Drainage and N-leaching in alluvial soils under agricultural land uses: Implications for the implementation of the EU Nitrates Directive

Alluvial areas are particularly vulnerable to drainage and N-leaching from agriculture, with a subsequent impact on the quality of alluvial aquifers. An analysis of the influence of the physical environment and soil usage on the vertical processes involved in nitrogen transport through the vadose zone of three different agricultural plots was conducted over 40 consecutive months. To achieve this, six monitoring stations were equipped with a Gee Passive Capillary Lysimeter, three FDR probes and a rain gauge and connected to a data-logger which recorded relevant data on an hourly basis throughout the experiment. The Gee Lysimeter is a relatively new device that allows the direct, continuous measurement of deep drainage in the field and also enables drainage water to be sampled for chemical analysis. The study area was located in the middle and lower sections of the Oja Alluvial Aquifer (Oja-Tirón catchment, Spain). This alluvial aquifer was selected because it 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 was declared a Nitrate Vulnerable Zone (NVZ), corresponding to 39% of the alluvial area of the Oja-Tirón catchment. A combination of factors associated with the physical substrate (soil texture, infiltration rate, field capacity) and the agricultural use (under conventional or ecological production systems) determined the final nitrogen leaching load from each plot. The plots located in the middle section of the aquifer (sites 1 and 2, with shallow, stony soils) generated more intensive drainage and higher N-leaching than the plot located in the lower section (Site 3, with a deep, fine textured soil), where there was no drainage, nor N-leaching. Paradoxically, the Site 3 plot was the only one found within the NVZ designated perimeter. It was also observed that when the degree of vulnerability associated with the textural characteristics of the substrate was high, drainage and N-leaching could be affected by the agricultural production model. These results support the proposal of to designate, as a general criterion, the entire alluvial area as a NVZ.