PROBLEMS - Hydrogeology: Objectives, Methods, Applications - page 334

The small peak in the F11 borehole corresponds to the leaching of
nitrates in the soil by natural precipitation; the large F11 peak and the
single F1 peak to leaching by irrigation water returning to the aquifer. The
natural peak is invisible on the F1, which has a higher level of background
noise than the peak.
The concentration gradient observed on the catchment fi eld can be
explained by the dilution of the water circulating below irrigated plots by
less charged water from the river.
Problem #8—Tracer tests
In the Chalk of the Pays de Caux (Lacroix et al., 2000), a tracer test was done
in the Bébec swallow hole. The Norville spring, located 4 km away as the
crow fl ies, enabled the monitoring of the restitution of the upper curve in
Figure 8: peak at 130 µg·L -1 , 2 days after injection. A nearby well, drilled into
18 m of Seine alluvium resting on 15 m of chalk, gave the lower restitution
curve, with a peak of 10 µg·L -1 , 2 and a half days after injection.
Question 1: What conclusions can you draw from these results on the
functioning of the environment?
Question 2: What conclusions can you draw on the vulnerability of the
two catchment points?
Figure 9 Restitution curves for the tracer test from the Bébec swallow hole to the resurgence
point and to the Norville well (from Lacroix et al ., 2000).