River restoration within a well capture zone, a pilot study

The study site, located in the groundwater recharge and production area Lange Erlen near Basel, Switzerland, is a major contributor to the city's drinking water and also serves as recreational area to the population of Basel. Because of its function as a recreational area there are initiatives to restore the channelized river to more natural conditions. These initiatives might conflict with the requirements of groundwater protection, especially during flood events. Therefore, a river section of 600 m in the vicinity of an unused and disconnected drinking water well was restored to study changes in the groundwater flow regime depending on hydrologic variations, water supply operation data, progress of river restoration, and subsurface heterogeneity. Two different groundwater models are used to simulate the well capture zone located near the infiltrating river. A deterministic, large-scaled groundwater model (1.8 x 1.2 km) is used to simulate the average behavior of groundwater flow and advective transport. It is also used to assign the boundary conditions for a small-scaled groundwater model (550 x 400 m) which relies on stochastically generated aquifer properties based on site-specific drill core and georadar data. The stochastic approach in the small-scaled groundwater model does not lead to a clearly defined well capture zone, but to a plane representation of the probability of a point on the ground surface belonging to the capture zone. The results of the groundwater models are compared with data from two tracer experiments and with physical, chemical, and microbiological data, sampled in monitoring wells between the river and the drinking water well. The groundwater models document significant variations regarding the dimension of the well capture zone depending on changing boundary conditions and on the variability of the hydraulic aquifer properties. In particular, at the small scale (10s to a few 100s of meters), knowledge of the subsurface heterogeneity is important to evaluate transport times and distances of microorganisms from the infiltrating river or the riverbank to the drinking water well. The data from the monitoring wells show that the chemical and microbiological processes predominantly occur in the hyporheic interstitial zone and the riverbank within a range of a few meters up to a few 10s of meters from the river. The methods presented here can be used to define and evaluate groundwater protection zones in heterogeneous aquifers associated with infiltration from rivers under changing boundary conditions, and under the uncertainty of subsurface heterogeneity, that is, for risk estimation of changing groundwater quality and site-specific evaluation of operational alternatives associated with river restoration.