Reclamation influence and background geochemistry of neutral saline

soils in the Po River Delta Plain (Northern Italy)

Di Giuseppe D.1,*

, Faccini B.1, Mastrocicco M.

1, Colombani N.

2, Coltorti M.

1

1Physics and Earth Science Department, University of Ferrara, Ferrara, Italy

2Department of Earth Sciences, ‘‘Sapienza’’ University of Rome, Roma, Italy

*e-mail: dgsdra@unife.it

Abstract

Reclaimed neutral saline sulphate soils constitute a large part of the eastern part of Po Plain

lowlands, where intensive agricultural activities take place. The knowledge of their geochemical

features is essential to develop the best management practices capable to preserve this threatened

environment. With this aim, three boreholes were drilled in an agricultural field and a typical

reclaimed soil profile has been characterized for major and trace element, pH, electrical

conductivity, redox conditions and water-soluble anions and ammonium. Statistical analysis (cluster

analysis and principal component analysis) has been used to understand the relationship between

elements and grain size. The soil profile is characterized by high salinity and high organic matter

contents responsible for high chloride, sulphate, and ammonium concentrations. Heavy metal

content is naturally high, since Po Plain sediments are the result of ultramafic rocks erosion; in

addition, organic matter tends to concentrate heavy metals by adsorption, mainly in peaty horizons.

As a consequence of chemical and zootechnical fertilization, high NO3- contents have been found in

the top soil, thus enhancing the risk of nitrate discharge in the water system, especially in relation to

extreme climatic events.

Keywords: Soil, Reclaim, Geochemistry, Agriculture, Redox, PCA

1. Introduction

Wetlands are critical and fragile environments were water salinization and accumulation of heavy

metals can easily occur. The increasing demand of land for agricultural purposes has led to the

reclamation of large wetland areas (Airoldi and Beck 2007) that may be affected by several

problems such as soil salinization (Mastrocicco et al. 2013a), decalcification (Van den Berg and

Loch 2000), gleying (Bini and Zilocchi 2004), and further increase in the amount of the heavy

metals (Bai et al. 2011; Molinari et al. 2013; Di Giuseppe et al. 2014). These processes can cause

extensive changes in the physical–chemical characteristics of the soil, such as redox conditions, pH,

and leaching of C, N, P, S and Fe (Pornoy and Giblin 1997). Moreover, to make this soil

productive, a large use of fertilizers, especially nitrogen compounds, is required, causing a

remarkable pollution of the superficial and ground water. These problems are rather common in

Italy (D’Antona et al. 2009; Mastrocicco et al. 2013b) and abroad (Rysgaard et al. 1996; DelAmo et

al. 1997; Bai et al. 2005; De Wit et al. 2005; Netzer et al. 2011; Statham 2012). To provide

information on the water/soil system that would be also useful for recognizing and interpreting

geochemical anomalies potentially induced by pollution processes, the Po River Delta Plain in

Northern Italy has been studied. This sedimentary basin, bordered by the Alps and the Apennine

chains, hosts about 25 % of the Italian population and most of the Nation’s agricultural activities. In

particular, the soil profile, hereafter reported as reclaimed soil profile (RSP), is located in the

Province of Ferrara, nextto Codigoro town (45° 500’ 330” N and 12° 050’ 400” E), where the

ZeoLIFE Project (LIFE+ 10/ENV/IT/00321; Coltorti et al. 2012; Di Giuseppe et al. 2012, 2013),

aimed at reducing nitrate pollution and correct agricultural soils, is currently developing. The delta

environment is characterized by high lateral mobility of the active channel belts, with recurrent

avulsion and channel bifurcation, which redistribute the water and sediment fluxes throughout the

system (Bondesan et al. 1995; Stefani and Vincenzi 2005). When the high-energy alluvial

deposition outranged the low-energy lacustrine conditions typical of organic deposition, the peat

levels were buried and incorporated into the stratigraphic sequence (Miola et al. 2006). The

sedimentological and geochemical features of the Po River sediments have been studied in detail,

mainly focussing on the spatial distribution of heavy metals in relation to provenance and

sedimentary facies (Amorosi et al. 2002, 2003; Amorosi and Sammartino 2007; Bianchini et al.

2002, 2012; Amorosi 2012). On the other hand, reclaimed saline soils are significantly different.

Their geochemical and pedological characteristics have been extensively studied for acid sulphate

soil (Unland et al. 2012; Burton et al. 2006; Ljung et al. 2009) but not so intensely for neutral saline

sulphate soils. These sediments evolve from backswamp deposits peculiarly rich in organic matter.

