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Hydrogeology of lowland karst in Ireland
D.P. Drew
Geography Department, Trinity College Dublin, Dublin 2, Ireland (e-mail: ddrew@tcd.ie)
Almost a half of the Republic of Ireland is under-
lain by Carboniferous limestone, most of which
is sufficiently pure to be karstified. Upland
outcrops of limestone are largely confined to
plateaux in the west and NW such as the Burren but the
great majority of the limestone underlies the undulating
lowland of central Ireland, which only locally exceeds
100 m in elevation. The lowland limestones are the
principal source of groundwater in Ireland and also
coincide with the most economically developed and
intensively farmed areas, unlike the situation in Britain,
where most karst limestone areas are relatively periph-
eral uplands. Such limited hydrogeological investigations
as have been undertaken of the lowland limestones of
Ireland suggest that karstic flow systems predominate,
that they are intimately related to surface drainage sys-
tems and that the groundwater and associated karst
features are often highly vulnerable to anthropogenic
influences.
In Europe the most extensive karstic areas are moun-
tainous or plateau in character and it is primarily
investigations in such terrains that have informed the
development of conceptual models of karstic hydrogeol-
ogy (Ford & Williams 2007). This is particularly the case
in Britain, where the most highly karstified aquifers are
the upland areas of northwestern England, the Mendip
Hills, the Peak District and the rim of the South Wales
coalfield, a total area of c. 1000 km
2
. These plateau
outcrops of Carboniferous limestone tend to be scat-
tered and fragmented (Gunn 1992, 1994; Waltham et al.
1997) and therefore there are no extensive groundwater
flow systems. The other significant carbonate aquifer
rocks that are in part karstified, the chalk and the
Jurassic limestones, also form relatively elevated ter-
rains. Lowland karstifed aquifers are small in extent and
widely scattered; for example, around Morecambe Bay
and perhaps the Gower Peninsula and areas of the
southern outcrop of Carboniferous limestone in South
Wales. The Permian limestone in the NE (Murphy 2003)
and the chalk of the west Hampshire Basin in Dorset
might be regarded as low-lying partly karstified aquifers.
In contrast, Carboniferous limestone is the outcrop or
near-outcrop rock across over 50% of the area of the
Republic of Ireland (Fig. 1). Probably all the limestone
is karstified to some extent (Drew et al. 1996), as karstic
drainage and karstic landforms are documented from
80–85% of the outcrop area. Extensive areas of upland
karst are restricted to the Burren in Co. Clare and the
plateaux of the NW, and more than 90% of the lime-
stones are in a lowland location with elevations of less
than 150 m a.s.l., and over much of the lowland less than
100 m a.s.l.
The Carboniferous limestone is the primary aquifer
rock in Ireland. In addition, because the limestone
underlies most of the lowland, it coincides with the most
productive agricultural land, the areas of most intensive
economic activity and the major centres of population.
Upland karst areas in Ireland, as in Britain, tend to be
thinly populated, devoted to extensive agriculture and of
recreational significance.
The controls on, and behaviour of, groundwater in
upland karst are well understood. Steep hydraulic
gradients, significant point recharge, conduit-dominated
hierarchical flow systems, low storage and concentrated
discharge via a small number of springs characterize
such karsts worldwide. Lowland karsts have been much
less studied and the extent to which to the accepted
conceptual models for elevated karst regions are appli-
cable is uncertain. In Ireland, an understanding of the
workings of the aquifer systems of the lowlands is of
particular importance so that groundwater resources can
be adequately assessed and protected and that the
functions of the karst groundwater in the wider environ-
mental context can be appreciated. This paper sum-
marizes present-day understanding of the lowland
karstic hydrogeology and the major problems associated
with the use and misuse of the groundwater. The loca-
tion of the counties mentioned and of the photographs
in this paper are shown in Figure 2.
