European land-based pollutant loads to the North Sea: an analysis of the Paris Commission data and review of monitoring strategies

Paris Commission (PARCOM) data have been used to investigate regional patterns in land-based pollutant loads to the southern North Sea for the period 1990–1993. Despite measures to standardise the procedures for obtaining river load estimates, important methodological discrepancies remain. These discrepancies relate to: (i) the location of sampling with respect to tidal and saline limits along the river reach, (ii) frequency of sampling, (iii) whether sampling is periodic or aperiodic and biased to particular flow conditions, and (iv) the choice between two load estimation algorithms. The PARCOM data indicate that riverine and total land-based loads of metals, total-N and total-P to the southern North Sea are dominated by the Rhine/Meuse and Elbe, which have the largest catchment areas and drain major centres of population and industry in central and western Europe. UK rivers have both the highest riverine loads per unit area and the highest direct discharge loads for many determinands. The high loads per unit area may be a function of the relatively small catchment areas and the proximity of major sources of pollution to the downstream monitoring sites. Higher direct discharges for the UK rivers may occur because UK direct discharge estimates include effluent inputs to the freshwater tidal reaches. For the other major European river systems, effluent discharges to the tidal freshwater reaches are included in the riverine load estimate. More thorough evaluation of load estimation methodologies is required if these data are to be used for comparative purposes in an international policy-making and legislative context.