# Denitrification in Intrinsic and Specific Groundwater Vulnerability Assessment: A Review
## 1 Introduction
They have been greatly enhanced since the advent and the improvement of geographic information systems (GIS), which made the interpretation and representation of groundwater quality easier than before [34]. Among all available methodologies, groundwater vulnerability assessment tools have attracted researchers' attention due to their relatively straightforward applicability and the production of understandable results by non-specialists. The concept of aquifer vulnerability inability [35; 36] aims to quantify the susceptibility of an aquifer to be adversely affected by any pollutant load imposed on the land surface. Groundwater vulnerability cannot be directly measured and quantified in the field since it represents a qualitative classification as a function of the main hydrogeological conditions of the study area, which could vary drastically from one place to another. The vulnerability can be classified in two types: (i) intrinsic and (ii) specific vulnerability [37]. The intrinsic vulnerability only depends on hydrogeological characteristics regardless the pollutant types. The specific vulnerability instead refers to the vulnerability of a given pollutant, involving in the evaluation processes both the pollutant's physical-chemical characteristics and its mass loading rates. In recent decades, different methodologies to assess groundwater vulnerability were proposed and tested around to world. They can be distinguished in four main categories: (i) deterministic or simulation models, (ii) overlay and index techniques, (iii) statistical tools, and (iv) hybrid methodologies [38]. All methodologies offer different advantages and, at the same time, they suffer from some drawbacks. For example, both statistical and deterministic models ask for a high-quality dataset and a massive quantity of information, especially for calibration and validation procedures, while overlay and indices methodologies could be affected by subjectivities in rates classification and weights assignment. Weighting and rating indices are easy-to-use methodologies, need relatively low amounts of data, are usually applicable to large domains, and can be easily modified and updated. On the other hand, they produce static snapshots of the aquifer vulnerability, which can be only used as a screening tool for regional and national scale management or planning. Conversely, process-based methods can produce a transient representation of groundwater quality, but they are mostly used on small domains (e.g., field or site scales) due to computational burdens and the requirement of high-quality and -frequency data. In general, the groundwater pollution status is controlled by a wide range of physical and biogeochemical processes (e.g., dilution, sorption, and degradation), which contribute to decreasing pollutants concentrations. As a result, N loads of rivers and aquifer systems often reflect only a minor part of the N surplus within the corresponding river basin area due to decay processes during the aquifer transit and within the surface water bodies themselves [39]. Many studies have highlighted that among these decay processes, DNT has a large potential to reduce the relative vulnerability of water bodies [40; 41]. Unfortunately, these attenuation processes are not always properly incorporated within the vulnerability assessment methodologies [42; 43], but only considered indirectly in lumped parameters. This paper deals with the last 20 years of literature on groundwater vulnerability and, specifically, it focuses on the introduction of NO3- attenuation processes within the evaluation assessment. The main difficulties in including, directly or indirectly, the DNT process in some methodologies, the improvements in results, and the effects on the consequent water management strategies are also discussed.