The sedimentation process causes great problems worldwide by raising the cost of operation and maintenance and complicating the design of water structures. The problems linked to sediments create difculties in managing water systems. About 1% of the world’s water storage capacity is lost every year through sediment deposition in reservoirs.
Many of the large international rivers such as, e.g., the Nile, Mississippi and Yellow Rivers carry huge amount of sediments. It is known that the Fraser River in Canada carries an average of 20 million tons of sediment each year, the Nile River carries an amount that exceeds 140 million tons annually and the Yellow River in China carries an annual amount of sediment that is several times more than that of the Nile River.
For the assessment and management of contaminated sediments in different countries chemical data have been used by state and federal regulatory agencies. For better dening the relationship between sediment chemistry and toxicity, providing regulatory agencies with additional insight into the importance of sediment chemistry data, Sediment Quality Guidelines (SQGs) [15, 16] have been developed over the last years. Increasing interest in the development of risk-based sediment assessment frameworks to guide assessments and management decisions has led to questions concerning the role of SQGs within a sediment assessment and management process that makes use of multiple Lines Of Evidence (LOE) [16] to reach management decisions based on Weight-Of-Evidence (WOE) [17]. At the moment nearly 20 sediment assessment frameworks have been proposed or used by regulatory authorities in different countries.
In Europe the Working Group AMPS has prepared a strategic paper, which in turn, was the basis for a recently prepared SedNet brochure entitled Sediment, a valuable resource that needs Europe’s attention [18, 19]. In brief, sediments are an essential, integral and dynamic part of river basins. Sediments in rivers are an important habitat as well as a major nutrient source for aquatic organisms. Sediments are also used as farmland and as a source of minerals and materials. Stimulated by the WFD, the view on sediments is changing to the recognition of the key role that sediments play naturally in the river systems. Sediment management should t into the holistic view of the role of sediments in river basin systems. This means that transboundary management is needed for river systems that cross water bodies and national borders.
Therefore, in the opinion of SedNet, the WFD represents a great opportunity and stimulus to come up with guidance for sustainable sediment management. The current scope of the WFD does not yet specically deal with this subject. Sustainable sediment management should eventually become an integrated part of the WFD. The requirements for a river basinwide sediment concept will be even more challenging [20] that the current WFD.
General assessment of the state of heavy metal pollution of European rivers is difcult, primarily because measurement of metals is rarely included in monitoring programmes, but also because concentration levels are usually so low that problems arise with sample preparation and analytical accuracy. Until recently, metals have generally been determined either for their total concentration in water or their concentration in the suspended particulate fraction. Recent evidence, however, suggests that anthropogenic inputs generally can be better evaluated from associated particulate metals [21]. Comparison and assessment of the state of heavy metals in European rivers is thus even more difcult than for most of the other water quality parameters (see Table 5). A general overview of the riverine transport of water, sediments and pollutants to the Mediterranean sea, as published by UNEP/MAP in 2003 (MAP Technical Reports Series no. 141) points to the fact that in Italian rivers, e.g., in the Po River, when pH = 7-8.5, most heavy metals are not dissolved: Cd, Cu, Hg, Pb, Zn are adsorbed on the surface of the suspended particles. In such cases particulate matter is normally by far the dominant transport form for heavy metals in these rivers. For example, the percentage of the load of heavy metals associated with particulate matter in the Po River is as follow [21-23]: Al, 98.4%; As, 23.0%; Cd, 41.0%; Co, 94.7%; Cr, 88.2%; Cu, 73.0%; Hg, 78.4%; Pb, 93.1%; Zn, 71.1%.
Recent investigations concerning the average dissolved trace metal concentration in Italian rivers have shown that elevated levels were as a rule found, e.g., in the Po (Cd, Cu, Ni, Zn), the Tiber (Pb, Zn) and the Arno (Cd) Rivers, although a direct comparison is not always possible because of differences in the reference periods corresponding to the values.
Table 6 summarizes the average heavy metals contents in the total suspended particulate matter for Italian rivers. Some interesting aspects can be seen in the relative abundance of the trace metals. In the Po River, the heavy metals concentrations are elevated for most of these elements, i.e., Cd, Cr, Cu, Hg, Ni and Zn. In the Tiber River there are high concentrations of Cd, Cu and Zn, while in the Adige and Bradano Rivers Cd may raise some concern. These values also should be assessed along with the geological and lithological patterns of the river drainage basins, but this is beyond the scope of such studies.
Also trace organic compounds play a major role in river pollution. In many cases, pollution in rivers is also accompanied by organic micropollutants. Traditional monitoring for organic micropollutants in the aquatic environment has focused on organochlorine compounds (e.g., DDT and PCBs) and Polycyclic Aromatic Hydrocarbons (PAHs). However, organic micropollutants have rarely been included on a large-scale basis in national monitoring programmes. Moreover, in recent years there has been a growing awareness of the problem of aquatic pollution by other dangerous compounds, such as non-organochlorine pesticides. Assessment of the environmental risk posed by pesticides must be based on a combination of studies of their impact on the aquatic biota and model calculations of potential pesticide runoff.
