*3.4. Point and Diffuse Pollution Sources*

The spatial analysis of pollution sources showed that intensive agriculture downstream of the district generates a high diffuse load and pollution in rivers and aquifers (Figure 7b). In general, citrus orchards and rice crops with irrigation are the main sources of diffuse pollution, as irrigated agriculture generates the most leaching compared to non-irrigated crops [36,50]. Nitrate surplus in soil for citrus orchards remains constant at an average of 217 KgN/ha/year from the years 2007–2015 [42]; however, nitrate pollution has been intensifying. The highest point loads are generated in the WWTP of urban areas of Almassora (10,000–50,000 inhabitants), Albacete, Valencia, Alcoi, and Elche (50,000–100,000 inhabitants) (Figure 7a), most of them located downstream of the district, where it is most overexploited. Nevertheless, the average load generated by the diffuse source is about 100 times greater than the point source, so the impact of the point sources on the district is comparatively low.

**Figure 7.** Diffuse (DP) and point (PP) pollution load (**a**). Nitrate status in the river-type surface water bodies (SW rivers) and spatial distribution of land uses (**b**).

Agricultural returns represent an important recharge in the water balance in the Júcar RBD [30]. The use of polluted aquifers to supply the main demands of the agricultural sector and the large amount of load discharged into rivers from irrigated crops explain the fact that locations with the highest nitrate pollution in the SW and GW are in irrigated agricultural areas. This is in agreement with previous studies in arid regions [54].

Different research papers in the Júcar RBD indicate that even if the rate of nitrogen fertilizers is reduced, leachate production remains high in areas irrigated with high nutrient concentration water [55–57]. However, the combined effect of the reduction in irrigation and nitrogen fertilization decreases nitrate leaching [58]. The source, quantity, and method of irrigation in conjunction with the fertilization plan have a major influence on the accumulation of nitrogen in the soil and the leachate generated [59–61].

SW rivers total loads are estimated at 2.39 KgN/ha/year (Table 4). Although the agricultural area covers 31% of the land use in the district, the pollution caused by diffuse load corresponds to 99% of the total load to rivers. Nevertheless, the total load obtained is lower compared to other basins in Europe with a similar percentage of agricultural land. For instance, in Portuguese basins with 44% of agricultural land, the estimated annual nitrate load average is 7.0 kgN/ha/year [10]; in the Sabor river basin (a tributary of the Duero River Basin, in the Iberian Peninsula), with 35% of the area occupied by agriculture, the nitrate load in the most critical areas is 4.26 kgN/ha/year [62]; and in the Danube River Basin, with 42% of agricultural land, the estimated average annual nitrate load is 6.14 kgN/ha/year [63].


**Table 4.** Nitrate balance in surface water bodies with river category in the Júcar RBD.

A nitrate load of 79 kgN/km2/year reaches the Mediterranean Sea. This load is lower than those obtained by Ludwig et al. [64] and Romero et al. [65] (233 kgN/km2/year between 1975 and 2000, and 100–200 kgN/km2/year between 2000 and 2010, respectively). Other studies around the world have assessed the discharge of nitrate into the sea. As representative examples: (i) Mitsch et al. [66] reported that in the Mississippi RB a load of 21.000 tN/year is generated, and about 1.600 tN/year (8%) reaches the Gulf (1990– 2000 period); (ii) the delivery from Danube RB to Black Sea was around 540–570 kg NO3 <sup>−</sup>/km2/year in the period 1995–2009 [63]; and (iii) nitrate loads delivered by the Po River to the Adriatic Sea in the period 2003–2007 were estimated at 86,295 tN/year [67].

Although several regulations have been implemented to reduce water resources nitrate pollution, the annual variation of the nitrate load in the SW rivers and nitrate discharges into the Mediterranean Sea in the Júcar RBD has remained constant from 1992 to 2017 (Figure 8a), which is in agreement with previous results obtained in other Mediterranean basins [64]. Nitrate loads have a similar behaviour to the streamflow in the basin (Figure 8a). This is because the most significant nitrate leaching events occur after periods of high rainfall, decreasing the mineral N in the soil, which is leached out [56,68].

Regarding seasonal variability in the SW rivers (Figure 8b), mean nitrate concentrations are low in the upstream and midstream without major differences between seasons. In contrast, a strong change in nitrate concentration was detected downstream. For instance, in winter, spring, and autumn 75% and 95% percentiles are in poor status. Compared to summer, the nitrate concentration increases 35%, 17%, and 16% in winter, spring, and autumn, respectively. As nitrate inputs are mainly from diffuse sources, rise of pollution takes place mainly in winter and spring, when water flows are high. This finding is consistent with the relationship between nitrate concentration and the rainfall reported by the Refs. [69,70], who studied the coastal region of the Júcar River, and also with other results previously reported in different basins [54,71–73]. The lower concentration in summer is influenced by the large number of dams that significantly modify river flows. Consequently, the main water sources in summer are dams and small channel discharges [74].

**Figure 8.** Annual load (tN/year), and discharge into the Mediterranean Sea in the Júcar RBD, and streamflow (hm3/year) (**a**). Seasonal nitrate concentration in the Júcar river Basin (**b**).

The integration of the SW–GW interactions in the hydrological planning of the river basins is of vital importance, since it allows for the identification of the main pressures, focuses actions to improve the status of water resources, and identifies sensitive areas to prioritize, in order to reach the environmental objectives of the WFD. Critical points were identified where further research is needed. For example, to support decision-making in the coastal zones of the basins where the most pollution is found, it is possible to measure the amount of groundwater used for irrigation and include in the fertilization plan the contribution of nutrients from the irrigation water, optimize soil management, and convert agricultural land to protection zones around the most critical rivers seeking to increase the buffer capacity of vegetation. On the other hand, in the smaller basins with a high contribution of pollution to Júcar RBD, it is possible to strengthen the monitoring network for nitrate concentrations, as well as to increase the nutrient gauging stations.
