Alistair Allen1,2, Ivor MacCarthy1,2, Sergei Kostic3, Dejan Milosevic3

1Dept of Geology, University College Cork, Cork, Ireland. Tel: +353 21 4902769; Fax

+353 21 4271565; e-mail: a.allen@ucc.ie

2Environmental Research Institute, University College Cork, Cork, Ireland

3Geofizica-Ing, Dr Ivana Ribara 182/2, Novi Beograd 11070, Serbia & Montenegro

Abstract: The Cork Harbour area in SW. Ireland is characterised by a series of upright E-W anticlines and synclines, cored respectively by Devonian Old Red Sandstone (ORS) sandstones and shales, and by karstified L. Carboniferous reef limestones, and cut by EW thrusts and steep N-S compartmental faults. A series of E-W gravel-infilled buried valleys, superimposed on the bedrock structure, formed during the late Pleistocene, 15,000-18,000 years ago, when sea level was ~130 m lower than at present, and were controlled by wide early Pleistocene U-shaped lowlands carved by glaciers which exploited the limestone-cored synclines. The buried valley ribbon aquifer systems represent a major source of groundwater in the area. Site investigation boreholes and geophysics indicate variable widths of the buried valleys, and electrical resistivity surveys using the vertical electrical sounding (VES) technique, have determined depths in excess of 100 m for several of the buried valleys. Granulometric analyses of gravel samples from boreholes and pump tests indicate hydraulic conductivities of the order of 5-7 x 10-3 msec-1, and transmissivities of ~ 0.25 m2sec-1 for the gravels in these aquifers. Groundwater yields from the buried valley aquifers range from 500-1500 m3sec-1, and groundwater reserves of the Lee Buried Valley, the largest and most studied of the buried valleys, which can be traced for at least 60km, has a width of 500-750 m and depths locally in excess of 140 m, are at an absolute minimum: VGWmin = 1.5 x 108 m3.

Groundwater from gravels of the Lee Buried Valley, was used for all or part of its municipal water supply by Cork city continuously from 1879-2001, collected by means of a shallow infiltration gallery constructed on the north bank of the R. Lee near the northern margin of the valley. Cork City Council is considering again using groundwater from this buried valley for the municipal water supply for Cork by drilling a well field of deep boreholes into the buried valley to the west of the city.

Groundwater from the Lee Buried Valley has more recently been employed as a source of geothermal energy for space heating/cooling buildings in Cork city, utilising geothermal heat pumps. The ‘heat island’ effect of urban areas, which raises the temperature of shallow groundwater in the aquifer by a few degrees is utilised, and this, together with the high groundwater yield and the shallow depth of the sand and gravel deposits in the buried valleys, make this an attractive easily exploitable resource. Currently the heating/cooling systems of five public buildings in Cork are employing this heating system and Cork City Council have plans to use this resource for heating/cooling systems for their major Docklands Redevelopment scheme, which will involve district heating systems for apartment complexes, hotels, shopping malls and commercial premises. As the River Lee is tidal within Cork City, a variable density groundwater flow modelling project is being conducted to determine the behaviour of the saline/freshwater interface beneath Cork in the light of large groundwater abstractions for the municipal supply and geothermal heating.

Key words: buried valleys, gravel aquifers; high yield, low enthalpy geothermal energy

Introduction

Intergranular ribbon aquifers of the buried valley type, i.e. deep gravel-infilled valleys formed during periods of reduced sea level usually associated with glacial epochs, are generally not considered to be of great importance in terms of areal extent and groundwater reserves compared to layered, intergranular bedrock aquifers, karst aquifers or alluvial plain aquifers. Consequently, they are usually classified as minor local aquifers, and are often completely ignored in groundwater resource assessments. However buried valley aquifers are much more widespread than currently appreciated and possess significant groundwater reserves. They have been largely neglected because they are less readily identified than other aquifer types, and are often difficult to trace laterally, as they may not necessarily underlie present day valleys. On the other hand, they may be of considerable width and depth, may extend laterally for several tens or even hundreds of kilometres and, since they occur at relatively shallow depths, once delineated, exploitation costs are relatively low.

