COMPARISON OF LOAD ESTIMATION FROM GRAB SAMPLES AND CONTINUOUS FLOW PROPORTIONAL SAMPLING
Report No FR0005
The principal objective of this study was to compare estimates of load based on discrete data (grab samples and instantaneous measurements of flow) with those based on continuous monitoring.
Subsidiary objectives were to demonstrate the effects of sampling frequency and of choice of method of calculation on the accuracy of estimates based on discrete data, and to check the validity of sampling from a single position in the cross-section of the river.
Under international commitments the UK is required periodically 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 uncertainties associated with the estimation of loads. The work described in this report is relevant to many situations where loads need to be calculated.
A method of load estimation based on continuous flow measurement and flow proportional sampling was shown to be feasible. It eliminates the uncertainty associated with the inability of grab sampling to account fully for temporal variations in concentration and flow.
Compared with continuous monitoring, estimates of annual load based on weekly grab sampling (at Kingston upon Thames) would be accurate only to within ±25%, while estimates based on monthly grab samples could easily err by as much as ±50%.
At Kingston upon Thames the river was well enough mixed in cross section for a single sampling point to be representative of the total flow. The possible error in load estimation arising from incomplete mixing was judged to be smaller than ±10%.
IV RESUME OF CONTENTS
The report describes the programme of sampling undertaken by the University of Essex on behalf of WRc and DoE.
The report explains the design of the studies, presents analyses of their results and discusses some of the difficulties and uncertainties associated with the estimation of loads. It also briefly considers the nature of possible relationships between the concentrations of different substances in the river and the river's rate of flow.
Copies of the Report are available from FWR, price £35.00 less 20% to FWR Members