The Choice of Major Pollution Impact Indicators: The first thing that comes to mind in the selection of major aquatic oil pollution impact indicators is an understanding of those environmental attributes which influence the survival of aquatic life, such that a shift in the ecosystem balance and biodiversity usually results from an alteration in their measurable proportions. Obviously, the boundaries of natural ecosystem are determined by the environment, that is, by what forms of life can be sustained by prevailing environmental conditions. In this light the choice of parameters has to do with maximum productivity of fish and other aquatic life as well as the well-being of human dwellers and their farmlands in cases where flooding are witnessed or recorded.

According to Damaskos and Papadopoulos (1983), the generally accepted indicators of water quality are dissolved oxygen (DO) and the biochemical oxygen demand (BOD). High oxygen depletion can be so severe as to affect fish life. If the DO value falls below the minimum oxygen requirement for the particular species of fish, they are subjected to stress, which can result in mortality. The oxygen content of natural waters varies with temperature, salinity, turbulence, the photosynthetic activity of algae and plants, and atmospheric pressure. Chapman and Kimstach (1992) noted that DO concentrations below 5mg/1 adversely affect the functioning and survival of biological communities, and below 2 mg/1 may lead to the death of most fish. The optimum concentration of DO for fish and other aquatic life is given in Table 1.

Table 1: Optimum environmental conditions for fish and aquatic life. 

Sources:  Chapman and Kimstach (1992); Chatopadhyay et al. (1988)

The BOD is estimated by the amount of oxygen required for the aerobic microorganisms (in the case of oil pollution, hydrocarbon degraders) present in the water body to oxidize the organic matter to a stable inorganic form. Thus, when we say that a water body has a BOD value of đ mg/1 we mean that the concentration of biodegradable organic matter in one litre of it is such that the micro-organisms need đ mg of oxygen in order to be able to oxidize it. The result of a study carried out by Chattopadhyay et al. (1988) indicated 10 – 20 mg/1 as the optimum BOD range for fish culture in effluent or polluted waters. The addition of significant quantities of crude oil to any water body causes an immediate rise in the BOD due to the activities of hydrocarbon degraders and the blockade of oxygen dissolution.

Changes in pH (or the hydrogen ion activity) can indicate the presence of certain pollutants, particularly when continuously measured and recorded, together with the conductivity of a water body. The pH of 1 unit could result in an increase of lead by a factor of 2.1 in the blood of an exposed organism (Sheehan et al., 1984). What is known, of course, is that pH changes can drastically affect the structure and function of the ecosystem, both directly and indirectly by, for example, increasing the concentration of heavy metals in the water through increased leaching from sediments. Helz et al. (1975) found out that cadmium, which is toxic to many organisms, could be readily remobilized from sediments.

It is important to note that the pH of any water body is dependent on its temperature. And temperature affects physical, chemical and biological processes in water bodies and, therefore, the concentration of many variables. According to Chapman and Kimstach (1992), increased temperature increases the rate of chemical reactions and decreases the solubility of gases (especially oxygen) in water. Respiration rates of aquatic organisms increase leading to increased oxygen consumption and increased decomposition of organic matter.

Crude oil is associated with some toxic heavy metals most of which contaminate the oil through underground deposits, especially lead and chromium. Iron is in great abundance in tropical and subtropical aquifers and is also associated with crude oil deposits. High iron concentrations in groundwater are widely reported from developing countries, where iron is often an important water quality issue. Some metals also get into oil due to pipeline ageing and corrosion. Metal-induced depression of productivity most certainly occurs and may persist in polluted aquatic systems. Certain organisms have been shown to have some ability to regulate levels of copper and zinc in muscle (Sheehan et al., 1984). However, bioaccumulation of metals such as lead and chromium by fish is expected, and this spells danger to the human populations consuming such fish. Results obtained by Rai and Chandra (1992) show marked accumulation of copper, manganese, lead, and iron by the alga Hydrodictyon reticulatum under both field and laboratory conditions. Fish usually preys upon algae and other planktonic and benthic organisms; and when there is bioaccumulation of heavy metals in fish, there is likelihood of morbidity and mortality in man along the food chain. Usually bioaccumulation of toxic metals can occur to certain extent before chronic-effects thresholds are reached.

