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Degradative potential of rivers as a key driver of the environmental fate and sink of organic pollutants

Final Report Summary - DEGRAPOLL (Degradative potential of rivers as a key driver of the environmental fate and sink of organic pollutants)

Rivers can be a significant sources and reservoirs of anthropogenic organic pollutants to the coastal environment. Discharge of domestic and industrial sewage water represents a major global source of chemicals, but indirect sources of water pollution also include contaminants that enter the water supply from soils or groundwater systems and from the atmosphere via wet, dry and gas deposition. With chemical factories, railways and highways in close proximity to natural waterways, unintentional releases of hazardous chemicals occur frequently. Once hazardous pollutants are in the aquatic system, they can have a number of detrimental effects that extend for considerable distances downstream from the pollutant source.
Even though the levels of organic pollutants in rivers are high, and the toxic influence of these pollutants is widely known, the impact of organic pollutants has been moderated despite the huge anthropogenic inputs to rivers. This modulation of riverine pollution is due to a number of attenuation processes that account for the riverine input-output paradox for organic pollutants (RIOPOP), and deserves further attention since it points out how natural ecosystems attenuate pollution, and provides clues for future man-made remediation strategies.
Multiple removal processes are at work in a river from upstream to downstream which must play an important role in the biogeochemical cycling and removal of man made organic pollutants. Many studies have focused on the vertical distribution and processes affecting pollutants in rivers and estuaries such as atmospheric deposition, accumulation and degradation in river sediments and by biofilms 1-9. However, processes that account for the importance of each removal process compared to the others in the direction of the flow (longitudinal transport) have been poorly studied. The processes involved are dilution dispersion, volatilization, sorption, degradation and transformation which collectively are referred to as natural attenuation 10,11; however the contribution of each removal process may change along a river as it strongly depends on the river characteristics 11. The potential synergies and cross relationship between the ecosystem, hydrology and pollution seems to result in a combination of different drivers of the biogeochemical cycling and removal of anthropogenic pollutants in rivers which are still difficult to calibrate. Depending on their physical-chemical properties, sources and use organic pollutants reaches rivers through different routes (WWTP, sewage treatments, direct discharge, efflents/tributaries, run off from soil, agricultural land, roads and soil erosion, groundwater and landfills leaching). Once in the river, organic pollutants will behave and impact depending on various environmental processes, the river-catchments environment and the chemical properties. They will be eventually removed by different processes including volatilization, sorption to particles/sediments and biofilms, photodegradation, biodegradation, biotransformation and leaching to groundwater. Figure 1 is a conceptual diagram summarizing these processes. Input processes are indicated with blue arrows while removal processes are indicated with red arrows. The development of the RIOPOP concept has led to the first publication of this project in the form of a peer-reviewed letter for the journal of Frontiers of Ecology and Environment (Rosalinda Gioia and Jordi Dachs. The riverine input-output paradox for organic pollutants. 2012. Frontiers of Ecology and Environment, 10, 405-406).

