as sediment with the metabolites of the plankton and other organisms. However, this situation radically changes when the suspended chemical(s) oil reaches the bottom – the decomposition rate of the chemical ceases abruptly especially under the prevailing anaerobic conditions, and any chemicals that have accumulated inside the sediments can be preserved for many months and even years. These products can be swept to the edge of the water system (the river bank, the lake shore, or the beach in the case of a spill into an ocean) by turbulent condition at some later time.