Abstract
Polder watercourses within agricultural areas are affected by high chemical oxygen demand (COD) and biological oxygen demand (BOD5) concentrations, due to intensive farming activities and runoff. Practical cases have shown that constructed wetlands (CWs) are eco-friendly and cost-effective treatment systems which can reduce high levels of organic and nutrient pollution from agricultural discharges. However, accumulated recalcitrant organic matter, originated by in-situ sources or elements of CWs (i.e., plants or microbial detritus), limits the fulfilment of current COD discharge threshold. Thus, to evaluate its relevance regarding rivers ecosystem health preservation, we analysed the response of bio-indicators, the Multimetric Macroinvertebrate Index Flanders (MMIF) and the occurrence of organic pollution sensitive taxa towards organic pollutants. For this purpose, statistical models were developed based on collected data in polder watercourses and CWs located in Flanders (Belgium). Results showed that, given the correlation between COD and BOD5, both parameters can be used to indicate the ecological and water quality conditions. However, the variability of the MMIF and the occurrence of sensitive species are explained better by BOD5, which captures a major part of their common effect. Whereas, recalcitrant COD and the interaction among other physico-chemical variables indicate a minor variability on the bio-indicators. Based on these outcomes we suggest a critical re-evaluation of current COD thresholds and moreover, consider other emerging technologies determining organic pollution levels, since this could support the feasibility of the implementation of CWs to tackle agricultural pollution.
Highlights
Throughout the years, the environmental limits imposed by the European Water FrameworkDirective (EU WFD) (2000/60/EC) have become stricter
The compliance with the European Water FrameworkDirective (EU WFD) goals by 2027 in EU-countries, urges policy makers and scientist to identify key ecological and water quality parameters used in combination to define the biotic and abiotic conditions of surface watercourses and setting appropriate standard limits
The relevance of chemical oxygen demand (COD) thresholds set for the Flemish polder watercourses and constructed wetlands (CWs) treating animal manure located near some of these polder watercourses, were evaluated
Summary
Throughout the years, the environmental limits imposed by the European Water FrameworkDirective (EU WFD) (2000/60/EC) have become stricter. The aim is to protect and prevent a further decline of the ecological and chemical water status of European fresh and brackish watercourses. By 2027, all surface watercourses should reach ‘good ecological and chemical conditions’ [1,2]. In Flanders (Belgium) these goals are far from being met; since a great majority of surface watercourses, especially polder watercourses have been affected by intensive agricultural practices, causing erosion and excessive spread of manure as fertilizer [3]. Organic matter (OM) concentrations in watercourses are analysed and measured by the biological oxygen demand (BOD5 ) and the chemical oxygen demand (COD). Their quantification is based on the amount of oxygen needed to degrade
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