Page 47 - Hydrogeology Principles and Practice

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30 Chapter Two

The larger springs are fed by conduits or flooded cave
systems. The conduits act as drains within the satur-
ated zone of the aquifer, and groundwater in fissures
and fractures flows towards the conduits. Within
the saturated zone the conduit flow has a turbulent
regime while the diffuse fissure flow obeys Darcy’s
law (Box 2.2).
Recharge to the aquifer can be characterized as
allogenic and autogenic. Allogenic recharge com-
prises sinking streams which collect on sandstone
and shale exposed in the core of the periclines. These
streams pass directly into the conduit system through
swallow holes. Autogenic recharge is either con-
centrated by closed depressions (dolines) or occurs as
diffuse infiltration through the soil. Closed depres-
sions are the first-order tributaries of the conduit
system and focus concentrated recharge into shafts
and caves. Weathering in the upper few metres of
bedrock produces dense fissuring that provides stor-
age for water in the unsaturated zone in what is
Fig. 2.11 Conceptual classification of karst aquifers from a
sometimes referred to as the epikarstic aquifer. The
consideration of recharge and groundwater flow mechanisms
epikarstic aquifer is recharged by infiltration and and the degree of saturated aquifer storage. After an original by
drains to the saturated zone via fractures and fissures, T.C. Atkinson.
but with frequent concentration of drainage into
shafts which form tributaries to cave systems.
Analysis of hydrographs (see Section 5.7), baseflow In karst aquifers where turbulent flow conditions
recession curves, water balances (see Section 8.2.1) can develop in solutionally developed conduits, rep-
and tracer tests (see Section 5.8.3) indicates that the resentation of the hydraulic behaviour of the system
diffuse flow component of the saturated zone in the is complicated by the difficulty in characterizing the
Mendip Hills has a storativity of about 1% and a hydraulic properties. A number of approaches are
−1
−3
−4
hydraulic conductivity of 10 to 10 ms (Atkinson commonly used to model the behaviour of karst
1977). About 70% of the flow in the saturated zone aquifers. The first is to assume that groundwater
is via conduits but these comprise less than one- flow is governed by Darcy’s law and then to use one
thirtieth of the active storage. From direct explora- of the models shown in Fig. 2.8. Further approaches
tion, the depth of conduit circulation beneath the to modelling flow in karst conduits is to adopt the
water table is known to exceed 60 m, implying a total Darcy–Weisbach pipe flow equation (eq. 5.22) or, for
storage in the saturated zone of at least 600 mm of mature karst landscapes, to use a ‘black box’ model in
precipitation, roughly equivalent to one year’s runoff. which empirical functions are developed based on field
Significant storage also occurs in the epikarstic aquifer observations of flow to reproduce input and output
although the total amount is not known. responses, in particular of recharge and spring flow.
Karst aquifers can be classified according to the These functions may or may not include the usual
relative importance of diffuse flow and conduit aquifer parameters such as hydraulic conductivity,
flow, the degree of concentration of recharge and the storativity and porosity. A third, hybrid approach is to
amount of storage in the aquifer as shown in Fig. 2.11. use the aquifer response functions developed as for
The Mendip Hills karst aquifer has high storage, the ‘black box’ approach and then make use of these
about 50% concentration of recharge into streams in an equivalent porous material model, although it
and closed depressions and 70% conduit flow in the must be recognized that large uncertainties remain,
saturated zone. requiring careful field validation.