Using Nitrogen and Boron Isotopes

Environmental Science and Pollution Research (2019) 26:2089–2104 https://doi.org/10.1007/s11356-017-0792-6 GROUNDWATER UNDER THREAT FROM DIFFUSE CONTAMINANTS: IMPROVING ON-SITE SANITATION, AGRICULTURE AND WATER SUPPLY PRACTICES Evaluation of sources and fate of nitrates in the western Po plain groundwater (Italy) using nitrogen and boron isotopes Manuela Lasagna1 & Domenico Antonio De Luca1 Received: 19 July 2017 /Accepted: 17 November 2017 /Published online: 24 November 2017 # Springer-Verlag GmbH Germany, part of Springer Nature 2017 Abstract Diffuse nitrate pollution in groundwater is currently considered one of the major causes of water quality degradation. Determining the sources of nitrate contamination is an important first step for a better management of water quality. Thus, the 15 18 11 isotopic composition of nitrate (δ NNO3 and δ ONO3) and boron (δ B) were used to evaluate nitrate contamination sources and to identify geochemical processes occurring in the shallow and deep aquifers of the Turin-Cuneo plain (NW Italy). The study area is essentially an agricultural zone, where use of synthetic nitrogenous fertilizers and organic manure is a common practice and the connection to sewer services is locally lacking. Also livestock farming are highly developed. A groundwater sampling campaign was performed on 34 wells in the shallow aquifer and 8 wells in the deep aquifers, to analyze nitrate, chloride, boron, 15 18 11 δ NNO3, δ ONO3 and δ B. Isotope data of nitrate indicate that nitrate contamination in the Turin-Cuneo plain originates from mixtures of synthetic and organic sources, slightly affected by denitrification, and manure or septic tank effluents. Moreover, boron isotopes were used to discriminate further among the main anthropogenic sources of pollution. The analyses results confirm that both animal manure and domestic sewage, especially under the city of Turin, can contribute to the nitrate contamination. The isotope analysis was also used for the evaluation of denitrification and nitrification processes: contrary to expecta- tions, a significant denitrification phenomenon was assessed only in the shallow unconfined aquifer, especially in the Poirino Plateau, the most contaminated sector of the study area. Keywords Nitrogen stable isotopes . Boron . Nitrate sources . Groundwater pollution . NW Italy Introduction groundwater, especially in agricultural areas. The correlation between agriculture and nitrate concentrations in groundwater Nitrate pollution in groundwater is currently one of the major was observed in many studies coming from different countries cause of water quality degradation. Starting from the second (Agrawal et al. 1999; Nolan and Stoner 2000;Harteretal. half of the past century, the global increase in the use of N- 2002;Debernardietal.2008; Capri et al. 2009; Petitta et al. fertilizers (synthetic and organic) has led to increased ni- 2009;Sacchietal.2013). The extensive use of fertilizers for trate leaching and runoff. Consequently, large regions of agricultural purposes, in fact, is considered to be the main non- the world suffered from nitrate contamination of surface and point source of nitrate contamination in groundwater (Liao et al. 2012;Postmaetal.1991;Baker1992; Chowdary et al. Responsible editor: Philippe Garrigues 2005). However, nitrate pollution in groundwater can also be caused by other sources, such as septic systems (Al-Agha * Manuela Lasagna 1999;MacQuarrieetal.2001). [email protected] The Nitrates Directive (European Commission 1991)is one of the earliest pieces of EU legislation aimed to protect Domenico Antonio De Luca water quality across Europe by preventing nitrates from agri- [email protected] cultural sources polluting ground and surface waters and by 1 promoting the use of good farming practices. One of the Earth Sciences Department, Turin University, Via Valperga Caluso B 35, 10125 Turin, Italy adopted action was the Designation of Nitrate Vulnerable 2090 Environ Sci Pollut Res (2019) 26:2089–2104 Zones^ (NVZs). NVZs are territories which drain into pollut- Boron (B) has generally a ubiquitous distribution in ground- ed waters or waters at risk of pollution and which contribute to water, because it commonly exists as a minor constituent nitrate pollution. As regards water resources, groundwater (Vengosh 1998). Naturally occurring B is mostly derived from containing or that could contain (if no action is taken to re- weathering of rocks; thus δ11B composition can be used to verse the trend) more than 50 mg/L of nitrates are defined as identify pollution sources if the contaminants have δ11B polluted water, or at risk of pollution. values distinct from the δ11B composition in groundwater In Piedmont, that represents the westernmost region in (Vengosh et