Geochemistry of a coastal karstic groundwater resource
located in Western Ireland
*
Barbara Petrunic , Garret Duffy, Tiernan Henry, & Rachel Cave
1
Earth and Ocean Sciences, National University of Ireland, Galway, Ireland
*
Corresponding Author: barbara.petrunic@nuigalway.ie
The karst groundwater resources located within the Burren and Gort lowland regions have important agricultural, ecological, commercial and domestic
significance in Ireland. The hydraulic connection between these groundwater sources and the coastal zone makes these systems particularly vulnerable to
saltwater intrusion, which may result in the salinization of the freshwater resource. The overall objective of this study was to build on previous
investigations and assess the groundwater chemistry in these coastal karstic regions. These data were used to understand the spatial variation in major ion
chemistry and nutrient levels of the groundwater sources in the region, and the extent of saltwater intrusion within the groundwater environment.
Figure 1. Location of borehole wells identified and used in this study.
Figure 2. Piper diagram for samples collected in August 2009.
Galway
l
+C
80
%
4
40
SO
%
20
geochemical sampling and unused
17
21
wells were monitored for water
% Seawater in samples (based on [Cl])
levels. The well locations are
20
%
3
O
%
Group
1
B
samples
Group
2
Nitrate concentrations are below the European
communities drinking water parametric value of 50 mg
NO3/L* and the European communities groundwater
threshold value of 37.5 mg NO3/L** in all but one
sample (not shown). Higher nitrate concentrations
may be expected in the groundwater due to poor
degradation conditions, but dilution is likely a major
control in the karst system.
S
samples
Aug 08
Feb 09
May 09
Aug 09
0.6
1.2
Salinity (ppt)
DOC values are generally higher in May 2009 samples,
likely due to agricultural activities.
Figure 5. Results from datalogger installed in well 50.
10
50
12
60
1
13
0
Well 50
16
11.5
14
16/02/2010
12:00:00
17/02/2010
00:00:00
17/02/2010
12:00:00
18/02/2010
00:00:00
Galway Bay Tide
Height above Malin Head (m)
-4
-3
-2
Temperature (°C)
Depth to water (m)
11.0
2
Galway Bay Tide
Well 50
1
0
13
-2
10.5
Neap
tide
data
Spring
tide
data
-1
Aug-09
%
%
10
0
6
5
4
3
2
1
0
18
16
14
12
10
8
6
4
2
0
2 3 4 5 8 9 10 12 14 17 21 23 24 30 33 34 35 38 41 42 45 48 49
Well ID
Seawater
affected
locations
1
Galway
S wells
B wells
B wells (limited data)
Tidally
affected
locations
Galway
Wells - tidal
Wells - not tidal
-3
14
23/12/2009
00:00:00
Figure 6. Maps denoting zones of seawater and tidal influence in the study area.
23/12/2009
12:00:00
24/12/2009
00:00:00
24/12/2009
12:00:00
25/12/2009
00:00:00
Date & Time
Galway
Bay
Six unused wells were monitored for water
levels and, if possible, conductivity and
temperature.
0
Galway
Bay
2
3
4
03/11/2009 23/11/2009 13/12/2009 02/01/2010 22/01/2010 11/02/2010 03/03/2010
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
Date & Time
May-09
20
3
-1
4400
Feb-09
30
Neap tide
Specific Conductivity
Temperature
*
European communities (Drinking Water) (No. 2) regulations, 2007
(S.I. 278 of 2007).
**
European communities environmental objectives (groundwater)
regulations, 2010 (S.I. No. 9 of 2010).
-3
16/02/2010
00:00:00
12
4600
-1
-2
14
70
4800
Specific Conductivity (µS/cm)
8
Height above Malin Head (m)
(recorded at 9AM,
cumulative value over
previous 24 h)
Depth to water (m)
Depth
to
water
6
Depth to water (m)
Rainfall (mm)
40
2
4
Rainfall
Although water temperatures may be regulated by
heat generated by downhole pumps, general trends
show lower temperatures in winter (Feb 2009)
compared to other months. Some wells show a
greater range in temperature (e.g., 2, 3, 4, & 5), likely
due to more unstable flow conditions.
