Origin and pattern of salinization in the Holocene aquifer of the southern Po Delta (NE Italy)

Journal of Geochemical Exploration 175 (2017) 130–137

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Origin and pattern of salinization in the Holocene aquifer of the southern Po Delta (NE Italy) N. Colombani a, E. Cuoco b, M. Mastrocicco b,⁎ a b

BiGeA - Biological, Geological and Environmental Sciences, University of Bologna, 48126 Bologna, Italy Department of Environmental, Biological and Pharmaceutical Sciences and Technologies, Second University of Naples, Via Vivaldi 43, 81100 Caserta, Italy

a r t i c l e

i n f o

Article history: Received 18 August 2016 Revised 7 January 2017 Accepted 18 January 2017 Available online 19 January 2017 Keywords: Coastal aquifer Factor analysis Hypersaline groundwater Salinization origin Transgressive-regressive cycle

a b s t r a c t The origin and patterns of groundwater salinity of a shallow coastal aquifer pertaining to a reclaimed subsiding zone of the Po Delta are examined in this study. The aim is to identify the source of the hypersaline groundwater residing in the basal portion of the aquifer and to infer the mechanism of salinization of the remaining portion of the aquifer. To disentangle the possible sources of salinity the molar ratio of environmental tracers like Cl− and Br− were used in combination with the classical geochemical analyses of major and minor cations ratios. Highresolution multi-level sampling (MLS) allowed obtaining a robust and self-consistent hydrogeochemical database, which was statistically analysed via factor analysis and proved to be log-normally distributed. Thus, a common origin could be inferred for the elevated salinity that characterize most of the groundwater samples, this can be recognized in the organic rich fine-grained sediments, deposited in salty back barrier and marsh environments during the last transgression phase. This study proves that a detailed analysis of groundwater geochemistry can be considered a valuable tool to assess the origin of salinity in coastal Holocene aquifers, where the traditional conceptual model of a simple fresh/seawater interface may not be adequate. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The salinity distribution in many coastal aquifers shows a complex pattern, resulting not only from current processes but also from past events, including sea-level fluctuation, subsidence, climate change, mixing and human activity. In general, groundwater salinization can take place via diverse mechanisms such as actual seawater intrusion due to intense aquifer exploitation and/or sea level rise (Andersen et al., 2005; Ghiglieri et al., 2012; Werner et al., 2013), mobilization of saline paleowaters due to past transgressions, (Han et al., 2011; Sola et al., 2014), water–rock interaction, like evaporite dissolution (Cendón et al., 2008; Lucas et al., 2010), and anthropogenic contamination, like return flow (Merchán et al., 2015; Perrin et al., 2011). Independently from the salt source, its identification is often puzzled by intensive groundwatersediment interactions amplified by the increased water ionic strength (Colombani et al., 2015a; Mondal et al., 2010) or by mixing processes enhanced by human activities as excessive pumping and land reclamation (Barlow and Reichard, 2010; Custodio, 2010). As these processes do not mutually rule out, discriminating between modern versus paleo

⁎ Corresponding author. E-mail address: [email protected] (M. Mastrocicco).

http://dx.doi.org/10.1016/j.gexplo.2017.01.011 0375-6742/© 2017 Elsevier B.V. All rights reserved.

seawater intrusion is vital to build a sound conceptual model of aquifer salinization (Chaudhuri and Ale, 2014; Werner and Gallagher, 2006). Coastal aquifers formed over the last 10,000 yr, have been subject to multiple phases of sea-level fluctuations. These sea-level changes provoked an extensive transgression of paleo seawater into previously freshwater coastal aquifers, affecting all delta and coastal areas in the Mediterranean sea (Geriesh et al., 2015; Giambastiani et al., 2013; Sola et al., 2014) and worldwide (Wang and Jiao, 2012; Tran et al., 2012). The aim of this study is to identify the origin and mechanism of groundwater salinization in an alluvial coastal aquifer in the northern Adriatic Sea that was broadly affected by Late Quaternary transgressive-regressive cycles. The study was performed by means of (i) a detailed reconstruction of the depositional environment that characterized this coastal aquifer in the Late Quaternary (Amorosi et al., 2015; Stefani and Vincenzi, 2005); (ii) using major elements and their molar ratios to distinguish different sources of salinity (Kim et al., 2002); and (iii) performing a statistical analysis to infer the relative relevance of different geochemical processes occurring in the coastal aquifer (Huang et al., 2013). The established conceptual model may be applicable to a large amount of similar coastal aquifers, with a proper incorporation of the local geological environments. The present study is furthermore fundamental for the understanding and future assessment of the impact of