The present investigation arises from an overall assessment of groundwater resources and quality in the Cork Harbour area of SW Ireland (Allen & Milenic, 2001; Milenic 2004), an extensive region of about 1,000 km2 (Fig. 1). Ribbon aquifers were recognised as constituting a major aquifer type in the area, with potentially considerable groundwater reserves, which are virtually untapped. Several buried valleys have been identified, and are currently being investigated, partly for their groundwater potential, and partly because of their potential as a low enthalpy hydrogeothermal energy source, particularly beneath cities (Allen et al, 1998; 1999; 2002; 2003; Leahy & Allen, 1998; Allen & Milenic, 2003). This has led to a detailed assessment of their groundwater reserves and quality, using as a case study the Lee Buried Valley, which underlies part of the course of the River Lee, a major Irish river flowing through Cork city.

Regional Setting

Cork, situated in the southwest of Ireland, at the mouth of the River Lee, is the second largest city in the Irish Republic with a population approaching 250,000 in its environs. The River Lee drains into Cork Harbour, one of the largest natural harbours in the world, a glacially eroded almost completely enclosed body of water, connected by a narrow entrance to the Atlantic Ocean. Cork has been for several centuries a major port exporting primarily agricultural produce, but in the past three decades has become highly industrialised and is now the principal centre for the pharmaceutical and chemical industries in Ireland, situated mainly around Cork Harbour. Nevertheless, farming is still the principal activity in the region.

Major bedrock and overburden aquifers underlie the Cork Harbour area, and are very vulnerable both to contamination from urban and industrial sources and from farming, and to salinisation due to the long coastline of the harbour which brings sea water into contact with both the bedrock and overburden aquifers (Allen & Milenic, 2006a). This vulnerability has been recognised by the Geological Survey of Ireland (GSI), who have initiated an aquifer protection scheme for the South Cork region (Kelly & Wright, 2000). The current investigation complements the above GSI programme and also dovetails with a previous EU Stride project, which dealt with the surface water quality of Cork Harbour.

Geology and Geomorphology of the Area

SW Ireland lies within the very low-grade Rheno-Hercynian fold-thrust terrane of the late Carboniferous Variscan Orogenic Belt. The geological structure of the area is characterised by a series of upright horizontal NE-SW to E-W folds, with the anticlines cored by U. Devonian sandstones and shales and the synclines by massive L. Carboniferous reef limestones or calcareous shales (Fig. 2). The folded sequence is cut by E-W thrusts and steep N-S compartmental faults. In the Cork area, two of these limestone-cored synclines, the Cork-Midleton Syncline and the Cloyne Syncline, separated by the Great Island Anticline are important aquifers, whilst further to the south on the southern flank of the Church Bay Anticline, the narrow Ringabella Syncline, is devoid of limestone, reflecting deepening water to the south during early Carboniferous times.

Formation and Evolution of the Buried Valleys

The topography of the Cork Harbour region is largely controlled by the geological structure, with the anticlines forming upland areas and the synclines occupied by valleys (Fig. 3). However, these topographic features developed during the Pleistocene glaciation (1.8 Ma-10 ka), and the present landscape has been mainly shaped by glacial action, although it also owes much to post-glacial modification, in particular to fluvioglacial deposition accompanying interglacials and interstadials during the Flandrian sea level rise (10,000ka-present).

Prior to the Pleistocene glaciation, intense peneplanation under tropical conditions during the Tertiary period (65-1.8 Ma), resulted in complete erosion of all pre-existing Mesozoic cover in the region, the establishment of a southwards sloping regional topography, a N-S drainage pattern and southerly flowing rivers (Nevill, 1963; see Fig. 4a). Erosion also led to exposure of the underlying Carboniferous limestones, highly fractured due to their massive nature and inability to undergo folding during the Variscan Orogeny, which were subjected to intense karstification under the tropical Tertiary conditions.

During the Pleistocene glaciation, the Tertiary N-S drainage pattern was truncated by glaciers advancing outwards from the mountainous regions of western Ireland, preferentially exploiting the weaker shales or karstified limestone coring the synclines (Fig. 4b), resulting in the development of a number of broad u-shaped lowlands coincident with the E-W Variscan synclines.