In these environments, due to the rapid subsidence enhanced by sediment compaction, the

coalescence of water pools has created large wetlands. In modern age, the anthropic reclamation

activity has drained off most of the marshes, causing the deposition of a fine-grained surface layer

where drainage is difficult (Stefani and Vincenzi 2005). These soils have been drained by humans

to take advantage of new land and they are different from the natural soil profiles existing

worldwide. Nowadays, the majority of the reclaimed lands is intensively exploited and widely

cultivated with highly nutrient-demanding crops (mainly corn) which require a large use of

fertilization and chemical treatment. The consequence of such agricultural practice in this

‘‘artificial’’ environment could lead to metal pollution (Borghesi et al. 2011) and eutrophication of

channels and coastal lagoons (Frascari et al. 2002). To preserve this fragile environment or develop

best management practices, it is compulsory to define the geochemical composition of the soils, i.e.,

the baseline on which future human activities will be overimposed. This study is aiming at

describing, evaluating, and comparing major and trace elements (Ni, Cr, V, Zn, Pb, Co, Cd, Cu, U,

As, Sr, Zr, S, Rb), organic matter, pH, electrical conductivity, and particle size vertical distribution

in a typical reclaimed saline soil. In addition, water-soluble cations and anions in the soil, with

particular emphasis to Nitrogen species, are also taken into account. A multivariate statistic

approach [cluster analysis (CA) and principal component analysis (PCA)] was adopted to assist the

interpretation of geochemical data, according to the procedure defined by Facchinelli et al. (2001)

and Tyler (2004).

2. Material and methods

2.1. The study area

The geological setting of the Delta Plain, extending for more than 730km2, is dominated by the

main branch of the Po river and by its ancient and recent alluvial and delta deposits (Fig. 1a).

Sediments belonging to this environment occupy an area extending from Ferrara to the Adriatic

coast, shaping up a fan-like delta limited to the North by the actual Po up to the mouth of Maistra

branch, and to the south by the Po di Primaro–Reno fluvial system located just south of the

Comacchio Lagoons. Within the delta system, three sedimentological groups can be distinguished:

the coarse deposits (gravel and sand) of the interdistributary channels and their banks (Fig. 1, Type

Deposit 1 and 2), and the fine deposits (silt, clay and peat) of brackish marsh and interdistributary

bays (Fig. 1, Type Deposit 3). The interdistributary bay is an environment characterized by low-

energy hydrodynamics, where clayey and organic matter-rich sediments such as peat (resulting

from burial of swamp vegetation) prevail. Moving eastward to the coast, the delta deposits are

interdigited with a series of sandy littoral stringy dune, mainly elongated North–South, marking the

ancient coastline (Fig. 1, Type Deposit 4).

Figure 1. Regional framework of the Po Delta system (modified from Stefani and Vincenzi, 2005).

The areas enclosed between topographic highs (created by paleochannels and paleodunes) are

topographically depressed areas (interdistributary bay) originally occupied by vast marshy basins

that are kept dry by the action of mechanical water pumps (Figs. 1, 2).

Figure 2. Site location. Three boreholes (C2, C3, C4). 3D view of soil profile.

In 1860, the surroundings of the actual city of Codigoro were almost entirely occupied by lagoons

that have been gradually dried up in the subsequent decades (Bondesan 1990); today the entire

Codigoro Municipality lays on dry land. Due to the soil lowering in the Po Plain that would cause

its flooding, the area belonging to the Municipality of Codigoro is kept dry artificially by a network

of draining and irrigation channels. The lowering of the soils is caused by the natural subsidence

plus an induced subsidence related to the human activity in the territory. The natural subsidence is

intrinsically linked to the general geological characters of the Po Plain, with variable rates, usually

\2 mm/year (Teatini et al. 2011). The induced subsidence is mainly related to water extraction from

aquifers at low or medium depth or gas at higher depth (Teatini et al. 2006). The drainage of damp

areas contributes also to increase this subsidence, due to the compaction of the sediments no longer

submersed and sustained by water. The mostimportant drainage pump in the area is the Codigoro

dewatering pump, which allows the contemporaneous drainage of two big channels conveying

water at different heights, the Acque Alte and the Acque Basse, located close to the study site (Fig.

2). The studied soil profile site is located between the main distributary channel of the ancient Po

Gaurus to the left and a minor distributary channel to the right (Fig. 1). With a height of ca. -3 m

a.s.l., it resides in an intensely agricultural area next to town of Codigoro, whose predominant

crops are corn and wheat. The site is only 13 km from the Adriatic coast. It is characterized by a

microclimate influenced by the sea. Coastal area extends from the sea up to 30–40 km inward,

and includes 2/3 of the entire delta territory, with a broad transitional zone where the sea mitigation

gradually disappears. Codigoro climate can be defined as ‘‘sub-coastal’’, in contrast with the ‘‘sub-

continental’’ climatecharacterizing the western part of the Ferrara Province. Rainfalls reach the

regional pluviometric minimum, represented by an average annual value varying between 500 and

700 mm. Temperatures are affected by the proximity of the sea. This is evident in particular during

the cold seasons, when marine thermoregulation contains the minima over zero, reducing the

number of night frosts (Mollema et al. 2012).