N. Colombani et al. / Journal of Geochemical Exploration 175 (2017) 130–137

projected climate change and sea-level rise on aquifers in vulnerable coastal flood plains. 2. Site description The study area is located in the coastal floodplain of the Po River near Ferrara (Northern Italy) and covers 750 km2 (Fig. 1). A great part of this territory are recently reclaimed lands, characterized by a flat topography and altitudes ranging from 5 to − 11 m above sea level (a.s.l.). The only topographic heights are dunes, paleodunes and riverbanks. In the Po Delta, fast subsidence and large sediment input allowed a full record of the Late Quaternary evolution (Fig. 1). The stratigraphic

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architecture was ruled by glacio-eustatic fluctuations. The latter gave origin to an alternation of alluvial plain deposits and coastal/shallow marine sediments. Alluvial plain deposits accumulated mainly during lowstand (Pleistocene) phases, while coastal/shallow marine sediments accumulated during transgressive-highstand (Holocene) phases (Amorosi et al., 2015). During the last glacial lowstand, from 125 ka B.P. to the last glacial maximum (about 18 ka B.P.), the study area was characterized by the sedimentation of well-drained middle alluvial-plain sands. Deglaciation and early transgression were associated with an erosive disconformity development and the fast inland movement of a barrier-lagoon-estuary system in response to rapid sea level rise and reduced siliciclastic influx

Fig. 1. Location map of the monitoring wells, the seawater samples, the Po River samples and the rainwater samples in the study area. The transect AA’ reports the general distribution and age of the depositional environments in the southern Po Delta area (modified from Amorosi et al., 2005). Locally deviations from the reported depth and thickness of the facies' distribution are possible. LST stands for lowstand system tract, TST for transgressive system tract and HST for highstand system tract.

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(Amorosi et al., 2005). In the study area, transgressive accumulation started between 10,000 and 9000 yr BP generating a wide coastal plain in the western part of the study area while seaward, in the Comacchio area, a back-barrier environment inundated by brackish waters was present. With continuing transgression, up to 5500 yr BP, the barrier-lagoon-estuary system migrated about 4 km landward and a shallow marine depositional environment developed across most of the study area. In the succeeding highstand phase, back-stepping fluvial and brackish marsh deposits were followed by Po Delta progradation under a combined influence of climate and subsidence. In this period, the complex interplay of sea level changes, subsidence, and sediment supply was the dominant control on facies architecture. Large sand spits and barrier islands grew, turning the previous bays into confined marsh lagoons with widespread organic clay intercalations and peat horizons. Afterwards, the depositional dynamics were modified by both climate changes and anthropic events (infrastructures, reclamation works and fluvial diversions) causing coastal progradation and a consequent regressive succession of the deposits (Stefani and Vincenzi, 2005). The shallow coastal aquifer in the study area is usually unconfined, but locally thin confining lenses of silt and clay sediments (often not spatially continuous) can be found. These lenses act as impermeable or semi-permeable barriers to the upward groundwater seepage and force the flow lines to focus at the lateral boundaries of these lenses. The aquifer is located inside the beach-ridges and transgressive barrier deposits and is underlie by fine-grained deposits of alluvial plain. The aquifer thickness ranges between 2 and 24 m and thins towards inland (Fig. 1). The hydraulic conductivity is variable, depending on the sorting of aquifer materials, and ranges from 1 × 10−6 m/s in coastal plain sediments up to 1 × 10−3 m/s in the beach-ridges (Colombani et al., 2016). The general horizontal groundwater flow direction is from the Adriatic Sea towards inland (Colombani et al., 2016), although the estimated horizontal groundwater flow velocity is quite slow, about 7–10 m/y (Mastrocicco et al., 2012). The aquifer is prevalently in reducing conditions, showing elevated concentrations of iron, manganese and ammonium (NH+ 4 ) and frequently hydrogen sulphide is detected too (Giambastiani et al., 2013). The unconfined aquifer in the study area is characterized by a wide range of salinity (from freshwater, typically found in the upper portion of the aquifer, to hypersaline in the basal portion) and was proved to have an active exchange with the surface water network (Colombani et al., 2015b, 2016). In fact, groundwater is recharged by the dune and paleodune systems, while it is forced to discharge into the surface water network. The hydraulic head difference between the aquifer and the surface waters constantly drained by pumping stations (about 1 m) causes a stable upward groundwater flux (Caschetto et al., 2017; Colombani et al., 2016; Giambastiani et al., 2013). The modern anthropogenic influence has determined a change in the aquifer hydraulic equilibrium. The need to acquire new areas to be allocated to agricultural activities led to a succession of remediation work, intensified from 1850s to 1960s. The resulting surface water system is very complex, being constituted by numerous natural and nonnatural water bodies (Fig. 1). Nowadays, the drainage system is necessary to lower the phreatic level, secure surface water discharge towards the sea, maintain the land dry and to provide water for agriculture. Here a wide variety of crops is cultivated, with the prevalence of maize and wheat. While the main fertilization practice is by using synthetic urea followed by poultry chicken and NPK fertilizers (Castaldelli et al., 2013). 3. Materials and methods Eleven monitoring wells were selected for this study (Fig. 1). The monitoring wells belong to the regional monitoring network of the Geological Survey of the Emilia Romagna Region. The 2 in. wells are fully screened from −1 m a.s.l. to a maximum of −22.5 m a.s.l. to cross the whole aquifer thickness. To minimize clogging and to prevent surface-

