Effective monitoring network design for evaluation of nitrate contamination in the Southern Campidano Aquifer

According to the Nitrate Directive, a nitrate vulnerable zone (NVZ) should be designed for the mitigation of nitrate (NO3−) contamination caused by agricultural practices, where the concentration of NO3- in groundwater exceeds the limit of 50 mg/L. The vulnerability of aquifers to nitrate pollution, sources of contamination, and the groundwater flow field should be carefully assessed before the designation of NVZ. Moreover, the effectiveness of the designation of NVZs depends on the adoption of effective monitoring strategies and the representativeness of monitoring wells. In this study, a methodological approach to identify an effective groundwater monitoring network for the designation of an NVZ is proposed. In the alluvial plain of the Campidano graben (Sardinia, Italy), the monitoring carried out by the regional agency for the Environmental Protection of Sardinia (ARPAS) revealed exceedances of 50 mg/L for nitrates. Therefore, a comprehensive hydrogeological survey was performed to examine the hydrogeological context and to provide a designation of the area. The Campidano graben developed during multiple tectonic phases starting from the OligoAquitanian and culminated during the extensional tectonics related to the opening of the South Tyrrhenian Basin during Plio-Pleistocene. The basin is filled by transitional to continental deposits structuring a multilayer groundwater system in which the alternations of several aquifer levels and aquicludes give rise to a complex hydrogeological system. Further complications can arise from improper construction of wells that can allow the mixing of groundwaters coming from different sources, which makes it difficult to identify the origin and extent of contamination. Some 345 points (wells and piezometers) were inspected to collect technical information on well construction characteristics, piezometric level, and field physicochemical parameters of groundwater. Several features of the inspected points were evaluated to define their suitability as monitoring points, like pressures near the points (i.e. stable or pile of manure at less than 50 m), their state of use (i.e. pump working), and their accessibility. A score was assigned to each feature, the sum of which determined the high, medium, or low suitability for each point to be sampled for chemistry and isotope analyses. Piezometric heads measured in the 345 wells allowed us to define the piezometric trends, which were consistent with previous studies. Drainage axes and main and secondary watersheds were identified. Nitrate values exceeding 50 mg/L were found in 170 samples distributed in points near the main drainage axis corresponding to the Flumini Mannu hydrographic axis, at areas occupied by greenhouse growing and in crop fields. The applied approach defines the most representative points in the multilayer groundwater system and serves to evaluate future suitable measures for the designation of new NVZ in the Campidano area. Regular monitoring will contribute to better management of NVZs, which should result in a reduction in nitrate pollution, protect drinking water sources, and improve water quality in water bodies.