Abstract
Several groundwater vulnerability methodologies have been implemented throughout the years to face the increasing worldwide groundwater pollution, ranging from simple rating methodologies to complex numerical, statistical, and hybrid methods. Most of these methods have been used to evaluate groundwater vulnerability to nitrate, which is considered the major groundwater contaminant worldwide. Together with dilution, the degradation of nitrate via denitrification has been acknowledged as a process that can reduce reactive nitrogen mass loading rates in both deep and shallow aquifers. Thus, denitrification should be included in groundwater vulnerability studies and integrated into the various methodologies. This work reviewed the way in which denitrification has been considered within the vulnerability assessment methods and how it could increase the reliability of the overall results. Rating and statistical methods often disregard or indirectly incorporate denitrification, while numerical models make use of kinetic reactions that are able to quantify the spatial and temporal variations of denitrification rates. Nevertheless, the rating methods are still the most utilized, due to their linear structures, especially in watershed studies. More efforts should be paid in future studies to implement, calibrate, and validate user-friendly vulnerability assessment methods that are able to deal with denitrification capacity and rates at large spatial and temporal scales.
Highlights
This review offers an overall explanation of how already known vulnerability methodologies incorporate the process of DNT without discussing every single application, focusing the discussion on methodologies that showed an implicit or explicit involvement of DNT in the vulnerability assessment
Logistic regression allows for the evaluation of those natural and anthropogenic factors that could impact the probability that NO3 − concentrations will exceed a specified threshold
The assessment of groundwater vulnerability to NO3 − is a valuable tool for the implementation of sustainable groundwater management plans
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The above-mentioned DNT controlling factors (Figure 1), both in the unsaturated and saturated zones, are usually considered in vulnerability assessment methodologies to indirectly account for DNT where direct measurements are not available. As will be further discussed in the following chapters, the main factor considered in most used methodologies (namely the rating and weights methods) is the soil texture, which is easier to measure and analyze compared to the other factors controlling DNT in the unsaturated or saturated zone Recent studies [53,54] reported DNT rates up to 320–390 Kg-N/ha/year, underlining the fact that the contribution of DNT processes within the saturated zone cannot be neglected in the definition of the vulnerability of a given territory
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