Assessment of sediment pollution levels from two artificial reservoirs in southern Italy

The sediment accumulation at the bottom of the reservoirs decreases the useful reservoir capacity, limiting the storage of water resources. Several studies estimated that, to date, the silting volume is about 50 percent of the usable capacity, so the reservoirs' ability to meet water demand in periods of low rainfall is compromised. Consequently, sediment removal from the bottom of the reservoirs is one of the most necessary actions to be implemented to ensure the recovery, at least partially, of the reservoir capacity. However, although dredging operations appear to be the natural way of limiting the problem, they produce a large quantity of materials that, according to current regulations, are a priori classified as waste and should be treated as such. It is clear that this option is not economically nor environmentally sustainable, so alternatives to disposal must be identified, which, following Agenda 2030 principles, could be identified in the reuse of sediments for environmental or material recovery. The scientific community has, in fact, long been wondering about possible alternatives for the reuse of dredged materials, trying to identify possible production fields that could benefit from the delivery of sediments to be used as a replacement for the virgin raw materials currently in use. However, as part of this purpose, it is necessary to ascertain the sediments' compatibility for reuse through in-depth characterization of the main parameters to determine, firstly, their suitability in terms of chemical-physical, mechanical, granulometric properties, etc., and, secondly, to verify the absence of pollution. Concerning this last focal point, several methodologies exist in the literature for estimating the sediment pollution level by calculating indices that relate the values detected by characterization analyses for each heavy metal investigated to its reference limits. These indices, each related to every characterized sample, are then compared with threshold values, resulting in the sample's pollution level. Therefore, this study aimed to assess the pollution level of sediment samples taken at two reservoirs in southern Italy, the Camastra and the San Giuliano, using three different indices found in the literature, the Nemerow integrated pollution index PN, the pollution load index PLI and the geoaccumulation index Igeo, also comparing them to identify which one best describes the samples under investigation. All the indices show that the samples taken from the San Giuliano reservoir and the Camastra reservoir are unpolluted and, therefore, suitable for potential reuse. The pollution load index and the geoaccumulation index always assume values that identify a 'no pollution' condition for all the samples; on the contrary, the Nemerow integrated pollution index identifies 1 sample from the San Giuliano reservoir and three samples from the Camastra reservoir in limit conditions ('warning limit' level, 0.7 < PN < 1). At the same time, referring to the Nemerow integrated pollution index, two samples taken at San Giuliano and four at Camastra show a 'low polluted' level, as the PN value is between 1 and 2. The discussion of the results suggests that for a more accurate definition of the sediment pollution level from the Camastra and the San Giuliano reservoirs, the Nemerow integrated pollution index is the most reliable index of all, as it can better characterize the contamination level of the examined materials and provide a more reliable assessment of their suitability for reuse.