Detection of Trends in Groundwater Quality: An Overview of Methods Developed in Europe
TNO Geological Survey of the Netherlands
Recent EU legislation, especially the Nitrates Directive (1991) and the Groundwater Directive (GWD, 2006), demand for the analysis of trends in groundwater quality. In fact, the detection of trends and the demonstration of trend reversal is one of the pillars of the new GWD. Therefore, trend approaches have been developed which enable us to observe changes in groundwater quality with time and to evaluate the effects of action programs which aim to reduce pollutant inputs into groundwater. The trend reversal obligation of the GWD requires that any significant and sustained upward trend will need to be reversed when reaching 75 percent of the values of EU-wide groundwater quality standards and/or threshold values, and such trend reversal has to be achieved through establishing programs of measures. We will present an overview of trend approaches being developed in some EU countries and discuss the monitoring objectives, the technical procedures that were applied and the effectiveness of the approaches. The overview is partly based on the European research project Aquaterra-Trend2 which tested trend analysis techniques at a wide range of hydrogeological conditions, including unconsolidated lowland deposits, chalk aquifers and fractured aquifers with a thick unsaturated zone (a). The presentation compares approaches of different monitoring and trend analyses techniques, focusing on 1. trends in upper groundwater under agricultural lands; 2. trends in national scale monitoring networks with observation screens at specific depths; 3. trends in spring waters and at abstraction sites and 4. the interaction between trends in groundwater and trends in receiving surface waters. A trend was defined as a change in groundwater quality over a specific period in time, over a given region, which is related to land use or water quality management. Trend analysis techniques aim to reduce the variability which is not related to the anthropogenic changes. Therefore, trend detection becomes more efficient when spatial and temporal variability are reduced by taking into account the physical and chemical temporal characteristics of the body of groundwater, including flow conditions, recharge rates and percolation times. The travel time to a monitoring screen and the travel time distribution of pumped wells or springs appeared to be a key factor in effectively demonstrating trends and trend reversal, which is why age dating appeared to be an effective tool for trend detection. Travel time distributions also appeared to be a key factor in relating trends in groundwater with corresponding temporal changes in surface water quality. The presentation will address modeling and monitoring approaches to unravel the dynamics of groundwater-surface water interaction and show how the slow movement of pollution fronts in the subsurface affected trends in surface water quality in the past and may continue to do so in the coming decades.