Fig. 4: Drainage patterns of southern Ireland: a) before; b) after; the Pleistocene

glaciation (Nevill, 1963)

Superimposed on these u-shaped lowlands are a series of buried valleys infilled with sand and gravel. At the peak of the last glaciation 15-18,000 years ago (Fig. 5), when much of Europe and North America were covered by ice, sea level fell to an estimated 130m lower than at present (Mitchell, 1976; Pirazzoli, 1996), so subglacial rivers eroded down to the new base level cutting deep steep-sided gorges in the process. In Cork, parallel vertical scarps outcropping along either side of the present day River Lee, at the margins of its floodplain, represent the flanks of the channel cut by the glacial River Lee during the rapid drop in sea level. Where it passes through Cork city, the Lee 'Gorge' has a width of 0.5-0.75 km, and a depth probably of the order of 140 m based on geophysical evidence from 4 km west of Cork city centre (Kostic & Milosevic, 2004).

When temperatures subsequently ameliorated towards the end of the Pleistocene glacial epoch about 10-12,000 years ago (Fig. 5), the ice sheets started to recede, and sea level began to rise again. The gorges became initially infilled by a succession of fluvioglacial sands and gravels either directly associated with the development of valley glaciers (tills and glacial gravels), or resulting from fluvioglacial outwash as the rivers responded again to changing base level.

Simultaneously, glacial till of variable thickness and lithology was deposited by the receding glaciers Although deposits from earlier glaciations had been eroded, this last glaciation, the Midlandian (26,000-12,000 ka; see Fig. 5), left behind significant deposits of glacial gravels and till (boulder clay), particularly within the synclinal lowlands. Thus, most of the karstified limestone bedrock became buried, sometimes to considerable depth, although locally upstanding resistant limestone pinnacles remain exposed.

There is also evidence that impeded river flow, due to ice dams, moraines or landslide debris, led to river diversion and deposition of layered, fine-grained fresh water sediments in ponds or small lakes during interglacial or interstadial periods. When the ice finally retreated and sea level rose rapidly, the buried valleys became infilled by fluvial gravels deposited by a braided river system, and later with alluvial clays and silts. It is likely that during this period, new river channels were partly cut into rock on the valley sides as the channels migrated laterally. Marshes, intersected by river channels, formed the final stage in the evolution of these buried valley systems, after the rate of sea level rise diminished some 6,000 years ago. However, the south of Ireland is still sinking, so sea level continues to rise relative to land in the region - by an estimated 16m over the last 8,000 years (Carter et al, 1989; Devoy et al, 1996).

The fluvioglacial gravels play an important role in the groundwater flow regime of this region. Their hydrogeological significance is largely due to their high permeability, thickness and extent. Conversely, the low permeability tills protect the underlying bedrock aquifers, but on the other hand restrict recharge of them.

In the Cork area, the geologically controlled topography consists of alternating ENEWSW to E-W ridges and lowlands, with present day rivers generally flowing eastwards along the synclinal structures, although post-glacial river capture by the entrenched preglacial N-S rivers has led to many of them, e.g. the Blackwater and the Lee, being diverted southwards to cut through anticlinal ridges to reach the sea. As a result the drainage pattern is trellised, with most rivers consisting of E-W and N-S stretches.

Underlying the E-W stretches are buried valleys, which are traceable for 10's of km and extend beyond these reaches. In the Cork Harbour area, at least four and perhaps five buried valleys exist (Fig. 6), the Lee, and the Tramore occupying the northern and southern margins respectively of the Cork-Midleton Syncline; the Owenaboy and possibly the Bandon, occupying the northern and southern margins of the Cloyne Syncline; and the Ringabella, which occupies the narrow Ringabella Syncline. To the north of Cork city, at least three other significant Irish rivers, the Bride, the Blackwater and the Suir are also thought to overlie buried valleys for at least parts of their courses.

Water Cycle in the Cork Harbour Area

The climate of the Cork Harbour area is dominated by the westerly atmospheric circulation of the middle latitudes and the Atlantic Ocean, which surrounds Ireland to the north, west and south. The maritime influence is strongest in the west of Ireland and decreases eastwards. Calculations of the major climatological parameters for the Cork Harbour area, are based on data over the period 1963-2000 (Table 1).

