Interpretation of a tracer test at Brévilles after 2 years of monitoring

To understand and predict the behaviour and transfer of diffuse contamination, a
small catchment is intensively studied in the vicinity of Paris. A multi tracer test
involving a new technique, the FVPDM method (Finite Volume Point Dilution
Method - Brouyère et al., 2008) has been performed in natural flow conditions.
Injections of four different tracers (uranine, sulforhodamine B, lithium chloride
and potassium iodide) took place in four piezometers involving different areas of
the aquifer and different depth. This tracer test follows a former test briefly
described. A particularly long monitoring (Nov-2005 to Feb-2008) demonstrates
the existence of several transport velocities within the sandy layer, which seems
linked to the decrease of hydraulic conductivity with depth. Absence of recovery
of two tracers confirms the results of the former test and identifies spatial
heterogeneities probably due to the geological structure. The new insight and
parameter quantification brought by interpretation of these tests contributes to a
better characterization of the saturated zone and shall be exploited through
modelling for transport predictions of pesticides.