Dissolved nitrates in the groundwater of the Cecina Plain (Tuscany, Central-Western Italy): Clues from the isotopic signature of NO3-.

Anthropogenic NO3- source is one of the most serious pollutants in the groundwaters from the multi-aquifer system of the Cecina Plain (Tuscany, central-western Italy). According to the EC Directive (91/676/CEE), the local administration of Tuscany has declared the Cecina Plain as "vulnerable by nitrates". Determining the origin and the source(s) of NO3-source contamination is an important step prior to any remediation program. In the present work a geochemical survey was carried out on 92 groundwater samples collected in June and October 2006 to highlight the natural and anthropogenic features that concur with the observed chemical composition. The geochemical facies of the water samples is dominated by Ca(Mg)-HCO3 and Ca(Mg)-SO4(Cl) and, subordinately, by Na(Ca)-Cl(SO4), likely resulting by water-rock interaction processes between meteoric-derived groundwaters with the sedimentary formations characterizing the Cecina Plain, at which ingression of seawater in the coastal area is also added. A mixing model, based on ionic ratios (NO3/Na, Cl/Na, Cl/Br), was applied to distinguish the anthropogenic sources (e.g. sewage, agriculture input) from natural sources (e.g. seawater intrusion). Selected samples were analyzed for the isotopic composition of NO3- (?15N-NO3 and ?18O-NO3) to formulate hypotheses on its origin and the processes this component may suffer in the water system. Nitrate concentrations range from ~0.01 to 354 mg/L. About 35% and 19% of the sampled wells in June and October 2006, respectively, had levels of NO3- higher that those recommended by the World Health Organization (<50 mg/L). The ?18O-e ?15N-NO3 values in the Cecina Plain groundwaters are between 7.5? and 22.6? and from -2.8? to 14.9?, respectively, indicating that synthetic fertilizers are the main source of NO3 pollution in the study area. Nevertheless, sewage/manure inputs and denitrification processes were also recognized.