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  Project background
  Mapping procedures
  Mapped valleys
  Origin of the buried valleys
  Preferred orientations
  Buried valleys and groundwater
  General conclusions
 
Buried valleys and groundwater

In areas where buried valleys are eroded into low permeable sediments e.g. clay, the aquifers in the buried valley typically constitute the only possibility for large-scale ground water abstraction. Surrounding clayey sediments define the lateral extent of the aquifers. There will be only limited water exchange between the aquifers in the buried valley and the surrounding layers. Groundwater flow and groundwater recharge in the aquifers then occurs inside the valley and in areas outside the valley via surficial sand layers.

Internal erosional surfaces may form sharp contacts between individual aquifers inside the valley. The individual aquifers tend to be elongate and follow the overall valley trend. Distinct hydraulic barriers in buried valleys indicated by large potentiometric surface discontinuities are found both parallel and perpendicular to buried valleys. The longitudinal barriers are supposed to be a result of erosional features inside the valley, whereas transverse barriers may be the result of cross-cutting and clay-filled valleys or glaciotectonic deformation within the valley itself.

Buried valleys filled with coarse-grained sediments eroded into a succession of predominantly sandy sediments will constitute one large aquifer system. Sand-filled valleys eroded into chalk or limestone will also constitute one aquifer system. Clay-filled valleys, on the other hand, may restrain the groundwater flow in the aquifer system. Accordingly, differences in the hydraulic properties of the infill and the surrounding layers either due to structural or textural differences are likely to affect the groundwater flow in the aquifer system.

The vulnerability of the groundwater resources to potentially harmful chemicals introduced at the ground surface is a complicated matter. On the positive side the chemical compounds may be degraded or retained before they reach the aquifer, or protective clay layers or an upward hydraulic gradient may prevent the contamination finding its way to the aquifer. However, in areas with buried valleys the physical conditions may be very complex because of the generally complicated architecture of the valleys and the surroundings. In this way preferred flow paths for transport of contaminated water from shallow aquifers to deeper aquifers may exist.

Due to the generally complex internal structure of the valleys potentially protective clay layers above the aquifers are likely to be discontinuous. The aquifers inside the valley will thus have a varying degree of natural protection. Even if laterally extensive clay layers are present, the protective effect will only have local importance if the surrounding sediments are sand dominated. In cases where small sand-filled valleys eroded into alternating clay and sand layers the buried valley in-fill may constitute highly vulnerable zones as the clay cover is penetrated. The valleys may therefore create short-circuits between the aquifers in the valley and the aquifers in the surrounding strata.

 

Vandglas


An example of a SkyTEM survey at Tønder in South-western Denmark. At the middle of the profile a deep buried valley is eroded more than 250 m into the subsurface (From Jørgensen et al. 2015).