2.2. Field sampling and laboratory analyses

Within the European project ZeoLife (LIFE+ 10/ENV/IT/00321), three boreholes (A, B, C; Fig. 2)

were drilled manually in an agricultural field of six hectares with an Ejielkamp Agrisearch auger

equipment at the end of October 2011. In all the boreholes, core samples were collected every 30–

50 cm down to a depth of 4 m. Samples were stored in a cool box at 4° C and immediately

transported in laboratory for sedimentological and chemical analysis. Each borehole was

georeferenced by a portable global positioning system (GPS). The total elements concentration of

soils was determined using both X-ray fluorescence (XRF) (major and trace elements) and ICP-MS

(trace elements) technique at the Department of Physics and Earth Sciences of the University of

Ferrara. The soil samples were air dried and sieved at 2 mm. An aliquot of each sample was

powdered through an agate mill in preparation for chemical investigations. Major (SiO2, TiO2,

Al2O3, Fe2O3tot, MnO, MgO, CaO, Na2O, K2O, P2O5, expressed in weight percent) and trace (V,

Cr, Cu, Zr, S) elements were analyzed by XRF on powder pellets, using a wavelength-dispersive

automated ARL Advant’X spectrometer. Accuracy and precision based on systematic re-analysis of

standards are better than 3% for Si, Ti, Fe, Ca and K, and 7% for Mg, Al, Mn and Na; for trace

elements (above 10 ppm) they are better than 10%. Additional trace elements (Co, Ni, Zn, Pb, Cd,

U, As, Rb, Sr, Ce, Pb) were analyzed using an X Series Thermo-Scientific spectrometer (ICP-MS)

after total dissolution with HF + HNO3. Specific amounts of Rh, In and Re were added to the

analyzed solutions as an internal standard, to correct for instrument drift. Accuracy and precision,

based on replicated analyses of samples and standards, are better than 10% for all elements, well

above the detection limit. As reference standards, the E.P.A. Reference Standard SS-1 (a Type B

naturally contaminated soil) and the E.P.A. Reference Standard SS-2 (a Type C naturally

contaminated soil) were also analyzed to crosscheck and validate the results. pH-H2O was

determined electrometrically in the supernatant after shaking 5 g of soil at field moisture for 1 h

with 25 ml H2O. Particle size distribution of the soil horizons was estimated by wet sieving. Each

sample inside the polythene bags has been adequately mixed. 150 g of samples were treated with

160 ml of hydrogen peroxide to eliminate the organic substance. The sand (>63 μm) was separated

by wet procedure through a mesh sieve. Silt and clay fractions were analyzed using a Micromeritics

Sedigraph 5100. The organic matter content expressed in weight percent (OM%) was measured by

dry combustion (Tiessen and Moir 1993). The soil water content was measured gravimetrically after

heating the samples for 24 h at 105° C (Danielson and Sutherland 1986). Distilled water

(resistivity>18 MOmh/cm) was used to extract the water-soluble cations (NH4+, K

+, Ca

+2, Mg

+2,

Na+) and anions (NO3

-, Cl

-, Br

-, F-, NO2

-, SO4

-2,PO4

-3) from the soil samples, using a sediment to

water weight ratio of 1:5. The sediment and water were mixed and sealed in bakers, then shaken for

1 h, and centrifuged for 1 h at 25° C to separate the sediment from the solution. Soil water and

groundwater samples were filtered through 0.22 μm Dionex polypropylene filters prior to anion

analysis. Anions in all water samples were analyzed using an isocratic dual pump ion

chromatography ICS-1000 Dionex. An AS-40 Dionex auto-sampler was employed to run the

analyses; quality control (QC) samples were run every 10 samples and the standard deviation for all

QC samples was better than 4%. NH4+ was measured with a double beam Jasco V-550 UV/VIS

spectrophotometer (Bower and Holm-Hansen 1980).

3. Results

3.1. Profile description

Sedimentological analyses in the three boreholes revealed a slight vertical and lateral variability of

silt and clay contents, whereas sand content is always very low. Samples from the first 80 cm ca.

shift from clayey silt to silty clay (see Table 1). From about 80 cm down to 150 cm, a layer rich in

clay and dark peat is observed. In the lower part of the sequence, (150–400 cm) silt returns the

prevalent grain size. In the Shepard (1954) ternary classificative diagrams, all samples fall in the

clayey silt and silty clay fields; the entire C2 log lay on the boundary between the two.

Table 1. Characteristics of the soil profile.

On average within the soil profile three main horizons can be recognized, according to FAO (2006)

(Table 1; Fig. 2): (1) ‘‘Ap’’: the upper well aerated silty clay unit, homogenized by anthropic

activities and characterized by the presence of carbonate inclusions and iron hydroxides, (2) ‘‘Oe’’:

a peaty silty clay layer, and (3) ‘‘Cg’’: the lower undisturbed clayey silt anoxic unit, rich of

yellowish undecomposed organic matter and carbonate inclusions. The age of RSP sediments is

historical as found in a deep core sampling carried out in Massafiscaglia (Fig. 3; Bondesan et al.

