# Denitrification in Intrinsic and Specific Groundwater Vulnerability Assessment: A Review
## 4 Results and Discussion
### Numerical Models  
With respect to the qualitative methods (rating methods), the numerically and physically based methods for the assessment of aquifer vulnerability to NO3\({}^{-}\) involve the analysis of all the relevant processes occurring within the unsaturated and the saturated systems. Numerical models subdivide the tridimensional domain into cells or elements (depending on the method used) where the flow, dispersive transport, and reactions among each cell are computed by differential equations of flux and mass conservation solved iteratively, starting from user-defined initial and boundary conditions. Thus, with numerical methods, an accurate temporal and spatial evolution of the fate and transport of NO3\({}^{-}\) in the subsurface can be achieved. In fact, these methodologies aim to determine the specific vulnerability for diffuse pollution sources rather than the intrinsic vulnerability of an aquifer, emphasizing the physical, chemical, and biological processes controlling the fate and transport of contaminants in both the unsaturated and saturated zones. Moreover, process-based models also provide the temporal variation of vulnerability, a concept often neglected by other methods. Among all the numerical models, the most used are: GLEAMS [134], SUTRA [135], MODFLOW [136] coupled with MT3 DMS [137], FEFLOW [138], PHT3D [139], HYDRUS [140, 141], and SWAT [142]. All of these models explicitly involve Nr attenuation processes, such as DNT, in their modules. For example, the N module of SWAT intrinsically considers various NO3\({}^{-}\) reduction processes, such as mineralization, volatilization, and DNT, which is simulated in the soil layer as a function of the amount of NO3\({}^{-}\), organic carbon, and temperature. The sink and source package of MT3 DMS was designed to simulate chemical processes, such as NO3\({}^{-}\) decay, following a first-order degradation reaction or fully integrating the reaction stoichiometries of the main terminal electron acceptors processes [143]. GLEAMS incorporates a N cycling algorithm, which considers N plant uptake and fixation among with fertilizers and manure applications. Due to their high versatility, despite these models are recognized as valuable tools for groundwater vulnerability assessment, they find application in a large variety of settings, such as: pollution transport in groundwater and surface waters, groundwater balance and depletion, seawater intrusion, and more. This picture shows that groundwater vulnerability assessment based on contaminant transport modeling can provide more quantitative evaluation than previously discussed rating methods. On the other hand, the model's large number of input parameters and the uncertainty associated with them may be considerable issues. In fact, to implement such simulations, an accurate characterization of all flow and transport processes acting at a given site is a prerequisite, and, in addition, there is also the need to deeply understand the geochemical environment [144]. Despite this, the possible applications of numerical models are quite numerous. Some examples of aquifer vulnerability to NO3\({}^{-}\) measured using MODFLOW/MT3 DMS models are found in USA [145], Europe [146], and Africa [147]. [PERSON] et al. [148], assessed NO3\({}^{-}\) groundwater vulnerability for an alluvial aquifer combining the output of three different numerical models: (i) groundwater recharge (GROWA), (ii) NO3\({}^{-}\) leached from the soil profile (SWAT), and (iii) groundwater flow velocities (FEFLOW). [PERSON] et al. [149] assessed the whole Florida shallow aquifer vulnerability through the Nr removal and transport rate in the unsaturated zone. [PERSON] et al. [150] developed the site-specific concept for aquifer nitrate vulnerability assessment (SCANVA) through the combination of a 3D geological reconstruction, a groundwater simulation based on MODFLOW-2000, and a complete hydro-geochemical assessment. [PERSON] et al.