water infiltration screens are surrounded by a geotextile sock and sealed with a mixture of cement and bentonite at the top. A straddle packers system Solinst® 800L was employed to isolate a window of 0.2 m within the fully penetrating wells. Each monitoring point was purged for three volumes (approximately 15 l). After that, one groundwater sample was collected using an inertial pump to attain a low-flow, which minimizes the risk of cross contamination. A single campaign was performed in October 2013 to collect 111 groundwater samples from the shallow coastal aquifer (see SI for details). The groundwater samples were collected after the stabilization of physical-chemical parameters monitored via a Hydrolab flow cell attached to the Hydrolab® MS-5 probe to acquire in situ pH, EC, Eh and DO. A Po River branch was sampled in four different campaigns from 2010 to 2014 near the monitoring well P4. The seawater samples were collected at the shoreline in five campaigns from 2010 to 2016 near the monitoring wells P1, P8 and P9. The bulk (wet and dry) atmospheric deposition was sampled in four campaigns from 2010 to 2016 using a HDPE bottle and a 20 cm diameter funnel equipped with a 100 μm Nitex® net to avoid external contamination. Samplers were installed near the monitoring wells P1 and P8 at a height of 1.5 m above ground. Samples were filtered through 0.22 μm Dionex® polypropylene filters, stored in a cool box at 4 °C and immediately transported in the laboratory to be analysed for the major ions using an isocratic dual pump ion chromatography ICS-1000 Dionex® equipped with an AS9-HC 4 × 250 mm high capacity column and an ASRS-ULTRA 4 mm self-suppressor for anions. An AS-40 Dionex® auto-sampler was employed for the analyses, Quality Control (QC) samples were run every 10 samples and the standard deviation for all QC samples run was lower than 4%. Alkalinity was measured via titration using the kit Alkalitats-Test (Aquamerck®). The Chloride excess (Cl− exc.) was calculated for saline and hypersaline groundwater using the following formula: −

