USING RADIUM AND ITS DECAY PRODUCTS AS ENVIRONMENTAL TRACER FOR THE ASSESSMENT AND DATING OF SUBSURFACE NON-AQUEOUS PHASE LIQUID (NAPL) CONTAMINATION

Abstract

The growing interest in environmental issues has generated a gradual change in the scientific approach to research in this field. The complexity of natural systems requires a deeper understanding of both various processes studied and their mutual interactions. In the recent past, the contribute of radioisotopes in such investigations has constantly increased due to their wide range of possible applications. Natural and anthropogenic radioisotopes decay can provide several information on natural processes and their evolution in time. Considering the aim of a research, the suitable radionuclides can be selected. In general, the dissimilarities in geochemistry and the complementarity in half-life time (such as for the members of Th and U radioactive series) enable the reconstruction and monitoring of environmental variations and anthropogenic contaminations in recent time as well as in more distant past. Moreover, the chemical properties and the wide natural distribution of some radioisotopes also allow their use as environmental tracers. In this thesis, the attention is focused on the application of environmental radioactivity in the study of contaminations caused by refine hydrocarbon leakages in soils and groundwater. In particular, the physical process called alpha recoil, involving radium and radon isotopes, is analyzed and used in dating groundwater (Chapter 1) and subsoil NAPL (Chapter 2), and also to trace it (Chapter 3). On the one hand radium isotopes and their progenies are well known in researches on hydrogeological topics: recharge time in reservoirs, marine water mixing, submarine groundwater discharge and so on. On the other hand the complexity of the processes studied requires even more investigations in multiphase systems. This statement assumes a major relevance in presence of a pollutant that modifies natural equilibrium and increases this complexity such as in case of a contamination by refine hydrocarbons. Oil and its refine products are non-polar substances, immiscible with water, better known as Non-Aqueous Phase Liquids (NAPLs) in the environmental field. The data contained in this thesis and their discussion aim to expand and improve scientific knowledge in the use of radioisotopes as natural tracers and in dating techniques. The main contributes can be considered: 1. A better understanding of the processes behind the retardation factors of 224Ra,228Ra and 226Ra in groundwater, proposing a method to estimate them with the use of non-polar liquids directly. 2. The development of a radiometric dating method based on environmental radioactivity to assess the age of NAPL contaminations. 3. The evaluation of limits in applicability of radon as tracer (Radon deficit technique) in old NAPL contaminations, using data collected during the monitoring of a site contaminated by Methyl Tertiary Butyl Ether (MTBE) in the city of Rome (Italy). In the first chapter, a new method, named NAPL method, for estimating retardation factor and recoil constant of radium isotopes in groundwater is described. The method is based on the evidence that alpha-recoiled radium ions, supplied by Th parent atoms occurring in phases immersed in NAPL are not adsorbed by solid phases. Experimental evidence is given that leucitic volcanic rock, zeolite 4A, clay minerals, monazite and manganese dioxide, all phases normally adsorbing radium from aqueous solutions, adsorb negligible amounts of radium when immersed in NAPL. This allows to use experimental data on rock samples, representative of porous aquifers, for estimating retardation factor and recoil constant of radium in groundwater without using radon data as a comparison term. The large difference in retardation factor estimation between the NAPL method and the method based on radon depends on the difference between the mechanism of recoil between radon and radium from aquifer rock into groundwater. Precise estimation of retardation factor and recoil constants of radium allows to apply equations ruling the temporal evolution of radium isotopes in groundwater and to determine its age. Implications are also described, useful for dating the contamination time of soils by NAPL fluids. The second chapter proposes a radiometric method to assess the age of refine hydrocarbons pollutions. In fact, dating NAPL leakages and spills still represents a critical issue with relevant consequences on legal attribution of responsibilities in case of environmental contamination. The mayor impact of this regards not only the environmental problem but also the economic effort required for remediation. Considering the ubiquitous presence of the environmental radioactivity and the natural physical process of alpha recoil related to some types of decay, the couple 228Th/228Ra is proposed to assess the age of NAPL releases. Recoiled 228Ra accumulates in NAPLs over time and then it decays in 228Th. A specific radiometric equation that calculates the temporal variation of their ratio is defined to date these contaminations in a range of maximum 20 years before present. After having verified the detectability of alpha-recoiled nuclides in kerosene and in distilled water by γ-spectrometry, a new specific radiometric method is developed by lab tests. Regards to possible constrains and limits, radium partition coefficient between water and kerosene, adsorption experiments of Ra-enriched fluids (distilled water and kerosene) on zeolite (4A type) and dating tests on artificially and naturally polluted samples are performed. On the one hand the use of Ra-enriched materials (monazite sand, pyroclastic rock and Welsbach mantles) overcomes the absence of reliable ages of real NAPL contaminations to verify the results of the proposed method. On the other hand the applications of the method on real polluted samples corroborates the suitability of this radiometric dating pair for soil and groundwater contaminations attributable to refine hydrocarbons. The method also comprehends a specifically developed extracting procedure to recover NAPLs from soils and groundwater. The results show the alpha recoil generates accumulation of radium in NAPLs, while chemical exchanges with soil and water are negligible. The ages measured in all the samples corresponds to the real ones (lab samples) or to the most probable time interval of contamination (real cases), suggested by site history. The applicability of radiometric dating based on alpha recoil to NAPL contaminated sites opens a new horizon in research and in the management of environmental remediation. The possibility to obtain an absolute age eliminates the previous uncertainties on timeline of NAPL contaminations, offering a useful tool in monitoring and studying of NAPL leakages. The third chapter presents the study of a real environmental contamination occurred in a fueling station in Roma (Italy). It was dismissed about 15 years ago. When underground tanks were removed, a subsoil NAPL contamination came out, showing gasoline leakage from the reservoirs bottom. Monitoring actions took place next and only recently radon dissolved in groundwater was measured and used as tracer of NAPLs in view of its high solubility in these substances. The relative deficit of radon in polluted groundwater compared to radon levels in background “clean” water allows to detect areas where residual gasoline is located. After 15 years of degradation and volatilization, only residual MTBE (a resistant additive introduced in place of lead) is still detectable. When groundwater table rises, removal of MTBE takes place, increasing its concentration in groundwater and generating a short and transient plume. MTBE concentration in groundwater is then progressively reduced because of natural attenuation processes. The half-life of this dissipation was estimated at about 23 days. Radon-deficit in groundwater from 12 monitoring piezometers was determined for a period of twelve months, from September 2018 to September 2019. The source of pollution, where former underground gasoline tanks were placed, is clearly evidenced by local low radon activity concentration. This finding is strengthen by direct measurements of higher contents of dissolved MTBE. A short and transient plume of contamination, elongated in the direction of groundwater flow, has been also periodically recognized using radon-deficit, as well as direct chemical analyses. Quantifying dissolved MTBE from radon-deficit equations is difficult and problematic when gasoline spillage is not recent, since only residual and strongly degraded NAPLs occur. Steady-state-radon partitioning model and one-dimensional model for radon transport with NAPL partitioning are not applicable in this case because basic assumptions are not respected. The aquifer is not homogeneous in terms of 226Ra distribution, porosity and emanation power and no equilibrium is reached for radon partition between NAPL and water. Furthermore, MTBE is soluble in water, but it is not representative of the mixture of substances presumably still sorbed onto the solid matrix of the aquifer. Additional information could be provided by in situ measurements of soil gases (radon, carbon dioxide and methane) and by studies on the natural bio degradation of gasoline.