Hydrogeological characteristics
General
Carboniferous limestones occupy in excess of
30 000 km
2
of the lowlands of the Republic of Ireland
but conditions are not uniform throughout. For
example, the impure limestones, mainly located in the
east and the Midlands (Fig. 1), exhibit relatively low
levels of karstification whereas the purest limestones,
predominantly west of the River Shannon, have devel-
oped a full suite of karstic landforms and a wholly
karstic drainage system. Extensive areas west of the
Shannon in eastern Co. Galway, south Co. Mayo, south
Co. Clare and Co. Roscommon are either devoid of
surface drainage systems or have an artificial drainage
system, mainly constructed between 1850 and 1950,
which was designed to alleviate flooding but which has
Quarterly Journal of Engineering Geology and Hydrogeology,41, 61–72 1470-9236/08 $15.00 2008 Geological Society of London
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effectively partially de-karstified large areas by imposing
river networks on areas that were naturally drained via
karst (Drew 1973, 1984; Coxon & Drew 1986; Drew &
Coxon 1988). Other areas of the limestone lowlands,
particularly in the east and south, support normal
surface drainage systems, although often with large
karstic springs flowing into the main rivers as in
Counties Tipperary and Cork. Some lowland lime-
stones, for example in Counties Kilkenny (Daly
1994) and Wexford, exhibit relatively little evidence of
well-developed karstification. However, most of the low-
lands are characterized by drainage systems that are
partly fluvial or lacustrine and partly karstic. Interaction
between the surface and subsurface waters is exten-
sive and complex; for example, the functioning of the
seasonal karstic lakes called turloughs (Coxon 1987).
Although karstification is widespread (Williams
(1970) suggested that karstification took place for long
periods in the Tertiary), the karst landscape is largely
blanketed by Pleistocene and Holocene deposits. The
Fig. 1. The distribution of Carboniferous limestone outcrop or near-outcrop in Ireland. Areas of impure limestone are indicated, as
are the upland (plateau) karsts. (Data from Geological Survey of Ireland.)
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primary landforms are those of glacial and fluvioglacial
deposition, and the widespread occurrence of karstic
landforms such as dolines is confined to areas where
superficial deposits are thin or absent, mainly west of the
River Shannon. It seems probable that many karstic
drainage systems on the lowlands were blocked and
rendered inoperative by sediment infilling and that
present-day groundwater flow systems are developed in
immature fissure or conduit systems of Holocene age, or
in reactivated ancient conduits from which sediment has
been eroded, or in a mixture of both (Drew & Daly
1993).
Recharge
Mean annual precipitation over most of the central
lowlands of Ireland is c. 1000 mm compared with
1500 mm or more in the upland karsts. It is estimated
that up to 50% of this may become recharge (Lee & Daly
2002) as opposed to values of <75% on the karstic
Fig. 2. County boundaries in Ireland, with those counties mentioned in the text shaded. Locations of photographs (Figs 3, 4, 6, 7
and 8) are shown.
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uplands with their thin and patchy soils. The modes of
recharge are: direct (diffuse) input from rainfall and
indirect recharge from allogenic sinking streams;
autogenic sinking streams; dolines; inflowing rivers and
recharge from turloughs. In plateau karsts such as the
Burren in Co. Clare (Drew 1990) recharge is dominantly
diffuse on the bare or partly vegetated limestone pave-
ments with the remainder (up to 30% in some ground-
water catchments but more commonly 5–15%) being
derived from allogenic streams, which sink underground
at the contact between the limestone and the non-
calcareous rocks. On the lowland, allogenic recharge is
uncommon and streams cross from non-limestone to
limestone strata whilst remaining on the surface, pre-
sumably because of the influence of the superficial
deposits. Exceptions occur, such as in the Gort area of
Co. Galway, where streams with flows of several cumecs
generated on adjacent Devonian sandstones sink under-
ground close to the limestone contact (Drew & Daly
1993).
