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Zones of untreatable water pollution call for better appreciation of mitigation limits and opportunities

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This opinion piece addresses subsurface legacy sources and their role in mitigation of large‐scale water pollution and eutrophication. We provide a mechanistic theoretical basis and concrete data‐based exemplification of dominant contributions from such sources to total recipient loads. We specifically develop a diagnostic test to detect such contributions, recognizing that they are inaccessible and associated with long transport times that tend to evade detection when homogeneous catchment models are calibrated to typically heterogeneous catchments. Dominant legacy‐source contributions are also in practice untreatable within the commonly short time frames given for compliance with environmental regulation. We therefore argue that, for considerable water quality improvements to be achieved within such short time frames, mitigation measures need to be spatially differentiated and directed to (sub)catchments without major legacy sources. The presented diagnostic test identifies dominant prevalence of such sources where there is linear temporal correlation between the nutrient/pollutant loads and the water discharges from a (sub)catchment. Confidence in this identification may be strengthened by independent quantification of long transport times and records of temporally extended presence of nutrient/pollutant sources in the same (sub)catchment. This article is categorized under: Science of Water > Water Quality Engineering Water > Planning Water
Data and model components and results for the Kringlan catchment. (a) Cumulative area‐normalized chloride input with precipitation and loading out from the catchment as function of time. (b) Travel time distributions g(τ) used in the modeling of chloride transport through the catchment, specifically g1(τ) and g2(τ) in this panel, which are obtained from Persson et al. () for the Swedish catchment Forsmark (see τ maps corresponding to g1(τ) and g2(τ) in Figure a,b, respectively); these are compared with other travel time distributions calculated for other pathway conditions in the same catchment in Darracq, Destouni, Persson, Prieto, & Jarsjö () and Bosson, Selroos, Stigsson, Gustafsson, & Destouni (). (c) Data‐ and model‐based area‐normalized chloride loading out from the catchment as function of time. (d) Data‐ and model‐based area‐normalized chloride loading from the catchment as function of area‐normalized water discharge (runoff); see further corresponding flux concentration results and additional result discussion in Figure S1 and Appendix S1
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(a) Schematic catchment illustration with quantification of land‐use shares for the Oxunda catchment, within the Norrström basin, Sweden. (b) Measured monthly concentrations of total phosphorous in the Oxunda stream of Oxunda catchment. The concentration time series in (b) is based on data and results reported in Andersson, Petersson, and Jarsjö (). Panel (b) also shows the stable average concentration since 1998 (dashed red line), the timing of implemented mitigation measures (orange line and text), and the WFD‐ related phosphorous concentration target for good water status (green line)
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Maps of simulated travel time τ under different pathway assumptions for catchments of different scales. The upper‐left map inserts show the locations within Sweden and in relation to the recipient Baltic Sea of the small Forsmark catchment (τ maps in panels a,b) and the major Norrström drainage basin (τ maps in panels c,d); red filled circles indicate catchment outlets to the sea. Travel time maps for the Forsmark catchment correspond to the distributions g1(τ) and g2(τ) in Figure b based on results in Persson, Jarsjö, and Destouni () and Darracq, Destouni, Persson, Prieto, and Jarsjö () (a,b), and maps for the Norrström basin are based on results in Darracq et al. (). The travel time simulations assume no contribution (a,c) or considerable contribution (b,d) of slow transport pathways through relatively deep and/or low‐gradient/conductivity subsurface zones in each catchment
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