An important indicator of significance in aquatic oil pollution monitoring and control is the actual volume of oil released, which is approximated by the milligrams of oil in a litre of the water. Oil pollution, apart from causing depletion of oxygen and suffocation of aquatic species, affects plants and cultivated crops in lowland areas characterized by seasonal flooding. Ilangovan and Vivekanandan (1992) working with blackgram (Vigna mungo) concluded that oil pollution in soil might deplete oxygen at the rhizosphere because of possible depletion of soil oxygen by hydrocarbon-degrading micro-organisms and, therefore, oil polluted soil directly affects the overall physiology of the plant as evidenced by lower levels of macro and micro-biomolecules of the plant as well as polarity, thereby reducing plant growth. According to these workers, as a result of continuous aqueous oil effluent irrigation in about 25 acres of crop field, oil compounds infiltrated up to 50cm depth of the soil. Also total phenolic content of the leaves in the plant increased significantly.

Limits of Major Indicators for the Freshwater Environment:

The activities of major oil field operators, especially in Nigeria, are such that unavoidable hydrocarbon releases into the immediate environment are expected. This is particularly the case in flowstations, gas plants and compressor stations where effluent are generated during normal production operations. Facilities located in inland areas, especially environments characterized by seasonal flooding, contribute immensely to environmental degradation thereby endangering both aquatic life and human settlers within the vicinity. Oil spillage obviously decreases aquatic productivity and impacts negatively on the economic and social lives of local fishermen. DPR (1991) has outlined a set of standards/limits of some major oil pollution impact indicators in oilfield effluent released into freshwater environments. The limits for physico-chemical parameters and mineral concentrations are presented in Tables 2 and 3.

Table 2:  The DPR Limits of major physico-chemical parameters in effluent inputs into freshwater and marine environment.

Source: DPR (1991)

Table 3: DPR Limits for Heavy metal concentrations in fresh water environments.

Source: DPR (1991)

And relative to the volume of oil pollution in the immediate and nearby environments, these parameters usually reveal impact magnitude by the degree of deviations in their measured values from set limits.

Examining the oil-grease input into the environment and comparing with DPR limit of 10 mg/1 can easily assess the environmental performance of oilfield operators in the inland areas. Figure 1 shows the oil-grease content of a flowstation effluent in the Niger Delta determined over a one-year period (December 1995 – November 1996). The presentation which is adapted from SEEMS (1997) show that produced waters as point sources have their compositions affected by factors other than separator efficiency. For instance, the percentage of lift-gas applied for oil-water separation may largely depend on the subjective assessment by production personnel.

It is expected that substantial dilution of the effluent will occur to such as extent that the recipient freshwaters when examined will conform to World Health Organization (WHO) standard. For example the WHO turbidity limit for freshwaters is 5 NTU (Chapman and Kimstach, 1992). Heavy metal content is also greatly reduced by algal growth. Of course, algae are know to concentrate many trace metals in their tissues, and as the algae die and sink, their metal content may be carried down to the sediments. However, the danger is in the consumption of the algae by fish which accumulate them to levels toxic to man.

Limits Of Impact Indicators In Estuarine And Marine Environments 

The brackish water and marine environments are generally characterized by high salinities, total solids and temperatures compared to freshwaters. Whenever there is a spill or oil influx into such environments, the main concern must be the volume of oil spilled as well as turbidity and suspended solids content. Heavy metal composition is of lesser interest, as sea waters are known to contain trace metals in significant concentrations (Helz et al., 1975).

DPR (1991) gave limits of some important physico-chemical parameters for anthropogenic inputs into estuarine and marine environments in Table 2. The maximum allowable oil-grease concentration is 20 mg/1 for effluent released into offshore environment. This is only for produced waters, especially for facilities in swamp locations. These are of course controlled releases, and do not include well flushing and equipment repair/maintenance waste waters.

Seawater salinities above 2,000 mg/1 show clearly the presence of pollutants which may impact estuarine fish species. Biodiversity sustainability is of utmost importance as a shift in ecosystem balance may result in stress and depression of certain susceptible species. It is important to note that for every salinity value of 35,000 mg/1, the sodium content is 10,770 mg/1 with sodium/salinity ratio put at 1:3.26 (SEEMS, 1997). This means that high salinity might not be favourable for aquatic life.

CONCLUSION

We have examined the important indices for evaluating the severity or otherwise of aquatic oil pollution impact. The choice of these variables is found to be governed by the consideration of aquatic productivity and socio-economic life of human settlers. The limits for anthropogenic or controlled effluent inputs have been assessed both for freshwater and estuarine/marine environments. From the foregoing, a conclusion can be drawn that regular environmental performance evaluation of oilfield activities must be the hallmark of any meaningful environmental policy. Continuous effluent monitoring and environmental evaluation are needful, as even the controlled inputs into the environment are not consistent in their composition of important parameters.  Regulatory agencies such as the Federal Environment Protection Agency (FEPA) and the Department of Petroleum Resources (DPR) have a task of seeing to the implementation of environment–friendly progrmames by the oilfield operators.

REFRENCES

Copyright 2004 - Journal of Applied Sciences & Environmental Management