Nicolò Colombani

Linking dissolved organic carbon, acetate and denitrification in agricultural soils

This study focuses on the factors affecting nitrate removal via microbial denitrification in agricultural soils, and particularly on the quantity and quality of dissolved organic carbon. To assess the relationship among dissolved organic carbon, nitrate and low molecular weight organic acids (acetate and formate), grids of ceramic suction cups were established in the four most representative soil types of the lower Po River floodplain, cropped with maize. Results highlighted a direct relation between acetate and dissolved organic carbon in all sites. The best fit was obtained in soils were the main source of organic carbon was the maize residues. By comparing dissolved organic carbon and acetate versus nitrate concentration revealed that acetate can be used as a better proxy for denitrification in the field with respect to dissolved organic carbon.

Assessment of the Intrinsic Vulnerability of Agricultural Land to Water and Nitrogen Losses via Deterministic Approach and Regression Analysis

A set of indices was developed in order to classify the vulnerability of agricultural land to water and nitrogen losses (LOS), setting a basis for the integrated water resources management in agricultural systems. To calibrate the indices using multiple regression analysis, the simulation results of Groundwater Loading Effects of Agricultural Management Systems (GLEAMS) model for combinations of different soil properties, topography, and climatic conditions of a reference field crop were used as “observed values.” GLEAMS quantified (1) the annual losses of the percolated water beneath the root zone, (2) the annual losses of the surface runoff, (3) the annual losses of the nitrogen leaching beneath the root zone, and (4) the annual losses of nitrogen through the surface runoff, which were used to calibrate the following indices LOSW-P, LOSW-R, LOSN-PN, and LOSN-RN, respectively. All the simulations to gain the LOS indices were carried out for the same reference field crop, the same nitrogen fertilization, and the same irrigation practice, in order to obtain the intrinsic vulnerability of agricultural land to water and nitrogen losses. The LOS indices were also combined to derive nitrogen concentrations in the percolated and in the runoff water. Finally, the connection of LOS indices with the groundwater was performed using an additional equation, which determines the minimum transit time of the percolated water to reach the groundwater table.

Reclamation influence and background geochemistry of neutral saline soils in the Po River Delta Plain (Northern Italy)

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.

The Importance of Data Acquisition Techniques in Saltwater Intrusion Monitoring

A detailed vertical characterization of a coastal aquifer was performed along a flow path to monitor the seawater intrusion. Physico-chemical logs were obtained by both open-borehole logging (OBL) and multilevel sampling technique (MLS) via straddle packers in piezometers penetrating the coastal aquifer of the Po River Delta, Italy. The open borehole logs led to a satisfactory reconstruction of the extent of the fresh-saltwater interface but provided a misleading characterization of the distribution of redox environments within the aquifer. On the contrary, good fits between sedimentological, stratigraphycal and physico-chemical data were obtained using the straddle packers devices. This study demonstrates that, within coastal shallow aquifers evenly recharged by irrigation canals, the simple and economical OBL technique can lead to misleading results when used to characterize density dependent groundwater stratification but is deemed adequate for preliminary assessments of the saltwater wedge location.

Evaluation of saline tracer performance during electrical conductivity groundwater monitoring.

Saline solutions are the most commonly used hydrological tracers, because they can be easily and economically monitored by in situ instrumentation such as electrical conductivity (EC) loggers in wells or by geoelectrical measurements. Unfortunately, these low-cost techniques only provide information on the total concentration of ions in solution, i.e., they cannot resolve the ionic composition of the aqueous solution. This limitation can introduce a bias in the estimation of aquifer parameters where sorption phenomena between saline tracers and sediments become relevant. In general, only selected anions such as Cl(-) and Br(-) are recognised to be transported unretarded and they are referred to as conservative tracers or mobile anions. However, cations within the saline tracer may interact with the soil matrix through a range of processes such as ion exchange, surface complexation and via physical mass-transfer phenomena. Heterogeneous reactions with minerals or mineral surfaces may not be negligible where aquifers are composed of fine alluvial sediments. The focus of the present study was to examine and to quantify the bias between the aquifer parameters estimated during model-based interpretation of experimental data of EC measurements of saline tracer relative to the aquifer parameters found by specific measurements (i.e. via ionic chromatography, IC) of truly conservative species. To accomplish this, column displacement experiments with alluvial aquifer materials collected from the Po lowlands (Italy) were performed under water saturated conditions. The behaviour of six selected, commonly used saline tracers (i.e., LiCl, KCl, and NaCl; LiBr, KBr, and NaBr) was studied and the data analysed by inverse modelling. The results demonstrate that the use of EC as a tracer can lead to an erroneous parameterisation of the investigated porous media, if the reactions between solute and matrix are neglected. In general, errors were significant except for KCl and KBr, which is due to the weak interaction between dissolved K(+) and the sediment material. The study shows that laboratory scale pre-investigations can help with tracer selection and to optimise the concentration range targeted for in situ multilevel monitoring by unspecific geoelectrical instrumentation.