Recharge from small autogenic streams, generated on
areas of low-permeability till or peat, are much more
common and although their baseflow discharges rarely
exceed 5–10 l s
1
such sinks occur at densities of
1–5 km
2
in some areas of lowland west of the River
Shannon. Dolines seem to be an important source of
point recharge. Although single dolines rarely exceed
30 m in diameter and the majority are 10–15 m in
diameter, they occur as clusters at high densities in some
areas; <20 km
2
in parts of Co. Roscommon, for
example. Some dolines contain a permanent stream,
presumably epikarst generated, that emerges and then
sinks within a few metres at the base of the doline. In
effect, there is a gradation from autogenic sinks to
dolines in terms of hydrological function (Fig. 3).
Well-defined zones in which river water flows to
groundwater occur along the channel of many rivers in
the western lowlands. In extreme cases, all the flow in
the Dunkellin and Lavally Rivers, which flow into
Galway Bay near Galway city, sinks underground dur-
ing dry periods, but more commonly only a proportion
of flow is influent.
Turloughs, of which some 135 are known, are over-
whelmingly located in the lowlands west of the River
Shannon, fill from groundwater in the autumn and
empty often from the same points (estavelles) to ground-
water in the spring, often over an extended period of
time, thus acting as a buffered source of point recharge
to the aquifer (Fig. 4).
Finally, diffuse recharge occurs where none of the
above-mentioned concentrating mechanisms for runoff
are operative. Areas with thick (>5 m) superficial
deposits often exhibit diffuse recharge, especially where
the subsoil is coarse textured, but concentration of flow
at or near the rockhead may still occur. Figure 5 shows
the distribution of autogenic swallow holes, turlough
inputs and influent streams which provide recharge for
the Millburn springs in northeastern Co. Galway, which
is typical of the region (Figs. 6–8).
Groundwater flow systems
Hydraulic gradients in the lowland aquifer range
between 0.001 and 0.01. Flow rates as determined from
water tracings range from 5 to 250 m h
1
. Modal values
for flow velocities are in the range 50–80 m h
1
.
Hydraulic gradients in the plateau karsts are typically an
order of magnitude higher, but this is not reflected in
flow rates of 20–300 m h
1
with a modal value of
100mh
1
. The similarity in flow rates may be due to
the fact that in the lowlands the gradient of the conduits
is relatively uniform compared with those of upland
conduits. There are no systematic difference detectable
in flow rates between different areas of the lowland or
between different limestone lithologies.
Relatively little information is available concerning
the nature of the groundwater flow systems; in particu-
lar, the relative proportions of conduit and distributed
flow and the relative proportions of deep and shallow
flow (Fitzsimons et al. 2005). Conduit flow is known to
Fig. 3. Doline field near Castlerea, Co. Roscommon.
Fig. 4. A turlough, almost dry, at Belclare, Tuam, Co. Galway.
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be present in areas of south and east Co. Galway
(Drew & Daly 1993) and geophysical evidence from
Co. Roscommon suggests that conduits with diameters
of several metres are present, although whether they are
water or sediment filled is not apparent (McGrath &
Drew 2002). Such data as are available suggest that
mixed groundwater flow regimes may be typical. For
example, Figure 9 (adapted from Coxon 1986) shows an
area of some 25 km
2
near Ballinrobe, south Co. Mayo,
for which a water table map was generated using bore-
hole data. Groundwater flow directions are shown,
inferred from the configuration of the water table. In
addition, flow lines (schematic) derived from water
tracing experiments are also shown. In the northern part
of the area the two sets of flow lines are compatible,
suggesting a degree of distributed flow as well as conduit
flow, whereas in the southern part of the area the traced
flow lines are not compatible with the supposed water
table gradients, suggesting that conduit flow is dominant
and that a coherent water table surface is absent.
