Boletín de la RSEHN. Sección Geológica

El procedimiento LU-IV es una nueva herramienta que permite cartografiar la vulnerabilidad específica de las aguas subterráneas a la contaminación por nitrato y delimitar las Zonas Vulnerables a la Contaminación por Nitrato (ZVN; Directiva 91/676/CEE). Combina un mapa de vulnerabilidad intrínseca (a partir del índice IV, que estima los riesgos asociados a parámetros del medio físico) y un mapa de riesgos asociados a los usos del suelo (LU). El procedimiento ha sido aplicado al territorio de la Comunidad de La Rioja (España).

El mapa de vulnerabilidad específica reveló que una superficie de 483 km2 presentaba niveles de vulnerabilidad de alto a extremo, mientras que otros 527 km2 mostraban niveles medios. Centrándonos en los territorios aluviales (los más vulnerables, atendiendo a factores del medio físico), el 53,3% presentó una vulnerabilidad específica de alta a extrema, el 23,8% niveles medios y el 22,9% niveles bajos o inapreciables. De estos resultados se desprende la necesidad de una protección integral de la totalidad de los territorios aluviales de La Rioja mediante la ampliación de las designaciones de ZVN. Las zonas que presentaron niveles medios de vulnerabilidad en territorios no aluviales también deberían ser susceptibles de seguimiento, dada su amplia extensión.

Nitrate pollution from diffuse sources is the main cause of groundwater quality degradation. The Nitrates Directive of the European Union (EU; 91/676/EEC) establishes that water resources should be considered affected by nitrate pollution when their nitrate contents exceed 50 mg L-1. The Directive defines Nitrate Vulnerable Zones (NVZ) as the areas of land draining into waters affected by nitrate pollution. In these areas farmers are required to comply with the measures laid out in Action Programmes designed to improve water quality. However, one major obstacle to a more efficient implementation of EU environmental policies for nitrate pollution control is the lack of consensus on the criteria to be used for designating NVZ. In this regard, the use of inappropriate criteria for the designations can seriously reduce the effectiveness of the Action Programmes implemented in the NVZ.

Bearing all this in mind, the new LU-IV procedure (Arauzo, 2017), for assessing and mapping groundwater vulnerability to nitrate pollution from diffuse sources, was applied to the territory of the Community of La Rioja (northern Spain; Fig. 1) in which, several aquifers (Table I) are chronically affected by nitrate pollution. More specifically, the study had the following objectives: (1) to analyse the distribution of nitrate contents in the 14 aquifers that, totally or partly, belong to the Community of La Rioja (Table I; Fig. 1), (2) to generate the thematic maps of the intrinsic groundwater vulnerability and the specific groundwater vulnerability to nitrate pollution, using the LU-IV procedure, and (3) to define the NVZ in the study area.

The LU-IV procedure combines a map of intrinsic vulnerability (based on the IV index) with a map of the risks associated with different land uses (using the Over tool from logical toolset of Spatial Analyst Tools in ArcGIS 10.3). The most basic formulation of the IV index uses four environmental parameters that are commonly related to intrinsic groundwater vulnerability: lithology of the vadose zone, depth to groundwater table, topography slope and annual precipitation (Table II). This method stands out as it meets the following requirements: (1) it uses readily available parameters that provide enough data to feed the model, (2) it excludes redundant parameters, (3) it avoids the need to assign insufficiently contrasted weights to parameters, (4) it assesses the entire catchment area that potentially drains N-polluted waters into the receptor aquifer, (5) it is implementable within a GIS, and (6) it provides a multi-scale representation (Arauzo, 2017).

The spatial modelling of nitrate contents in the 14 aquifers under study (Fig. 2) showed large nitrate polluted areas in all of the alluvial aquifers, which exhibited the worst conditions. It was observed that the alluvial aquifers located in first- and second-order basins (aquifers no 044, 045 and 047) showed significant negative correlations between nitrate concentration and water table elevation (Table I). This result was consistent with the existence of stagnant groundwater zones in the lower sections of these aquifers (Fig. 2), where nitrates tend to accumulate.

The map of the intrinsic groundwater vulnerability (based the IV index; Fig. 3) showed high to extreme levels of intrinsic vulnerability across 12.3% of the territory of La Rioja (617 km2) and moderate levels in 21.6 % (1.084 km2). All the alluvial areas were within the range of high to extreme intrinsic vulnerability. The map of the specific groundwater vulnerability to nitrate pollution (according the LU-IV procedure; Fig. 4) showed high to extreme levels of specific vulnerability across 483 km2 and moderate levels in 527 km2. To date, the Community of La Rioja has officially designated a total of three NVZ within the study area, jointly covering a surface of 124 km2 (Fig. 1).

It was estimated that 53.3% of the alluvial areas in La Rioja exhibited high to extreme vulnerability to nitrate pollution, while 23.8% was subject to moderate risk (Fig. 4). This high vulnerability can be attributed to the combined effect of some typically alluvial characteristics, including: the shallow water table in the alluvial areas, the interconnections between rivers and alluvial aquifers, the permeability of the alluvial deposits and the typically high concentration of irrigated agriculture on the lower terraces and floodplains along river banks.

These results highlight the need to redefine what have been the officially designed NVZ in the study area until now. First of all, it seems necessary to ensure the effective protection of the alluvial aquifers, by extending the designations of the NVZ to all the alluvial areas (this high level of protection should be generalized for all the alluvial areas in Europe). On the other hand, territories with moderate vulnerability to nitrate pollution (beyond the alluvial areas) should also be monitored and controlled, given their wide extension. In other words, if we want to effectively implement Action Programmes designed to restore and protect water quality, in line with Directive 91/676/EEC, it is first necessary to review and redefine the current criteria for the designation of NVZ.