Trends in groundwater quality in relation to groundwater age

Abstract

Groundwater is a valuable natural resource and as such should be protected from chemical pollution. Because of the long travel times of pollutants through groundwater bodies, early detection of groundwater quality deterioration is necessary to efficiently protect groundwater bodies. The aim of this work was to develop and improve tools to detect trends in groundwater quality considering the reactive transport of pollutants from the ground surface to the monitoring screen. The study area of the research presented in this thesis was the province of Noord-Brabant, the Netherlands. Noord-Brabant is one of the areas in Europe which is most affected by agricultural pollution. Time-series from selected wells of the groundwater quality monitoring network were used in this thesis, in conjunction with groundwater ages determined with 3H/3He. Trend reversal of groundwater quality was demonstrated by relating concentrations of pollutants in groundwater to the time of recharge determined with 3H/3He. The quality of recharging groundwater deteriorated before 1980 but has been improving since 1990. Three types of solute specific trends were distinguished by a combination of reactive transport modeling and the analysis of concentrations related to groundwater age. These are the result of: (1) the anthropogenic trends in historical concentrations in recharging groundwater unaltered during conservative transport through the groundwater body (e.g. chloride), (2) anthropogenic trends in recharge concentrations altered by reactions during transport (e.g. retardation of potassium), and (3) geochemical reactions in the subsurface that are triggered by anthropogenic changes in the recharge concentrations of other solutes (e.g. release of sulfate, iron, and arsenic by nitrate-induced pyrite oxidation). Comparison of trend detection methods showed that there is no single optimal method to detect trends in groundwater quality across widely differing catchments. The selection of the method should firstly be made on the basis of the specific goals of the study, and secondly on the system under study, and the available resources. For trend detection in groundwater quality, the most important difference between groundwater bodies is whether the characteristics of the subsurface or the monitoring system causes mixing of groundwater with different travel times. Noble gas concentrations measured in groundwater samples, required for 3H/3He dating, showed that the production of nitrogen gas in groundwater by nitrate reduction and pyrite oxidation, leads to re-partitioning of noble gases between water and gas phase, thereby distorting groundwater dating with 3H/3He. Groundwater travel times can be estimated from degassed samples, if the total dissolved gas pressure (TDG) is used to estimate the depth and timing of degassing. The uncertainty of degassed 3H/3He ages, assessed by two-phase flow and transport simulations, had a standard deviation of 6 years with a bias of 2 years. As CFCs appear to be subject to significant degradation in anoxic groundwater and SF6 is highly susceptible to degassing, 3H/3He appears to be the most reliable method to date degassed groundwater in the anoxic parts of an aquifer. The main conclusion of this thesis is that an accurate estimate of groundwater age is of utmost importance in the detection of trends in groundwater quality.