Tri-modal flow may also occur. Figure 10 shows
breakthrough curves derived from a water tracing
experiment (tracer injection into swallow holes) con-
ducted in the lowland karst between Gort and the
coastal springs at Kinvara, Co. Galway. In addition to
the breakthrough curve for the tracer at the springs,
concentration curves at points intermediate between
input and output are also shown. These comprise data
from water samples taken from known conduits, from
shallow epikarst sites and from deeper (<100 m deep)
borehole locations. The character of the breakthrough
curve and the absolute tracer concentration differs
between types of site, suggesting the existence of at least
Fig. 5. Modes of groundwater recharge in a part of the catchment for the Millburn springs, eastern Co. Galway (adapted from Drew
1973).
Fig. 6. A small sink (discharge c.5ls
1
) typical of the lowland
karst, near Tuam, Co. Galway. Fig. 7. The sink of the Gort River at Cahergassaun, Co.
Galway. Underground flow from the sink is assumed to be in
a major conduit.
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three groundwater flow regimes, of which locally the
conduit system is dominant (Drew et al. 1997).
Data on karstic flow systems derived from borehole
pumping tests are difficult to interpret. Figure 11
(adapted from Wright 2000) shows a series of histo-
grams for borehole productivity (determined from well
discharge v. specific capacity relationships) for a variety
of Carboniferous limestone environments and compares
them with values for high-yielding sand and gravel
(intergranular) aquifers. The considerable variation in
borehole productivity within and between limestone
areas supports the idea of a range of flow types being
operative. It is noticeable that the impure (Calp) lime-
stone is characterized by low-productivity boreholes,
and this may reflect the dominance of fissure rather than
conduit flow in this less karstifiable limestone.
Discharge
The majority (perhaps 80%) of springs and therefore of
groundwater catchments in the lowlands are small, with
discharges of only a few litres per second (Drew &
Chance 2007). Many of these springs may not be karstic
in whole or part but may be derived from the over-
lying unconsolidated materials. There are in excess of
200 springs with discharges exceeding 10 l s
1
and two
springs (those at Kinvara and at Cong) with discharges
of 5000–30 000 l s
1
. Variations in discharge are com-
monly in the range 1:5 compared with ranges of <1:60
for plateau karst springs. It is not known whether this
lesser variation in outflow is a function of the nature
of the flow system (a small range of hydraulic heads
and hence hydraulic gradients) or a result of storage and
slow leakage of water in the overlying glacial and
fluvioglacial deposits. In some areas of the lowlands
groundwater catchments can be delimited with reason-
able confidence. For example, in Co. Roscommon, most
of the springs with discharges in excess of 10 l s
1
are
located on the periphery of areas (10–150 km
2
) that are
elevated some 10–30 m above their surroundings. As is
Fig. 8. Estavelle on the edge of Lough Coy, Co. Galway. At the
time of the photograph the estavelle was functioning as a sink
to drain the turlough, but at higher groundwater levels it acts
as a spring to fill Lough Coy.
Fig. 9. Groundwater flow directions in the Ballinrobe area,
Co. Mayo, based on (a) the presumed configuration of the
water table surface and (b) water tracing experiments (adapted
from Coxon 1986).
Fig. 10. Tracer breakthrough curves at Kinvara springs Co.
Galway (pfu (plaque-forming units) = bacteriophage per ml)
(adapted from Drew et al. 1997).
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the case with many true plateau karsts, these peripheral
springs have discrete catchments. However, in areas with
virtually no relief spring catchments are less easy to
define. Figure 12 shows possible catchment areas (based
on the recharge area required to support spring outflow
and the result of one water tracing to each spring) for
three adjacent springs in NE Co. Galway. Although the
approximate area of each catchment is known its loca-
tion is much less apparent, as topographic catchments
for each spring cannot be defined and piezometric
surface maps provide insufficient resolution to allow
flowlines to be drawn with confidence. Thus the desig-
nation of the location of a catchment area for a spring is
somewhat arbitrary. For the most southerly spring
(Aucloggeen) the delimitation of the catchment is fur-
ther complicated by the fact that some water from a
surface river sinks underground and feeds the spring,
and so in theory the catchment for the surface river
upstream of the zone of sinking forms a part of the zone
of contribution for the spring. This aspect is discussed
further below.