1995), a locality close to the study area. Surface sediments are younger than 1,500 years ago

(Stefani and Vincenzi 2005). This soil is classified as a Humi Thionic Fluvisols Thapthohistic

according to WRB classification (WRB 2007).

Figure 3. Massafiscaglia borehole stratigraphy and calibrated radiocarbon age (modified from Bondesan et al., 1995)

On average, the texture is silty clay in the upper layers (Ap and Oe) and clayey silt in the lower

layer (Cg) of the profile (Fig. 5). In Ap horizon, the clay fraction (<2 μm) makes up to the 50% ca.

of bulk dry weight, decreasing to about 25% in Cg level (Fig. 4). The contents of silt (2–63 μm)

increase slightly downwards. The content of sand (>63 μm) is only 0.8–7% in Ap horizon, whereas

it increases slightly above 10% in the Cg horizon. The pH-H2O ranges between 6.3 and 7.6 in the

Ap horizon; it decreases in the organic Oe layer (5.8–6.9), and varies between 6.1 and 7.4 in the Cg

horizon (Fig. 5).

Horizon Man depth (cm) Colour moist Structure Skeleton Texture CaCO3 %

Ap 0 – 50 Olive gray 5Y 4/2 incoherent angular silty clay 8

Oe 50 – 140 Dark brown 10YR 3/3 polyhedral sub-angular silty clay 3

Cg 140 – 400 Gray 5Y 5/1 massive Clayey silt 10

Figure 4. Texture of soil profile

Figure 5. Average ECe, pH, organic matter, and Eh of soil profile

The vertical distribution of soil horizons is quite peculiar and it is typical of a developed fluvisols,

having a buried histic horizon between 80 and 140 from the surface (Table 1).

Ap horizon has skeleton, texture and structure influenced by plowing, with strong concentration of

plant roots and rhizomes. Oe horizon has a thickness of about 60 cm and an average content of 14%

of partly decomposed organic matter (Fig. 5). The transition between Ap and Oe is sharp and

characterized by a change in the organic matter content (average in horizon: from ca. 14 to <8%)

and in colour from olive gray to very dark brown or black.

The Cg horizon is always saturated with water, for the presence of the water table near the surface.

The transition between Oe and Cg horizon is gradual and somewhat variable among the tree

boreholes. The soil salinity increases rapidly from Ap to Cg horizon (Fig. 5). The source of

salinization is represented by soluble salts accumulated during the depositional process, this is also

confirmed by groundwater salinity that in the area is extremely elevated (up to 20 mS/cm). Eh

shows positive values (+310/+110 mV) in the upper regions of the RSP (Ap and Oe horizons),

whereas it rapidly changes towards negative values (-12/-200 mV) with depth (Cg horizon) (Fig. 5).

3.2. Major and trace elements distribution in RSP

The extended data set of 33 chemical analyses carried out on the ‘‘bulk’’ sample is reported in the

journal repository (Supplementary Table 1), while the maximum, minimum, mean, standard

deviation, and correlation matrix are reported in Tables 2 and 3. Ap and Og show different

composition with respect to Cg horizon. As already shown by Bianchini et al. (2012), the chemical

composition of the soil is strongly influenced by the grain size, and possibly by the elementary

complexation of the organic fraction (Twardowska and Kyziol 2003). The anthropic factor should

also be taken into account since: (1) generally the upper layer (Ap) contains most of the anthropic

contamination, while the underlying layer essentially represents the lithogenic input, (2) the upper

layer (Ap) is affected by annual plowing to an average depth of 40 cm, (3) after the reclamation, a

series of drains were installed in the field at an average spacing of 10 m and at a depth varying from

0.85 m in the central part of the field to 1.35 m b.g.l. at the northern and southern boundaries of the

field. On the other hand, Cg horizon has not been affected by human activity, in fact, the chemical

composition of the deeper samples is similar in all three holes. The vertical distribution of major

elements is shown in Fig. 6. The Ap horizon is characterized by low SiO2, TiO2, Al2O3, K2O, Na2O,

Fe2O3 and high MnO, CaO and P2O5. Oe horizon has the highest TiO2, Al2O3, K2O, Fe2O3 and the

lowest MnO, CaO and Na2O contents of the whole profile. SiO2, CaO, MnO, Na2O increase, while

K2O, Fe2O3, Al2O3 and TiO2 decrease in Cg. Trace elements taken into account in this study were

chosen on the basis of their environmental relevance. Some of them, although considered

micronutrients essential for the plant growth, become harmful contaminants above critical

concentrations (Hermanescu et al. 2011); others, such as As and Cd, are particularly toxic and

undesired in soils and waters (Kapaj et al. 2006; Bernard 2008). Oe horizon is remarkably enriched

in Ni, V, Zn, Co, Cu, Pb, Cd, Cr, As, U, and Rb. Ap is characterized by Ni, V, Zn, Co, Cu, Pb, Cd,

Cr, As, content intermediate between Oe and Cg. This latter has the lowest heavy metal and U

concentrations, with the highest Zr, Sr and S contents (Fig. 7).

