Effectiveness of Riparian Zones in Contaminant Mitigation Project acronym: AQUISAFE 1


The Aquisafe project aims at mitigation of diffuse pollution from agricultural sources to protect surface water resources. The first project phase (2007-2009) focused on the review of available information and preliminary tests regarding (i) most relevant contaminants, (ii) system-analytical tools to assess sources and pathways of diffuse agricultural pollution, (iii) the potential of mitigation zones, such as wetlands or riparian buffers, to reduce diffuse agricultural pollution of surface waters and (iv) experimental setups to simulate mitigation zones under controlled conditions. The present report deals with (iii) and has the purpose to provide a brief overview of the current state of knowledge related to the role of riparian zones as best management practices for water quality improvement at the watershed scale. Research indicates that landscape hydrogeological characteristics such as topography and surficial geology influence both riparian zone hydrology and biogeochemistry. Topography, depth to a confining layer and soil hydraulic conductivity all affect groundwater input to riparian zones and the water table fluctuation regime throughout the year. Research also indicates that although most biologically mediated reactions in soil are redox dependant, landscape hydrogeology, by affecting riparian hydrology, affects the redox conditions in the soil profile. In turn, microbial processes and changes in element concentrations are predictable as a function of the redox state of the soil.Variations in biogeochemical conditions directly affect the fate of multiple contaminants in riparian systems. In particular, variations in soil redox potential in riparian zones can affect the evolution of numerous contaminants and solutes within riparian zones like pesticides, phosphorus, NO3-, N2O, NH4+, SO42-, CH4, Fe2+/Fe3+ or Dissolved Organic Carbon (DOC). Of all the solutes/contaminants mentioned above, nitrate is one of the most important concerning water quality in many areas of the US and Western Europe. Consequently, many studies have investigated nitrate removal in riparian systems. Depending on site conditions, nitrate retention generally varies between 60 and 90 %; however, there are situations where nitrate removal is less or even where a riparian zone becomes a source of N to the stream. Although the riparian literature is clearly dominated by nitrate removal studies, many studies also focus on phosphorus, sediments, pesticides, chloride, bromide and bacteria. Although there are situations where riparian zones have been shown to be sources of P, Atrazine, bromide, E. coli or E. streptococci bacteria, riparian zones generally contribute to the reduction of most contaminants in subsurface flow and overland flow. Nevertheless, although conditions favorable to the reduction or oxidation of a given contaminant at the microbial level are often known, more research needs to be conducted to determine the variables controlling the fate of contaminants other than nitrate in soil at the riparian zone scale.Finally, although many studies have investigated the hydrological and biogeochemical functioning of riparian zones in the past few decades, much research remains to be conducted in order to quantify and predict the impact of riparian zones on water quality at the watershed scale in a variety of climatic and hydrogeological settings. In particular, better strategies and/or tools to generalize riparian function at the watershed scale need to be developed. Particular areas where research is needed to achieve this goal include: 1) the development of strategies to quantify and model the cumulative impact of individual riparian zones on water quality at the watershed scale; 2) a better quantification of the importance of spatial and temporal variability in hydrologic and biogeochemical riparian functioning relative to annual nutrient transport; 3) a better understanding of the role of vegetation in terms of its impact on riparian biogeochemical processes and the response of these processes to manipulations of vegetative cover; 4) a better understanding of the impact of human activities and infrastructure on riparian zone function in both urban and rural landscapes; 5) a better understanding of the fate of emerging contaminants in riparian systems.

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