Groundwater resources interact more or less directly with both surface and underground ecosystems (water, soil, rocks and biota). Physical-chemical and biological properties of solid, liquid and biological matrices exert their influence on water percolation from surface to unsaturated layers feeding the aquifers.
Pollutants released on top soil may migrate through unsaturated and saturated porous media to reach the aquifer. The migration rates are dependent on the characteristics of the chemical compounds and of the matrix (soil/rocks) they cross through. Complex phenomena that result from hydrogeological, geochemical, chemical and microbiological processes control the modes and rates of migration, assuring attenuation levels that may differ for different pollutants in different aquifers. At local scale, geochemical peculiarities may change significantly the background concentration levels of chemical species in aquifers overlapping possible effects by external pressures, thus seriously complicating the management actions to plan.
Methods and devices for the estimation of hydrogeological parameters
The low availability in terms of quantity and quality of groundwater requires a detailed knowledge of fluid dynamics processes, which occur in the vadose zone and in deepest zones of subsoil. Studies on the groundwater recharge, on the preferential flow and on the pollutants transport, require both direct and indirect estimation of the soil and rock hydraulic properties of saturated/unsaturated media. IRSA utilizes innovative approaches in laboratory and field experiments to estimate the hydrogeological parameters. New devices are tested in laboratory pilot plants by setting up prototypes, testing new probes and sensors, with the aim to identify new methods and theories that allow to determine the hydraulic conductivity and water retention functions in unsaturated porous media and in fractured rocks, by including also the clogging phenomenon. Researches at IRSA are carried on to evaluate the possibility to obtain a spatial distribution of physical and hydrological properties of the subsoil, even when it consists of rocks, by using field tests and innovative non-invasive geophysical techniques. The study of groundwater flow and of the interactions between surface waters and groundwaters is addressed through tracer tests by using both a direct method based on the monitoring of a tracer in observation wells and indirect methods based on geophysical surveys. Methods for estimating water evaporation from bare or vegetated soil, based on the measurement of vertical gradients of temperature and moisture, were also tested at field scale.
Groundwater ecosystem vulnerability and natural pollution attenuation
Chemical or microbiological pollutants retained in subsoil may be released over time according to complex sorption/desorption, diffusion, decay or inactivation phenomena. The study of the kinetics of bio-geochemical processes is required to fit field data and forecast future scenarios. The efficacy of the natural wastewater filtration through soil and groundwater for improving the microbiological characteristics of effluents from treatment plants and enhancing the potentialities for wastewater reuse are investigated. Advanced bio-molecular methods to test the efficacy of soil/subsoil filtration on the reduction of pathogens in groundwater were used.
The migration of organic pollutants, in particular, is dependent on physicochemical factors as well as on the presence/absence of sub-soil microbial communities able to reduce or remove them by biodegradation processes. Field and laboratory IRSA studies show the fundamental role of natural bacterial communities, occurring at different depths, in the degradation of several herbicides, such as terbuthylazine, simazine, diuron, linuron. These studies highlight the role of abiotic factors (temperature, organic matter content, pH, etc.) on the biodegradation rates and disappearance time (DT50) of these pollutants, and on their consequent likelihood to be leached to groundwater. IRSA has implemented mathematical models based on advanced algorithms (Lagrangian approach), which are capable to simulate groundwater flow and pollutants transport in subsoil accounting for their biodegradation rates. The models, after appropriate field calibrations, allow the visualization of the apparent trajectories of pollutants in saturated/unsaturated zones of the subsoil and the estimation of pollutant residence times and expected concentrations at observation wells. These results allow a sustainable management of groundwater resources as well as the implementation of measures and remediation actions in order to protect the sources of groundwater supply.
Threshold values and natural background levels in ground waters
Background level for a substance in ground waters is defined as the concentration corresponding to no, or only very minor, anthropogenic alterations to undisturbed conditions. European Directive 2006/118/EC on groundwater, as well as the Italian National Act D.Lgs. n.30 of 2009, require chemical “Status” of ground water bodies to be assessed against defined “threshold values”.
This process firstly requires the correct identification of ground water bodies, secondly the design of a monitoring network able to capture both natural and anthropic processes, the choice of substances and parameters as a function of the pressures on ground waters, and finally the assessment of substances concentration in respect to the threshold values established at National level. The latter can be modified by Regions in the case of naturally-occurring substances, taking into account peculiar hydrogeochemical and hydrogeological settings which could be responsible for different background levels. Methods applied are generally ascribed to statistical techniques, in which the water samples affected by human activities are singled out from the total population analysing the probability distribution of the concentration of the considered substance, or considering anthropic markers as nitrates. IRSA has recently tested a methodological approach on a case study in northern Latium in which the possible background levels for arsenic, fluoride and vanadium have been evaluated. Currently IRSA is involved in a research project financed by Regione Lazio for the assessment of the natural background levels in ground water bodies where municipal dumps are located.