Additional sources of salinity and heavy metals from plant residues of peaty horizons in the Po River lowland (Italy)

Abstract

The Po River lowland (Italy) is suffering of soil and surface water salinization due to seepage of saline groundwater into drainage canals of agricultural lands. The main drivers are paleo-saline porewaters trapped in peaty and silty clay lenses, deposited in back barrier and lagoonal environments. To study this phenomenon, a 2-ha tile drained agricultural field, experiencing dramatic crop yield decrease in the last years, was selected. The field is located in a reclaimed area at approximately 3 m below sea level in the Ferrara province, 24 km from the Adriatic Sea. To delineate the salinity gradients, 10 high resolution vertical profiles of soil cores were collected with a manual auger corer and analysed in laboratory for EC, pH, grain size, organic matter content, major ions and trace elements. The composition of porewater's leaching fraction (LF) was gained via deionized water batch extraction with solid liquid ratio 1:5, while the plant available water (PAW) was obtained via microwave hot water extraction in three selected profiles. Linear correlation matrix, principal components analysis (PCA), LF and PAW depth profiles were employed to gain insights on the salinity sources. Stratigraphical cores were almost homogeneous with the most permeable layers constituted by peats. An average porewater salinity of 8.2 g/L with a downward increasing trend was found, with slight acidic pH and very high content of organic matter. PCA highlighted the strong connection among some trace elements (like As, Cd, Cr, Li, Pb, Se), NH4+ and the saline peaty horizons. The PAW salinity was up to two times the LF, highlighting that peaty aquitards are the major saline source in these reclaimed lands. A new salinizing mechanism was then postulated, since plant tissue fragments can store much more salts than the mineral fraction and can act as a long-term source of diffusion driven salinization, so far neglected.