Agricultural activities have been recognized as one of the main causes of groundwater degradation worldwide. In the European Union (EU), the Nitrate Directive (Directive 91/676/CEE) set the threshold of nitrate concentration to declare water bodies as affected at 50 mg/l. After that, the states should designate as Nitrate Vulnerable Zones (NVZ) those areas contributing to the pollution. However, several researches have called into doubt the efficiency of this protocol, due to the way that NVZ are designated in some countries, the voluntary basis in the application of the good agricultural practices and the ambiguous interpretation of the action programmes.
In this study, the effectivity of one of the first NVZ designated in order to control nitrate pollution in Spain is assessed. Gallocanta Groundwater Body (GGB) is located within the Gallocanta Lagoon catchment (540 km2) an endorheic basin located in north-east Spain and subject to intense agricultural activity. Due to the high levels of nitrate, a significant section of GGB was declared as affected by nitrate pollution by the water authority. Afterwards, the regional government designated part of the GGB as a NVZ in 1999. This NVZ has been modified in a couple of occasions, being the current extension 208 km2. For this analysis, we compiled available water quality data from 1979 to 2018 (water authority monitoring programs) from 74 stations across the study area. Statistical tests were applied to the 29 stations with at least ten measurements during the study period. In addition, we obtained a proxy of the whole GGB through annual interpolations.
Before 1999, 20% of the stations presented increasing trends (p<0.05), and no decreasing trends were detected. The proportion of stations with increasing trends was 16% after the NVZ implementation, and 21% of the stations presented decreasing trends (p<0.05). Our estimation for the whole GGB showed an increasing trend (p=0.018) in nitrate throughout the study period. Prior to NVZ designation, the estimated increasing trend was 0.3 mg/l per year. After the designation, the estimation is negative (-0.51 mg/l per year), although this value is not statistically significant (p=0.48). Considering actual values rather than trends, we found that 60% of the stations presented nitrate concentrations higher than 50 mg/l in 1999 whereas in 2018, 57% of the stations remained above that threshold.
Even though the rate of increasing nitrate concentration has shown a slight decrease, nitrate concentrations are still high in the area and are expected to remain high in the near future. Our observations are in line with previous studies and help to highlight the inefficiency of the current designation methods for NVZ and/or action programmes implemented to reduce nitrate pollution and comply with the environmental goals of the Water Framework Directive.