A simple stochastic model of point source solute transport in rivers basedon gauging station data with implications for sampling requirements

A simple point-source water quality model was constructed to explore hypoth etical scenarios of linear alkylbenzene sulphonate (LAS) load variability, chemical degradation kinetics, river discharge and river velocities within the context of planned water quality monitoring in the river Lambro, northe rn Italy. LAS is an anionic surfactant and a commonly used ingredient of ho usehold detergents. Input loads were derived from flow and concentration me asurements at sewage treatment works (STW). River travel times are approxim ated using hydraulic geometry concepts based on existing records of stage, discharge and velocity measurements. The,model proved to be especially usef ul in predicting the impact of overflows from an undersized STW near the to p of the catchment on diurnal variations in water quality downstream. Proba bility density functions (pdfs) of LAS concentrations for a number of point s on the river were approximated by running the model stochastically (using a Monte Carlo procedure) over independent frequency distributions of disch arge, temperature and input load. The predicted concentration pdfs were the n used to estimate the number of random grab and 24 h composite samples, wh ich would be required in order to estimate mean concentrations with confide nce. This is especially important in situations where the analysis is expen sive arid the sample budget is limited. The model results showed that grab sampling was not a viable strategy in the Lambro largely due to the very st rong diurnal variability in concentrations induced by the STW overflow. Thi s conclusion was corroborated by field data. (C) 1999 Elsevier Science Ltd. All rights reserved.