VARIABILITY OF PESTICIDES IN RIVER WATER AND ITS EFFECT ON ESTIMATION OF LOAD
Report No FR0152
P A H VAN DIJK*, A SAGE** & R M HARRISON** * WRc ** University of Essex
The objectives of this study were to investigate the temporal variability of the concentrations of twelve Red List compounds, mainly pesticides, in the River Thames, and to compare the estimates of loads of these substances based on discrete data (grab samples and instantaneous measurements of flow) with those based on continuous monitoring.
Under international commitments, the UK is required to provide information on the loads of pollutants discharged to its coastal waters. It is important, especially for trend analysis, that these submissions take into account the inherent seasonal variation in concentration and flow, and also the uncertainties associated with the estimation of loads.
The data gathered for this one-year study showed very strong temporal variations in the flow of the River Thames, in the concentrations of the pesticides and hence in the pesticide loads. A large proportion of the total load was discharged in a short period following the floods of 1990.
The pattern of variation of concentration for the organochlorine insecticides and the chlorinated aromatics was different from that for the herbicides.
Estimates of annual load based on weekly grab samples could differ from those achieved by continuous flow-proportional sampling by as much as ñ50%. Confidence intervals for estimates based on discrete sampling were often very wide.
These results confirm the conclusions of previous reports (FR0005, FR0092) that methods of load estimation based on discrete sampling suffer from large uncertainties due to their failure to account fully for variations, particularly peaks, in concentration and flow.
A programme of sampling was undertaken by the University of Essex under contract to WRc. The work was centred on the continuous flow-proportional sampler previously established at Kingston upon Thames.
Over a period of about a year a sequence of continuous samples was taken from the flow-proportional sampler and a comparative sequence of grab samples at weekly intervals. Corresponding flow measurements were recorded. The samples were analyzed for 12 organic compounds, mainly pesticides, on the Red List. The report presents a statistical analysis of this data, concentrating on the questions of temporal variability and the estimation of river-borne loads.
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