Problems with lowland karst
Surface water–groundwater interactions
An important difference between the lowland and the
upland karsts in Ireland is the existence over most of
the lowland of a surface system drainage, sometimes
skeletal, sometimes fully integrated. Fluvial and ground-
water systems may coexist with limited interaction
Fig. 11. Borehole productivity for selected aquifers in Ireland (adapted from Wright 2000). I represents the highest productivity, IV
the lowest.
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(e.g. springs), as is the case in much of the limestones of
eastern and southern Ireland. In areas of the western
lowlands, however, the two systems are intimately
linked, with water being exchanged between surface and
subsurface in a variety of ways and in large quantities.
Thus, for successful water management it is essential
that the conjunctive workings and interactions of both
surface water and groundwater be appreciated. An
example of complicated exchanges of water is given in
Figure 13 (Coxon & Drew 2000) in the lower section of
the River Fergus catchment in south Co. Clare. In this
area flow directions and fluxes vary with stage con-
ditions. The western lowlands to which the example
above belongs is also the area in which turloughs are
most abundant. Turloughs are priority habitats under
the EU Habitats Directive of 1992 and some 80 have
been designated as Special Areas of Conservation.
Turloughs are Groundwater Dependent Ecosystems
(GWDEs) and hence vulnerable to changes in ground-
water inputs, duration of flooding and water quality.
Land use or hydrological changes that may affect these
hydrological variables must therefore be carefully con-
sidered. At the time of writing, particular attention
is being paid to determining the catchment area for
turloughs, if such a term is appropriate.
Geotechnical issues
Examples of human interaction with karstic ground-
water in the lowlands have been documented by Jones &
Fig. 12. Zones of contribution to three adjacent springs in east Co. Galway (adapted from Drew & Daly 1993).
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Gunn (1982) and Gunn (1984). Reports of subsidence or
similar geotechnical problems are relatively few despite
the fact that lowland limestones are the area of most
concentrated economic activity. However, accompany-
ing the rapid economic growth experienced by Ireland in
the period since the 1990s, there has been a commensu-
rate increase in infrastructural development, including
the building of a series of motorways linking the major
urban centres. The motorways planned to link Dublin
with Galway and with Cork both have routes that are
largely over karstified limestones, and the proposed
motorway between Galway and Limerick crosses some
of the most highly developed karsts in Ireland in its
northern stretch. Although issues of ground stability are
relevant, the effects of disposal of road runoffinto the
karst may be of more significance. Changes in the
location of, and the volumes of point recharge that
result from road drainage can modify existing ground-
water flow regimes or can reactivate inactive conduits by
eroding sediment fills (Stephenson & Beck 1995; Hoetzl
1999; Knez et al. 2004).
The rate of expansion of urban areas for housing and
commercial developments has increased dramatically
throughout Ireland. Many of these urban expansions are
in highly karstified environments (in south Co. Galway,
for example) but possible interactions with the karst
systems, other than in terms of groundwater protection,
are rarely considered, although impacts have been well
documented elsewhere; for example, by Crawford (1984)
in Bowling Green, Kentucky.
Defining zones of contribution to springs
Springs are the sole water supply source for many
settlements in the lowlands, including towns such as
Ennis, Fethard and Roscommon, and protection of such
supplies is therefore a priority. Determining what should
be protected, what is a ‘sensible’ estimate of the Zone of
Contribution (ZOC), can be much more difficult for a
spring on the lowland karst than for a spring draining
upland karst or a borehole. Figures 12 and 13 show
springs that derive water both from inflowing rivers and
from more distributed discharge and in which a con-
siderable part of the catchment of the river is a part of
the contributing area to the spring. In the case of the
Ennis area (Fig. 13) the area contributing some water to
the spring (under consideration for use as a public
supply) at Pouladower under high water conditions, can
be 10 times greater than the area theoretically required
to sustain spring flow. The contributing area reduces as
stage falls but is always much larger than the theoretical
minimum. It is difficult to assign a viable and enforce-
able protection areas to springs with such variable
contributing areas (Deakin 2000).
