High‐frequency water quality monitoring in an urban catchment: hydrochemical dynamics, primary production and implications for the Water Framework Directive

Abstract

Abstract:This 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.

Using high-frequency water quality data to assess sampling strategies for the EU Water Framework Directive

“…Flow in the Kennet was estimated from gauging stations located approximately 2 km upstream. Flow in The Cut was estimated from a gauging station at Binfield (gauging 50 km 2 of the catchment), plus measured discharges from the sewage treatment works, plus an estimate of discharge from the lower part of the catchment based on that from the upper (Halliday et al, 2015).…”

“…1c) was sampled near its confluence with the River Thames at Bray (Wade et al, 2012;Halliday et al, 2015). The catchment geology is predominantly London Clay and Reading Beds (Palaeocene clays and sands), giving an impermeable catchment with a baseflow index of 0.46.…”

“…Improved grassland covers 30 % of the catchment and 26 % is classed as arable, mostly in the northern half, and woodland occupies 15 %, mostly in the south. River flows are substantially increased by abstraction from the Thames for drinking water (Halliday et al, 2015) and its subsequent release through the STWs, increasing the specific runoff (Table 1). The WFD classification is mostly "poor", being "moderate" only in the upper reaches above the major conurbations.…”

“…The Cut was sampled from April 2010 to February 2012 (Wade et al, 2012;Halliday et al, 2015). Sampling frequency was hourly and measurements of dissolved oxygen, pH and water temperature were made by a YSI multiparameter sonde as above.…”

Prediction of storm transfers and annual loads with data-based mechanistic models using high-frequency data

“…An optimal DBM model and associated parameters are identified using statistical measures, but a model is only accepted if it has a plausible physical explanation (Young, 1998(Young, , 2003Young and Beven, 1994;Young et al, 2004). With the increasing availability of high temporal resolution datasets for additional variables alongside stream discharge (Bieroza and Heathwaite, 2015;Bowes et al, 2015;Halliday et al, 2015;Outram et al, 2014), this technique has been used effectively for relating rainfall to hydrogen ion concentration in rivers , and rainfall to dissolved organic carbon .…”