−

Clexc: ¼ Cl −Naþ −K þ

ð1Þ

All concentrations were expressed in meq/l. Statistical elaborations were performed trough Statistica 12, ProUCL and SigmaSTAT softwares. Frequency distributions were tested with both Shapiro-Wilk and Lilliefors tests at 95% confidence interval (CI). Factor Analysis (FA) was computed in order to identify and separate the geochemical processes governing the behaviour of each measured chemical parameter. The considered number of factors extracted was based on the Kaiser criterion, i.e. only factors with eigenvalues N1 are retained (Kaiser, 1958). In order to maximize the variance of the principal axes, the Varimax normalized rotation was applied. 4. Results and discussion 4.1. Gibbs diagrams The Ferrara coastal aquifer has a variable TDS from minimum values of 0.5 g/l up to a maximum of 75 g/l. The lowest TDS are found near the Po River watercourse and in the paleodunes, while the highest TDS are found in the Comacchio area (Fig. 1). More than a half of the groundwater samples have TDS higher than 10 g/l, suggesting elevated water-sediment interactions and evapoconcentration processes. In general TDS values increase with depth, because of the interaction between groundwater and the fine sediments of the deepest part of the coastal unconfined aquifer, that were deposited in a back-barrier environment (Caschetto et al., 2017). In the study area, back-barrier facies have a thickness of 2–6 m, and include inner-lagoon deposits with abundant organic matter and outer-lagoon deposits with less organic matter, associated to marine and brackish waters. Fig. 2 shows that evaporation played an important role in the majority of the groundwater samples collected via MLS techniques. In particular, samples collected in P33 and P34 are grouped in the top right area

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Fig. 2. Gibbs diagrams for anions (left panel) and cations (right panel).

of the Gibbs diagrams testifying a considerable role of evaporation in the geochemical evolution of groundwater that leads to concentration of TDS higher than the actual seawater. Samples from P1, P2, P8 and P10 also have a clear evaporative trend, especially evident in the deepest samples (see Fig. 1 for location and SI for the complete dataset). All the above monitoring wells are located in back-barrier environments characterized by bay or lagoonal deposits with typical shallow water fauna association and pioneer vegetation (Amorosi et al., 2015). In these environments, it is renowned that transpiration could concentrate solutes in the residual porewater, so that if the source water is seawater, then the residual porewater may become highly saline (Fass et al., 2007; Ridd and Stieglitz, 2002; Wood et al., 2003). On the other hand, P4 and the top part of P5 and P7 samples fall in the water-sediment interaction area of the Gibbs diagrams. The samples affected by the supply of freshwater either from the Po River or from irrigation, have a Na+ + K+ / (Na+ + K+ + Ca2 +) molar ratio up to 0.6 mg/l and a low Cl− / (Cl− + HCO− 3 ) molar ratio. This suggests that cation exchange with the clay minerals of the matrix plays a role, increasing Na+ and K+ and decreasing Ca2+, according to the water-sediment interaction dominance. The low values of Cl− / (Cl− + HCO− 3 ) molar ratio are imputable to an excess of bicarbonate due to the organic matter mineralization processes in peaty lenses (Colombani and Mastrocicco, 2016), which cause an abrupt increase in alkalinity. The dissolution of carbonates present in the aquifer matrix could also help to increase the alkalinity, but a recent study on stable isotopes in two monitoring wells has shown that the major source of alkalinity is due to organic matter mineralization processes (Caschetto et al., 2017). The latter group of monitoring wells, pertain to the early growth of the modern Po Delta system, characterized by delta front arcuate ridges laterally connected to prograding strandplains (Amorosi et al., 2003) and made up of rectilinear beach ridges with associated aeolian sands (Stefani and Vincenzi, 2005). 4.2. Cl−, Br− and their molar ratio as environmental tracers To investigate the origin of salinity, Cl− and Br− have been used as environmental tracers. In particular, their molar ratios plotted versus Cl− concentrations could be a good indicator of groundwater origin as recently proposed by Kwong et al. (2015) and Hoque et al. (2014). Dissolved Cl− and Br− in groundwater are tracers close to ideal conservative behaviour due to their hydrophilic character and small ionic size. Moreover, they do not take part in ion exchange reactions at low temperatures, nor they can be adsorbed onto mineral surfaces. The major

