Hydrochemical processes in lowland rivers: insights from in situ, high-resolution monitoring

摘要

This paper introduces new insights into the hydrochemical functioning oflowland river systems using field-based spectrophotometric and electrodetechnologies. The streamwater concentrations of nitrogen species andphosphorus fractions were measured at hourly intervals on a continuous basisat two contrasting sites on tributaries of the River Thames – one draining arural catchment, the River Enborne, and one draining a more urban system,The Cut. The measurements complement those from an existing network ofmulti-parameter water quality sondes maintained across the Thames catchmentand weekly monitoring based on grab samples. The results of the sub-dailymonitoring show that streamwater phosphorus concentrations display highlycomplex dynamics under storm conditions dependent on the antecedentcatchment wetness, and that diurnal phosphorus and nitrogen cycles occurunder low flow conditions. The diurnal patterns highlight the dominance ofsewage inputs in controlling the streamwater phosphorus and nitrogenconcentrations at low flows, even at a distance of 7 km from the nearestsewage treatment works in the rural River Enborne. The time of samplecollection is important when judging water quality against ecologicalthresholds or standards. An exhaustion of the supply of phosphorus fromdiffuse and multiple septic tank sources during storm events was evident andload estimation was not improved by sub-daily monitoring beyond thatachieved by daily sampling because of the eventual reduction in thephosphorus mass entering the stream during events. The results highlight theutility of sub-daily water quality measurements and the discussion considersthe practicalities and challenges of in situ, sub-daily monitoring.