**1. Introduction**

Water crises are not only caused by droughts and shortages of the resource, but also by pollution and water quality deterioration, which reduce the quantity of safe water in many regions of the world [1]. As a result, a challenge faced by all countries is a reduction in the concentrations of pollutants in surface water (SW) and groundwater (GW) [2–4]. Several measures have been implemented to decrease the concentration of nitrates in water bodies around the world. The European Union has implemented some legislative instruments designed to protect water quality [5], such as the Nitrate Directive (1991), Urban Waste Water Treatment Directive (1991), and Water Framework Directive (WFD) in 2000. Despite the measures that were taken, in many areas, the water quality did not reach a good status [6,7].

The most important source of nitrate is the agriculture, which generates diffuse pollution followed by point pollution with urban and industrial discharge [8–10]. The nitrogen accumulated in soil leaches to water bodies through runoff or percolation, and then, hydrology is the means of transport until it is seen as a pollutant [11]. Nitrate

**Citation:** Dorado-Guerra, D.Y.; Paredes-Arquiola, J.; Pérez-Martín, M.Á.; Tafur Hermann, H. Integrated Surface-Groundwater Modelling of Nitrate Concentration in Mediterranean Rivers, the Júcar River Basin District, Spain. *Sustainability* **2021**, *13*, 12835. https://doi.org/ 10.3390/su132212835

Academic Editors: Alban Kuriqi and Luis Garrote

Received: 29 September 2021 Accepted: 16 November 2021 Published: 19 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

transport in water is influenced by the interaction between SW and GW, as this interaction forms the link between land activities and aquatic ecosystems [12–14].

Monitoring pollution sources and nitrate loading with a high spatial and temporal resolution is challenging, and as a result, integration of large-scale hydrological models of rainfall runoff and water quality have been widely used. Among these are SWAT [15], MODFLOW [16], SHETRAN [17], QUAL2E & QUAL2K [18], STICS-MODCOU [19], and PRZM-GW [20]. A complete review of models used in pollution estimation in Europe was conducted by the Ref. [5]. Many of the hydrological models only consider the base flow component of the aquifers, or river–aquifer interactions are not represented. This introduces further uncertainty in the runoff calculation. However, the discharge of GW into the rivers is considered important in arid and semi-arid areas, as it is part of the non-stationarity of the rain–runoff relationship, and it influences the quality of SW and the well-being of aquatic ecosystems [11,21,22].

Several studies have evaluated the SW–GW interactions in watershed management and their impact on water quantity and quality. As a result, GW discharges with high nutrient levels are considered as a source of SW pollution and ecosystem damage [23,24]. Understanding the effects of SW–GW interactions is a key factor in the management of water resources in GW-dependent areas to supply the demands; however, it is not always considered in decision-making [25,26]. For this reason, it is a challenge to determine how GW discharges can impact the nitrate concentration in SW bodies.

Hydrological variability and water scarcity in the Júcar River Basin District (RBD) in Spain have made necessary the joint use of GW and SW to satisfy water demands, in some cases leading to the overexploitation of water resources [27]. In general, the total contribution of the Júcar RBD fluvial network comes mostly from GW runoff. Although nitrate concentration in GW bodies is stabilized without upward trends except for some deep aquifers [28], 33% of the aquifers have a nitrate concentration above the threshold of good status (NO3 − < 50 mg/L) [29]. As a consequence, Júcar RBD has water quantity and quality problems.

Accordingly, the main objective of this study was to estimate the influence of the SW– GW interactions on nitrate concentration and to determine the sources of nitrate pollution in the Júcar RBD SW bodies. The following research questions were covered: (1) How nitrate transfer from the aquifer affects spatial–temporal variation of the concentration of nitrates in the rivers, and (2) what the sources of pollution in the Júcar RBD are. To answer the above research questions, two models that integrate the SW–GW interactions and water quality were linked together. With the combination of the models, it is expected that the contrast of results will provide less uncertainty. First, the PATRICAL model (Spanish acronym for "Precipitation Input in Network Sections Integrated with Water Quality; [28,30]) integrates river–aquifer interaction for a medium-large watershed. The PATRICAL output is the starting point for the second large-scale surface water quality model, RREA (Spanish acronym for "Rapid Response to the Ambient State"; [31]). The RBD authorities in Spain have extensively employed PATRICAL and RREA in the construction of the hydrological plans and in the implementation of the WFD. Additionally, it has been used to evaluate climate change impacts on water resources [32], to improve the drought's indicators in the Júcar RBD [33], and to observe changes in the hydrology in the Mediterranean side of Spain [27]. In previous works, RREA was used to quantify the effects of the main existing pressures on the receiving waters in the Middle Tagus Basin in Spain [34]. Among the multiple benefits of these models, they can be used to identify pollution sources, simulate nitrate concentration in surface and groundwater, and assess the efficiency of management measures to prevent water degradation.

