Groundwater management: effectiveness of mitigation measures in nitrate vulnerable zones - a Portuguese case study

Abstract

Groundwater vulnerability depends on hydrogeological characteristics, land use dynamics and pollutant sources. In Europe, agriculture is the main source of diffuse groundwater pollution. The over-application of nitrogen (N) soil inputs promotes nitrate leaching to surface and groundwater and associated impacts on human health and biodiversity. Mitigation programs have been implemented across the Europe Union (EU) to reduce N loads to water bodies. The EU Nitrates Directive (1991/676/EEC) aims to reduce water nitrate pollution associated to agriculture. The reduction implies the implementation of Good Agricultural Practices and National Action Programs with mandatory measures for the Nitrate Vulnerable Zones (NVZ). The Esposende-Vila do Conde is one of the nine Portuguese nitrate vulnerable zones. The application of organic and chemical fertilizers, especially on intensive agricultural farms, without the implementation of good agricultural practices and effluent management, will promote an increase in groundwater nitrate concentration. A total of 1728 groundwater sampling points were collected to determine nitrate contents, from 2007 to 2020. A space-time database including farmer questionnaires, field data and groundwater nitrate contents were prepared. The groundwater points located in artificialized territories and agricultural areas show the highest nitrate concentration. This situation is mainly due to a decrease of surface water recharge, associated to the increase of pollutant sources, as well as inadequate agricultural practices. The areas with considerable agricultural productivity and located away from surface water show an increase in nitrate concentration. The occurrence of total organic N on the soil is not an influence of groundwater nitrate contents and there is a casual relation between groundwater nitrate contents and mineral N. The methodology allows the identification of higher groundwater nitrate content, as well as pressure, state, and response indicators. However, despite the global decrease in nitrate contents, the measures remain insufficient and appropriate mitigation will be required to reduce groundwater contamination.