reservoir of water (the ocean) has relatively uniform Cl− and Br− concentrations and their molar ratio is around 655 ± 4 (Alcalà and Custodio, 2008), but for the study area the observed Cl−/Br− Adriatic Sea molar ratio is 696 ± 47. This value is slightly higher than the ones reported by other authors, but this fact can be explained by seawater dilution near the delta due to freshwater input from the Po River branches. Groundwater data have been compared with the mixing line of seawater (Adriatic Sea) and freshwater end-members (Po River). Given the inherent variability of TDS in both the end-members, a mixing area rather than a simple mixing line was constructed using all the samples collected from 2010 to 2016 and plotting the 99.99% confidence interval (CI) in both the plots of Fig. 3. In spite of the high variability in Cl− and Br− concentrations found in the actual Adriatic coastal seawater samples, due to the mixing effect with the Po River waters discharging immediately North of the study area, the groundwater samples exhibit an extremely regular linear trend with a R2 of 99.5%. A possible explanation of such a behaviour is the common source of groundwater salinity for the whole aquifer, although to better elucidate this hypothesis other elaborations are needed. For instance, the Cl−/Br− versus Cl− plot (Fig. 3, right panel) shows that most of the samples are above the actual freshwater/seawater mixing line with freshwater samples showing a high variability in the Cl−/Br− molar ratios (from 192 to 1440), while brackish, saline and hypersaline samples show an almost constant Cl−/Br− molar ratio of 870 ± 70. The origin of this constant Cl−/Br− molar it is not likely to be identified in the local bulk deposition (wet and dry), since the latter gives Cl−/Br− molar ratios highly variable (from 433 to 2060) due to chemical fractionation during evaporation of sea droplets generated by the wind. The highest values of Cl−/Br− in the freshwater samples belong to the monitoring well P9 (see Fig. 1 for location and SI for the complete dataset), which is located within the residential area of Lido di Spina, suggesting that such values may be imputable either to rainfall recharge in coastal area or to urban wastewater (Alcalà and Custodio, 2008). The lowest values of Cl−/Br− in the freshwater samples belong to P7 (see Fig. 1 for location and SI for the complete dataset) and could be imputable either to rainfall recharge in inland area or to agricultural pollution due to rice cultivation. Thus, for the monitoring wells located in areas where the impact of human activity may not be neglected, Cl−/Br− molar ratio cannot be considered a self-standing screening tool to discriminate the groundwater origin. On the contrary, the mean values of

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Fig. 3. Binary plot of Br− versus Cl− (meq/l) for MLS, Adriatic Sea, Po River and rainfall samples (left panel). Binary plot of Cl−/Br− molar ratio versus Cl− (meq/l) for MLS, Adriatic Sea, Po River and rainfall samples (right panel). Black lines indicate the ideal mixing line between actual freshwater and seawater, while the grey shaded areas represent the mixing area depicted by the 99.99% confidence interval.

Cl−/Br− in P4 of 392 are typical of recharge waters from inland areas, e.g. a field study located near Ferrara town reported Cl−/Br− values around 400 for non-contaminated groundwater (Giambastiani et al., 2015). In addition, the Cl−/Br− values of P4 show a close correspondence with a mean value of 430 recorded for the Po River freshwater end-member (see SI for the complete dataset) confirming the inflow from the Po River. Moreover, from the general trend of all the MLS samples collected, it is evident that salinization is not due to intrusion of modern seawater, even in monitoring wells located close to the coastline, like P1. In fact, the Adriatic Sea samples and the mixing line between freshwater and the modern seawater end-members plot below the trend individuated by groundwater samples, and even no MLS sample falls within the grey shaded area of the 99.99% CI. This pattern indicates that salinization in this coastal aquifer derives from the mixing between other two end-members, in accordance with the Cl−/Br− molar ratio. The Cl−/Br− ratios of groundwater samples (greater than actual marine signatures) confirm that evapoconcentration via salts precipitation and/or plant transpiration, rather than water-rock interaction, was the dominant mechanism in the formation of hypersaline residual porewater (Fass et al., 2007). This was also recently confirmed by isotopic analyse of δ18O and δ2H in two monitoring wells of this aquifer (Caschetto et al., 2017). Here hypersaline groundwater showed values enriched respect to the actual seawater (approximately + 2‰ for δ18O and + 5‰ for δ2H) confirming that evapotranspiration was the prevailing mechanism of salt concentration (Li et al., 2016). Physical processes taking place in soil (dilution, evapotranspiration and mixing) do not significantly modify the Cl−/Br− molar ratio but can change the absolute concentrations of Cl− and Br− (Alcalà and Custodio, 2008; Edmunds, 2001). This can be seen in both shallow and deep groundwater samples; in fact, as well as the P1 shallow samples show molar ratios up to 888 and absolute concentrations of Cl− up to 529 mmol/l and Br− up to 0.6 mmol/l, so also samples from the deep portions of P33 and P34 show molar ratios up to 925 but with absolute concentrations of Cl− up to 1270 mmol/l and Br− up to 1.38 mmol/l. Indeed, Cl− versus Br− plot (Fig. 3, left panel) has a good correlation, indicating a similar origin between them. Assuming that the proposed mechanism may be responsible for the high salinity of groundwater samples collected at the base of the aquifer, there is a need to verify whether the paleo depositional environment was favourable for the onset of the abovementioned conditions. In fact, recent reconstructions of the depositional history in the Po Delta

