Delineating spring recharge areas in a fractured sandstone aquifer (Luxembourg) based on pesticide mass balance

A simple method to delineate the recharge areas of
a series of springs draining a fractured aquifer is presented.
Instead of solving the flow and transport equations, the
delineation is reformulated as a mass balance problem
assigning arable land in proportion to the pesticide mass
discharged annually in a spring at minimum total transport
cost. The approach was applied to the Luxembourg Sandstone,
a fractured-rock aquifer supplying half of the drinking water
for Luxembourg, using the herbicide atrazine. Predictions of
the recharge areas were most robust in situations of strong
competition by neighbouring springs while the catchment
boundaries for isolated springs were extremely sensitive to the
parameter controlling flow direction. Validation using a
different pesticide showed the best agreement with the simplest
model used, whereas using historical crop-rotation data and
spatially distributed soil-leaching data did not improve predictions.
The whole approach presents the advantage of
integrating objectively information on land use and pesticide
concentration in spring water into the delineation of groundwater
recharge zones in a fractured-rock aquifer.