Abstract
AbstractThis paper describes the hydrochemistry of a lowland, urbanised river‐system, The Cut in England, using in situ sub‐daily sampling. The Cut receives effluent discharges from four major sewage treatment works serving around 190 000 people. These discharges consist largely of treated water, originally abstracted from the River Thames and returned via the water supply network, substantially increasing the natural flow. The hourly water quality data were supplemented by weekly manual sampling with laboratory analysis to check the hourly data and measure further determinands. Mean phosphorus and nitrate concentrations were very high, breaching standards set by EU legislation. Although 56% of the catchment area is agricultural, the hydrochemical dynamics were significantly impacted by effluent discharges which accounted for approximately 50% of the annual P catchment input loads and, on average, 59% of river flow at the monitoring point. Diurnal dissolved oxygen data demonstrated high in‐stream productivity. From a comparison of high frequency and conventional monitoring data, it is inferred that much of the primary production was dominated by benthic algae, largely diatoms. Despite the high productivity and nutrient concentrations, the river water did not become anoxic, and major phytoplankton blooms were not observed. The strong diurnal and annual variation observed showed that assessments of water quality made under the Water Framework Directive (WFD) are sensitive to the time and season of sampling. It is recommended that specific sampling time windows be specified for each determinand, and that WFD targets should be applied in combination to help identify periods of greatest ecological risk. © 2015 The Authors. Hydrological Processes published by John Wiley & Sons Ltd.
Highlights
With the development of new analytical equipment facilitating the collection of in situ sub-daily hydrochemical data the last decade has seen the expansion of high-frequency water quality monitoring schemes (e.g. Jordan et al, 2005; Palmer-Felgate et al, 2008; Rozemeijer et al, 2010; Cassidy and Jordan, 2011; Bowes et al, 2012a; Cohen et al, 2012; Bieroza et al, 2014)
The data derived from these technologies offer new insights into the chemical and ecological dynamics of a highly regulated and sewage impacted river, which would be unattainable from traditional low-frequency monitoring data
The hydrochemical data revealed that the effluent discharges being received by The Cut exert significant influence on the water quality dynamics, with 50% of the annual P catchment loads attributed to effluent discharges
Summary
With the development of new analytical equipment facilitating the collection of in situ sub-daily hydrochemical data the last decade has seen the expansion of high-frequency water quality monitoring schemes (e.g. Jordan et al, 2005; Palmer-Felgate et al, 2008; Rozemeijer et al, 2010; Cassidy and Jordan, 2011; Bowes et al, 2012a; Cohen et al, 2012; Bieroza et al, 2014). Jordan et al, 2005; Palmer-Felgate et al, 2008; Rozemeijer et al, 2010; Cassidy and Jordan, 2011; Bowes et al, 2012a; Cohen et al, 2012; Bieroza et al, 2014) These new datasets have the potential to revolutionise our understanding of hydrochemical processes and reveal complex instream nutrient dynamics never before seen Within the UK, where urbanisation is already significant (79% of the population—49 million), it is anticipated that by 2050, 86% of the population will live in urban areas (63 million) (United Nations, 2012). In this study we expand high-frequency monitoring to an urban river, The Cut, which is situated in the Thames catchment, southeast England. Modern sewage treatment prevents the gross organic pollution of the past, it is important to evaluate the effects of urban growth on water quality and compliance with legislative requirements, and to understand the hydrochemical functioning of such systems
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