Surface run-off (Q) is dependent on topography and estimated at 10% for the synclines (0-30 metres above sea level - mASL) and 75% for the anticlines (up to 200 mASL), respectively. The general water balance equation has been used to calculate effective infiltration (ief.):

ief. = P - (E + Q + DW)

Thus, the calculated value for effective infiltration into the buried valley aquifers is: ief. = 650 mm/year

Types of Aquifer

In the Cork Harbour area, major aquifers occur in both bedrock and overburden. The Cork-Midleton and Cloyne Synclines are cored by intensely karstified limestones, which have significant storage capacity and transmissivity properties, and so are excellent aquifers. The anticlinal structures are cored by Devonian sandstone and shale sequences (Old Red Sandstone), which due to folding and recrystallisation during the late Carboniferous Variscan Orogeny no longer has any primary porosity. Associated fracturing has led, however, to a good secondary fracture porosity, and although well yields are not particularly high (0.1-6.7 l/sec), these rocks can be regarded as classical fractured aquifers (Allen & Milenic, 2006b) Overlying these bedrock aquifers, are overburden aquifers represented by the buried valleys. These latter aquifers are a major resource.

Intergranular Buried Valley Ribbon Aquifers

The buried valley intergranular aquifers are a series of ribbon aquifers ranging in width from a few tens of metres to nearly a kilometre. They are predominantly infilled with variably sorted fluvioglacial gravels. The lateral extent of several buried valleys in southern Ireland, including those in the Cork Harbour area can be delineated over distances of tens of kilometres (e.g. Kostic & Milosevic; 2004; 2005; Davis et al, 2004; 2006). The digital elevation map (Fig. 6) illustrates the locations of these E-W trending

buried valleys

The Lee Buried Valley (Fig. 6, No. 1) is a typical example showing a lateral extent of at least 60 km (Allen et al, 1999). It underlies the Cork-Midleton Syncline for all of this distance, but only underlies the present course of the River Lee for 10 km, where the River Lee passes through Cork city before turning southwards to enter Cork Harbour. Late glacial damming and diversion of the River Lee, and its subsequent capture by a N-S consequent stream has brought about this deviation from its original course.

Where it underlies Cork city, an abundance of bedrock outcrops and site investigation borehole data has made it possible to delineate the extent of the Lee Buried Valley fairly precisely (Davis et al, 2004; 2006) and it is approximately 0.75 km wide. Although most site investigation boreholes extend no deeper than 20 m, a few boreholes drilled to depths of up to almost 50 m without encountering bedrock, and a borehole of just under 60 m at Carrigtohill, 10 km to the east of Cork which also failed to encounter bedrock, indicated a depth of at least 60 m for the Lee Buried Valley.

In view of the need for accurate depth estimates, geophysical surveys of the Lee and other buried valleys in the Cork area were undertaken (Kostic & Milosevic, 2004; 2005). In all 24 electrical resistivity (ER) traverses or partial traverses utilising the Vertical Electrical Sounding (VES) technique were undertaken across six buried valleys. Results indicate depths of the buried valleys ranging up to 147 m, with the deepest values encountered in the Lee Buried Valley, which was studied in most detail. Depths in excess of 100 m were encountered in some of the other buried valleys. To confirm these values, a dipole-dipole electrical resistivity survey was employed at a site which recorded one of the deepest VES measurements (Kostic & Milosevic, 2005), and an almost identical depth value was measured. One of the deeper geophysical profiles from the Lee Buried Valley (Kostic & Milosevic, 2004) is presented in Fig. 7, whilst Fig. 8. shows a schematic profile of the Lee Buried Valley (Allen & Milenic, 2003), illustrating the stepped margins postulated for the buried valleys.

In the light of the postulated geometry, and taking a minimum width of 250 m, an average depth of 50 m and a very conservative value for porosity of 0.20, the absolute minimum volume of groundwater reserves of the Lee Buried Valley aquifer is of the order of

VGWmin = 1.5 x 108 m3.

Fig. 8: Schematic cross-section through the Lee Buried Valley (modified after Allen & Milenic, 2003)

Other buried valleys in the Cork Harbour area are narrower and less deep than the Lee Buried Valley, but they too hold considerable groundwater reserves and also represent significant resources, not only of groundwater, but also of sand and gravel. For instance, the Tramore Buried Valley at the southern margin of the Cork-Midleton Syncline has to date been traced for about 40 km, and has a width ranging from 300-500 m and maximum depths of the order of 80 m.