In the calibration of the models, the database of nitrate concentration and the evaluation of the status of the water bodies carried out by the Júcar RBD were used. To evaluate the simulated capacity of the nitrate status, an analysis was made from the perspective of detection of the water bodies that do not comply with a good status, using a 2 × 2 contingency table for dichotomous events [35]. The median variation of nitrate concentration in

the main fluvial course of the Júcar and Turia rivers is presented, and the pollution sources are identified. This study provides a comprehensive analysis considering most of the elements that affect the contribution of nitrates to SW bodies in the Júcar RBD. Understanding how the SW–GW interactions influence the nitrates concentration is critical to manage the conjunctive water use of SW and GW. In addition, the results will allow the identification of key points to focus on mitigation measures and will be used in hydrological planning for the 2022–2027 cycle.

#### **2. Materials and Methods**

#### *2.1. Study Location*

The Júcar RBD is located in the east of the Iberian Peninsula (Spain) on the Mediterranean side, with an area of 42,735 km2. The hydrographic network is made up of nine water resource systems (WRS or system) that drain into the Mediterranean Sea, and are divided into 303 river water bodies (SW-river) (Figure 1a). The WRS of the Júcar and Turia rivers cover nearly 69% of the total area of the district. The climate varies from humid to semi-arid, with the presence of droughts and a concentration of approximately half of the annual rainfall in autumn on the coastal strip [33]. The average annual pluvial precipitation is 485 mm/year, with a spatial range of 339 mm/year in the Vinalopó-Alacantí (hereafter Vinalopó), and 743 mm/year in Marina Alta.

The total contribution (4070 hm3/year) of the Júcar RBD fluvial network comes mostly from GW runoff (2983 hm3/year), hence the importance of GW in this district [27]. This can be explained due to the surface area covered by GW bodies (40,822 km2), 72% of which are permeable. The predominant material in 90% of the district geological formations is carbonated, with substantial subterranean drainage. However, quaternary detrital formations predominate in the coastal plains of the area, which contributes to pollution problems due to the lower rate of transportation [36]. SW–GW interaction in the SW rivers is classified as follows: 78% receives discharges from the aquifer, considered as gaining stream; 18% are SW rivers where the river infiltrates resources into the aquifer, considered as losing; the remaining are considered as variable, where one situation or another occurs depending on the time of the year. A detailed description of the SW–GW interaction in the Jícar RBD can be found in the Ref. [37].

The land in the Júcar RBD is occupied by 49% of forest areas and open spaces, and agriculture represents 36% of land use, where 3% are artificial surfaces and 12% are wetland and water bodies (Figure 1b) [38]. Agriculture is the activity with the highest water resource requirement (80% of total demand) and the third most important economic activity in the district [39].

The Pressure Inventory of the Júcar Hydrological Plan (HP) [40] reports that 63% of the SW rivers are under significant pressure from organic, urban, and landfill discharges. The pressure of diffuse pollution by land use in which large areas are found in irrigation crops, urban areas, and also livestock, affect 60% of SW bodies. On the other hand, aquifers with good nitrate status (NO3 − < 50 mg/L) represent 77% of all GW bodies, while 33% are impacted GW bodies. Pollution problems in the rivers and aquifers are located along the coastline and of the adjacent inland strip [29].

Characteristics of the Júcar RBD were collected from the following sources: land use (CORINE Land Cover System 2018); geology map (Spanish Geological Survey lithographic map); 100 × 100 m2 digital elevation model (Spanish Army Geographic Centre); water hydrographic network and water demands (Water Information System for the Júcar RBD, "SIA Júcar" in Spanish, Available online: aps.chj.es/siajucar/, accessed on 26 March 2021); and identification of losing and gaining rivers in the Júcar RBD (Geological and Mining Institute of Spain; [37,41]).

**Figure 1.** Water resource systems in the Júcar RBD, surface water bodies and water quality gauging stations (**a**) and land use map (**b**). SW rivers: surface water bodies with river category; SW-QGS: surface water quality gauging stations.
