Groundwater inputs to the coastal zone
of South Galway and Clare, west coast of Ireland
Rachel R. Cave and Tiernan Henry, Department of Earth and Ocean Science, School of Natural Sciences, National University of Ireland, Galway.
Email: rachel.cave@nuigalway.ie Tel: +353 91 492351 Web: www.nuigalway.ie/biogeoscience
Introduction
3114
(a)
Submarine groundwater discharge (SGD) is now recognised as a
significant source of nutrients and contaminants to coastal waters (1-6).
3115
In some regions, the magnitude of fluxes from SGD are comparable
to riverine fluxes. So far however, they are not taken account of when
measuring fluxes to coastal waters under the EU Water Framework
Directive. This poster presents results from an on-going
study of SGD along the west coast of Ireland by researchers at NUI,
Galway, funded by a Millennium grant from NUI, Galway and subsequently
(c)
(b)
by the Geological Survey of Ireland (GSI) under the Griffiths Programme.
Kinvarra bay is surrounded and underlain by the Burren karst limestone(7).
There are no surface water flows into the bay, due to the porous nature
of the karst and the thinness of the soil cover in the area (Fig. 1, box
outlines Kinvarra Bay) The poster background shows the Burren hills, and in
the foreground, rafts and boats belonging to local fisherman Rainier Krause,
who very kindly gave us permission to use them for our study in the bay.
Two Microcat conductivity-temperature-depth loggers were placed in
Kinvarra bay from Nov 2006 to Jul 2007, recording at 10 minute intervals. Each
was hung 1m below a floating raft placed in the bay for rope mussel cultivation.
5
4
3
2
1
04 Feb 2007
03 Feb 2007
03 Feb 2007
02 Feb 2007
31 Jan 2007
02 Feb 2007
31 Jan 2007
01 Feb 2007
30 Jan 2007
30 Jan 2007
29 Jan 2007
28 Jan 2007
28 Jan 2007
27 Jan 2007
27 Jan 2007
26 Jan 2007
25 Jan 2007
25 Jan 2007
24 Jan 2007
23 Jan 2007
23 Jan 2007
22 Jan 2007
22 Jan 2007
21 Jan 2007
20 Jan 2007
20 Jan 2007
19 Jan 2007
19 Jan 2007
18 Jan 2007
17 Jan 2007
17 Jan 2007
16 Jan 2007
downloaded, and returned to the water. Discharge rates were calculated using
the tidal prism method (see below). Samples of water from 2 groundwater
resurgences flowing directly into the bay were taken over the winter and
analysed for nitrite, nitrate and phosphate using a flow injection analyser (Lachat
16 Jan 2007
0
14 Jan 2007
As the rafts rise and fall with the tide, the Microcats were maintained at ~1m
depth at all times (Fig.1a-c). They were recovered approximately once a month,
6
30
25
20
15
10
5
0
15 Jan 2007
Salinity
(d) 35
Tide height (m)
Methods
Fig. 1 (a) Location map of known groundwater resurgences on the southern Galway Bay
coastline. Box outlines Kinvarra Bay, with Microcat positions
(b) Microcat being deployed from a raft for rope- mussel aquaculture
(c) Downward looking view of Microcat hanging ~1m below the water surface
(d) Example of correspondence between tide height and salinity
FIA QC8000). Nutrient fluxes were calculated as discharge x concentration.
(Table 1). Effective rainfall data are from NUI Galway, northwest of Kinvarra, and
from Mellowes College, Athenry, inland in the catchment.
Fresh water Outflow from Kinvarra
Fresh Water outflow at Kinvarra
Fresh
ater Outflow
from Kinvarra
RainfallW
volume
over catchment
2
100
1
26 Apr 2007
28 Apr 2007
Fig. 2 Calculated fresh water outflow per day (2 ebb tides) from Kinvarra Bay (columns). Daily rainfall volumes calculated from stations at NUI Galway and Athenry (black line).
Calculation of discharge using the Tidal Prism
Data from Microcat 3115 was used to calculate the fresh water outflow from the
bay, using the tidal prism method, where Vprism = Area x height
Vprism = vol of water removed from the bay on a falling tide in m3
Area = the surface area of the bay in m2
Height = the height difference between high tide and low tide in m
Proportion of fresh water in the tidal prism = 1- Sprism/Ssea
The salinity observed at high tide on each tidal cycle (Fig. 1d) is taken to
represent the salinity of seawater entering the bay from outside, Ssea. This takes
into account that some of the outgoing brackish water will be returned on the
incoming tide. The average of high and subsequent low tide salinity values is
taken to be the salinity of the tidal prism, Sprism. The volume of fresh water
removed on each ebb tide is then calculated and each pair of tides for a day is
summed, then divided by 86400 to get an average flow in m3 s-1 for each day.
For other methods of calculating submarine groundwater discharge see (8).
References
1. Johannes, R. E., 1980. The Ecological Significance of the Submarine Discharge of Groundwater. Marine Ecology-Progress Series 3, 365-373.
2. Giblin, A. E., Gaines, A. G., 1990. Nitrogen Inputs to a Marine Embayment - the Importance of Groundwater. Biogeochemistry 10, 309-328.
3. Moore, W.S., 1999. The subterranean estuary : a reaction zone of groundwater and sea water. Marine Chemistry 65(1-2),pp.111-125.