Figure 6. Maximum, minimum, and average vertical distribution of major elements. The Y-axis shows the depth below

ground in cm (cm b.g.l.) and the X-axis represents the element concentration expressed in weight percent (wt%)

Table 2. Descriptive statistic and correlation matrix for major elements.

The concentration is expressed in weight percent

* L.O.I loss on ignition

Element Min. Mean Max. S.D.

SiO2 51,74 55,3 60,84 2

TiO2 0,62 0,7 0,81 0,1

Al2O3 12,32 16 19,6 2,1

Fe2O3 5,68 7,15 8,6 0,9

MnO 0,03 0,09 0,15 0

MgO 3,93 4,56 4,98 0,3

CaO 0,9 5,69 9,43 2,7

Na2O 0,48 0,92 1,42 0,3

K2O 2,26 2,99 3,75 0,4

P2O5 0,11 0,15 0,21 0

L.O.I. 3,39 6,44 9,25 1,5

Sand% Silt% Clay% SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 L.O.I.

Sand% 1,00

Silt% 0,39 1,00

Clay% -0,76 -0,89 1,00

SiO2 -0,19 0,17 -0,02 1,00

TiO2 -0,67 -0,60 0,75 0,41 1,00

Al2O3 -0,67 -0,69 0,81 0,17 0,92 1,00

Fe2O3 -0,65 -0,57 0,72 0,08 0,81 0,91 1,00

MnO 0,56 0,46 -0,60 -0,52 -0,85 -0,85 -0,73 1,00

MgO 0,21 0,34 -0,34 0,02 -0,34 -0,28 -0,14 0,13 1,00

CaO 0,63 0,45 -0,63 -0,51 -0,91 -0,91 -0,83 0,92 0,18 1,00

Na2O 0,45 0,62 -0,66 0,35 -0,56 -0,73 -0,77 0,44 0,18 0,46 1,00

K2O -0,67 -0,71 0,82 0,14 0,91 0,98 0,85 -0,83 -0,38 -0,88 -0,68 1,00

P2O5 -0,11 -0,44 0,36 -0,49 0,13 0,33 0,46 -0,17 0,01 -0,08 -0,62 0,29 1,00

L.O.I. 0,51 0,28 -0,44 -0,80 -0,77 -0,62 -0,55 0,77 0,03 0,77 0,14 -0,56 0,09 1,00

Figure. 7 Maximum, minimum, and average vertical distribution of trace elements. The Y-axis shows the depth below

ground in cm (cm b.g.l.) and the X-axis represents the element concentration expressed in ppm

Table 3. Descriptive statistic and correlation matrix for trace elements.

Element Min. Mean e Max. S.D.

V 86,6 132 183 26,6

Cr 156 221 315 35,2

Co 14,0 19,3 29,9 3,4

Ni 95,7 142 226 27,7

Cu 24,3 50,6 76,3 14,3

Zn 77,5 105 147 18,4

As 7,4 13,7 22,2 3,5

Rb 72,5 134 202 33,1

Sr 103 198 285 56,0

Cd 0,7 1,3 2,4 0,4

U 1,2 3,1 9,5 2,1

Ce 0,0 22,9 78,1 21,7

Pb 12,4 20,9 31,4 5,0

S 1138 6816 17466 4950

Zr 82,5 134 202 31,2

V Cr Co Ni Cu Zn As Rb Sr Cd U Ce Pb S Zr

Cr 0,73 1,00

Co 0,53 0,21 1,00

Ni 0,62 0,49 0,86 1,00

Cu 0,96 0,72 0,50 0,61 1,00

Zn 0,86 0,55 0,74 0,78 0,83 1,00

As 0,50 0,28 0,18 0,22 0,56 0,26 1,00

Rb 0,95 0,73 0,54 0,61 0,91 0,88 0,39 1,00

Sr -0,78 -0,68 -0,55 -0,60 -0,74 -0,79 -0,29 -0,86 1,00

Cd 0,70 0,44 0,61 0,78 0,74 0,71 0,58 0,66 -0,57 1,00

U 0,67 0,69 0,19 0,41 0,65 0,68 0,11 0,72 -0,57 0,51 1,00

Ce 0,50 0,15 0,62 0,59 0,48 0,71 0,14 0,62 -0,46 0,55 0,36 1,00

Pb 0,87 0,64 0,56 0,59 0,88 0,77 0,39 0,81 -0,65 0,57 0,50 0,39 1,00

S -0,25 -0,18 -0,12 -0,16 -0,27 -0,09 -0,28 -0,05 -0,13 -0,02 0,09 0,13 -0,36 1,00

Zr -0,87 -0,56 -0,51 -0,51 -0,88 -0,72 -0,54 -0,76 0,58 -0,56 -0,50 -0,32 -0,80 0,37 1,00 The concentration is expressed in weight percent