al. 1994; Bassett et al. 1995). More specifically, Italy, NVZs correspond to the 37.6% on utilized agricultural detergents coming from evaporites, synthetic fertilizers, ma- area and 54% on regional plain areas (Ministry for nure and sewage have high content of B and different δ11B Environment et al. 2010). signature (Vengosh et al. 1994;Komor1997;Widoryetal. In the NVZs action programs must be implemented by 2005). farmers on a compulsory basis, including limitation of fertil- Komor (1997) and Bassett et al. (1995) first used B iso- izer application (mineral and organic) and other measures at topes as comigrating tracers of NO3, but only Komor (1997) farm scale. The application of these measures can have nega- and Widory et al. (2004) referred the B isotope composition of tive impact on farm economics, especially for small farms input from agriculture (e.g., hog manure, cattle feedlot runoff, (ADAS 2011). Consequently, the definition of the different synthetic fertilizers) and combine N and B isotopes in order to nitrate sources in groundwater (sewage, animal manure, distinguish between different NO3 sources in groundwater and chemical fertilizer, natural soil mineralization) has major im- surface water. portance, especially for the stakeholders that must implement Microbial processes and groundwater redox that govern N management actions for groundwater protection. isotope evolution (denitrification/nitrification) have no conse- To define the sources of nitrates in groundwater, the stable quence on B (Bassett et al. 1995). The key factors that affect 15 18 nitrogen (δ NNO3) and oxygen (δ ONO3) isotope of nitrates the B isotopic composition in groundwater are the B sources have been extensively used (Aravena et al. 1993;Seiler2005; and adsorption to clays; only in extremely saline environ- Sacchi et al. 2007;ReandSacchi2017). ments, mineral precipitation affects the B isotopic composi- Nitrates from mineral fertilizers are characterized by the tion (Seiler 2005). 15 atmospheric values of δ NNO3 (approximately 0‰)and A recent study (Pennisi et al. 2013) compiled a database of 18 δ ONO3 (approximately 23.5‰), whereas nitrates derived isotopic data on potential boron contaminants of natural wa- from organic fertilizers, livestock effluents and sewage are ters (ISOBORDAT database). Their isotopic composition, 15 15 usually enriched in N(δ NNO3 varies from +8 to +20‰) which spans over a wide range, helps to identify boron con- because of ammonium volatilization (Kendall 1998). The cor- tamination sources and geochemical processes in water. 15 18 relation between the δ NNO3 and δ ONO3 isotopic indicators The purpose of this paper is to define the nitrate contami- can also provide data on the nitrification and denitrification in nation sources in the shallow and deep aquifers of Turin- an area. Aravena and Robertson (1998) demonstrated a con- Cuneo plain (NW Italy) using isotopic composition of NO3 15 18 15 18 11 current increase in the δ NNO3 and δ ONO3 of groundwater (δ NNO3 and δ ONO3) and boron (δ B). Furthermore, the nitrates during denitrification. During nitrification, the investigation aims to identify the processes affecting nitrate 18 δ ONO3 ratio changes to lower values because of the incor- concentrations in groundwater, such as nitrification and/or 18 poration or the exchange with δ OH2O of groundwater. The denitrification. 18 typical range of δ ONO3 for nitrified synthetic fertilizers generally is 5‰ to 15‰; it also depends on the groundwater isotopic composition (Sacchi et al. 2013). Furthermore, nitri- Study area fication process produces a positive correlation between the nitrate concentrations and δ15N of the nitrates (Choi et al. The study area is located in Piedmont (northwestern Italy) and 2003). Thus isotopic composition of nitrate can help to dis- corresponds to the plain between Turin and Cuneo cities criminate between synthetic fertilizers, mineralized synthetic (Turin-Cuneo plain). The study area can be further distin- fertilizers, manure/septic system effluents and soil organic guished in different sub-areas: the Po Plain area in Turin matter as nitrate sources in groundwater. However, the use Province (Turin plain), a plain area in Cuneo Province of these isotopes is not useful to further discriminate between (Cuneo Plain) and a plateau area in Turin and Asti Province manure and septic system effluents that display the same ni- (Poirino Plateau). The minimum altitude is 200 m above sea trate isotopic composition. level (a.s.l.) in the eastern sector of the plain, corresponding to In recent studies boron isotopes (δ11B) were used coupled the confluence of the Stura di Demonte River and Tanaro with δ15N of the nitrates to discriminate between synthetic River, and the maximum

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