Application of a groundwater quality index as an assessment and communication tool in agro-environmental policies – Two Portuguese case studies

SummaryA simple methodology based on multivariate analysis is developed to create a groundwater quality index (GWQI) and a composition index (GWCI), with the aim of monitoring the joint influence of agriculture on several key parameters of groundwater chemistry and potability. The methodology is based on the definition of two standard water samples of high and low quality that, together with the actual data, are run through a statistical algorithm known as correspondence factor analysis. The applicability of the constructed indices as an assessment and communication tool is evaluated in two case studies in the south of Portugal. Index maps are created, providing a comprehensive picture of the contamination problem and easily interpretable for people outside the scientific domain. Such maps could be particularly useful in agro-environmental policies such as Nitrates Directive 91/676/EEC. Drawn up in order to reduce the environmental load of nutrients from agriculture, its implementation continues to be very problematic in most EU member states. In the case studies, the GWQI maps reveal that groundwater quality in the upper aquifers is extremely low, with an almost complete absence of potable water. However, the impact of agricultural activity on the groundwater composition shows a large spatial variability, which is accurately depicted by the GWCI maps and mainly related to crop type and aquifer lithology. In addition, contamination-freshening sequences occur and are displaced by groundwater flow, partly affected by faults that form either preferential flow paths or flow barriers.