Nitrate occurrence in groundwater hosted in hard-rock aquifers: estimating background values at a regional scale

This study was aimed at assessing the occurrence of nitrate in poorly anthropized areas at a regional scale, as an attempt to estimate the nitrate background in areas far from intense agriculture, farming, industrial activities and urban areas. Nitrate concentrations, together with physical-chemical parameters and major anions and cations, were determined in 49 spring waters flowing out of granitic and metamorphic rocks in Sardinia (Italy). Nitrate in the spring waters was in the range of <0.1 mg/L to 47 mg/L, a range similar to that observed in 404 rain samples collected in Sardinia, with highest values mostly occurring in spring waters located downstream villages. The median nitrate might be considered an estimate of background values in the study area. The median nitrate in waters from metamorphic aquifers was 1.9 mg/L, close to the 1.3 mg/L median value observed in the Sardinian rain. The median values of nitrate in waters from granitic aquifers was higher, i.e. 7.0 mg/L. These results were compared with an historical dataset consisting of 183 spring water samples collected between 2000 and 2012 in the same area. Again, nitrate in spring waters from granitic rocks was higher (maximum value 80 mg/L, median: 7.6 mg/L) than nitrate in spring waters from metamorphic rocks (maximum value 48 mg/L, median 1.5 mg/L). Geogenic factors may affect nitrate occurrence in groundwater and might explain the higher median values observed in springs flowing out of granitic rocks. In the study area, the soils developed on granitic rocks had higher permeability and lower cation exchange capacity, as compared to soils developed on metamorphic rocks. Such characteristics may favor a rapid leaching of nitrate to groundwater. Moreover, the redox potentials in granitic springs higher than those in metamorphic springs might have favored the persistence of nitrate in granitic environments. Overall results of this study showed that the granitic aquifers were more susceptible to nitrate contamination as compared to the metamorphic aquifers.