Consequences of diffuse pollution on the water quality of rivers in the watershed of the Lagoon of Venice (Italy)

The results of three water quality monitoring programs implemented in the watershed of the Lagoon of Venice are presented. The programs focus on nutrient discharge because of its critical effect on eutrophication of the Lagoon. One of these programs is concerned with the long term variation of water quality. It shows the positive effect induced by the interventions on point source pollution of urbanised areas. It results in the reduction of ammonia concentration and in the increase of dissolved oxygen downstream of major cities. It also shows the negative effect of increases in corn crops in upland areas on the nitrate nitrogen concentration at the sources of major rivers. The short term monitoring program shows the quick variation of nutrient concentrations during rain events in the rivers, and the close relation between the diffuse pollution sources and the deterioration of water quality. The total nutrient loads measured at the monitored river outlets confirm the loads predicted by mathematical models for the whole basin, and the importance of the diffuse pollution loads on the global loads discharged into the Lagoon. The progress in reduction of point source pollution increases the role of the diffuse sources, and demands for urgent and strong intervention on those to reach the final goal of stable mesotrophic conditions in the water body. Finally, data analysis of the monitoring programs also indicates an effective strategy for further monitoring of the river basin focused on the effectiveness of the pollution abatement program, and the real time central of water quality for the management of the Lagoon of Venice. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved.