The VIOLA project: geochemical characterization and natural background levels in a coastal groundwater body of the Apulia Region (southern Italy)

Coastal areas worldwide are often highly impacted due to the concurrence of aquifer exploitation for irrigation, human consumption exacerbated during touristic seasons and industrial activities. In order to meet the objectives of the GWD, European groundwater bodies’ status (chemical and quantitative) is evaluated every 6 years. Criteria for good status include chemicals exceeding standards and threshold values, saline intrusion and others. Apulian region features a very high ratio coastline/area (44 m/km², for Italy is 25 m/km²) thus seawater intrusion is a very common phenomenon, due to both natural and anthropogenic drivers.

In this contribution, the first results of the VIOLA project (Natural Background Values for the Apulian groundwater bodies) are presented, supplying a preliminary geochemical characterization of the coastal Murgia groundwater body. This is part of a fractured and karstified calcareous-dolomitic aquifer with groundwater naturally flowing to the Adriatic sea. Exceedances were reported for nitrates, as well as for EC, Cl, SO₄, Fe, Mn. The main objective for this groundwater body is to assess the natural background levels for the aforementioned parameters, and discriminate between the natural and anthropogenic origin of saline intrusion. Four sampling campaigns have been planned, and we present here the result of the first two sampling rounds carried out in spring and fall 2019.

Groundwater sampling was performed at 47 wells with submergible pumps or with a water depth sampler. Field parameters (T, EC, pH, DO, ORP) were measured with a multiparametric probe in a flow through cell. Ammonia, cyanides and nitrites were measured in the field (UV-VIS). Laboratory analysis were performed for major anions, major cations, minor and trace elements, environmental isotopes, DOC and microbial parameters.

Natural background values (NBLs) for the critical parameters have been provisionally assessed using the preselection method. Sampled waters show neutral/weakly alkaline and mostly oxidizing conditions, with conductivity values between about 700 and 20.000 µS/cm. The high salinity detected in some water points (chlorides up to 10 g/L, sulphates up to 1 g/L) is clearly linked to mixing with seawater. On a Piper diagram, the samples show a clear transition from earth alkaline-bicarbonate water towards mean seawater composition. A clear trend from coastline to inland can be recognized, with the most extended contamination in the northern and southern sectors. Trace elements (B, Sr, Ba), show a similar pattern. From spring (beginning of the irrigation season) to fall (end of the irrigation season) only a slight increase in salinity/chloride concentration can be observed. Nitrates are widespread in the study area, as well as the agricultural pressures, with values even higher than 100 mg/L without a specific spatial pattern.

As for the NBLs derivation, classic indicators of anthropogenic contamination (e.g. nitrates, ammonia, NaCl) for the pre-selection of uninfluenced samples failed, due to the extensive distribution of saline groundwaters in the study area. Thus, new solutions, including environmental isotope analysis, for discriminating the anthropogenic and natural origin of the salinity are under evaluation.