Hydrodynamic Characteristics

Hydraulic conductivity (k) and transmissivity (T) have been calculated using particle size distribution methods from borehole samples, and by pumping tests on wells and an infiltration gallery. For the particle size distribution, the USBR method was used, providing the following mean value for k (Milenic & Allen, 2005): k = 6.87 x 10-3 m/s

Pumping test were carried out on several horizontal and vertical water abstraction structures, including an infiltration gallery at Cork Waterworks and a well in the car park at Cork County Hall, which capture the upper levels of the aquifer. For the k and T calculation, the recovery level method (s = f (log t/t-t1)) was used as the most reliable for these examples (Fig. 9). Characteristic values are in the range (Milenic & Allen, 2005):

k = 4.0-4.8 x 10-3 m/s

T = 1.22-1.43 x 10-2 m2/s

Note that the pump test which gave these results was conducted on a well drilled very close to the southern margin of the Lee Buried Valley, and that only 3 m of gravels were encountered before bedrock was intersected. Given that hydraulic conductivity values for the various methods show excellent correlation, taking an average hydraulic conductivity value of 5 x 10-3 m/s and a mean depth of 50 m for the Lee Buried Valley (see above), a more realistic transmissivity value of 0.25 m2/s would be indicated.

Fig. 9: Water recovery data for the calculation method s = f(logt/t-t1)(Milenic & Allen, 2005)

In view of the excellent correlation of results by the various methods, the above hydrodynamic values may be considered as representative.

Effective porosity for the sand and gravel is estimated to be around 25%. The porous media is homogeneous in horizontal extent with a slight vertical heterogeneity. Part of this heterogeneity has resulted from washing down of fines from the overlying alluvium. Due to the low elevations of the Cork-Midleton and Cloyne Synclines, particularly in the Cork Harbour area the buried valley aquifers are characterised by subartesian pressures. The general groundwater direction is E-W, typified by a steady flow gradient (10m of head difference for water levels of 10 km in length), and a characteristic laminar flow regime. The static water level is close to the surface, and adjacent to Cork Harbour is tidally influenced ranging from -1.8m at high tide, to -3.8m at tidal lows.

Aquifer Recharge and Discharge

Assessment of modes of recharge to the buried valley aquifers suggests that recharge occurs mainly through the following processes (Milenic, 2004):

1         Direct infiltration of rainfall in areas of open gravel extraction pits

2         Percolation through overlying alluvial sediments, ranging in thickness up to 4-5 m

3         Hydraulic connection with various surface streams including the River Lee -brackish within tidally influenced areas adjacent to Cork Harbour

4         Hydraulic connection with the adjacent karst and ORS aquifers

The buried valley aquifers are recharging by direct infiltration only over very small areas, apart from the Lee Buried Valley, where large scale sand and gravel extraction is ongoing to the west of Cork city. An area of about 2 km2 is estimated to be involved, with a calculated recharge volume of approximately 1.3 x 106 m3 /year. Percolation has calculated as the drainage through a 5m thick, fully saturated layer of clay in flat terrain, and is within the range of 0.05-0.3 cm/h depending on the intensity and duration of precipitation. For the Lee Buried Valley, aquifer recharge by this process over an area of 27 km2, will give rise to a recharge volume about 2.6 x 106 m3/year. Total groundwater replenishment of the Lee Buried Valley by rainwater through direct recharge and percolation, has been estimated to take approximately 19 years (Milenic, 2004).

Recharge of the buried valley aquifers from surface streams will only take place during and immediately after heavy and prolonged rainfall, when the streams are in spate (Meybeck et al, 1996; Allen & Chapman, 2001). During dry spells when low flow conditions obtain, groundwater feeds streams, and discharge/recharge relations between streams and groundwater of the shallow buried valley aquifers will be dependent on local precipitation and evapotranspiration totals and on rainfall distribution. That hydraulic connection exists between the streams and buried valleys is indicated by the observation that in Cork city, where the River Lee is tidal, groundwater levels show a strong tidal influence (Allen, 2006).

The buried valleys are excavated into bedrock, usually at the margins of the synclinal structures, where they have exploited the boundary between the ORS and the limestones. Recharge from the adjacent limestones will be considerable, due to the highly karstified nature of the latter, but recharge will also take place from the fractured ORS.

No natural occurrences of groundwater discharge from the buried valleys have been observed in the study area, Discharge to streams during low flow conditions has been mentioned above and this probably represents the most important natural discharge mechanism. Aquifer discharge takes place artificially through a number of drilled wells, and also through an infiltration gallery located at Cork Waterworks. The latter was constructed in 1879 as a source of potable water for Cork city and was in operation up until 2001. At the time of construction, the gallery had a capacity of around 260 l/s, but by the time use of the gallery was discontinued, capacity has been reduced to 50 l/s, mainly due to clogging of the porous brick screen. The groundwater from the gallery is now discharged into the River Lee.