Modelling the fate of styrene in a mixed petroleum hydrocarbon plume

Severe petroleum hydrocarbon contamination (styrene and the BTEX compounds: benzene, toluene, ethylbenzene and the isomers of xylene) from leaking sewers was detected in a Quaternary aquifer below a chemical plant in the Padana Plain, Italy. From 1994, active pump and treat remediation has been employed. The site is bordered by canals which, in combination with variable pumping rates and groundwater flow directions, control groundwater levels. In this study we sought to determine the fate of styrene at the site within a mixed styrene/BTEX plume where the hydraulic boundaries induced strong seasonal variations in flows. In order to determine the fate of styrene, detailed field investigations provided intensive depth profile information. This information was then incorporated into a staged flow and reactive transport modelling. Three sets of measurements were obtained from sampling multilevel samplers (MLSs) under different hydraulic conditions at the site. These included measurements of BTEX, styrene, all major ions, pH and redox potential. A three-dimensional transient flow model was developed and calibrated to simulate an unconfined sandy aquifer with a variable flow field. Subsequently a reactive, multi-component transport model was employed to simulate the fate of dissolved BTEX and styrene along a selected flow line at the site. Each petroleum hydrocarbon compound was transported as independent species. Different, kinetically controlled degradation rates and a toxicity effect were simulated to explain the observed, selective degradation of pollutants in groundwater. Calibration of the model was accomplished by comparison with the three different sets of measurements obtained from the MLS devices. The results from various scenarios show that the detailed simulation of geochemical changes can be very useful to improve the sites conceptual model.

Assessing the Effect of Saltwater Intrusion on Petroleum Hydrocarbons Plumes Via Numerical Modelling

A contamination by petroleum hydrocarbons was detected in a sandy aquifer below a petrochemical plant in Southern Italy. The site is located near the coastline and bordered by canals which, together with pumping wells, control submarine groundwater discharge toward the sea and seawater intrusion (SWI) inland. In this study, a three-dimensional flow and transport model was developed using SEAWAT-4.0 to simulate the density-dependent groundwater flow system. Equivalent freshwater heads from 246 piezometers were employed to calibrate the flow simulation, while salinity in 193 piezometers was used to calibrate the conservative transport. A second dissolved species, total petroleum hydrocarbons (TPH), was included in the numerical model to simulate the plumes originating from light non-aqueous-phase liquid. A detailed field investigation was performed in order to determine the fate of dissolved hydrocarbons. Fifteen depth profiles obtained from multilevel samplers (MLS) were used to improve the conceptual model, originally built using a standard monitoring technique with integrated depth sampling (IDS) of salinity and TPH concentrations. The calibrated simulation emphasises that density-dependent flow has a great influence on the migration pattern of the hydrocarbons plume. This study confirms that calibration of density-dependent models in sites affected by SWI can be successfully reached only with MLS data, while standard IDS data can lead to misleading results. Thus, it is recommended to include MLS in the characterization protocols of contaminated sites affected by SWI, in order to properly manage environmental pollution problems of coastal zones.

