Karst Aquifers - Hydrogeology: Objectives, Methods, Applications - page 56

• the unsaturated zone in the strict sense primarily allows fl ow paths
with a strong vertical component;
• in the epikarst, water circulates primarily horizontally, due to the
permeability contrast between the epikarst and the unsaturated
zone;
• between low water and fl ood periods, the piezometric inversions
between passageways and the surrounding rock change the direction
of fl ux exchanges (Figure 29): in low water periods, the passages drain
the surrounding rock. In fl ood periods, the main passage, accepts
infl ows of infi ltrated water and recharges the adjoining rock, which
water infi ltrating vertically is otherwise slower to reach.
Figure 29 Hydraulic behavior of karst: drainage-fracture exchanges.
3.2 Qualitative consequences
The qualitative consequences of this structure are that:
• the hazard of pollution does not carry the same risk everywhere,
depending on whether it is located on the draining system or on a
poorly transmissive area;
• the environment being very heterogeneous and very open in terms of
drainage passageways, there is no mechanical fi ltration, and, in fl ood,
turbidity from the upstream watershed, as well as from sediment
stirred up by turbulent fl ow in the conduits, passes downstream out
to the emergence point. This turbidity can serve as a vehicle for heavy
metals or adsorbed organic micropollutants, as well as for bacteria and
viruses;
• the residence time of water in the network of passages, often less than
a week, does not allow for a signifi cant decrease in the bacterial load
derived from the upstream basin (swallow holes, sewage dumped
directly into the subterranean environment).