08 August 2022
Concern rises about groundwater pollution
Published online 19 May 2021
Contamination by short-lived pollutants is a larger problem than previously thought in areas where rapid flow occurs from the surface to the underlying groundwater.
The risk of degradable pollutants rapidly seeping into drinking water may be widely underestimated, according to a multi-institutional study.
Carbonate rock aquifers represent one of the most important sources of drinking water for nearly a quarter of the world’s population. The fast flow of water from the surface to the groundwater through cracks and fissures, often referred to as focused recharge, is known to transport surface-borne pollutants to the groundwater over time periods as short as weeks or days. That means even relatively short-lived pollutants can reach the groundwater before being degraded. The extent to which focused recharge impacts groundwater quality, however, had not been systematically assessed.
A group of international investigators, including Lhoussaine Bouchaou at Ibn Zohr University in Morocco, developed a model to quantify the risk of groundwater contamination by pollutants in the carbonate rock regions of Europe, North Africa and the Middle East.
“Most large-scale models to date have only considered slow or diffuse percolation in the soil,” says team leader, Andreas Hartmann, of the University of Freiburg, Germany. “Our carbonate rock model allows for a dynamic interplay of slow or diffuse and fast or focused recharge, which is a typical characteristic of weathered carbonate rock, or karst systems.”
The team compared the travel times of water from the surface to the subsurface with the degradation times of three pollutants — a veterinary antibiotic called salinomycin, a pesticide known as glyphosate, and the bacterial pathogen E. coli. They found that fast flow into the groundwater increases the risk of groundwater contamination with all three types of pollutants still intact, particularly in the Mediterranean region, where thin soils and large seasonal rainfall can promote rapid transport of pollutants into the groundwater.
Their findings suggest up to 50% of pollutants that have not yet degraded can still reach the groundwater through focused recharge. This is substantially higher than the figure of less than 1% estimated when only diffuse recharge is taken into account. A worst-case scenario simulation revealed that, with focused recharge, the level of glyphosate in groundwater can exceed the permitted value in Europe by up to 19 times.
The findings add weight to the argument that legal frameworks for water management and the use of fertilisers and pesticides need to be revised based on clear delineation of areas with a higher contamination risk.
Hydrogeologist, Andy Baker of the University of New South Wales, Australia, who was not involved in the study, says, “The use of a continental-scale groundwater recharge model allows the authors to identify regions with the greatest amount of rapid recharge… In this study it was the European Mediterranean, because its thin soils have limited water storage capacity. Hartmann’s modelling approach will help inform sustainable aquifer management.”
With a view to better inform future water governance, Hartmann says: “We are planning to use the hot spots of groundwater contamination risk identified by our model to help develop local and site-specific mitigation approaches.” His team is also working on a global karst hydrology model, which would allow them to compare groundwater contamination risk across all karst regions in the world.
Hartmann, A. et al. Risk of groundwater contamination widely underestimated because of fast flow into aquifers. PNAS 118(20), e2024492118 https://doi.org/10.1073/pnas.2024492118 (2021).