# Denitrification in Intrinsic and Specific Groundwater Vulnerability Assessment: A Review
## 4 Results and Discussion
### Rating Methods for Nitrate Vulnerability Assessment  
Among all methodologies to assess aquifer vulnerability, the rating methods are surely the most widely utilized [38]. The main advantage lies in their straightforward applicability and in their highly understandable results. The pioneer methods of this class are DRASTIC [77] and GOD [78], followed by many other methodologies specifically created to assess the intrinsic aquifer vulnerability, such as AVI [79], EPIK [80], SINTACS [81], SEEPAGE [82], and many more. A comprehensive review of these methodologies is available in [PERSON] et al. [38]. These methods have been widely applied both on watershed, regional, and even global scales, producing reliable results [83; 84; 85; 86; 87]. They consist of the combination of thematic layers, representing the different physiographic attributes that control the groundwater vulnerability to pollution of a given area; the most utilized factors are: (i) depth to the water table, (ii) recharge, (iii) unsaturated and saturated media, (iv) soil texture, (v) topographic slope, (vi) hydraulic conductivity, (vii) hydraulic resistance, and (viii) presence of phreatic or confined aquifers. Each parameter is classified using a range of rates (generally 1 to 10) and multiplied for a specific weight, indicating the individual parameter importance and magnitude. These lumped parameters incorporate most physical attenuation processes (such as dilution and travel time) but disregard biogeochemical processes (such as DNT) that may occur along the pathway between the pollution sources (usually at land surface) and the considered target, namely the saturated portion of the aquifer and/or the surface water bodies towards which groundwater discharges (Figure 3). Figure 3: DNT rates as employed in the different vulnerability assessment methods. Figure 2: Conceptual organization of this review. It should be mentioned that some biogeochemical processes are often a function of soil properties (such as texture and water content), therefore, many end users of rating methods claim that natural attenuation processes are accounted for, even though not explicitly. Specifically, soil texture could influence the redox values suitable for DNT, which were found to be more favorable in clayey textured soils than in sandy textured soils [88, 89]. Nevertheless, other important factors influencing DNT, like organic carbon substrates and Nr mass loading rates, are generally completely neglected. The same assumption is made for the unsaturated and saturated zone where the pollutant attenuation can be regulated by several processes that depend on their physical and hydraulic properties (e.g., thickness, hydraulic conductivity, and presence of organic matter). Both saturated and unsaturated zones are commonly classified considering only the lithological characteristic and the thickness of the media. Moreover, all methods offer a large range of rates, especially for saturated and unsaturated media classification, implying an inherent subjectivity (expert judgment). In any case, the classification is usually made considering dispersion and diffusion capacity, neglecting the main biogeochemical factors that can occur within the saturated and unsaturated zones. The SEEPAGE methodology [82] tried to overcome this problem, introducing a potential attenuation parameter referring to the soil and subsoil characteristics. Specifically, the introduced attenuation parameters based its classification on several factors that can be easily related to DNT, such as: (i) soil surface and subsoil texture, (ii) topsoil pH, (iii) organic matter content of topsoil, (iv) soil drainage class, and (v) soil permeability. Unfortunately, the methodology has not been so widely applied in comparison to DRASTIC and SINTACS methods and only a few applications have been documented so far [90, 91].