Integrity of fine-grained layers to DNAPL migration in multilayered aquifers: assessment in a pce-contaminated alluvial system, using high-precision simulations

Abstract

Chlorinated organic compounds widely contaminate aquifer systems worldwide (Doherty, 2000). They belong to the so-called DNAPLs (dense non-aqueous phase liquids), are denser than water, and have very low water solubility. Thus, they can migrate under pressure and gravity forces through the unsaturated and saturated porous medium until they reach a bottom aquiclude (Mercer & Cohen, 1990). They are usually detected in industrial and urban areas, persist in the environment, and are linked to toxic effects. The behavior of chlorinated organic compounds in the subsurface has been studied since the early 1980s, e.g., (Parker et alii, 1987; Kueper et alii, 1989). Some numerical models, including migration and remediation of alluvial aquifers (Guadano et alii, 2022), using the MT3DMS numerical code (Zhou et alii, 2023; Zheng & Wang, 1999), have been written to simulate their migration in aquifer systems. The dynamics of a spilled DNAPL migration in a variably saturated zone can be described using numerical simulations for the governing equations of immiscible phase fluid flow in a porous medium. These are coupled with conserved partial differential equations for each fluid flow, based on the Darcy equation, together with the conservation of mass and an equation of state. They are written as a function of each fluid flow’s saturation, capillary pressure, density, viscosity, permeability, and porosity. Since each phase’s capillary pressure and permeability are a function of the saturation, these equations are non-linear, with a dominant hyperbolic advection term proportional to gravity and the pressure gradient. It is responsible for forming sharp (shocks) front and rarefaction, which can create significant errors in the output results if not treated with conservative numerical solutions methods. The present study deals with the three-dimensional (3D) numerical model implemented to analyze the expected impact of perchloroethylene (PCE) releases in multilayered aquifer systems characterized by the juxtaposition of more permeable layers (gravel and sand) with low-permeability layers (silt and clay). A 3D numerical model is developed using the numerical code CactusHydro, introduced in (Feo & Celico, 2021; Feo & Celico, 2022). It is based on the high-resolution shock-capturing flux (HRSC) conservative method to follow sharp discontinuities accurately and temporal dynamics of a three-phase immiscible fluid flow in a porous medium. CactusHydro resolves the governing equations that describe the migration of an immiscible phase fluid flow in a porous medium composed of non-aqueous (n), water (w), and air (a), and a variably saturated zone. The migration of the spilled DNAPL is considered immiscible, and the effects of the volatilization, biodegradation, or dissolution are not considered. CactusHydro treats the vertical and horizontal movement of the contaminant in the variably saturated zone as a whole and is numerically resolved as a unique zone (not separating the vertical movement from the horizontal one since the flow equation includes both zones). The model presented here simulates the DNAPL migration with the aim of predicting (i) the free-product distribution in multilayered aquifers (up to several hundred meters thick) and then (ii) the distribution of possible long-term pollution sources for shallow and deep groundwaters. A first scenario was simulated in detail, with the scope of understanding whether the interposition of a low-permeability layer between two higher permeability horizons can influence the vertical migration of DNAPLs. The test site is the alluvial aquifer of Parma Plain (Northern Italy), where perchloroethylene (PCE) was recently detected in groundwater (Zanini et alii, 2019; Zanini et alii, 2021), therefore suggesting the existence of DNAPLs sources. The results demonstrated that the PCE can migrate downward through low-permeability layers, even though a very low velocity was estimated.