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KARST IN THE CONTEXT OF REGIONAL GROUNDWATER FLOW
driven flow systems may still predominate in deep parts
of basins. The basinal groundwater system may be even
more complicated, heterogeneous and heterochronous,
when a basin goes through two or more hydrogeologic
cycles, and/or where a basin is deformed and subdivided
by differential tectonic movements and/or intruded with
magma, with sub-systems of different magnitude and
origin, superimposed and mingled (Pinneker, 1982; Tóth,
1995).
Most known karst systems develop in continental
domains dominated by gravity-driven flow systems.
Epigenic unconfined karst is exclusively formed by
gravity flow, but hypogenic speleogenesis is often a part
of mixed flow systems, where groundwater flow is a
result of different energy sources acting simultaneously,
most commonly topography-driven flow and flow driven
by temperature or solute density gradients. Flow driven
by sediment compaction and tectonic compression can
also contribute to mixed systems relevant to hypogenic
speleogenesis, although the former is commonly
volumetrically limited and the latter is temporally limited.
A common misconception about hypogenic karst is that it
is believed to be unrelated or contrasted to meteoric
circulation (e.g. Budd, Saller, and Harris, 1995). Although
non-meteoric waters (such as connate or magmatic
waters) may be involved in some cases, most hypogenic
speleogenesis is produced by predominantly meteoric
waters, even where non-gravity drives for flow are
involved. Meteoric waters in intermediate and deep
(regional) flow systems, coming from distant recharge
areas, can maintain or rejuvenate aggressiveness when
entering a soluble formation from below in discharge
areas to generate features that fall into the hypogenic class
as it is defined above.
Segments of groundwater flow systems are
characterized by three distinctly different flow regimes:
the recharge, midline or throughflow, and discharge, with
respective distinct physical, chemical, and hydrokinetic
conditions. Hence, the rates and products (solutional
porosity styles and patterns) of speleogenesis differ
accordingly in karst systems associated with respective
situations. In regions with substantial relief, composite
flow patterns develop where flow systems at local,
intermediate, and regional scales (types) are recognized.
Figure 1, which is an adopted and modified version of
Figure 1 of Tóth (1999), illustrates these flow regimes
and types, with epigenic and hypogenic karst systems
shown in the context of regional hydrogeology. In terms
of regional hydrogeology, epigenic and hypogenic karst
systems are regularly associated with different types and
segments of flow systems. Epigenic karst systems are
predominantly local flow systems, and/or parts of
recharge segments of intermediate and regional flow
systems. The recharge regime is characterized by
relatively high hydraulic heads, decreasing with depth,
and by downward and divergent flow. Hypogenic karst
systems are associated with the discharge regimes of
regional or intermediate flow systems, with largely the
opposite energy and flow conditions. In discharge areas,
hydraulic heads are relatively low and decrease upward,
resulting in converging and ascending flow. However, in
intermediate to regional confined systems, crossformational
communication causes recharge and
discharge regimes (areas of correspondingly descending
and ascending cross-formational communication) to
laterally alternate in the throughflow area, largely
following the gross topography (Shestopalov, 1981,
1989). For particular aquifers in multiple-aquifer systems,
the relationships between recharge and discharge regimes
are even more complex, with vertically superimposed
recharge and discharge regimes (Section 3.1). From the
perspective of a single formation or a bed, recharge
includes all the ways that fluids enter the strata (Sharp
and Kyle, 1988).
Recharge and discharge areas of basinal groundwater
flow systems also have characteristic distinctions in
groundwater chemistry and thermal regime.
Groundwaters in recharge areas are typically chemically
aggressive and promote dissolution, have low TDS, and
are characterized by oxidizing conditions and negative
anomalies of geothermal heat and gradient. In basinal
groundwater flow systems there are systematic changes in
hydrosomes with depth, from HCO3 through SO4 to Cl.
Discharge areas have largely opposite characteristics:
high TDS, chemical precipitation, accumulation of
transported mineral matter, reducing conditions, and
positive anomalies of geothermal heat and gradient (Tóth,
1999).
While recharge areas display highly variable input
parameters in the groundwater regime (both hydraulic and
chemical), in basinal discharge areas these parameters
vary little over time and have low dependence on climate.
The general characteristics of the discharge regime, in
geochemical terms, may not seem to favor speleogenesis.
However, as demonstrated throughout this book,
hypogenic speleogenesis is commonly associated not with
terminal discharge regimes of basinal groundwater flow
systems but with intermediate discharge limbs of these
systems. Even more importantly, the fundamental feature
of hypogenic speleogenesis is that it is driven by upward
cross-formational hydraulic communication, so that
relatively deep fluids of intermediate and regional
groundwater flow systems interact with contrasting
regimes of shallower aquifers and local systems. This
causes disequilibrium conditions and favors various
dissolutional mechanisms. These aspects are discussed
elsewhere throughout this book.
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