The results of this study demonstrate that the application of multivariate analysis was particularly useful for evaluating and interpreting the results in an integrated manner, especially when there are a large number of parameters analyzed in an environmental assessment. The use of multivariate statistical techniques applied to these data provides a useful tool for pre- and post-dredging studies. (Casado-Martínez et al.,2009).
In pre-dredging studies, it is important to determine the particularities of the port environment in the so-called normal conditions of maritime traffic and establish a monitoring that will be carried out during operations, like was done in the port of Genova (Italy). Dredging must consider the proximity to areas that must be protected from the negative effects of sediment movement (bathing beaches, mammals setting). On the other hand, dredging operations needs constant and voluminous sand bypassing across the entrance of the port (Capello et al., 2010).
The results of this study lead the authors to take special care about the sediment that should be extracted in future dredging works. Multiple anthropogenic stressors are likely to affect the environmental health of the entire ecosystem. The silt-clay sediments and their high organic matter content found in the inner docks of the port, related to the extensive use of hydrocarbons (e.g., fuels, motor oils, maritime traffic, moorings for pleasure craft, the shipyard, etc.), residual discharges from the Arroyo del Barco, and the historical use of biocides in antifouling paints, can have ecological and environmental consequences related to resident biota and deposition of particulate contaminants. In general, silt-clay sediments, function as traps for toxic substances, which could be easily suspended by maritime traffic and storms, eventually making them more available to benthic biota (Yebra et al., 2004; Muniz et al., 2015; D` Alessandro et al., 2020).
Studies in the internal sectors of the port of Quequén (Necochea, Buenos Aires) show very high values of organic contamination due to the management of cereals in the port area, which confirm that grain size and total organic carbon (TOC) affect the habitat, feeding behavior and survival of benthic organisms (Godoy et al.,2011). Caplat et al. (2005) have identified Total Organic Carbon (TOC) levels between 5% and 10% in dredged sediments from the harbor of Port-en-Bessin, northwest of France. Those sediments are characterized by a high proportion of reducing materials (oxides, oxyhydroxides) that can give rise to metallic sulfides through diagenetic processes, and therefore are sinks for trace metals (Douglas and Adeney, 2000).
Large volumes of polluted sediments from dredging efforts make remediation difficult. Any remediation strategy must include simultaneous programs to identify and reduce inputs of these pollutants to the marine environment, and hence to the bottom sediments (Birch and Taylor, 1999). Therefore, for the regulation of massive, long-lasting and intense dumping activities at landfills, a number of specific actions should be considered such as the action of dump-induced shock waves to estimate the dispersion of the dumped material and the spatial and temporal evolution of the turbidity characteristics, even beyond the landfill boundaries, during and after the disposal of the dredged material. With more emphasis on low dispersion environments hydrodynamic processes do not allow a rapid dilution of the dumped material (Palanques et al., 2022).
Considering the results of this study, recommendations can be postulated for the sustainable management of the dumping zone. Mar del Plata port needs a dumping site zone for submerging the accumulated wastes and today attached at some docks (mostly abandoned ships). These types of dumping sites zones are deserved for a port of great activity generating great volumes of waste (Chin and Ota 2000; Karl et al. 2001; Bolam and Rees 2003; Dufour and Van Lancker 2008). However, for these purposes it is necessary to locate this type of dumpling site at a certain location either at the province jurisdiction (3 nautical miles) or at the national domain (offshore from 3 nautical miles).
Hydrodynamics, sediment sources and the compartments between docks conditioned the sediment dispersal. The port was constructed when the volume of littoral drift was not known nor the engineering alternatives to avoid the sand accumulation (e.g. sand traps or sand bypassing).
Other ports on the Argentine-Uruguayan coastal plain have presented similar results in relation to sedimentation problems. The Montevideo port (Uruguay), at the outlet of the Río de la Plata, received important volumes of silt from the episodic inputs of the Río de la Plata discharge (Muniz et al. 2015, Venturini et al., 2004). The Quequén Port (Buenos Aires) is also conditioned by higher inputs of sand transported by longshore drift and causing a huge amount of sand at the extreme of its dock (Godoy et al., 1988).
In Mar del Plata, the obstruction of the longshore drift current has increased the erosion problem over the last 100 years, affecting the operation and economy of the port, as well as the degradation of the tourist resource and the bathing quality of the beaches. The beach of the Punta Mogotes touristic Complex, located immediately south of the port, is artificial. The sedimentary accumulation was caused by the construction of the southern breakwater and has become the most crowded touristic beach in Argentina (Isla, 2001). However, all the tourist beaches to the north of the port have suffered significant erosion processes.To solve this problem, the National Government invested in dredging and replenishment works during the end of 1998. The dredging of the sand bank at the extreme of the southern dock was analyzed in terms of the grain sizes and the nourishment of the beach of Playa Grande (Isla and Schnack 1986). Sediment was pumped from the sand bank in the port access channel to three importantic tourist beaches located north of the port. It is estimated that about 1.6 million m3 of sand were redistributed along these three beaches (Bértola, 2001; Marcomini and Lopez, 2006). These jobs permitted the increase of 300% of the beach's surface (Bértola, 2001; Marcomini y Lopez, 2006; Padilla y Eraso, 2012). However, the nourished sand was too fine and there was a rapid loss during the first stages post-nourishment (Isla 2006).
Coastal protection systems have undergone a major change on a global scale in recent decades; “hard" techniques, such as groins and breakwaters, have gradually been replaced by "soft" works. Artificial beach replenishment and sedimentary by-pass are now considered as two very effective soft methods for beach protection and restoration (Pontrelli Albisetti et al., 2015).
It is very important to evaluate the material to be used for beach fill; natural selection processes act on it, redistributing the finer part offshore and the coarser debris to the surf zone, which does not provide a functional use in controlling beach erosion. One option is the case study known as Guardialfiera Reservoir, southern Italy, which used sand dredged from reservoirs instead of marine sand. Using marine sand could alter the bathymetry, inducing a variation in wave climate and exposing marine life to serious risks (De Vincenzo et al., 2018).
However, these aproximations s can only be applied to the sandbank accumulated at the mouth of the port, and not to the sediment inside the port, as these sediments contain not only high values of organic matter, but also many trapped contaminants, whose dredging can spoil environmental conditions. Benthic indicators, most used ecological indicators, also showed very poor environmental conditions in docks and the central part of the Mar del Plata port, while better condition was present in the mouth (Rivero et al., 2005). In addition to benthic organisms, which are directly related to sediment quality, there are other species to be taken into account in the environment of the Port of Mar del Plata.
Along the southern breakwater Mar del Plata harbor there is a colony of sea lions with normally more than 100 specimens (Rodríguez and Bastida 1998). At the same time, a population of seahorses (Hippocampus patagonicus) is stable along the northern breakwater (Pujol, 2014). Future dredging plans should consider the faith of these communities into account.
In summary, the dredged material, after adequate treatment, can be reused in a positive way by depositing it on land for construction, public works, building materials (such as bricks, blocks without structural purposes and in combination with construction waste), beach nourishment, dike rehabilitation, among others (Mymrin., et al 2016). The industrial utilization of this type of waste can replace traditional raw materials, minimizing the extraction of natural resources and ultimately reducing the degradation of coastal areas by reducing the current practice of these discharges into the marine environment.