Groundwater Uses

Despite its potential, groundwater in the buried valley aquifers, even that of the Lee Buried Valley is severely underutilised. Few industries in the Cork area use groundwater, mainly the chemical and pharmaceutical industries, and breweries and a distillery at Midleton 15 km east of Cork, the largest in the Irish Republic. However most of these groundwater supplies come from the karstified limestones of the Cork Syncline.

The present water supply systems for both Cork city and the Cork Harbour area employ surface water sources, using River Lee water either from Inniscarra Reservoir 10 km to the west of Cork city or from an intake point at Cork Waterworks on the western edge of the city. As mentioned above, Cork city previously derived its water supply from groundwater of the Lee Buried Valley via an infiltration gallery approximately 0.5 km in length, constructed at a depth of 5 m in the gravels along the northern bank of the River Lee, and ending in Cork Waterworks,. This basically horizontal well, 1 m in diameter, was lined with porous brick and initially provided the whole water supply system for Cork city. However, as requirements increased and the yield from the infiltration gallery declined, river water began to be used to augment the groundwater supply and eventually replaced it completely. This was partly due to the high Mn and Fe contents of the groundwater derived from the gravel matrix of the aquifer. Elsewhere in County Cork local water supplies are derived from buried valley aquifers, such as at Fermoy in NE Cork.

Replacement again of all or part of the surface water sources for the municipal water supply by groundwater from the Lee Buried Valley is under consideration by Cork City Council. This is being driven by the escalating costs of surface water treatment, and in particular the considerable cost of the future EPA requirement of disposal to landfill, of all sludges derived by coagulation and settling processes during water treatment, which currently are disposed of to the River Lee. The city council proposes to utilise buried valley groundwater for the municipal water supply by drilling a well field of deep boreholes into the floodplain of the River Lee just to the west of the city. A consideration, however, is the effect of extracting several thousand cubic metres of groundwater per day, on the freshwater/saline water interface in Cork city.

An increasingly important use of groundwater from the buried valley aquifers in Cork city is for space heating and cooling, utilising heat pump technology (Allen et al, 1999; 2003; Allen & Milenic, 2003). Shallow groundwater beneath Cork city exhibits slightly enhanced temperatures due to the 'heat island' effect of urbanisation, which gives rise to a reduction in evaporation of soil moisture due to replacement of natural surfaces by artificial surfaces characteristic of a city (Allen et al, 1999). This results in greater absorption of solar radiation by buildings and by concrete and tarmac surfaces, which have a much lower reflectivity (albedo) than natural surfaces. In addition, heat absorbed by and generated in buildings is transferred to the ground via their foundations. The combination of these effects leads to heating of the underlying soil or rock, and of any fluids contained within them. Shallow groundwater beneath cities will therefore register temperatures a few degrees higher than average groundwater temperatures in the surrounding countryside (Allen et al, 2003). In Cork city, groundwater temperatures rise from 9℃(outside the city) to >13℃in the city centre. A vertical temperature profile (Milenic & Allen, 2005) is illustrated in Fig. 12.

Fig. 10. Vertical temperature profile determined in a well in central Cork city(Milenic & Allen, 2005)

The nominal available geothermal energy resource (EG) in kilowatts (kw) can be calculated using the equation (Allen et al, 2003):

EG = H x F x DT

where: H = specific heat of water (kJ/kg.℃); F = flux of water [well yield (kg sec-1)]; and DT = temperature reduction in the heat pump (℃). Based on a specific heat of 4.2 kJ.kg-1℃-1, a well yield of 20 kg sec-1, urban groundwater temperatures of 13℃ and a temperature reduction in the heat pump of 8℃, a nominal geothermal heating resource of 672 kW may be available from wells within urban areas (Table 1). When used as a source of heating in a heat pump working at a COP of 4.5:1 the heat produced is approximately 865 kW. This can supply space heating for buildings with a footprint in excess of 12,000 m2 with a peak heating intensity of 70 W/m2 (Allen et al, 2003).