Abnormal trace element concentrations in a shallow aquifer belonging to saline reclaimed environments, Codigoro (Italy)

Geogenic trace element (TE) contamination is an upcoming concern. The present study reports the temporal and spatial variation of major ions and TEs in a shallow unconfined aquifer belonging to a complex marsh saline environment reclaimed in modern age and intensively cultivated. The use of intensive depth profiles in five different locations gave insights into groundwater and sediment matrix interactions. Data indicate that the dominant factor involved in determining the spatial variability of TE is the sediment–water interaction, while the temporal variation of TE is due to the organic matter content and to the water table oscillation, which in turn drive the groundwater redox status and the mobilization of some inorganic microconstituents, such as Fe and Mn. Despite that the anthropogenic input of TEs in groundwater from fertilizer sources cannot be undoubtedly ruled out, given the elevated TE background concentrations, the combined use of high-resolution sediment profiles, seasonal groundwater sampling and end-member analyses is a promising procedure to distinguish between anthropogenic metal contamination and geogenic contribution in reclaimed deltaic environments. Finally, this study underlines the need of having a dense piezometer network and to perform several monitoring campaigns to ensure that the temporal and spatial variability could be correctly represented and background values of TE confidently determined.

Fate of arsenic, phosphate and ammonium plumes in a coastal aquifer affected by saltwater intrusion

A severe groundwater contamination with extensive plumes of arsenic, phosphate and ammonium was found in a coastal aquifer beneath a former fertilizer production plant. The implementation of an active groundwater remediation strategy, based on a comprehensive pump and treat scheme, now prevents the migration of the dissolved contaminants into the marine environment. However, due to the sites proximity to the coastline, a seawater wedge was induced by the pumping scheme. Additionally the groundwater flow and salinity patterns were also strongly affected by leakage from the sites sewer system and from a seawater-fed cooling canal. The objective of this study was to elucidate the fate of arsenic and its co-contaminants over the sites history under the complex, coupled hydrodynamic and geochemical conditions that prevail at the site. A detailed geochemical characterisation of samples from sediment cores and hydrochemical data provided valuable high-resolution information. The obtained data were used to develop various conceptual models and to constrain the development and calibration of a reactive transport model. The reactive transport simulations were performed for a sub-domain (two-dimensional transect) of an earlier developed three-dimensional flow and variable density solute transport model. The results suggest that in the upper sub-oxic zone the influx of oxygenated water promoted As attenuation via co-precipitation with Al and Fe oxides and copper hydroxides. In contrast, in the deeper aquifer zone, iron reduction, associated with the release of adsorbed As and the dissolution of As bearing phases, provided and still provides to date a persistent source for groundwater pollution. The presented monitoring and modelling approach could be broadly applied to coastal polluted sites by complex contaminant mixture containing As.

A modified SINTACS method for groundwater vulnerability and pollution risk assessment in highly anthropized regions based on NO3− and SO42− concentrations

Groundwater vulnerability and risk assessment are worldwide tools in supporting groundwater protection and land planning. In this study, we used three of these different methodologies applied to the Campanian Plain located in southern Italy: SINTACS, AVI and LOS. However, their capability to describe the observed chemical pollution of the area has resulted quite poor. For such a reason, a modified SINTACS method has been then implemented in the area in order to get a more reliable view of groundwater vulnerability. NO3- and SO42- from more than 400 monitoring wells were used for specific vulnerability assessment. Land use was chosen as key parameter to infer the risk of groundwater pollution in our area. The new methodology seems to show a higher correlation with observed NO3- concentrations and a more reliable identification of aquifers pollution hot spots. The main sources of NO3- were found in sub-urban areas, where vulnerability and risk are higher than in other areas. Otherwise due to reducing conditions triggered by the presence of elevated sedimentary organic matter and peat, concentrations below agricultural areas were lower than in sub-urban areas. The SO42- specific vulnerability map showed a positive correlation with observed concentrations, due to geogenic and anthropogenic SO42- sources present in the area. The combination of both NO3- and SO42- derived risk maps becomes essential to improve the conceptual model of aquifer pollution in this severely anthropized area. The application of this new and original approach shed light on the strengths and weaknesses of each of the described previous methods and clearly showed how anthropogenic activities have to be taken into account in the assessment process.