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

Abstract

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 and Environmental Change Engineering Water > Planning Water.