4. Discussion

4.1. Statistical analysis of data.

Statistical analysis using CA and PCA methods was carried out to identify groups of variables

which are correlated and highlight their possible genetic relationships. R-mode cluster analysis was

performed on chemical parameters, grain size (clay %, silt % and sand %) and organic matter

content (O.M.) using the between-groups linkage based on Pearson correlation coefficients. This

method is the most appropriate to evidence correlation between variables (Facchinelli et al. 2001;

Le Maitre 1982). The results of CA (dendrogram) are shown in Fig. 8.

Figure. 8 Cluster dendrogram highlighting relationships between distinct parameters of RSP

The distance axis represents the degree of association between groups of variables. The lower the

value on the axis, the more significant the association. Weighted pair group (WPG) average linkage

methods (MVSP software, demo version) have also been used, for validation. Results are very

similar and in both cases, two distinct main groups can be envisaged: Group A, associated with the

finer fraction of the soil, and Group B, representative of the coarser fraction.

Group A is further subdivided into three sub-groups of variables defined as cluster 1, 2 and 3. Al2O3

and K2O (principal elementary components of the clay minerals) are key factors, since all the other

elements of the group are strongly correlated with them. In turn they are related to clay, indicating

that the Group A represents clay minerals. Cluster 1 probably identifies Illite and Smectite

(potassium-rich clay minerals), as V, Cu, Rb, Zn, Pb, Cr, and U are associated with these mineral

phases. On the contrary, Ni, Co, and Cd do not correlate well with major elements (cluster 2),

suggesting the paucity of serpentine, a mineral commonly found in Po plain soils from other sites

(Bianchini et al. 2002, 2012, 2013). This probably reflects the progressive serpentine destabilization

in supergene environment observed by Kierczak et al. (2007). Cluster 3 is constituted by As, P2O5

and S, which are relatively more distant from the key factors. This suggests that these elements are

not related to the genesis of clay minerals but have been associated with clay sediments at a later

stage well after their deposition. As in fact could have been chelated by the abundant organic

matter, which typically includes P2O5 and S among their constituents (Chou and De Rosa 2003);

alternatively, it could derive from the agricultural use of the soil after its reclamation (Chou and De

Rosa 2003). Group B is divided into two sub-groups corresponding to the silty (cluster 4) and sandy

(cluster 5) soil fraction. Zr and Na2O correlate slightly with silt suggesting the persistence of zircon

and alkali feldspar in the coarser grain size. The presence of zircon in the coarse fraction is common

in soils derived sedimentary rocks. On the other hand, the presence of alkali feldspar has already

been identified in the soils of Po Plain (Bianchini et al. 2012). Calcium and magnesium are

positively correlated with sand. This indicates that CaO and MnO are mostly contained in the

carbonate fraction (mainly of biogenic origin), and that their content is higher in the sand rather

than in the finer fraction of the soil. PCA was carried out grain size, O.M., major and trace

elements. Parameters used are: (1) extraction method: PCA, and (2) rotation method: Varimax with

Kaiser normalization; rotation converged in four iterations. The results of PCA are reported in

Tables 4 and 5. Given the results of the initial eigenvalues, five principal components were

considered, which account for over 85% of the total variance. The eigenvalues of the five extracted

components are greater than one. F1 positively correlates clay %, TiO2, Al2O3, Fe2O3, K2O, V, Cr,

Cu, Rb, and Pb, whereas sand %, CaO, L.O.I. and Sr are negatively correlated with this component.

P2O5 has positive loading in F2, where SiO2, Na2O, S, and Zr are negatively correlated. Component

F3 includes Co, Ni, Zn, and Cd. U and O.M. are positively loaded in F4 and correlate negatively

with silt % and MgO, while As is isolated in the fifth component (F5). The statistical treatment (CA

and PCA) suggests a lithogenic control over the distribution of SiO2, TiO2, Al2O3, Fe2O3, K2O,

Na2O, V, Cu, Rb, Pb, Cr, Zr.

Table 4. Total Variance explained.

These elements have in fact the largest weight in F1 and F2 and are very well correlated with clay,

silt or sand fractions, belonging to minerals which are direct products of parental rock weathering.

