Developing a nitrogen load apportionment tool: Theory and application
Introduction
Human activities frequently lead to the impairment of surface waters and marine ecosystems, and the contamination of groundwater (Jordan and Weller, 1996; Malagó et al., 2019a). Intensive agriculture is the main source of nitrogen (N) pollution (Tilman et al., 2002; Malagó et al., 2019a). In 2008, the Organization for Economic Co-operation and Development (OECD) reported how the problem of nutrient input/output imbalance had become common in all industrialised countries (OECD, 2008). Several studies have been conducted on the balance, export and consequent alterations in the natural cycle of N (Bouwman et al., 2005; Bouraoui et al., 2010; Grizzetti et al., 2011; Balestrini et al., 2013; Tzoraki et al., 2014; Mockler et al., 2017; Pinardi et al., 2018; Viaroli et al., 2018; Soana et al., 2019). Such studies have increased the awareness that environmental policies are fundamental to managing the nutrient cycle. In this context, the European Water Framework Directive (WFD; Directive, 2000/60/EC) and the Nitrates Directive (Directive 91/676/EEC, 1991) stand as the most significant European legislation. The main objective of the WFD is the achievement of a "good ecological status" in all water bodies across Europe. Meanwhile, the Nitrates Directive requires the Member States of the European Union (EU) to control and reduce the water contamination by nitrates from agricultural sources that are polluting surface water and groundwater by promoting sustainable farming practices.
The implementation of both EU Directives requires quantification of the anthropogenic pressures, the estimation of nutrient riverine export and identification of the critical source areas (Malagó et al., 2019b). These tasks are difficult to manage, especially in large areas where the heterogeneity of the environment and nutrient sources (i.e. hot-spots, sinks) is added to the complexity of hydrological and physical processes (De Girolamo et al., 2017a).
In recent years, several methodologies and tools have been developed to perform these tasks. These methods include either simplified approaches, such as statistical methods (Grizzetti et al., 2005), export coefficient models centered on export coefficients or unit loads (Drewry et al., 2006), and more complex approaches, such as ecohydrological models (Schoumans et al., 2009; Lam et al., 2010; Sharifi et al., 2019) or Bayesian networks (Ticehurst et al., 2007; Nash et al., 2010; Lu et al., 2013).
The most suitable spatial unit for estimating diffuse pollution and developing nutrient balance is the basin scale (Barceló and Sabater, 2010; Bartoli et al., 2012; Lassaletta et al., 2012; Castaldelli et al., 2013), even if the variability of the environmental features entails a great uncertainty in the nutrient input and output estimations, especially in data-limited regions (Oenema et al., 2003; De Girolamo et al., 2019). Currently, studies focusing on the total N (TN) budget in Mediterranean basins are still scarce (Romero et al., 2016). In these basins, characterised by fragmented land use, different management practices, and high variability in climate, as well as hydrological processes and environmental features, the nutrient budget calculation is more difficult than in basins where the agronomic practices and environmental features are uniform (De Girolamo et al., 2017a). Indeed, the nutrient load that leaves the basin and reaches the river is the result of a complex interaction between the crop production system (types and amounts of fertiliser, and timing of their application) and environmental factors (slope, soil properties, hydrology and climate).
The overall objective of this paper was to develop a nitrogen load apportionment tool (NAT) for basin-scale application. The specific objectives were the following: i) to estimate the annual TN budget at the basin scale; ii) to assess the TN riverine export through measurements of flow and nutrient concentrations; iii) to estimate the TN load in leaching and runoff, by combining environmental features and monitoring data; and iv) to calculate the TN load apportionment and to identify the areas contributing most of the TN in the basin.
The proposed model constitutes an alternative approach to complex conceptual models for estimating nutrient load apportionment, which generally require time and experienced staff, in addition to specific knowledge of the processes acting in the basin. The novel approach can be useful to support effective water management and long-term, sustainable land-use planning in data-limited areas characterised by intermittent river networks, such as those that are typical of the Mediterranean region. The model was tested in the Canale d’Aiedda Basin (SE Italy), where the TN inputs and outputs were calculated using multiple data sources combined with field measurements. It can be easily exported to other basins, since it requires only a spreadsheet program, GIS software and data that are generally available.
Section snippets
Study area
The Canale d’Aiedda stream (basin area about 360 km2) is located in the Apulia region (SE, Italy, Fig.1), and flows into the Mar Piccolo. The basin area and its surroundings are subject to a considerable concentration of industries and mussel aquaculture, in addition to agricultural activities. Due to the extent of the environmental impact, in terms of health and ecological risk, the area is included in the list of Contaminated Sites of National Interest (Sito di Interesse Nazionale, SIN), with
Total nitrogen soil system budget
The ΔN was assessed on a yearly time scale at the basin level (area 35,849 ha), and also with reference to the productive land only (area 19,608 ha). The results are summarised in Table 2. The sum of the TN input from DSs (Σ Input) at the basin level was quantified as 2359.0 t yr−1, which, divided by the basin area, gave a specific TN of approximately 66 kg ha−1yr−1. The main sources of TN were NSF (1740.9 t yr−1) and NAF (251.5 t yr−1), while NBF (169.5 t yr−1) and NAD (197.2 t yr−1) represent
Discussion
The NAT model quantifies the pressures caused by PS and DS on water bodies, and identifies the areas contributing most of the TN in the basin via load apportionment. The model can be considered to be an alternative approach to complex models (De Girolamo et al., 2019) because it is designed to be applied in cases of limited resources (i.e. economic, time and data) by staff without advanced skills in ecohydrological modelling. It requires data that is generally available, and measurements of
Conclusions
In this study, a NAT was developed and proposed as an alternative approach to more complex models, aimed at quantifying the pressures on water bodies from PSs and DSs and identifying critical source areas through TN load apportionment. The model’s effectiveness was tested in the Canale d’Aiedda, a basin characterised by limited data availability, and representative of the rural areas of the Mediterranean region.
The main findings of this study include:
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The model requires limited experience with
Acknowledgments
The Authors wish to thank Dr. Vera Corbelli, Special Commissioner for Urgent Interventions of Environmental Requalification of Taranto, and her staff for the financial support in setting up the monitoring plan. Thanks are also due to Civil Protection Service – Puglia Region, Assocodipuglia Consortium, Arpa Puglia, Acquedotto Pugliese (AQP) and Reclamation Consortium of Stornara and Tara for providing data. The authors wish to thank Giuseppe Pappagallo for the analytical determination of
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