# Denitrification in Intrinsic and Specific Groundwater Vulnerability Assessment: A Review
## 4 Results and Discussion
### Nitrate Vulnerable Zones  
A step forward has been made with the introduction of several methodologies aimed at evaluating nitrate vulnerable zones (NVZ). Conserving the same structure of a rating method, specific parameters connected to NO3\({}^{-}\) leaching and attenuation were introduced in the evaluation procedure. An example is the agricultural NO3\({}^{-}\) hazard index named IPNOA, which was proposed by [PERSON] and [PERSON] [97]. IPNOA facilitates the assessment of hazard factors contributing to NO3\({}^{-}\) load and control factors able to amplify or decrease the NO3\({}^{-}\) load. Specifically, the hazard factors are chosen among agricultural sources of Nr input which could have a potential impact on groundwater resources, such as: (i) applied organic fertilizers, (ii) synthetic fertilizers, and (iii) wastewaters, while the control factors used to evaluate NO3\({}^{-}\) behavior in site-specific conditions and farming practices are: (i) Nr content in soil, (ii) climate characteristic, (iii) agronomic practices, and (iv) irrigation techniques. For example, tillage operations could greatly influence DNT [98, 99], as well as the soil Nr content and the fertilizer application rate [47]. The methodology was applied by several authors [33, 100, 101, 102] and was also coupled with classical rating methods, such as SINTACS, with the aim of improving NO3\({}^{-}\) vulnerability assessment [103, 104]. Another approach for NVZ delineation is the calculation of N budgets and surpluses on the farmland scale [105], watershed scale [106, 107, 108, 109, 110], or even at the subcontinental scale [111, 112, 113]. It consists of differentiating between the soil Nr inputs and outputs, where inputs usually are: (i) synthetic fertilizers, (ii) sewage sludge and animal manure, (iii) biological N fixation, and iv) atmospheric deposition, while the outputs are: (i) the amount of crop harvested and removed from the field, (ii) crop residues, (iii) leached Nr, and (iv) degassed N. This methodology offers a mappable output of the available Nr content for leaching. [PERSON] et al. [114] presented an overlay index method for NVZ identification through the analysis of three groups of factors: the first was related to intrinsic vulnerability (water table, saturated hydraulic conductivity, and aquifer type), the second considered the attenuation factors acting in the soil and specifically related to DNT capacity (organic carbon, pH, and texture), and the third indicated the pollutant infiltration capacity (slope, precipitation, and evapotranspiration). Rates and weights were then assigned using Pearson's r correlation and analytical hierarchical process (AHP) to limit subjectivity. [PERSON] et al. [115] combined the N-surplus approach with a rating methodology specially developed for NVZ. Similarly, [PERSON] et al. [116] developed a global risk index (GRI), integrating the N budget with water surplus (irrigation and rain), soil permeability, unsaturated zone residence time, and surface morphology. In these methodologies, DNT is related to different parameters such as soil texture, soil pH, Nr input, and unsaturated zone residence time. Indeed, DNT is more likely to occur when the travel time through the unsaturated zone is long [44]. Following the same concept, [PERSON] et al. [117] proposed a rating methodology for NVZ using all those parameters directly connected to NO3\({}^{-}\) concentration in groundwater, combining the classical parameters of the unsaturated zone media, surface morphology, and recharge with the natural soil Nr content and the amount of fertilizers recommended for different kind of crops. The methodologies described so far have been applied in several case studies and compared to canonical vulnerability assessment methods and were found to deliver more realistic results [117; 118; 119].