Tracing vertical distribution of NAPL contaminations: two case studies

Radon deficit technique was applied to study the vertical distribution of the local NonAqueous Phase Liquid (NAPL) contamination in two different sites, in Rome. The higher solubility of 222Rn in NAPLs than in water or air is wellknown and already used to localize the spatial distribution of a NAPL plume [De Miguel et al., 2020; Schubert, 2015]. In two pump stations Rn deficit was periodically measured in groundwater to monitor the areal distribution of NAPL [Briganti et al. 2021; Mattia et al. 2020]. The first site is characterized by a shallow aquifer and the vertical Rn distribution was determined by inserting 12 passive accumulators at different depths in two different wells. These PDMSAC (polydimethylsiloxane and activated carbon) accumulators selectively absorb 222Rn and they are impermeable to groundwater and humidity occurring in the wells [Voltaggio & Spadoni, 2013]. After reaching equilibrium in two weeks, the Rn accumulated by PDMSAC was counted using high resolution ?spectrometry. The second site is characterized by volcanic deposits with a groundwater level about at 18 meters and equipped with a soil venting system. After purging, this equipment was used to sample soil gas at different intervals of depth from 1 to 18 m. Rn values were determined by Durridge RAD7. The method was optimized to collect and analyze the gas directly. A parallel test was performed to measure Rn content by PDMSAC method at the same levels. Preliminary data indicates the possible application of the two methods to assess vertical distribution of NAPL contaminant, considering local geological setting. Main differences are related to the kind of produced data (a relative value integrated in time or an instantaneous absolute value) and dependence on infrastructural equipment. Determining vertical Rn distribution, either in groundwater or in soil gas, generates a clear impact on remediation of NAPLs pollutions and a better reconstruction of the plume and of the pollutant. References Briganti A., Voltaggio M., Tuccimei P. and Soligo M., (2021). Using 222-Radon as tracer for areal and vertical distribution of hydrocarbon contaminations. In: Abstract Book, Geology without borders90° Congresso della Società Geologica Italiana, 1416 September, Trieste, 175. De Miguel E., BarrioParra F., IzquierdoDíaz M., Fernández J., GarcíaGonzález J.E. & Álvarez R., (2020). Applicability and limitations of the radon-deficit technique for the preliminary assessment of sites contaminated with complex mixtures of organic chemicals: A blind field-test. Environment International, 138. Mattia M., Tuccimei P., Soligo M. and Carusi C., (2020). Radon as a Natural Tracer for Monitoring NAPL Groundwater Contamination. Water, 12, 3327. Schubert M., (2015). Using radon as environmental tracer for the assessment of subsurface Non- Aqueous Phase Liquid (NAPL) contamination - A review. Eur. Phys. J. Spec. Top. 224, 717730.