Do fine calcium carbonate particles affect water quality at an artificial coarse-clastic beach characterized by high abrasion rate of the filling pebbles?

Artificial coarse-clastic beaches are increasingly used as a form of protection scheme to counteract the negative effects of coastal erosion processes. A decent outcome of such a replenishment is often measured in terms of time duration of the intervention, as beach filling is a practice that needs subsequent integrations over time. On sandy beaches the refill material may be entrained and transported further away long- and cross-shore; on coarse-clastic beaches the problems are mainly related to sediment mass loss, which eventually leads to a significant volume decrease of the body of the beach. In this sense, the right refill grain-size is not the only element to consider; the toughness to impact and friction of the selected lithology is as much important. A recent analysis of the abrasion rate measured on individual marble pebbles at an artificial coarse-clastic beach at Marina di Pisa (Italy) showed that in just 13 months the marked sediments underwent an average mass loss of about 60%, either on initially rounded and on angular pebbles. Although marine surface waters are reported to be supersaturated with respect to calcite, preliminary geochemical data during different surveys revealed significant changes in seawater ionic strength and chemistry in some of the sampling stations, possibly reflecting freshwater admixing. This allows carbonate minerals to dissolve. These observations suggest that a large quantity of calcium carbonate was abraded and distributed in the seawater by both mechanical abrasion and seawater-clast reactions. As the artificial beach is bound at both edges by large boulder groynes that hinder water exchange and circulation, a tough problem about water quality might arise on that site. As beach fruition and water quality are crucial aspects for the economy of coastal areas where tourism is the main revenue stream, the results of the present research may be helpful in supporting coastal managers to take conscious decisions to improve the management of the sites in need of protection or restoration achieved by implementing artificial coarse-clastic beaches.