4. Burnett, W. C. et a. 2003. Groundwater and pore water inputs to the coastal zone. Biogeochemistry 66, 3-33.
5. Slomp, C.P & Cappellen., 2004. Nutrient inputs to the coastal ocean through submarine groundwater discharge: controls and potential impact. J. of Hydrology 295 pp. 64-86.
6. UNESCO, 2004. Submarine groundwater discharge: management implications, measurements and effects. UNESCO. Report no. IHP-6 Series on Groundwater no 5/IOC
Manuals and Guides no. 44.
7. Drew, D. P., Daly, D.,1993. Groundwater and karstification in mid-Galway, south Mayo and north Clare. Geological Survey of Ireland. GSI report series RS93\3. Dublin.
8. Burnett W. C. et al. 2006. Quantifying submarine groundwater discharge in the coastal zone via multiple methods. Sci. Tot. Environment, Volume 367, Issues 2-3, 31
August 2006, p 498-543
Acknowledgements
We were assisted in part of this work by undergraduate students Mirek Matras, Brian Clancy and Joey O’Connor. We
thank Mr Rainier Krause for allowing us access to his aquaculture rafts, and for ferrying us to and fro. David Drew
provided much of the background knowledge of the karst system in this area. Ronan O’Toole walked the coastline in
summer 2005 marking all the freshwater outflows to the intertidal zone. This work was funded initially by a Millenium
grant from NUI Galway, a much larger follow-up programme was funded from 2007 by the Geological Survey of Ireland
as part of its Griffiths programme, see www.gsi.ie
Table 1: N and P concentrations and loads from the two principal known resurgences in Kinvarra Bay. The loads are
calculated from their averaged concentrations. Between them, these resurgences account for a discharge of ~10m3/s.
Table 1
Date
09/11/2006
21/11/2006
21/12/2007
10/01/2007
18/01/2007
25/01/2007
31/01/2007
Flow
3
m /s
8
64
27
15
37
22
25
Castle
Arch
Average
(NO2+NO3)-N (NO2+NO3)-N (NO2+NO3)-N
g/L
3377
5478
3100
2900
2870
2680
2520
g/L
1166
2442
938
1080
1020
1050
722
kg/d
1579
22050
4754
2562
6180
3605
3534
Castle
Arch Average
PO4-P
PO4-P
PO4-P
g/L
10.9
20.7
5.51
5.7
7.89
6.27
4.96
g/L
30.7
18.2
9.04
8.74
9.86
8.72
9.19
kg/d
14
108
17
9
28
14
15
Results
Due to natural storage in the catchment in turloughs and subsurface, the
restricted nature of groundwater outflow, and the range of conduits active at
different water table levels, the daily volumes of fresh water discharge do not
closely follow the rainfall pattern. Rainfall minus evaporation data give an
expected average discharge from the catchment of ~30m3 s-1. About 8m3 s-1 are
accounted for by man-made surface rivers entering the sea in the northern end of
the catchment. The average discharge calculated by the tidal prism method for
Kinvarra Bay for Nov-June is 22m3 s-1. This indicates that the bay is the focal
point for a significant proportion of the total discharge from the catchment, and
will therefore bear the brunt of any nutrient or contaminant input. The nutrient
concentrations measured so far indicate that the load of Nitrogen (as NO2 and
NO3) entering the bay (and exiting it in winter) is of the order of several tonnes
per day, while the Phosphate load is a few tens of kg per day. Related work
indicates that ammoniacal-N loads are comparable to the oxidised nitrogen loads.
Based on research grant-aided by the Department of Communications, Energy and Natural Resources under the National Geoscience
Programme 2007-2013. The views expressed in this study are the author's own and do not necessarily reflect the views and opinions of the
Minister for Communications, Energy and Natural Resources.
30 Apr 2007
19 Apr 2007
21 Apr 2007
23 Apr 2007
16 Apr 2007
14 Apr 2007
12 Apr 2007
09 Apr 2007
05 Apr 2007
07 Apr 2007
02 Apr 2007
31 Mar 2007
29 Mar 2007
26 Mar 2007
24 Mar 2007
22 Mar 2007
19 Mar 2007
17 Mar 2007
15 Mar 2007
12 Mar 2007
10 Mar 2007
08 Mar 2007
05 Mar 2007
03 Mar 2007
28 Feb 2007
01 Mar 2007
26 Feb 2007
23 Feb 2007
21 Feb 2007
19 Feb 2007
16 Feb 2007
14 Feb 2007
12 Feb 2007
09 Feb 2007
07 Feb 2007
05 Feb 2007
02 Feb 2007
31 Jan 2007
29 Jan 2007
26 Jan 2007
24 Jan 2007
22 Jan 2007
19 Jan 2007
17 Jan 2007
15 Jan 2007
8 Jan 2007
12 Jan 2007
10 Jan 2007
1Jan 2007
3 Jan 2007
5 Jan 2007
27 Dec 2006
28 Dec 2006
30 Dec 2006
23 Dec 2006
25 Dec 2006
18 Dec 2006
19 Dec 2006
21 Dec 2006
12 Dec 2006
14 Dec 2006
16 Dec 2006
08 Dec 2006
10 Dec 2006
03 Dec 2006
05 Dec 2006
07 Dec 2006
28 Nov 2006
29 Nov 2006
01 Dec 2006
22 Nov 2006
24 Nov 2006
26 Nov 2006
17 Nov 2006
19 Nov 2006
20 Nov 2006
11 Nov 2006
13 Nov 2006
0
15 Nov 2006
50
Rainfall m3 x 107
3
150
09 Nov 2006
m3/s of fresh water
200