# Denitrification in Intrinsic and Specific Groundwater Vulnerability Assessment: A Review
## 1 Introduction  
Around the world, several water bodies do not meet the minimum quality standards necessary to ensure their utilization for anthropic activities [1, 2, 3] and groundwater quality is even declining over time [4]. This phenomenon is attributable to the presence of different pollutants in both surface and groundwater resources [5], due either to anthropogenic activities or natural processes [6]. Specifically, the subsurface pollution related to nitrogen surplus is considered a persistent and widespread issue, especially in agricultural areas [7], where the excess of nitrate (NO\({}_{3}\)-) represents a major concern [8, 9, 10, 11]. NO\({}_{3}\)- is simultaneously: (i) an indispensable nutrient for agricultural production, necessary to properly meet the increasing food demands for communities, and (ii) the most widespread groundwater pollutant worldwide [12], which can be linked to numerous severe human health threats [13]. Significant increases in groundwater NO\({}_{3}\)- pollution can be attributed to several processes such as: (i) overuse of fertilizers and/or animal waste, (ii) sewage leaks, (iii) improper management of wastewater effluents, and (iv) wastewater treatment without denitrification (DNT) [14]. Driven by this worrying scenario, the United States Environmental Protection Agency [15] and with the European Union Council [16] have established a maximum NO\({}_{3}\)- concentration of 50 mg/L in drinking water. The natural processes of water dilution and NO\({}_{3}\)- decay represent the main processes that could attenuate NO\({}_{3}\)- concentration in water bodies. The degradation of NO\({}_{3}\)- through DNTcan reduce the reactive nitrogen (Nr) levels in both deep and shallow aquifers [17; 18]. The process occurs both in the saturated and unsaturated zones under mildly reducing conditions [19], with dissolved oxygen usually below 2 mg/L. It generally refers to the heterotrophic DNT, operated by facultative anaerobic bacteria, using a carbon source as electron donor and NO3- as electron acceptor to produce atmospheric nitrous oxide (N2O) and nitric oxide (NO), which can be further reduced to dinitrogen (N2). Unfortunately, the DNT byproducts that are generated when DNT is incomplete are also considered greenhouse gases and ozone-depleting substances [20]. While heterotrophic DNT is the most widely known pathway to reduce NO3- in the subsurface, other processes can help to diminish NO3- concentrations, like dissimilatory NO3- reduction to ammonium [21]. This process is an anaerobic respiration of organic matter by chemoorganoheterotrophic microbes using NO3- as electron donor, but unlike DNT, it conserves bioavailable Nr in the system as ammonium. Other relevant processes ubiquitously occurring in subsurface environments are anammox [22] and autotrophic DNT via different inorganic electron donors [23], includign dissolved reduced species such as ferrous iron and sulfide or solid phases such as pyrrhotite and pyrite. Despite the many biogeochemical processes that can attenuate NO3-, fertilization often occurs at amounts exceeding both crop demand and the natural attenuation capacity of soils. This generates a NO3- surplus which can be readily transported by percolating waters through the soil profile and can promote NO3- accumulation in groundwater bodies, if oxic conditions prevail [24; 25]. Moreover, a reliable identification of NO3- sources may be difficult due to the occurrence of multiple releasing sources and the complementarity of several geogenic and anthropogenic processes acting in the same area [26; 27; 28; 29]. In addition, the occurrence of NO3- may suffer from consistent temporal variations [30], depending on: (i) the precipitation regime [31], (ii) the hydrogeological conditions [32], and (iii) the land use [33]. Several tools and techniques have been proposed to estimate groundwater pollution, ranging from analytical, graphical, and statistical methods.