(Amorosi et al., 2005, 2015; Stefani and Vincenzi, 2005) show that the study area during the Holocene transgression (between 10,000 and 8000 yr BP) was interested by the rapid landward migration of a barrier-lagoon-estuary system in response to sea level rise. This created a shallow back-barrier inter-basin inundated by brackish waters in the Comacchio area, which was a perfect environment to form hypersaline waters. In general, after large variations in sea level at the end of the Last Glacial event, the saline to hypersaline residual porewater has been washed away. Although, in low-permeability coastal aquifers, part of this water trapped into fine-grained sediments could have been preserved from rapid flushing leading to the formation of saline and hypersaline groundwater (Lorenzen et al., 2012). The inverted head gradient created by the constant pumping of the drainage network, have then mobilized and slowly transported upward the highly saline groundwater, previously preserved in the deepest portion of the aquifer within low permeability sediments (Caschetto et al., 2017). This is causing the salinization even of the overlying prodelta and beach-ridges sediments (see Fig. 1). To further prove that the origin of groundwater salinization is due to the hypersaline porewater trapped in the fine-grained sediment in the basal portion of the aquifer, a recent column experiment by Colombani and Mastrocicco (2016), has shown that samples collected from a buried peaty lens at a depth of − 21 m a.s.l. in P33, produce values of Cl−/Br− molar ratio between 900 and 1100 when flushed with Po River water. Again this information combined with the very constant Cl−/Br− molar ratio values in saline and hypersaline groundwater samples, point out that the origin of salinity is owing to organic-rich sediments deposited in back-barrier environments at the base of this aquifer during the Holocene transgression (Scarponi et al., 2013). 4.3. Major cations and their molar ratios The ternary plot in Fig. 4 indicates that, the main process affecting the groundwater samples is evapoconcentration, confirming the abovementioned hypothesis about the origin of the salinity. This is explained by the increasing linear trend of the Na+/Ca2+ ratio, since all samples plot along a line moving from rainwater towards seawater and further up to hypersaline samples. Since the predominance of the evapoconcentration process in concentrating salts in porewater, the other two ratios do not prevail over the Na+/Ca2+, while in the case

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Fig. 6. Binary plot of (Mg2+/Ca2+)/Cl−exc. versus Cl−exc. for saline and hypersaline MLS and Adriatic Sea samples. Fig. 4. Ternary plot of cation molar ratios for MLS, Adriatic Sea, Po River and rainfall samples. The black line indicates the ideal mixing line between actual freshwater and seawater.

of other sources of salinization the samples would not plot along a straight line but eventually will distribute in different clusters. Fig. 5 shows that the evaporative process was likely to take place in an organic rich environment. Organic N mineralization is the main 3− source of NH+ 4 and phosphate (PO4 ) in groundwater, which are by far the prevailing N and P species in the deep brackish to hypersaline portion of the aquifer, where reducing conditions prevail (Mastrocicco et al., 2013). Here, NH+ 4 concentrations has an average concentration of 4.27 mmol/l and up to 9.21 mmol/l, while PO3− concentrations are 4 generally lower, with an average concentration of 0.22 mmol/l and up