Figure 14 shows the surface catchment (c.90km
2
)
of the Smaghraan River in eastern Co. Roscommon.
Embedded within this surface catchment is a set of
groundwater catchments feeding springs (mean dis-
charge 10–120 l s
1
), three of which are used for public
water supply. In places, all or part of the flow in the river
system sinks and flows to a spring, and the outflow from
these springs then contributes to river flow. Because the
Smaghraan River loses water to groundwater in its
lower course and this water flows to Spring I, under
normal or high water levels the catchment for Spring I
includes all of the Smaghraan River catchment upstream
of the spring but at low water all flow in the river sinks
at the swallow hole feeding Spring IV and the apparent
contributing area to spring I is reduced by 30%. Also,
some of the water discharging from groundwater catch-
ments II and III recharges groundwater catchment IV
but the amounts of water involved and their contribu-
tion to the overall discharge of Spring I are not known.
Fig. 13. Groundwater–surface water interactions in the Ennis
area of south Co. Clare (adapted from Coxon & Drew 2000).
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The difficulties involved in assigning meaningful ZOCs
to springs in these circumstances are considerable.
Discussion and conclusions
There are similarities between the upland and lowland
karsts in Ireland. Groundwater flow rates are com-
parable, as are the proportions of point and diffuse
recharge. Discrete, coherent groundwater catchments
exist in areas of lowland even if they are only slightly
elevated above the surrounding areas. There are also
major differences between the two types of karst. The
extensive interaction between surface water and ground-
water systems in the lowlands means that it is often
difficult to unambiguously assign source areas for
springs or to accurately quantify areal recharge.
Contributing areas may vary greatly in areal extent
temporally, according to water levels, and in many cases
are much greater than the land area required to generate
the outflow at the spring. Contributing areas in terms of
the area needed to sustain spring discharge are usually
only tens of square kilometres in extent. Typically,
upland karsts are characterized by a thin or patchy or
wholly absent cover of superficial deposits, whereas the
lowland limestone is mainly blanketed with superficial
deposits, which may reach tens of metres in thickness.
The hydrogeological effect of these deposits depends
upon their character. The effect may be to confine the
limestone aquifer, or, at the other extreme, to function
as an intergranular aquifer with high storage, feeding
water into the highly transmissive limestone aquifer
beneath. Hydrogeological conditions in the lowland
limestones are not uniform. Surface fluvial systems are
widespread in the east and south but almost entirely
absent in parts of the west. Conduit flow systems are
Fig. 14. Groundwater catchments within the Smaghraan River (surface) catchment, eastern Co. Roscommon.
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poorly developed, if at all, in the impure limestones but
dominate in areas such as south Galway. A conventional
water table surface is identifiable in the folded limestone
lowlands of the SE but seems absent from the limestone
plains of the west.
Comparable extensive areas of lowland karst are
uncommon in Europe, the Monfalcone karst in north-
eastern Italy and the karsts near Poitiers in west–
central France being exceptions. The extensive lowland
limestones of eastern Canada are largely mantled and
function only partially as aquifers. In the USA the
Pennyroyal Plain in Kentucky and the Mitchell Plain in
Indiana are comparable with the Irish lowland in terms
of their topography and in the predominance of doline
recharge (Magdalene & Alexander 1995; Palmer 2004),
but unlike the Irish lowland they are underlain by
extensive networks of active conduit systems (caves).
Perhaps the area most similar to the Irish limestone
lowland is the low-lying karstified plain that extends
over parts of Minnesota, Wisconsin and Iowa (Dalgleish
& Alexander 1984). This mantled fluvio-karst developed
in Devonian limestones has a precipitation regime, topo-
graphic setting, covering of superficial deposits and
hydrogeological system comparable with that of the
Irish lowland karst.
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