Sewage sludge impact on sediment quality and benthic assemblages off the Mediterranean coast of Israel - A long-term study

The distributions of benthic assemblages, heavy metals and organic carbon (C_org) in sediments were examined during a long-term study at a sewage sludge disposal site off the Mediterranean coast of Israel. The disposal of sewage sludge has a marked but localized, seasonally dependent, impact on the benthic assemblages and sediment quality. Elevated concentrations of C _org, Hg, Cd, Cu, Zn, Pb, and to a lesser degree Ni in the sediments were detected mostly northward of the sewage outfall, in the direction of the prevalent longshore current. High concentrations of C_org and metals were reflected by elevated populations of tolerant and opportunistic polychaetes in spring and by an azoic zone in fall. The impacted area extended mainly towards the north (up to ca. 4 km) and to a lesser extent south of the outfall (up to ca. 2.5 km). No evidence of increased accumulation of sewage sludge with time was found, nor of pollutants associated with it. Principal component analysis (PCA) grouped the anthropogenic metals and C_org with infaunal abundance for the spring surveys, while biotic diversity was negatively correlated with the pollutants. In the PCA of fall surveys, abundance was negatively correlated with the pollutants, decreasing with increased concentration of C_org and anthropogenic metals. We suggest that the seasonal pattern shown by infaunal abundance, anthropogenic metals and C_org is due to the stratification of the water column from spring to fall on one-hand and winter storms on the other. Winter storms resuspend and disperse the fine organic particles, sweeping the site clean of sludge; accumulation of sludge takes place throughout the quiescent periods of the year, when stratification is reestablished. The disposal site is dispersive and the spatial extent of the impacted area varies seasonally and interannually. This monitoring study, in addition to addressing specific questions about sewage sludge impact, represents an unusually large and unique set of long-term measurements that will serve as a basis to evaluate the site recovery following the cessation of disposal.