to 0.55 mmol/l. A recent study (Caschetto et al., 2017) has explored the stable isotopes behaviour in two monitoring wells; confirming that in the deep and saline portion of the aquifer, the source of elevated NH+ 4 concentrations is the mineralization of organic N-rich fine sediments. On the contrary, shallow and fresh-brackish portion of the aquifer, the source of elevated NH+ 4 concentrations is due to anthropogenic input from sewage in urban environments and from mineral fertilizers in agricultural environments. 3− NH+ ratios are extremely variable, ranging from 0.7 to 820 4 to PO4 (Fig. 5), and consistent with Redfield ratios found in similar studies on nutrients in modern (Slomp and Van Cappellen, 2004) and paleo back-barrier environments (Bratton et al., 2009). From Fig. 5, it can also be noted that part of the brackish to saline groundwater samples show a certain degree of mixing with Ca-bicarbonate waters that tend to increase the Ca2+/Sr2+ molar ratio.

Table 1 Extracted Factors and related explained variance for untransformed (FA1) and log-transformed data (FA2). The most significant extracted factors (≤|0.8|) are shown in bold.

FA1

2+ 3− Fig. 5. Binary plot of NH+ versus Ca+ molar ratios for brackish to hypersaline 4 /PO4 2 /Sr groundwater samples.

F− Cl− Br− NO− 3 PO3− 4 SO2− 4 HCO− 3 + Li + Na NH+ 4 K+ Mg2+ Ca2+ Sr2+ TDS Eigenvalue Expl. var. % Tot. expl. var. %

Factor 1

Factor 2

Factor 3

0.18 0.92 0.92 −0.11 0.16 0.86 0.48 0.03 0.93 0.28 0.97 0.89 0.58 0.80 0.93 8.90 59.6 80.1

−0.22 0.34 0.35 0.06 0.83 −0.18 0.72 0.23 0.32 0.88 0.09 0.39 0.34 0.49 0.33 1.77 11.9

0.53 0.07 0.09 −0.11 0.16 −0.01 −0.15 0.90 0.06 0.20 −0.01 0.14 0.34 0.19 0.07 1.28 8.6

FA2 F− Cl− Br− NO− 3 PO3− 4 SO2− 4 HCO− 3 + Li + Na NH+ 4 K+ Mg2+ Ca2+ Sr2+ TDS Eigenvalue Expl. var. % Tot. expl. var. %

Factor 1

Factor 2

Factor 3

0.58 0.95 0.96 −0.32 0.21 0.13 0.76 0.40 0.97 0.82 0.95 0.95 0.58 0.90 0.96 9.28 61.9 80.4

−0.48 0.21 0.21 −0.73 0.25 0.70 0.24 0.09 0.19 0.09 0.13 0.27 0.68 0.31 0.24 1.74 11.6

0.11 0.07 0.08 0.05 −0.80 −0.39 −0.35 0.48 0.01 −0.22 0.01 0.09 0.31 0.04 0.02 1.04 6.9

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Fig. 7. Factor plots from untransformed (FA1) and logtransformed (FA2) data. Factor 1 in both elaborations accounts for variables related to the saline source. Factor 2 in FA1 and Factor 3 in FA2 account for variables related to the interaction with organic rich sediments.

