Problematiche connesse con la gestione del servizio idrico nella città di Napoli: situazioni di non conformità del parametro manganese

In 2006, tests undertaken on the water supplied to the city of Naples by the Water Supply Provider of Naples (ARIN S.p.A.) revealed the presence of manganese concentrations close to, and sometimes exceeding , the levels permitted by law (maximum permissible concentration: 0.05 mg/l, Legislative Decree 31/01). The phenomenon obviously worried users who, in many cases, saw water of a reddish colour gushing out from their taps, which can hardly be considered a typical characteristic of drinkable water. This created numerous problems for ARIN which, immediately, took action in order to tackle to the problem, washing the water pipes and tanks of the principle holding tanks. However, the aqueduct of Naples is not the only one to have had to deal with such problems. Other water supply providers have had to face similar problems: one such example is the communal aqueduct of Vercelli which had to deal with levels of manganese of approximately 0.12 mg/l in its water supply. In fact ARIN is not new to the phenomena of manganese contamination. Towards the end of the 1980s, in the midst of a water crisis due to a fall in effective precipitations, tests on the waters supplied to the city of Naples revealed the presence of concentrations of manganese, nitrates and iron which were well above the limits established by law (Mn: 0.05 mg/l; NO3: 50 mg/l; Fe: 200 mg/l, Presidential Decree 236/1988). The re-emergence of the phenomena is unfortunately attributable to the presence of manganese in the stratum: in the Neapolitan area, in fact, manganese has a mostly natural genesis, being present, as an oxide, in the piroclastic-alluvial filling deposits of the Campania Plain. These deposits house the stratum from which water is drawn from the one hundred and fifty-six wells of the Integration and Reserve well field of Lufrano and Acerra, located to the east of the city of Naples (A.I.R.: Aqueduct of Integration and Reservoir) which is managed by ARIN S.p.A.. Completed precisely in the eighties, the A.I.R is subdivided into various subsystems of which four are currently in use. It is a system of integrated conduction of the water source capacities of Serino (Acquaro Pelosi and Urciuoli, situated in the province of Avellino) and of those coming from other regional sources (more precisely from the Aqueduct of Western Campania: source A.C.O.), which constitute the main contributions to the Neapolitan aqueduct. With a mean concession capacity of approximately 700 l/s, the A.I.R well field takes over in the conduction of water when there is a reduction of the capacities from the sources of Serino and the A.C.O. Unfortunately, increasing the uptake of water from the A.I.R., linked to the periods of water shortage crisis due to natural and/or anthropic factors, involves the arrival, in the holding tanks, of waters rich in manganese, as well as in nitrates. In fact, the mobilization of deep and insufficiently oxygenated water, connected with the necessity to increase the uptakes from Lufrano and Acerra, is the principle cause of the solubilization of manganese in the stratum waters. Based on the above outlined details, the authors have concentrated on the A.I.R conduction system, extending their study to also include nitrate parameters. Elaborating a considerable number of data, both of a hydro-chemical as well as managerial nature (capacities taken from Lufrano and Acerra, subsystems in use, number of wells of the single subsystems in use and the relative uptake capacities, variation in the turnovers of the wells) the area was physically circumscribed and, consequently, the relative subsystem of conduction which mainly affects the contribution of manganese in the drawn waters; in contrast, the wells from which qualitatively better contributions are drawn for the aforementioned parameter are highlighted. The research has underlined, however, that the use of the latter involves, unfortunately, the contemporary increase of nitrates in the drawn waters. Therefore, the authors have defined a management model of the A.I.R well field which, based on the knowledge of the capacities of Serino and the A.C.O., permits defining the maximum possible uptakes from the single subsystems with the aim of obtaining manganese and nitrate parameters which conform to the legislative dispositions in force.