Declines in the dissolved organic carbon (DOC) concentration and flux from the UK

Abstract

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The importance of sewage effluent discharge in the export of dissolved organic carbon from U.K. rivers

“…However, it does imply that at individual works, sharp changes can occur. Worrall et al () found that the median step change in DOC concentration, for those river records where a significant step decrease was observed, was 41% decline.…”

“…The least squares mean difference between sewage discharge and control reaches was 0.19 (±0.04), and this would mean that a sewage discharge increases the downstream DOC concentration of the receiving river water by 19%. Therefore, if the entire source of DOC from the sewage discharge were shut off, then the concentration observed in the river downstream of STWs would decrease by 16%, far lower than the average 41% observed for step changes at tidal limit sites as measured in Worrall et al ().…”

“…Worrall et al () found a median decline of 41% in DOC concentrations at the tidal limit, which was coincident with improvements in sewage treatment work discharges. However, they could not ascribe an exact mechanism to this decline, and they proposed two possibilities: first, that the STWs could be the source of the DOC and improved treatment has removed this source of DOC, and second, that the STWs are a source of nutrients that stimulated river flora and fauna resulting in an increased autochthonous production of DOC.…”

“…As shown by the previous studies of Finlay et al (), Noacco et al (), and Worrall et al (), the extensive monitoring records maintained in the United Kingdom over time and space mean that the United Kingdom is the ideal location for considering STWs as a source of DOC to rivers.…”

“…Recently, Noacco, Wagener, Worrall, Burt, and Howden () have ascribed a long‐term trend (130 years, 1884 to 2005) in DOC concentrations for the River Thames to increases in sewage discharges as the population of the catchment rose from 900,000 to 3,200,000. Further, Worrall, Howden, Burt, and Bartlett () have shown that there have been recent declines (the period 2003 to 2015) in the DOC concentration of U.K. rivers at their tidal limits. The declines were dominated by significant step decreases that could be associated with the implementation of the Urban Waste Water Treatment Directive (European Commission, ); that is, step declines in the DOC concentration time series could be ascribed to changes in discharges from STWs.…”