In fact the ratio of (Mg2++Ca2+)/Cl−exc. over Cl−exc. (Fig. 6) for the saline and hypersaline groundwater samples shows a clear trend of dolomite dissolution for the hypersaline samples, with the ratio reaching an asymptotic value of 0.35. This is also supported by the Mg2+/Ca2+ molar ratio that increases abruptly from 4 up to 20, indicating the increasing presence of Mg2+ over Ca2+ in hypersaline groundwater. The results found are agreement with the ternary graph of Fig. 4, since the ternary gives the relative abundance of the three selected variable, with the Mg/Ca which is prevalent over Na/Ca in freshwater samples while the opposite occur for saline samples. 4.4. Factor analysis Despite to the significant information on the origin of salinization processes that the classical geochemical tools have provided, a statistical analysis could give more insights on the relative relevance of different geochemical processes occurring in a coastal aquifer (Huang et al., 2013). From the Factor Analysis (FA) a clear lognormal trend can be distinguished in the samples population, confirmed by statistical tests at 95% CI. The presence of high saline samples in the dataset generates parent normal populations at the right tale of frequency distributions, producing the detected lognormal trend (Vistelius, 1960). This is a strong evidence that if this frequency distribution is preserved, it brings in the statistical elaboration the information related to the overall mineralization processes from which it is derived. However, a correct FA computation requires normal distribution of data (Reimann et al., 2002), for this reason the data have been also log-transformed, after which the tests verified that the data attain normal distribution. Factors were extracted both for lognormal (FA1) and normal distributed (FA2) dataset. FA results are reported and compared in Table 1, where significant loadings (≥±0.8) are marked in bold type. Factors extracted following the Kaiser criterion are three for both FA1 and FA2 and their total explained variance is 80.1% and 80.4%, respectively. Comparing the loadings of each factor of FA1 versus FA2, factor 1 clusters in both FA. In fact in this factor are enclosed all the conservative and/or highly soluble ions (i.e. Na+, K+, Sr2+, Br−, Cl−, Mg2+) and the TDS, which are all variables related to groundwater mineralization processes due high saline sources. Differences in the outcomes between the two elaborations are detected with the factors 2 and 3. In the FA1 the 3− factor 2 clusters NH+ 4 and PO4 , giving a clear statistical fingerprint of mineralization processes by interaction with peaty lenses. This outcomes in the FA2 is pointed out by factor 3 with significant PO3− load4 ings, while NH+ was effaced or at least minimized by log4 transformation and enclosed in the factor 1. This proves that the peaty lenses located at the base of the coastal aquifer play the major role as evidenced by Mastrocicco et al. (2013) in the field and quantified by Colombani and Mastrocicco (2016) in laboratory experiments. To better understand this finding, in Fig. 7 the factor plots of FA1 and FA2 are compared. The factors separated the mineralization processes

related to highly saline waters (factor 1 in both FA) and the interaction with peaty lenses (factor 2 in FA1, and factor 3 in FA2). Through the plots, it can be quickly observed the significant loadings (N±0.8) of conservative ions related to factor 1 and the significant loading of PO3− re4 lated to factor 2 (FA1) and 3 (FA2). In FA1 most of the variables enclosed in factor 1, have loading related to factor 2 comprised between 0.3 and 0.5, thus not close to zero. Such a situation generates a clustering of variables in the first quadrant located far from the Y-axis. This behaviour suggests the interdependence of the involved variables in different mineralization processes (Cuoco et al., 2015), since the more a variable plots close to a specific axis (factor) the more it depends on the mineralization process that the specific factor (axis) represents. With normalization, this evidence is almost lost, even though in the FA2 NH+ 4 plots with factor 3, with a loading of −0.22 towards the PO3− variable, pro4 viding also in this case a statistical evidence of the source of these solutes from peaty lenses in the deeper part of the aquifer. 5. Conclusions The origin and patterns of groundwater salinity in a shallow coastal aquifer pertaining to a reclaimed subsiding zone of the Po River lowland were examined in this study. To disentangle the possible sources of salinity the molar ratio of environmental tracers like Cl− and Br− were used in combination with the classical geochemical analyses of major and minor cations molar ratios. High-resolution multi-level sampling allowed obtaining a robust and self-consistent hydrogeochemical database, which was statistically analysed with the factor analysis and proved to be log-normally distributed. A common origin was identified as the driver of groundwater salinization in the whole aquifer, and the source of salinity was recognized in the basal organic rich sediments deposited during the last transgression phase in back-barrier and salty marsh environments. Hypersaline porewaters entrapped in these finegrained sediments were generated by a history of sea-level fluctuation and evaporative enrichment, and subsequently forced to migrate upward by the intense drainage activities put in place in this fertile coastal plain to ensure the development of agricultural practice. This process can pose an equal or greater water quality threat than seawater intrusion, causing a more rapid salinization of coastal groundwater resources due to the higher salinity characterizing the entrapped porewater. A comprehensive analysis of coastal groundwater geochemistry together with a detailed paleogeographic reconstruction is therefore extremely important to infer the origin of salinization in lowlying coastal aquifers, as the traditional conceptual model of a simple fresh/seawater interface may not be sufficient. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.gexplo.2017.01.011.

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