The “riverine input-output paradox” suggests that rivers can potentially be natural reactors and bioreactors, and substantially remove and reduce by means of different mechanisms the bargain of organic pollutants that finally reach the sea. Degradation by biofilms in rivers is believed to be one of the most important attenuation mechanism in rivers, although very little is known on factors driving this process. The main aim of the research proposed here is to contribute in the rise of knowledge about the degradative potential of rivers as driver of environmental fate and transport of organic pollutants during longitudinal transport. Biodegradation can be carried out by one specific species or by an entire microbial community, however factors driving the activity of competent degraders are still not clear, as it is not clear whether the community needs to adapt to the chemical or is already producing enzymes to degrade the chemical. Stream biofilms are dynamic matrices that undergo changes in composition of photosynthetic periphyton, heterotrophic microbes and extra cellular polymers in response to environmental conditions. The stream biofilm plays an important role in surface water attenuation. As compounds sorb to biofilm they can be potential exposure to higher trophic levels organisms.
Antibiotics are widely used and often abused in human medicine and stockbreeding operations, for both infectious disease therapy and growth promotion. Common veterinary and aquaculture antibiotics include tylosin, -lactams, tetracycline, and sulfonamides. The occurrence and spread of antibiotic-resistant bacteria (ARB) are pressing public health problems worldwide, and aquatic ecosystems are a recognized reservoir for ARB and antibiotic resistance genes (ARGs). Naturally occurring ARB and ARGs in the aquatic environment are selected for and enriched for by antibiotics found in sewage and agricultural runoff, which result from the widespread and increased use of antibiotics. A Nature review (2010) by Allen et al12., identifies mechanisms of antibiotic resistance as: a) Impermeable barriers. Some bacteria are intrinsically resistant to certain antibiotics (blue squares) simply because they have an impermeable membrane or lack the target of the antibiotic. b) Multidrug resistance efflux pumps. These pumps secrete antibiotics from the cell. c) Resistance mutations. These mutations modify the target protein, for example by disabling the antibiotic-binding site but leaving the cellular functionality of the protein intact or d) Inactivation of the antibiotic. Inactivation can occur by covalent modification of the antibiotic, such as that catalysed by acetyltransferases (purple) acting on aminoglycoside antibiotics, or by degradation of the antibiotic, such as that catalysed by β-lactamases (brown) acting on β-lactam antibiotics. So degradation can be a mechanism of resistance. Therefore the main question the laboratory experiment is aiming to answer is: “What resistant really means? Does it mean that the bacteria can degrade the antibiotic or that they are “inert” to it?”

Description of the laboratory experiment.
River water and biofilm (river stones) were collected at the source of the Llobregat river (upstream) and at the mounth of the Llobregat river (downstream). The stones and the river water from the were used to prepare mesocosms at two different concentrations (5000 ng/L and 1000ng/L) of sulphonamides (sulfamethazine and sulfamethoxhazole) for the two sampling locations. The experiment has last for 8 weeks (July-September 2012) and samples were collected every 2 weeks for:
a. chemical concentration to investigate whether there is any change in chemical concentrations as well as the presence of metabolites. Samples were processed for analyse of sulphonamides and metabolites by LC-MS ).
b. microbiological bulk measurements such as chlorophyll, biomass, nutrients and DNA estraction were also collected.

Controls for chemical degradation, sorption to the glass wall and microbial activity without the chemical were also deployed along the mesocosms.
Preliminary results show a decrease of chemical concentration during the experiment. However, results from analysis of metabolites and microbial activity my help to shed the light on what has caused the decrease in concentration of sulfonamides. Currently, we are finishing the scrutiny of all the biological results and preparing a manuscript to be published in a peer-reviewed scientific journal.
The characterization of attenuation processes such as biodegradation and biotransformation in rivers can provide insights for possible bioremediation strategies. These bacteria can proliferate in the environment as a result of the selection of pressures created by pollutants, adapting by mutating or acquiring degradative genes13. The study of the pollutant cycling, and bacterial response to organic pollutants will provide important benefits in current remediation strategies, and better tools for managing pollution in rivers and its impacts, which besides the RIOPOP, are far from being negligible.
This research and its outcome will implement awareness of toxic chemicals and the results of their consequent release in rivers in terms of degradation from upstream to downstream. This is veryimportant for the objectives of REACH whose purpose is to make industry responsible for assessing and managing the risks posed by chemicals and providing appropriate safety information to their users. Based on the results of this project, the degradation processes in rivers could be considered in the assessment process of a chemical under the REACH legislation. The development of models which can find applications in risk assessment and can serve as supporting tools for decision makers at international level is one of the main objectives of EU REACH. EU REACH represents the vanguard in the policies on sustainable chemical management. An important "step-forward" will be done by introducing a novel multidisciplinary key to interpret the relationship between the degradation and the cycling of organic pollutants in rivers. This aspect will be extremely important for the global fate modelling community.

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