Geothermal heating/cooling systems utilising groundwater from the Lee Buried Valley have been installed in a number of public buildings in Cork city. A typical example is the Glucksman Art Gallery at University College Cork (UCC), where groundwater extracted from a 10 m deep borehole into the gravels supplies two heat pumps, which generate a 200 kW load that maintains constant humidity and temperature of 19±1/2°C year round in the building. Other major building projects utilising groundwater for geothermal energy are the Environmental Research Institute, UCC; the Lifetime Lab, the old 19th century Cork City Waterworks buildings refurbished for use as an environmental educational facility for schools by Cork City Council; the Cork City Hall extension, and the Share Hostel for battered women. Several other projects of this nature are under consideration. The most visionary is the Docklands Development Project, a redevelopment programme for the Docklands area of Cork city, which will utilise geothermal energy derived from groundwater of the Lee Buried Valley for district heating systems for apartment complexes, hotels, shopping malls and commercial premises. The groundwater can additionally be used as grey water in the buildings for flushing and washing purposes after it has left the heat pump. (Allen et al, 1999).

The Docklands area is located to the east of Cork city centre, adjacent to the estuary of the River Lee. Groundwater levels in the Cork area fluctuate with tidal maxima and minima and in the docklands area and city centre the water table is only a few centimetres below the ground surface at high tide, with a range of about 2 m under normal conditions and 3 m during spring and neap tides. Groundwater maxima and minima are usually slightly offset by about 30 minutes relative to tidal maxima and minima, and electrical conductivity values exhibit a parallel variation with groundwater levels (Milenic & Allen, 2004).

The recognition that large abstractions of groundwater for geothermal energy may have

an impact on the saline/freshwater interface beneath the docklands area and may also have consequences for deep boreholes in Cork city where groundwater is abstracted for industrial purposes has led to initiation of an investigation of this phenomenon. Variable density groundwater flow modelling will be employed to assess the behaviour of the fresh/saline water interface beneath the Docklands in response to continuous large abstractions of groundwater for geothermal heating and cooling. Results of this study will determine the maximum volumes of groundwater that can safely be abstracted without inducing intrusion of saline water at levels where it will interfere with groundwater abstractions from industrial wells in Cork city. In addition the investigation will aid the location of reinjection wells for the spent water after it leaves the heat pump, which will assist in the control of the saline/freshwater interface.

Conclusions

Intergranular buried valley gravel aquifers are important groundwater bodies which may have widespread distribution in northern latitudes affected by the Pleistocene glaciations. Many European buried valleys are thought to have formed during a particularly cold spell in the Late Pleistocene about 15,000-18,000 years ago, which led to a drop in sea level to about 130 m lower than that of the present day. In southern Ireland, the courses of the buried valleys were controlled by the pre-existing bedrock structure, which was exploited by the glaciers as they spread out from their mountain sources, so in the Variscan Orogenic Belt of SW Ireland the buried valleys tend to trend W-E or WSW-ENE, and occupy the margins of lowland areas defining synclinal fold structures.

In the Cork Harbour area, the buried valleys are infilled by fluvioglacial gravels, overlain by up to 5 m of alluvium. Locally, close to Cork Harbour the gravels may contain lenses of fine sand, silt or mud, possibly of marine or lacustrine origin, but away from the harbour, the gravels tend to be more homogeneous, although relatively unsorted.

Detailed hydrogeological investigations of the Lee Buried Valley, the best known of these palaeorivers, indicates a length of at least 60 km, a width of 500-750 m and depths up to in excess of 140 m. Groundwater reserves for the aquifer are at an absolute minimum 1.5 x 108 m3, and groundwater yields range from 500-1500 m3sec-1. Hydraulic conductivities are about 5 x 10-3 m/sec, and transmissivities may be of the order of 0.25 m2/sec. The River Lee is tidal through Cork, and groundwater levels fluctuate with the tides indicating a hydraulic connection between the river and groundwater, which is only a few cms below ground level in central Cork city at water table maxima.

Groundwater in these buried valley aquifers is severely underutilised. Although in the past groundwater was abstracted via an infiltration gallery for the Cork city municipal supply, this was discontinued in 2001. However, plans are afoot to resume use of groundwater for municipal supplies by installing a well field in the buried valley to the west of Cork city. In addition, groundwater from the Lee Buried Valley is currently being employed to supply low enthalpy geothermal energy for space heating/cooling in a number of buildings in Cork city and there are plans to extend this usage in the Cork Docklands Redevelopment. One major consideration in all extractions of groundwater from the buried valley aquifers in Cork, is the impact this will have on the saline/freshwater interface beneath the city.