Relating wellfield drawdown and water quality to aquifer sustainability – A method for assessing safe groundwater abstraction

Planning for sustainable groundwater abstraction requires realistic and reliable assessments of groundwater drawdown in aquifers as well as impacted groundwater-dependent terrestrial and aquatic ecosystems. Present-day impact assessments allocate water for the environment and ecosystems in the form of environmental groundwater requirements (EGWRs). However, securing sustainable groundwater abstraction also requires stable groundwater quality for consumers and receptors (e.g. streams and groundwater-dependent ecosystems). We suggest dividing EGWR into two parts, namely EGWR_flow, related to impacts on surface water, and EGWR_wq, related to impacts on groundwater quality. We then propose a method for modelling maximum safe groundwater abstraction rates based on a relationship between water quality and piezometric heads. The method provides estimations of sustainable abstraction rates that secure stable water quality and maintain water security. Using hydrological modelling, we estimated spatially differentiated EGWR_wq, for Zealand, Denmark, based on three different conditioned drawdowns of 3, 6 and 9 m. We found that depending on the boundaries and the different conditioned drawdowns EGWR_wq poses a 2.5–83% share of the groundwater recharge. We found that a redistribution of water abstraction would make 27 million m³ available compared with actual abstraction, dependent on scale, while keeping within a conditioned drawdown of 3 m. Our results demonstrate how conditioned drawdown can be implemented with the objectives of evaluating actual abstraction and securing stable water quality, hence comprising a step towards more reliable and relevant groundwater impact assessments.