To evaluate the chemical status of groundwater bodies (GWB) according to the European Groundwater Directive, EU Member States should take into account natural background levels (NBLs) where needed, defined as "the concentration of a substance or the value of an indicator in a groundwater body corresponding to no, or very minor, anthropogenic alterations to undisturbed conditions". The NBL assessment in coastal GWBs is complicated by the overlapping of natural and anthropogenic processes leading to the salinization of groundwater and the recognized methodologies do not face these widespread situations. In order to assess the NBLs for the compounds linked to a possible saline contamination, a robust threshold value (TV) for chlorides is needed. We present a case study in the Apulian peninsula (southern Italy), where we assessed the NBLs for Cl, F, SO4 and B in two GWBs (one inland and one coastal) located on the Adriatic side of the Murgia aquifer, a karst fractured aquifer hosted in the JurassicCretaceous carbonate platform, sustained by saltwaters of marine intrusion. Data related to 113 water points of the regional groundwater monitoring network sampled over almost thirty years (1994-2021) were used. We applied a double pre-selection (PS) method. The first PS, "static", based on a fixed threshold for anthropogenic contamination markers (37.5 mg/L for NO3 and 0.375 mg/L for NH4), allows for the elimination of groundwater samples impacted by human activities. On these, the second PS, "dynamic", based on the identification of anomalous values in the Cl distributions through statistical techniques (box and whiskers plot, Huber's non-parametric test), allows for the identification of additional water points affected by saline contamination. The residual dataset of water points was used for the definition of NBLs, calculated as the maximum value for normal distributions or the 95th percentile for non-normal distributions. The percentage of water points eliminated by the double PS vary between 24% and 38%. Interestingly, the Cl NBL calculated for the coastal GWB was well above the TV of 250 mg/L established in Italy. A statistical comparison with historical data dating back to the period 1951-1975 has shown that Cl concentration in the two GWBs has not changed significantly over the last few decades. The current statistical populations are hence representative of a pristine condition and therefore, with reference to the salinity parameters, the investigated GWBs do not show any evident anthropogenic impact since the historical period. A dynamic PS seems more suitable for NBLs calculation, in particular for coastal aquifers affected by saline contamination, where the use of a fixed Cl threshold (e.g. 250 mg/L as Cl or 1000 mg/L as NaCl) appears unjustified and counterproductive.
A double pre-selection method for natural background levels assessment in coastal groundwater bodies
Daniele Parrone;Eleonora Frollini;Rita Masciale;Giuseppe Passarella;Elisabetta Preziosi;Stefano Ghergo
2022
Abstract
To evaluate the chemical status of groundwater bodies (GWB) according to the European Groundwater Directive, EU Member States should take into account natural background levels (NBLs) where needed, defined as "the concentration of a substance or the value of an indicator in a groundwater body corresponding to no, or very minor, anthropogenic alterations to undisturbed conditions". The NBL assessment in coastal GWBs is complicated by the overlapping of natural and anthropogenic processes leading to the salinization of groundwater and the recognized methodologies do not face these widespread situations. In order to assess the NBLs for the compounds linked to a possible saline contamination, a robust threshold value (TV) for chlorides is needed. We present a case study in the Apulian peninsula (southern Italy), where we assessed the NBLs for Cl, F, SO4 and B in two GWBs (one inland and one coastal) located on the Adriatic side of the Murgia aquifer, a karst fractured aquifer hosted in the JurassicCretaceous carbonate platform, sustained by saltwaters of marine intrusion. Data related to 113 water points of the regional groundwater monitoring network sampled over almost thirty years (1994-2021) were used. We applied a double pre-selection (PS) method. The first PS, "static", based on a fixed threshold for anthropogenic contamination markers (37.5 mg/L for NO3 and 0.375 mg/L for NH4), allows for the elimination of groundwater samples impacted by human activities. On these, the second PS, "dynamic", based on the identification of anomalous values in the Cl distributions through statistical techniques (box and whiskers plot, Huber's non-parametric test), allows for the identification of additional water points affected by saline contamination. The residual dataset of water points was used for the definition of NBLs, calculated as the maximum value for normal distributions or the 95th percentile for non-normal distributions. The percentage of water points eliminated by the double PS vary between 24% and 38%. Interestingly, the Cl NBL calculated for the coastal GWB was well above the TV of 250 mg/L established in Italy. A statistical comparison with historical data dating back to the period 1951-1975 has shown that Cl concentration in the two GWBs has not changed significantly over the last few decades. The current statistical populations are hence representative of a pristine condition and therefore, with reference to the salinity parameters, the investigated GWBs do not show any evident anthropogenic impact since the historical period. A dynamic PS seems more suitable for NBLs calculation, in particular for coastal aquifers affected by saline contamination, where the use of a fixed Cl threshold (e.g. 250 mg/L as Cl or 1000 mg/L as NaCl) appears unjustified and counterproductive.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
