applied sciences Article  
Combining Radon Deficit, NAPL Concentration, and Groundwater Table Dynamics to Assess Soil and Groundwater Contamination by NAPLs and Related Attenuation Processes  
[PERSON] 1, [PERSON] 1,*, [PERSON] 2, [PERSON] 1, [PERSON] 3, [PERSON] 3 and [PERSON] 2  
1 Dipartimento di Scienze, Universita \"Roma Tre\", Largo San Leonardo Murialdo 1, 00146 Rome, Italy; [EMAIL_ADDRESS] (M.S.)  
2 Istituto di Geologia Ambientale e Geoineggenia, Consiglio Nazionale delle Ricerche, via Salaria km 29,300, 00146 Rome, Italy; [EMAIL_ADDRESS] (G.C.)  
3 Protezione Ambiente, Mares Sr.L., via Fiume Gallo, 3, 00144 Rome, Italy; [EMAIL_ADDRESS] (C.C.); [EMAIL_ADDRESS] (E.R)  
*Correspondence: [EMAIL_ADDRESS]  
Footnote 1: email: {martina.martina.  
- remedial strategies [1]. The efficiency of both approaches relies on environmental monitoring [2] and on a suitable geochemical tracer of ongoing transport, dilution, degradation, and volatilization phenomena [3,4]. Radon gas can be used to identify NAPLs in soil and groundwater because of its preferential partitioning in organic phases [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]. The resulting radon deficit (regarding the site-specific radon background concentration in soil or groundwater) can be utilized as an NAPL indicator [17].  
Moreover, it is possible to estimate the saturation of the residual NAPLs either in the vadose or saturated portion of an aquifer. NAPL saturation (\(S_{NAPL}\)) in the saturated aquifer, on top of the \"NAPL source zone\", or downgradient in the plume, can be calculated using the following equation [17]:  
\[S_{NAPL}\left(\%\right)=\left[1-\Delta C_{\infty}/\left(\Delta C_{\infty} \times K_{NAPL/W}-\Delta C_{\infty}\right)\right]\times 100 \tag{1}\]  
where:  
\(\Delta C_{\infty}\): radon deficit = equilibrium radon concentration in NAPL polluted aquifer/equilibrium radon concentration in clean aquifer.  
\(K_{NAPL/W}\): radon partition coefficient between NAPL and water at Roma average temperature (70, [17,22])  
It is worth noting that this equation, Equation (7) in ref [17], is affected by a typo in the numerator where \"\(\Delta S_{\infty}\)\" is reported in place of \"\(\Delta C_{\infty}\)\".  
Although many works applied this approach to estimate the amount of the residual NAPLs in soil and groundwater (see for example, [8,17,24,25]), to our knowledge no references are available for a statistical treatment of radon and NAPL concentration, in terms of principal component analysis. This research is a first example of the potential of such a kind of approach to infer processes accounting for relationships among relevant parameters (radon, NAPLs, water table fluctuations) to successfully manage the reclamation of a contaminated area. Since groundwater monitoring and sampling is routinely carried out once every two-three months by companies responsible for remediation actions, it is particularly useful to have a tool to establish relations among variables and understand the evolution of contamination with standard sampling times.  
Two study sites, with different hydrogeological features, age of the spillage, composition of residual NAPLs, and clean-up procedures are employed here to show the advantages and drawbacks of this method. The factor analysis is here applied only to groundwater and not soil variables' data, which were not available with the same frequency; however, the same approach can be extended to soil contamination in case of a larger dataset.