A comparison of river water quality sampling methodologies under highly variable load conditions

When river water quality fluctuates over relatively short periods of time with respect to the sampling frequency, the collection of grab samples may be inappropriate for characterising average water quality. This paper presents the results of a water quality monitoring study carried out on a stretch of the river Lambro (northern Italy) dominated by a periodically overloaded sewage treatment works (STW) located near its upstream end. Water quality was strongly influenced by a pronounced diurnal cycle in pollutant loads caused by the regular emission of untreated waste water during periods of high domestic flow (daytime). Two different sampling techniques were employed: grab sampling and 24-h composite sampling using automatic samplers. Samples were collected at the plant overflow and at several sites along the river and analysed for two common ingredients of household detergents, linear alkylbenzene sulphonate (LAS) and boron (B) and for routine water quality variables. The results obtained show that: (1) The diurnal variability of point-source-derived chemical concentrations in the river downstream of the undersized STW increased with increasing removal efficiency in sewage treatment. (2) The shape of the diurnal concentration signal remained relatively intact for a considerable distance downstream of the STW for several water quality variables, suggesting that hydrodynamic dispersion plays a relatively minor role in controlling concentration patterns in this river. (3) In-stream degradation of LAS was consistent with first order kinetics with a rate constant of 0.05–0.06 h^(-1). (4) Grab sampling is a relatively inefficient methodology for capturing mean concentrations for rivers subjected to highly variable loads, especially when it is restricted to office hours. The inefficiency of grab sampling is more marked for substances (e.g. LAS) which are effectively removed during sewage treatment than for substances which are not. (5) For LAS, diurnal variability in the concentration signal decreases with distance downstream, making grab sampling an increasingly reliable methodology for estimating mean concentrations. (6) 24-h composite sampling is an efficient way of eliminating the effect of diurnal variations in load strength.