Recharge systems for protecting and enhancing groundwate

TOPIC2
Geochemistry during infiltration and flow 287
dence time of 10 days does not allow for a dilution of mobile, persistent toxic chemical contaminants. To this end,
restrictions in the zone of contribution (ZOC) around a well are introduced in the planning concept. Within the
ZOC of a well, many contaminating industrial activities are banned, which includes high agricultural fertilizer and
pesticide applications. Many wells near downwelling rivers do not meet with the requirements of the Swiss Law on
Water Protection. Their ground water has mixed residence times of < 10 days, and the percentage of hyporheic
ground water is too high for the dilution of contaminants.
GROUNDWATER FLOW FIELD NEAR DOWNWELLING RIVERS
Infiltration of river water into layered alluvial aquifer sediments of glaciofluvial origin is a three-dimensional (3-D)
process. Investigations in systems of downwelling rivers with well clusters of screened sections at different depths
have shown a stratification of at least three different types of ground water (Fig. 1): Hyporheic ground water stratifies
on the top of the saturated aquifer (Hoehn et al., 1983). The vertical component of hydraulic conductivity
(H.C.) is orders of magnitude lower than the high horizontal component. The aquifer material has a very high variability
in grain size, and less permeable layers can be observed at a scale of 0.1–10 m, along distances of 1 – 100 m
(e.g., Huggenberger et al., 1998). Because of the difference in H.C., downwelling of cold river water
(0 – 5 °C.) in winter does not result in a density-driven groundwater flow. The warm summer water (20–25 °C.)
results in an increase of H.C. of a factor of about two. Contrary to the high variability of H.C. in aquifers of
glaciofluvial outwash, the permeability of river beds and banks does not seem to be as variable: specific infiltration
rates of 0.05 - 5 m 3 /m 2·d are reported by Hoehn (2002). Given permeable beds and banks, rivers are downwelling
or upwelling, depending on the hydraulic gradient between the water level of the river and the water table.
At downwelling flow conditions, the
river is hydraulically connected through
a saturated zone up to a difference in
water-level elevation of about 2 m, and
seepage through an unsaturated zone
occurs at greater differences.
Below the freshly infiltrated hyporheic
ground water, alluvial ground water
predominates, which infiltrated from
the river more upstream in the flood
plain. The chemical composition of this
groundwater of greater depths may be
similar to that of the river, but it is older
and the temperature varies less in time.
The groundwater at the bottom of the
aquifer may not be of bank filtration origin,
but result from seepage of precipitations
and inflow from the slopes to the
flood plain valleys. As shown in Fig. 1,
the flow directions of the different portions
of the groundwater body must not
necessarily be the same. In phreatic
groundwater systems, the flow direction
of the top layer is also dependent on the
position of the water table space and the
Figure 1. Groundwater stratification near downwelling rivers
in alpine and perialpine flood plains. 1, River water;
2, Hyporheic groundwater (τ, < 30 d; τ, subsurface residence time);
3, riverborne alluvial groundwater (τ, 1 – 20 months);
4, Alluvial ground water not of river origin (τ, years).
A, Well tapping exclusively hyporheic ground water;
B, Well tapping groundwater mixture
10 – 16 June 2005, Berlin ■ 5th International Symposium ■ AQUIFER RECHARGE ■ ISMAR 2005