3
Spring tide
Galway Bay Tide
2
20
30
12
Height above Malin Head (m)
0
Nitrate (mg NO3/L)
30
Aug-08
Dissolved Organic Carbon (mg/L)
S samples are in the shaded zones.
0
Wells 3 & 48 are B samples in May 2009.
Higher [K] in well 48 (not evident in Piper
diagram) may be due to anthropogenic
contamination. Seawater contribution
is small in most wells, therefore
difficult to detect using these
methods.
40
Temperature (°C)
0
250
0
10
80
%
80
%
Dissolved Chloride (mg/L)
Figure 4. Nitrate concentrations, DOC
concentrations and temperature values
collected on groundwater samples.
0
0
60
%
40
%
Mg
40
60
%
Cl
Well 45
S samples in Group 2 are wells
that are intermittently influenced
by seawater (wells 3 & 48). The
extent of seawater influence in
these wells will depend on local
conditions such as precipitation,
groundwater recharge, and water
extraction rates.
SO4
%
%
20%
Ca
16000
Other trends
40%
80
%
%
20
60
%
500
40
80
Figure 3. Groundwater salinity and chloride concentrations.
60%
%
20%
40
60%
40%
80%
60
10 km
Wells 2, 3, 48: < 0.5% (May & August 2009); Well 4:
~ 2% (May & August 2009); Well 45: ~ 25% (May
2009); ~ 70% (August 2009).
20
N18
+K
N67
80%
m.
Gort
Na
36 34
%
rounded to the nearest 1000
HC
24
14
S samples in Group 1 are
influenced by seawater (based on
[Cl]), but the well water has low
total meqs. As a result, salinity is
not an accurate indicator of
seawater influence in this well
(well 2).
Higher [Na] & [Cl] compared to B samples; Near coastal wells
affected (2, 3, 4, 45, & 48); Well water at one location
deviates from mixing line suggesting additional geochemical
processes are affecting the water chemistry.
Functioning wells were used for
30
20
23
35
Saltwater-influenced (S) samples
g
12
42
& 43
54 2
+M
9
49
51 29
Ca
8
25
41
%
10
50
48
60
5
Mixing
line
%
3&4
20
38
33
Bicarbonate type waters; Mg and Ca content varies in B waters; Higher
Mg generally found in samples from Gort lowlands, likely a result of
contact with dolomitic layers.
Seawater
from the coastline.
45
%
Ballyvaughan
40
Clare and approximately 10 km inland
Kinvarra
%
%
Unused well
Galway to the town of Ballyvaughan, Co.
Functioning well
Galway
Bay
60
80
of Galway Bay near the town of Kinvarra, Co.
%
Background (B) samples
The study area extends along the southern coast
Tidal influence: wells 29, 50, 51, 54
Seawater influence: well 50
No tidal influence: wells 25, 36
Water levels respond to Galway Bay tides. The time lag, calculated during periods of low
precipitation, is <2 h. Rainfall events take approximately 2 to 3 days to impact water
levels in the well.
Assistance in the field by Eoin MacAleer, Catherine Elder and
John Coyne is gratefully acknowledged. Rainfall data was
collected by Frank Gaffney. The authors also wish to thank
all the well owners that participated in this study.
Includes Ordnance Survey Ireland data reproduced under
OSi Licence number NUIG200803.
Unauthorised reproduction infringes Ordnance Survey
Ireland and Government of Ireland copyright.
© Ordnance Survey Ireland, 2011.
N67
Gort
N67
N18
N18
10 km
Gort
Wells used for geochemical
analysis were not monitored
for water levels, but those
within the saltwater zone are
likely tidal. Similarly,
geochemical evidence is
required to determine if the
unused wells are within the
seawater affected zone.
However, based on measured
conductivity values only one
unused well is clearly within
the seawater affected zone
(Fig. 5).
10 km
Galway Bay tidal data provided by the Marine Institute, Ireland.
Funding for the project was provided by the NUI Galway Griffith
Geoscience award. 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.
Petrunic, B., Duffy, G., Henry, T., and Cave, R., 2011, Geochemistry of a coastal karstic groundwater resource located in Western Ireland, Environ 2011, 21st Irish Environmental Researchers’ Colloquium, Cork, Ireland.