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ASCENDING HYPOGENIC SPELEOGENESIS
A new theory of speleogenesis in a confined multipleaquifer
system has been developed during the last two
decades (Klimchouk, 1990; 1992; 1997a, 2000a; 2003a;
2004). It is based on:
1) Views about close cross-formation communication
between aquifers in multi-story systems,
2) Ideas of hydrostratigraphic conversion of soluble
formations in the system,
3) The concept of ascending transverse speleogenesis, and
4) Recognition of the ultimate control on confined
speleogenesis by transmissivities of adjacent non-soluble
aquifers.
3.1 Cross-formational communication and
basinal hydraulic continuity
The older simplistic notion of artesian flow assumed
that recharge to confined aquifers occurs only in limited
areas where they crop out at the surface at higher
elevations (e.g. at basin margins), and that groundwaters
move laterally through separate aquifers within the
throughflow area with no appreciable communication with
adjacent aquifers across confining beds. Until recently,
such views were commonly adopted in karst literature,
resulting in one of the interpretative problems about
artesian speleogenesis, mentioned above. This notion does
not allow placement of artesian speleogenesis into the
category of hypogenic speleogenesis as defined earlier.
However, the most essential problem with it is that the
implied substantial flow distances and travel times through
soluble rocks generally preclude the possibility for
significant conduit development in the confined flow area
due to dissolution capacity constraints.
Since the middle of the 20th Century, close crossformational
communication between aquifers and basinwide
hydraulic continuity have been acknowledged in
mainstream hydrogeology. It is well recognized that there
are virtually no completely impervious rocks or sediments,
just large contrasts in permeabilities. In modern
hydrogeology the term “confined aquifer” is not used in
the absolute sense of hydraulic isolation; a notion of semiconfinement
is more appropriate as separating aquitards
are commonly leaky at certain time and space scales.
Where there is a vertical head gradient between
aquifers in a layered aquifer system, flow in highpermeability
beds is predominantly lateral but flow in the
separating low-permeability beds is predominantly
vertical, if permeabilities differ by more than two orders of
magnitude (Girinsky, 1947). Further developing these
ideas, Mjatiev (1947) recognized that recharge areas of a
confined aquifer are not just the uplifted marginal
outcrops, but include all of the areas within the basin
where the head is lower than in any adjacent aquifers. In
the western literature, it was the work of Hantush and
Jacob (1955) that introduced a “leakage factor” to account
for hydraulic communication across confining strata and
replaced the “confined aquifer” with the “multiple
aquifer.” The concept of basin-wide hydraulic continuity
has since become well accepted; the importance of crossformational
communication between aquifers has been
recognized on a local scale from numerous aquifer and
well data, and on a regional scale from basin hydraulics
and water-resources evaluations. Shestopalov (1981, 1988;
1989) and Tóth (1995) provided important reviews and
discussion of these characteristics.
This concept implies complex flow patterns in artesian
basins and complex recharge-discharge characteristics for
particular aquifers in the system. Besides marginal
recharge areas and lateral flow components, this pattern
includes laterally alternating recharge and discharge areas
(areas of correspondingly descending and ascending cross
communication) in the confined flow region, juxtaposition
of recharge-discharge regimes for particular aquifers in a
system, and flow systems at different scales.
Figure 3. Flow pattern in a multi-story artesian aquifer system
(from Shestopalov, 1989).
Figure 3 illustrates flow patterns in a typical multipleaquifer
system. Recharge to, and discharge from, a given
aquifer may take place across intervening aquitards
throughout the whole throughflow area. The amount and
direction of hydraulic communication across homogenous
dividing beds of low permeability depends on the
relationships of heads between adjacent aquifers, which, in
turn, are guided significantly by surface topography. For a
given aquifer, there is a gradual vertical transition between
net recharge and discharge, both of which occur
simultaneously. At the regional scale, the respective areas
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