Measures to protect drinking-water wells near rivers from hydraulic engineering operations in peri-alpine flood-plains

摘要

In alpine and peri-alpine flood plains, rivers can lose water to the ground water (infiltrating conditions), or gain water from it (exfiltrating conditions). Since sediments within these flood plains are highly permeable, drinking water wells near infiltrating rivers are often highly efficient. To reduce flood risks, engineers couple hydraulic flood protection measures with an enhancement of lateral connectivity. Eventually, the residence time of the infiltrating ground water is reduced and the fraction of infiltrated water rises. Floods lead to the breakthrough of freshly infiltrated water. This can lead to groundwater contamination, if the river is loaded with wastewater. Where measures of hydraulic engineering are necessary, drinking-water wells must therefore be protected from contamination. We discuss three cases of contamination risk: i) low risk under exfiltrating conditions or where river and ground water interaction is weak, ii) increased risk during or after hydraulic operations under infiltration conditions; and iii) generally high risk at wells with low residence times and high fractions of infiltration water. We suggest that the risk is reduced when a thick unsaturated zone separates the river from the well (> 10 m), compared to direct water loss through saturated media. For wells at risk, we propose technical protection measures to reduce the danger.