Nitrate leaching into groundwater: the role of soil physical environment
and agricultural management in controlling nitrate diffuse pollution
M. Arauzo, 1* M. Valladolid, 2 F.X. Fernández-Santos 1
1 Departamento de Contaminación Ambiental, Instituto de Ciencias Agrarias CSIC, Serrano 115
dpdo., 28006 Madrid, Spain 2 Departamento de Biodiversidad y Ecología Evolutiva, Museo
Nacional de Ciencias Naturales CSIC, José Gutiérrez Abascal 2, 28006 Madrid, Spain.
* Presenting author contact: mercedes.arauzo@csic.es
The reduction and prevention of water pollution caused by nitrate from agricultural sources in the
European Union was addressed by the Nitrates Directive (91/676/EEC). It establishes that both
surface freshwater and groundwater should be considered affected by nitrate pollution when they
contain more than 50 mg L-1 of nitrate. The Directive also defines Nitrate Vulnerable Zones (NVZs)
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 recover water
quality. However, recent research studies have shown that inadequate NVZs designations, which
are often too restrictive, are giving rise to results which fall short of expectations.
Alluvial areas are particularly vulnerable to nitrate leaching from agriculture, with a subsequent
impact on the quality of alluvial aquifers.
The Oja Alluvial Aquifer (Oja-Tirón catchment, Spain) constitutes a good example of a mass of
water chronically affected by nitrate pollution from agricultural sources. In accordance with the
requirements of the EU Nitrates Directive, a surface area of 94.3 km2 has been declared a Nitrate
Vulnerable Zone (NVZ), corresponding to 39% of the alluvial area of the Oja-Tirón catchment.
In the present study, an analysis of the influence of the physical environment and soil usage on the
vertical processes of nitrogen transport in the vadose zone of three agricultural parcels (located in
the middle and lower sections of the Oja Alluvial Aquifer) was conducted over 40 consecutive
months.
A combination of factors associated with the physical substrate (soil texture, infiltration rate, field
capacity) and the agricultural use (ecological and conventional production) determined the nitrate
leaching load from each parcel. The parcels located in the middle section of the aquifer (Sites 1 and
2, with soils of a stony texture) generated more intensive drainage and higher nitrate leaching than
the parcel located in the lower section (Site 3, with soil of a fine texture), where there was no
drainage, nor nitrate leaching. Paradoxically, the Site 3 parcel was the only one found within the
NVZ designated perimeter.
The most abundant drainages, the highest total-N concentrations in drainage water and the largest
nitrate leaching loads took place at Site 2, managed in accordance with a conventional production
model. The lesser impact of the ecological production at Site 1 gave rise to results which were much
more sustainable despite the higher degree of intrinsic vulnerability.
It is concluded that the occurrence of drainage events in the Oja alluvial areas depended, in the first
instance, on the degree of vulnerability associated with the textural characteristics of the vadose
zone. When the degree of vulnerability was low (the case of Site 3), no drainage events were
recorded and there was no risk of nitrate leaching. When the degree of vulnerability associated with
the textural characteristics of the substrate was high, drainage was notably affected by the
agricultural use of the land (the cases of Sites 1 and 2), which also affected the total-N contents in
the drainage water and the nitrate leaching load.
These results lead us to question the suitability of the present demarcation of the NVZ in the Oja
alluvial area and add support to extend the NVZ to cover the entire alluvial area. This information is
essential to ensure that the technical and economic resources of the Action Programmes
concentrate on those areas with the highest risk of nitrate leaching.
Extracted from Arauzo et al., 2013. Drainage and N-leaching in alluvial soils under agricultural land uses: Implications for
the implementation of the EU Nitrates Directive. Agriculture, Ecosystems & Environment, in press.