F1 includes CaO, Sr and L.O.I., indicating a carbonatic fraction, found as shell fragments along the

whole soil profile. S and P2O5 are represented in F2. As described below, sulphur present in the

RSP clearly influences soil salinity and is associable to the large organic matter present in the whole

RSP profile, since the marsh peat is the principal pool for organic C, N, and P and other elements,

e.g., Fe and S (Portnoy 1999). F3 can be identified as a secondary lithogenic factor, probably

related to further degradation of clay minerals (Serpentine), mainly happening during transport and

after deposition. F4 may represent the fraction of the soil, where U is complexed with organic

matter (Bednar et al. 2007 and references therein). The fifth factor (F5) identifies an element with

different origin, unrelated to lithogenesis. Arsenic shows higher concentrations in the first 20 cm of

the Ap horizon, clearly indicating an anthropogenic input. Common sources of As in soil are

arsenate pesticides and industrial pollutants (Sparks 2003; Nriagu 1994).

Table 5 Component matrixes (five factors selected) for grain size, O.M., major, and trace elements

3.2 Major and trace element distribution

The vertical distribution of major elements is concordant with the results of the statistical analysis

(Fig. 7; Table 2) Correlation matrix highlights that Al2O3 and K2O correlate significantly with TiO2

(r>0.9) and Fe2O3 (r>0.8), indicating the presence of metal-rich phyllosilicates (e.g., chlorite,

smectite) within the fine fraction of these soils. In RSP, high concentrations of these elements are

found in surface horizons, where the grain size is finer (Figs. 4, 6). The increase of Na2O and S

downward clearly demonstrates a salinity gradient due to soluble salts accumulation during the

depositional process of the salt marshes. Profile tends to be enriched in P2O5 and As toward the

surface, probably due to the contribution of fertilizers, especially raw pig slurry (Jin and Chang

2011). It is thus possible that the upper layer is contaminated by anthropogenic inputs: on the other

hand, heavy metals and Rb enrichment factor is correlated to the grain size difference between the

horizons.

It has to be noted that the absolute concentrations of some potentially toxic heavy metals, such as Cr

and Ni, are high if compared with the limits established by local regulations (Italian Legislative

Decree 152/06) for agricultural and residential land use (Cr: 150 ppm and Ni: 120 ppm). This fact

has to be interpreted as a natural–geogenic–anomaly, typically observed in soils evolved from the

Po River alluvial sediments, which derive from the weathering of parent rocks including

femic/ultrafemic lithologies (Amorosi et al. 2002; Bianchini et al. 2002, 2012; Bonifacio et al.

2010). This conclusion is supported by the high Ni and Cr content of ancient bricks (and mortars)

from historical buildings of the region made with local sediments analogous to those considered in

this study and manufactured in times preceding any significant form of anthropogenic pollution

(Bianchini et al. 2004, 2006). The vertical distribution of heavy metals shows a marked enrichment

in the Oe horizon (Fig. 7). Comparing the heavy metals of Oe with the values found by Amorosi

et al. (2002) in neighbouring areas, all the elements are significantly higher (Table 6). This confirms

that deposits rich in organic matter formed in anaerobic and waterlogged ecosystems are effective in

trapping metals from the interacting waters (Syrovetnik et al. 2007).

Table 6 Maximum, minimum and average values of heavy metals. Comparison between RSP and Amorosi et al. (2002)

3.3 Soluble anions and cations distribution

The determination of the soluble anions and cations in distilled water is important to provide

information useful for the agricultural activities, in fact they can be considered representative of the

mobile water phase (Ure 1996). The concentrations of dissolved NO3-, Cl

-, Br

-, F

-, NO2

-, SO4

-2, PO4

-

3, NH4

+, K

+, Ca

+2, Mg

+2, and Na

+ are reported in Table 7 and Fig. 9. In this figure, inverse distance

interpolation (IDW, ArcGIS 9.3 software) method was also applied to study the vertical spatial

variability of NO3-, Cl

-, NH4

+ and Na

+. This interpolation is based on a simple principle of

geography that things close to one another are more alike and is often used to create a continuous

surface from sampled point values (Cheng et al. 2007). All samples display a remarkably high

chloride (Cl- up to 1,788 ppm) and sulphate (SO4-2

up to 6,158 ppm) contents, coupled with very

high sodium concentration (Na+ up to 1,495 ppm) that prevails over the other cations (Fig. 9).

Figure. 9 Vertical distribution of selected anions and cations

‘‘Sodium adsorption ratio’’ (SAR) is an index widely used to define the soil salinity (Sposito and

Mattigod 1977), expressed as the ratio between the concentrations of sodium (Na) and the sum

among the magnesium and calcium (Ca + Mg):

SAR =Na

(Ca + Mg) / 2

The studied RSP soil displays extreme SAR values (up to 127), with the highest values typically

recorded in the deep horizons. The concentration of soluble components is comparable with those

measured in soils and sediments from coastal sectors of the Po River plain (Marinari et al. 2012;

Cidu et al. 2013), but seems to be a natural (geogenic) feature inherited from the original

depositional environment, i.e., a wetland characterized by highly saline brackish water (Mastrocicco

et al. 2013b). Further source of salinization is represented by the groundwater that in the area is

extremely saline (up to 13 mS/cm), taking into consideration that the depth of the water table (and

the related capillary fringe) of the phreatic aquifer is extremely superficial and sometimes (for

example in July and August, when the channel levels are artificially rose for irrigation purposes)

tends to approach the surface. In this context, the general decrease of EC (Fig. 5), Cl-, and Na

+

toward the surface (Fig. 9) is probably related to the percolation of rain and irrigation waters that

‘‘wash’’ and desalinize the superficial horizons. NH4+ content is negligible in Ap and Oe horizons,

but increases significantly from about 150 down to 400 cm (Fig. 9).

