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Ecohydrology, back to the roots

When ecological awareness meets the research process, new scientific fields are developed, such as ecohydrology. The idea is simple: to stimulate the regenerative capacities of an ecosystem in order to improve the filtration of polluting substances present in its waters. Cleaning up the pollution emitted by centuries of industrialization is no easy task, so we might as well rely on tools already existing in nature.


Death rows on the Thames, claiming the lives of victims who have not paid to have the river cleaned up, during the Great Stink. 10 July 1858. Cartoon from Punch Magazine.

Since the first industrial revolution, river water has been perceived by entrepreneurs as a free and inexhaustible resource. Whether as a driving force, washing agent or fuel solvent, factories have become accustomed to settling on its banks, mining and metallurgical operations to dump large quantities of solid matter into it, and cities, which are becoming increasingly large, to evacuate their wastewater there. Although some law articles were already restricting the discharge of rubbish into river waters, they did not prevent, for example, the Bradford Canal in England from catching fire several times during the 19th century because of the flammable gases emanating from it during the summer period. Such events, like the ʺ Great Stinkʺ that struck London in 1858, prompted European governments to take sanitary measures to counter pollution that had increased incredibly in recent decades.

At the end of the 1990s, the European Union authorities wanted to harmonize the policies of the member countries on this issue. They adopted, in the first months of the 21st century, their Water Framework Directive, also called WFD. This requires the Member States to take measures aiming at preserving and restoring aquatic ecosystems on their territory, with an objective of “good status or good potential” of surface and underground water bodies planned for 2015. Unfortunately, this ambition will soon prove to be unattainable. The Commission’s report on the implementation of the WFD in 2012 proposed to extend this deadline until 2027. A date which was again called into question by the committee’s last report, published last year. Nevertheless, the latter document shows clear improvements in water quality over the last few decades. While less than half of the surface water bodies achieve the “good status” defined by the directive, a “large majority” of groundwater basins now have this status. Moreover, according to the report, after thirty years of regulation in this area, the steady decline in water quality in Europe is finally beginning to reverse.


Understanding ecosystems as a whole

The Glen Canyon Dam, on the Colorado River in northern Arizona, USA. Located in a high desert climate, the lake it creates causes huge evaporation and seepage losses. An increasingly valuable amount of water in an arid land for both humans and the animals and plants that live along the river.

The website of Natural Water Retention Measures, a program motivated by the WFD, keeps an eye on a list of 129 projects that promote the use of natural methods and green infrastructures to help preserve aquatic ecosystems in Europe. At the heart of these projects, the emphasis is on the fields of ecoengineering and ecohydrology. At the end of the 20th century, several researchers began to point out the inadequacy of conventional methods in the sustainable management of water resources. Since the degradation of ecosystems stems from a variety of factors, a localized and short-term approach (e.g. the construction of basins or canals) generally proves too limited to be an effective solution.

“Environmental scientists […] seemed to focus their research on single problems, single factors or single species. In reality, the environment is very complex”, said applied ecology professor Maciej Zalewski in 2002. In an article for the Hydrological Sciences Journal, he uses the amount of spring water in a basin as an example. This determines the reproduction of fish, which also affects zooplankton density, filtration levels and, finally, the appearance and intensity of algal blooms. “To solve problems, environmental scientists must not only be integrative but also provide a bridge between pure science and society.”

This hybrid system in Norway is using Flowforms in a treatment pond to filtrate pollutants, as would do a natural wetland.

Thus, at the end of the 20th century, the concept of ecohydrology began to take shape, stemming from an observation: the reduction in biodiversity due to anthropization diminishes the potential for restoring and regulating natural ecological processes. In short, the poorer the environment, the more limited the number of routes that energy and nutrient flows can take. Eco-hydrology, therefore, considers ecosystems as “super-organisms”, endowed with resilience and adaptive capacities that allow them to absorb, to a certain extent, the environmental impacts caused by humans. Thus, the best approach would be the use of ecological engineering techniques, such as reforestation, the creation of dikes or the planting of specific species, to boost these natural capacities, to increase the number of ʺroutesʺ. According to Zalewski:

The water and climate-soil-vegetation interactions […] should be considered as a fundamental context for the above concept and its implementation. From an ecological perspective, the questions focus on the role of hydrology in regulating the life cycles and life strategies of freshwater organisms. Their consequences for water quality have, up to now, been largely neglected.


Cucurbits vs. hydrocarbons

Polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs). These three hydrocarbons are among the 45 substances on the WFD list that pose a serious threat to aquatic environments and humans. The first two are dioxins unintentionally released during industrial thermal processes (metallurgy, steelmaking or incineration). The third is a substance with toxicological properties similar to dioxins, and which was massively used in Europe between the 1930s and 1980s for its insulating properties and its resistance to heat (in electrical transformers, capacitors, etc.) According to INSERM and EFSA reports, the regulation of the use of PCBs and filters limiting the passage of PCDDs and PCDFs in factory fumes have greatly reduced the impregnation of populations with these substances in recent years. However, these three names also appear in the Stockholm Convention of 2011, listing persistent organic pollutants. Their high chemical stability goes hand in hand with a strong resistance to biodegradation. Clinging to mineral particles in the water, these substances end up at the bottom of rivers and act as a source of continuous pollution, particularly difficult to eradicate.

“The Cucurbitaceae family seems to have the astonishing property of capturing highly hydrophobic substances present in the soil, such as hydrocarbons, and isolating them in their stems and leaves.”

A Polish study published last December focuses on this issue. Analyses of the water composition along the Pilica River showed that the concentration of PCDDs, PCDFs, and PCBs in the water downstream of the Sulejów reservoir was reduced by almost 80%. When arriving at the dam, the current slows down and the hydrocarbon residues, attached to the mineral particles, settle to the bottom of the basin. The authors of this study, therefore, looked for an ecohydrological method to recover them from these sediments, and finally opted for a phytoremediation technique, literally “recovery by plants”. Indeed, the Cucurbitaceae family (zucchini, cucumbers and other types of squash) seems to have the astonishing property of capturing highly hydrophobic substances present in the soil (like our polluting acronyms) and isolating them in their stems and leaves. For example, a cucumber crop fertilized by sediment from the Sulejów reservoir saw the toxicity of its soil reduced by half. This method could also capture hydrocarbons present in sewage sludge, which is discharged from water treatment plants that are not designed to filter these elements. When this sludge is used as a fertilizer (of excellent quality by the way, because it is rich in organic matter and various chemical elements that promote plant growth), the toxic concentration can be reduced by up to 70%. The presence of other polluting elements in this sludge makes it difficult to use it in agriculture. However, a first mission is fulfilled, to clean up in an ecological way a large part of the dioxins present in these soils, in order to avoid the contamination of the surrounding waters.

            This research provides a glimpse of the potential of eco-hydrology. Environmental scientists are truly inspired to come up with methods that are both ecological and economical in terms of water pollution control. To continue in the field of phytoremediation, the use of specific grass species in the filtration of bisphenol A, an endocrine disruptor present in the composition of plastic, has recently been studied. And outside the field of eco-hydrology, we can mention the recent development of nano-silicone sponges with a considerable capacity to absorb oil residues escaping from underwater drilling; the use of graphene in the capture of pharmaceutical substances present in water; or the use of polymer electrodes to oxidize arsenic molecules and thus reduce their toxicity by more than 90%. Knowing that all these examples come exclusively from studies published since the beginning of the year, we can be sure that the scientific community will continue to surprise us in 2020.



Etienne Morisseau

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