Understanding the impacts of groundwater abstraction through long-term trends in water quality

It is vital to understand long-term trends in water quality parameters when assessing the sustainability of groundwater abstraction. Withdrawal-to-availability analysis is still used widely in groundwater management considering quantities and utilization rates based on in- and outflows. In this study, we took a step further than the water balance approach and carried out a detailed investigation of trends in long-term time series of groundwater quality, in order to analyze the sustainability of groundwater abstraction. We assessed trends, links, and patterns in abstraction, potentiometric surface, and water quality parameters for 28 well fields around Copenhagen, Denmark. Groundwater monitoring data from 1900 until 2014 were investigated for each well field. During this period, the well fields experienced a 2-14 m decrease in the nearby potentiometric surface compared to the first-or pre-pumping-potentiometric surface recordings. Sulfate concentrations increased in 25 out of 27 well fields after the maximum abstraction period, compared to the earliest water quality measurements. The results indicate that in the 1980s, when water consumption (abstraction) and drawdown were at their highest, water abstraction caused a steady increase in sulfate and calcium, which we consider unsustainable. In contradiction, the abstraction in 24 well fields show almost steady sulfate levels in the aquifer after decreased water consumption since 1995. Only four well fields showed more than a 20 mg/L increase in sulfate concentration, which indicates that the recent abstraction does not interfere with sulfate levels in the aquifer. Our method and results show how long-term water quality trends can support the management of aquifer exploitation and evaluate sustainability on the well field scale.