Table 7 Concentration of dissolved anions and cations

As the permeability of the soil is extremely low (few cm/day, Mastrocicco et al. 2013a), the

percolation of ammonium derived from fertilization in depth can be ruled out; moreover,

nitrification processes in the upper part of the RSP, where oxidizing conditions are met, would

quickly transform ammonium into nitrate thus preventing NH4+ transfer into the lower levels of soil

column. The high ammonium values found in Cg horizon can be related to the natural presence of

considerable amounts of organic matter in reducing conditions (Mastrocicco et al. 2013a) which

slowed down its rate of decomposition. High ammonium contents are also found in groundwater

(48.6–64.6 ppm) during autumn and winter, when the level of channels is artificially lowered and

surface water income through the drains does not affect the overall groundwater composition. A

reverse trend is observed for nitrates that are higher, close to the surface, as an effect of agricultural

fertilization, but tend to decrease with depth (Fig. 9). The drastic decrease of NO3-, at a depth of ca

100–120 cm, can be due to the denitrification processes mediated by biological activity, i.e.,

biochemical reactions triggered by soil bacteria that participate to the decomposition of the organic

matter (Rivett et al. 2008). These data indicate that the investigated zone is less vulnerable to

nitrates than expected (Castaldelli et al. 2013; Mastrocicco et al. 2013b), and that the fertilizers

nitrogen load is metabolized along the soil profile. However, in concomitance with extremely dry

and hot seasons (like the period September 2011–August 2012) the precipitation and accumulation

of NO3--soluble salts within the first 0–30 cm of soil, leading to salinity stress for plant roots, with

crop loss and local N concentration, that can result in a massive nitrate discharge in the channels

after heavy rainfalls, when NO3- is suddenly mobilized, percolates through soil cracks and is

conveyed in the drainage system (Mastrocicco et al. 2013a).

4. Conclusions

A typical RSP has been characterized through grain size and geochemical analyses together with

data statistical treatment. This kind of soil is artificial, as it would be naturally submersed, and has

very peculiar features. It is characterized by high salinity, presence of peat levels and/or horizons

and by high organic matter contents responsible for high chloride, sulphate and ammonium values

in the soil. Redox conditions are reducing, beside the first 150 cm where tillage, cracks and the

presence of a sub-irrigation drainage system allows air circulation. Ferrara Province reclaimed soils

lay below sea level: their hydrology is totally regulated by anthropic interventions; the whole area is

mechanically kept dry with drain pumps and the water table undergoes seasonal variations linked to

agricultural cycles and land use. RSP heavy metal content is naturally high, since these soils

originate from Po plain sediments derived from the erosion of ultramafic rocks; moreover, heavy

metal adsorption by organic matter tends to further concentrate them, especially in peaty horizons.

Reclaimed soils undergo intensive agricultural exploitation, as testified by the high NO3- content

due to fertilization. For all the above-mentioned features, the reclaimed soil constitutes a

particularly vulnerable and fragile environment. Further studies are needed to check the amount of

heavy metals and metalloids effectively adsorbed by the cultivated crops, as well as the solubility of

these elements in groundwater and in the surface hydrological system. Sediment low permeability

and bacterial de-nitrification processes could prevent groundwater from high level of nitrate

pollution. However, the actual meteorological conditions are strongly affected by the ongoing

climatic changes, with long dry season alternated to temporally concentrated heavy rainfall. The hot

and prolonged spring/summer temperatures can favor nitrate salt deposition and accumulation in

soil, quickly flushed away by flash floods and massively input into the channels through the drains.

In spite of the application of the Nitrate and Water Framework Directives (91/676/CEE;

2000/60/CE), nitrogen pollution is still a major menace to lowlands and coastal lagoons and further

studies are required to better understand the complex processes of nitrate release into the

environment. Swamp reclamation supplied inhabitable land and cultivable soils; however, they are a

delicate system whose management is difficult both from environmental (nitrate pollution),

economic (high needs of power for water pumps), and agricultural (highly saline soils subject to a

quick depletion of nutrients) points of view. An accurate and continuous monitoring of the

reclaimed areas is recommended for a correct conservation of the territory and the limitation of

heavy pollution phenomena.

Acknowledgements

Authors thank Dr. Umberto Tessari and Dr. Renzo Tassinari for their analytical support. This work has been supported

by EC LIFE+ funding to ZeoLIFE project (LIFE+ 10 ENV/IT/000321).

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