Impact of combined sewer overflows on water quality of rural canals in agro-urban environments

ABSTRACT: In many parts of the world, rural canals in agro-urban environments represent the water bodies intended to receive stormwater runoff from urban, industrial and agrarian surfaces. The risk of acute and chronic contamination of riparian vegetation and sediments, and the deterioration of water quality, make it problematic to use these canals to convey water for irrigation purposes and, more generally, to maintain environmental and ecological equilibriums of peri-urban areas. In this work, the spatio-temporal impact of Combined Sewer Overflows (CSOs) on water quality of a rural canal located in the metropolitan city of Milan, which is one of the most anthropized areas in Europe, was investigated through periodic water samplings. The presence of both chemical and microbial contaminants was evaluated over dry and wet periods along canal stretches that cross urban, industrial and agricultural areas. The results show that CSOs contribute to an increase in the chemical and microbial loads during rainy periods. Maximum contamination values are found downstream the collection of all CSOs, nevertheless the canal proves to be effective in the dilution of pollutants along its path. The impact of CSOs appears to be influenced by seasonality. Chemical and microbial pollutant loads gradually increase from winter to spring in the urban and industrial canal stretches, whereas in agricultural strength chemical gradually decrease from winter to spring and microbial remains stable. These preliminary findings provide useful information for managing the anthropic impact on the water quality of rural canals in agro-urban settings.