High phospate bearing sediments in a salinized coastal aquifer.

To understand the possible sources of high phosphate (PO43-) concentration in the unconfined coastal aquifer of the Po River Delta (Italy), a detailed characterization of groundwater and chemical fluxes was carried out. The unconfined aquifer is connected with the drainage system, consisting of a dense network of canals and pumping stations that continuously drain the soil. The land use is essentially agricultural and for this reason nutrients like PO43- could leach in the unconfined aquifer. At first PO43- concentrations were analyzed in 32 shallow piezometers screened at the top of the unconfined aquifer, then PO43- and chloride (Cl-) concentrations were also analyzed every meter in four piezometers located along a flow line via multilevel sampling technique. PO43- and Cl- were analyzed via an ion chromatography system Dionex ICS-1000. In order to calculate horizontal and vertical specific discharge fluxes within the aquifer and between surface and groundwater, a detailed characterization of equivalent freshwater heads and intrinsic permeability was performed in the selected piezometers. In addition core samples were extracted for water soluble PO43-. Milly-Q water was used to extract the PO43- from the sediment samples, using a sediment:water weight ratio of 1:5; the sediment and water were mixed and sealed in beakers, shaken for 1 h, and centrifuged for 1 h at 25 °C to separate the sediment and the solution, then analyzed using the ICS-1000. PO43- concentrations in shallow piezometers was in general below detection limits (0.1 mg/l), except for peaty soils where concentration up to 22 mg/l were detected. PO43- concentrations along the selected flow line range between 12.3 mg/l to below detection limits, the average concentration was 3.9 mg/l generally increasing with depth. PO43- peak concentrations of groundwater and sediments overlapped with recent and buried peaty layers, emphasized that in this lowland coastal aquifer the high PO43- concentration is associated with elevated organic matter content and also related with the groundwater ionic strength, which favors the PO43- mobilization. In addition, the drainage system generates an upward flux of salinized groundwater mitigated only by irrigation canals during the cropping season and by natural recharge during winter time. This could threaten crop yields during years of low recharge as salinity could reach level which is not tolerable by crops like cereals. Moreover surface waters could suffer of eutrophication induced by geogenic nutrients like PO43-. Thus, it is concluded that the elevated PO43- concentrations found in groundwater come principally from sediments originated in paleo-marsh environments and cannot be “a priori” attributable to agricultural inputs. To distinguish between anthropogenic PO43- input and natural presence a detailed characterization of both aquifer’s hydraulics and geochemistry is required.