According to the European and Italian regulation, the threshold for nitrates contamination in groundwaters addressed to human consumption is set to 50 mg/L. Identification of nitrates sources is not easy to assess and detailed hydrogeological investigations are required to delimitate Nitrate Vulnerable Zones (ZVN). Miocene limestones and calcarenites from the Mores Formation, which widely outcrop in the Logudoro basin (NW Sardinia, Italy), host a karst aquifer that gives rise to several springs. One of these, the Funtana Nurighe spring, supplies the Cheremule’s municipal aqueduct, a small village of about 400 inhabitants. Results of the hydrogeochemical monitoring carried out by the Regional Agency for Environmental Protection of Sardinia (ARPAS) since 2013 have highlighted continuous exceedances of the nitrate threshold value in the spring waters. This implies a significant impact on the water supply of the municipality and on the inhabitant’s health. Based on the ARPAS monitoring results and the reconstruction of a detailed geological model of the area, a preliminary ZVN has been defined, which currently constrains over a wide area. The main goal of the present research is to deepen the hydrogeological knowledge of the study area and to provide useful information for the redelimitation of a more effective ZVN. The study includes i) a comprehensive review and interpretation of chemical analysis from the ARPAS monitoring, ii) the collection of new samples from springs located in the nearby area (June 2020 and June 2023), and iii) a new monthly monitoring of flow rates and the sampling of the spring waters since June 2023 for the determination of nitrates, major ions, trace elements, and water isotopes (δ2H and δ18O). Multi-temporal geochemical analyses from ARPAS show that chemistry of the F.na Nurighe spring is characterized by a bicarbonate alkaline-earth type during the winter/ spring period and a chloride-bicarbonate alkaline-earth type during the summerautumn period. Compared to nearby springs, which are also fed by marls and basalt formations, the compositional ratios of the Funtana Nurighe waters (i.e., Mg/Ca vs. SO4/Cl; Mo/U vs. SO4/Cl) indicate significant mixing processes, and therefore possible lateral recharge from other hydrogeological units, especially during the dry season. Flow rate measurements collected so far are representative of the baseflow recession curve (BRC), with yield decreasing from 5.4 to 2.9 L/s, and the initial part of the subsequent rising limb where yields have reach values of 18 L/s. Nitrates show a progressive increase from 71 to 120 mg/L during the BRC, followed by an abruptly collapse to 49 mg/L; this value remains almost constant during the recharge period and then rise sharply to over 100 mg/L in the late April measurement. Such prelimi-nary results have provided useful information about the possible alimentation sources of the spring. However, the lack of a clear correlation between flow rates and nitrates concentration requires further investigations and monitoring.
Evaluation of recharge sources and nitrates origin in a karst spring: A case study from the F.na Nurighe spring (NW Sardinia, Italy)
Arras C.Primo
;Calia M.Secondo
;Biddau R.;Da Pelo S
Ultimo
2025-01-01
Abstract
According to the European and Italian regulation, the threshold for nitrates contamination in groundwaters addressed to human consumption is set to 50 mg/L. Identification of nitrates sources is not easy to assess and detailed hydrogeological investigations are required to delimitate Nitrate Vulnerable Zones (ZVN). Miocene limestones and calcarenites from the Mores Formation, which widely outcrop in the Logudoro basin (NW Sardinia, Italy), host a karst aquifer that gives rise to several springs. One of these, the Funtana Nurighe spring, supplies the Cheremule’s municipal aqueduct, a small village of about 400 inhabitants. Results of the hydrogeochemical monitoring carried out by the Regional Agency for Environmental Protection of Sardinia (ARPAS) since 2013 have highlighted continuous exceedances of the nitrate threshold value in the spring waters. This implies a significant impact on the water supply of the municipality and on the inhabitant’s health. Based on the ARPAS monitoring results and the reconstruction of a detailed geological model of the area, a preliminary ZVN has been defined, which currently constrains over a wide area. The main goal of the present research is to deepen the hydrogeological knowledge of the study area and to provide useful information for the redelimitation of a more effective ZVN. The study includes i) a comprehensive review and interpretation of chemical analysis from the ARPAS monitoring, ii) the collection of new samples from springs located in the nearby area (June 2020 and June 2023), and iii) a new monthly monitoring of flow rates and the sampling of the spring waters since June 2023 for the determination of nitrates, major ions, trace elements, and water isotopes (δ2H and δ18O). Multi-temporal geochemical analyses from ARPAS show that chemistry of the F.na Nurighe spring is characterized by a bicarbonate alkaline-earth type during the winter/ spring period and a chloride-bicarbonate alkaline-earth type during the summerautumn period. Compared to nearby springs, which are also fed by marls and basalt formations, the compositional ratios of the Funtana Nurighe waters (i.e., Mg/Ca vs. SO4/Cl; Mo/U vs. SO4/Cl) indicate significant mixing processes, and therefore possible lateral recharge from other hydrogeological units, especially during the dry season. Flow rate measurements collected so far are representative of the baseflow recession curve (BRC), with yield decreasing from 5.4 to 2.9 L/s, and the initial part of the subsequent rising limb where yields have reach values of 18 L/s. Nitrates show a progressive increase from 71 to 120 mg/L during the BRC, followed by an abruptly collapse to 49 mg/L; this value remains almost constant during the recharge period and then rise sharply to over 100 mg/L in the late April measurement. Such prelimi-nary results have provided useful information about the possible alimentation sources of the spring. However, the lack of a clear correlation between flow rates and nitrates concentration requires further investigations and monitoring.
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