Spatio-temporal dynamics of nitrogen in river-alluvial aquifer systems affected by diffuse pollution from agricultural sources: Implications for the implementation of the Nitrates Directive

Abstract

Reducing nitrate pollution from diffuse agricultural sources is the major environmental challenge in the two adjacent catchments of the Oja–Tirón and Zamaca rivers (La Rioja and Castilla y León, northern Spain). For this reason, part of their territory was designated a Nitrate Vulnerable Zone (NVZ) according to the Nitrates Directive. The Oja Alluvial Aquifer, the Tirón Alluvial Aquifer and their associated rivers are particularly vulnerable to nitrogen pollution due to the shallow water table, the high permeability of alluvial deposits, interconnections between the alluvial aquifers and surface waters and pressures from agriculture. To this end, nine sampling campaigns, organised on a semi-annual basis and focused on the rivers and alluvial aquifers of the two catchments, were carried out from April 2005 to April 2009. The main objectives of the study were: (1) to investigate the chemical forms of nitrogen in river-alluvial aquifer systems of the Oja–Tirón and Zamaca catchments, (2) to improve our understanding of the spatio-temporal patterns of nitrogen distribution in the alluvial aquifers and associated rivers by integrating hydrochemical data and hydrogeological and environmental parameters, (3) to estimate the amount of nitrogen exported from the rivers and alluvial aquifers to the River Ebro, and (4) to evaluate the suitability of the current method of designating NVZs in the area. High groundwater flow velocities in the upper alluvial zones favoured the advective transport of nitrate and generated a dilution effect. In these areas, inter-annual variations in nitrate concentrations were observed related to precipitation and N-input from agriculture. However, low flow velocities favoured processes of accumulation in the lower alluvial zones. Our results demonstrated that the entire alluvial surface was highly vulnerable, according to dynamics of the nitrogen in the river-alluvial aquifer systems being studied. The amount of nitrogen exported from these river-alluvial aquifer systems to the River Ebro was estimated at 2.4 ± 0.2 kt year−1. Findings from this investigation highlight the need to include the alluvial area corresponding to the Tirón aquifer as a NVZ, particularly as the Tirón sub-catchment provides more than half of the nitrogen exported from the River Tirón to the River Ebro. Based in these results, at least the entire alluvial surface in the study area should be considered a NVZ in order to address the recovery of water quality.