It is widely acknowledged that sediments play a fundamental role for the health of the aquatic ecosystems as they offer a habitat to many organisms and support to all the aquatic fauna. The sediments have been often dened as the sink of persistent substances to accumulation in this compartment. Therefore, also for sediments, a sounder approach ensues from the integration of chemical, biological and ecotoxicological data.
Sediments also represent the compartment where the vast majority of the most dangerous, persistent and accumulation-prone pollutants are deposited. They not only exert effects on the benthic organisms, but can also endanger all the aquatic life at its various levels through the food web because of their re-suspension in the water column [25]. Toxicological analyses of sediments can supply very important information about the state of health of water bodies, rst of all in the case where the plain analysis conducted on the water column does not point to a signicant risk.
Biological and toxicological investigations of sediments constitute a complementary approach in the appraisal of their quality with peculiar advantages and limits [26-28]. The benthic communities react to changes in the quality of water, sediments and the habitat in general. The variations in the structure of the communities supply important information directly relevant for the assessment of the contamination impact. In other words, they are important for the effects in situ, although they are not diagnostic for contamination or toxicity themselves. In fact, the integration of all the biotic and non-biotic factors, implicit in biological analyses, cannot distinguish among stressful factors of chemical, physical and biological type.
The integration of the biological and toxicological indexes allows the limits of these two approaches to be circumvented, thus reducing the uncertainty of the risk assessment and allowing the interventions to be planned for the attainment of objectives of ecological quality.
Among the various sediments toxicity testing phases, interstitial water plays a major role for assessing the effects in situ since the benthic organisms are exposed to it directly. Studies have shown that the toxic concentrations in sediments were correlated to the concentrations in interstitial water and the dissolved fraction of the pollutants is considered in many cases to be bioavailable. Tests on pelagic and benthic species exposed to several phases of sediments of various nature and contamination degree have demonstrated that interstitial water represents a suitable means to forecast global sediment toxicity [29].
River sediments are saturated with interstitial water, which favours mobilization and separation of the polluting substances. Particularly, lipophilic (or hydrophobic) substances characterized by high values of the octanol-water partition coefcient (logKow≥ 3), are preferentially absorbed onto suspended material and, after sedimentation, are embodied within the sediment. Due to these characteristics, such substances tend to be signicantly bioaccumulated along the trophic chain and therefore are potentially very dangerous for their ability to produce toxic effects in the biota.
River sediments represent the main sink for micropollutants in the river environment. Sediments release the micropollutants in the water environment and this process depends on the environmental conditions. Changes in the environmental conditions can lead to signicant changes in the pattern of the release of micropollutants in water from sediments. Such phenomena can take place in strongly polluted areas or when environmental changes are dramatic.
As regards Italian legal provisions on water protection, the Decree DL.vo. 152/99, in Paragraph 3.2.1.3, Annex 1, prescribes that the analysis of river sediments should be considered as additional and optional, as it is useful to elucidate chemical parameters and ecological descriptors.
Pesticides have become an important part of modern agriculture and their use has been growing exponentially every year after the Second World War because of the increasing food demand. In spite of the undeniable advantages that pesticides have brought to modern economy, the phytosanitary products can generate a series of problems for untargeted animal species as well as for human health. Many pesticides and also some of their products of degradation can be found today in surface as well as in groundwaters. Some compounds, through the processes of erosion and streaming or drifting, can come in contact with surface water bodies and contaminate the sediments. The linkage of pesticides to sediment particles delays their migration and increases their persistence with potential risks to ecosystems [30]. Natural or xenobiotic substances released into the environment follows a specic biochemical cycle that determines their transport and distribution, which in their turn depend on the partition processes. Obviously the biogeochemical cycle of a substance and its environmental fate depend on the characteristics of the various compartments as well as from environmental parameters.
On the other hand, when environmental conditions are stable, the partition and the fate of an ionisable substance depend on its inherent properties, such as solubility, KH, Kow, Koc, DT50 and pH [31]. As already said, the sediments of an aquatic environment are saturated with interstitial water, which favours the transport and partition of pesticides. Moreover, this kind of water contains Dangerous Organic Materials (DOMs), i.e., colloids and other macromolecules that can bind these substances, thus facilitating their transport and bioavailability. It has been demonstrated that pesticide particles of small dimensions are distributed at greater concentrations in the sediment fractions containing small particles (
For the time being, a relation has been found between the contents of organic material of sediments and their adsorption ability for chemical substances, in particular for non-ionic organic compounds. Another parameter that inuences the adsorption ability is pH, this being particularly obvious in the case of s-triazine-type herbicides. The maximum adsorption takes place when pH is close to the values of pKa (1.7-2.6). At this pH half of the s-triazine compounds is in the cationic form and the other half is in the not-ionic state. The increase in pH results in a decrease of the cationic fraction in the solution and, therefore, in lesser adsorption. Other pesticides can be adsorbed as a consequence of weak bonds like the hydrogen bonds and the Van der Waals’ forces. Non-ionic compounds probably interact with sediments through hydrophobic connections little affected by pH.
Answers to all your Biology Questions
