Journal of Environmental Radioactivity 130 (2014) 68e71
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Journal of Environmental Radioactivity
journal homepage: www.elsevier.com/locate/jenvrad
Short communication
Application of 234U/238U activity ratios to investigations of
subterranean groundwater discharge in the Cádiz coastal area
(SW Spain)
J.F. Rodrigo a, M. Casas- Ruiz a, *, J. Vidal b, L. Barbero c, M. Baskaran d, M.E. Ketterer e,1
a
Departamento de Física Aplicada, Universidad de Cádiz, Av. República Saharaui s/n, 11510 Puerto Real, Cádiz, Spain
Departamento de Construcciones Navales, Universidad de Cádiz, Av. República Saharaui s/n, 11510 Puerto Real, Cádiz, Spain
Departamento de Ciencias de la Tierra, Universidad de Cádiz, Av. República Saharaui s/n, 11510 Puerto Real, Cádiz, Spain
d
Department of Geology, Wayne State University, Detroit, MI 48202, USA
e
Department of Chemistry and Biochemistry, Northern Arizona University, Flagstaff, AZ 86011-5698, USA
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 May 2013
Received in revised form
26 November 2013
Accepted 3 January 2014
Available online 23 January 2014
The activity ratios of 234U/238U were used to investigate processes of subterranean groundwater
discharge into coastal marine waters in a study location at Bay of Cádiz (southwest Spain). Marine waters
in the bay and surrounding open ocean exhibited U concentrations of 3.4 0.1 mg/L and activity ratios of
1.15 0.01, in agreement with the expected composition of seawater (234U/238U activity
ratio ¼ 1.148 0.002). Three water samples obtained from the discharge zone of the Guadalete River
exhibited activity ratios of 1.17e1.22 along with slightly lower U concentrations compared to seawater,
which is likely due to mixing between seawater and a groundwater end-member. One possible source of
groundwater was characterized by sampling and analyzing a well water sample collected in the
neighboring village of El Puerto de Santa María; this water sample exhibited an activity ratio of
1.34 0.03 and a U concentration of 1.22 mg/L. Water from the Guadelete River estuarine zone can be
explained to result from a two-component mixture of seawater and groundwater from the El Puerto de
Santa María well; however, if there are several groundwater reservoirs with different U activity ratios
that discharge to the coastal water, then, it may be difficult and more studies are being conducted to
address this issue.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
ICP-MS
234
U/238U activity ratios
Coastal aquifers
Submarine groundwater discharge (SGD)
1. Introduction
A significant fraction of the world’s population lives in locations
near the coastal ocean where the populace relies upon fresh water
from coastal aquifers. The eventual fate of coastal aquifers is to mix
with the sea through subterranean discharge. In many coastal
areas, groundwater is over-drawn, as is the case in Florida (USA).
With over-pumping of groundwater, the water table lowers, leading to entry of seawater into the aquifer and destruction of the fresh
water resource. Accordingly, a key problem in groundwater hydrology in coastal areas is to understand the possible hydrological
connections between groundwater and coastal seawater.
Submarine groundwater discharge (SGD), has been recognized
as an important component of the hydrological cycle. This
* Corresponding author. Tel.: þ34 956016080.
E-mail address: [email protected] (M. Casas- Ruiz).
1
Present address: Metropolitan State University of Denver, PO Box 173362,
Campus Box 52, Denver, CO 80217-3362, USA.
0265-931X/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jenvrad.2014.01.004
discharge includes meteoric water from land drainage as well as
seawater that has entered coastal aquifers. Burnett and Dulaiova
(2003) define SGD as any and all flow of water on continental
margins from the seabed to the coastal ocean, regardless of fluid
composition or driving force.
The process of SGD is increasingly being recognized as an
important factor in the understanding and sustainable management of coastal freshwater aquifers, especially in many highly
populated areas of the world. In addition, SGD represents a significant pathway for transfer of disolved solutes between the land
and the sea (Moore, 1999; Charette and Sholkovitz, 2002), which
can lead to contamination of coastal marine waters.
Many studies have applied a variety of methods to estimate SGD
(Burnett et al., 2006). Isotopic measurements such as 222Rn, 226Ra,
228
Ra, 224Ra, 223Ra, 87Sr, 3He, 4He, 3H, 14C, 234U, and 238U can serve as
key indicators of fluxes across the groundwater/marine interface.
Of these, the U- and Th-series decay products 222Rn, 226Ra, 228Ra,
224
Ra, and 223Ra have been used most extensively (Charette et al.,
2008).
J.F. Rodrigo et al. / Journal of Environmental Radioactivity 130 (2014) 68e71
69
Fig. 1. Hydrogeological map of the area and positions of sampling stations.
In this work, we have investigated U isotopes as a possible
means of assessing mixing between groundwater and seawater.
The basis of this method is that 234U and 238U commonly exhibit
disequilibrium in open-system aqueous systems. Under closedsystem conditions, 234U and 238U are in secular equilibrium with
an activity ratio (AR234/238) of unity and an atom ratio of
0.00005472 0.00000011 (Chen et al., 1986). In contrast, waterrock interactions in the Earth’s surficial environment tend to
result in activities of 234U exceeding that of 238U primarily due to
recoil of 234U, preferential leaching of 234U compared to 238U.
When 238U undergoes alpha decay in mineral grains, some of the
recoiled 234Th can be released into the surrounding aqueous
phase. This 234Th atom undergoes decay to 234U via 234Pa. The 234U
that is present on mineral surfaces resulting from the decay of
234
Th is more readily leachable, and the recoiled 234U is also more
easily leachable from the recoil-damaged latticed sites. The cumulative effects of 234U - 238U disequilibria are evident by
considering that the oceans exhibit a w15% excess of 234U, with
AR234/238 ¼ 1.148 0.002 (Shen et al., 2002). The oceans are wellmixed, and dissolved U has a residence time of w500 ky; hence,
this activity ratio as well as the dissolved U concentrations are
both very consistent in seawater. Nevertheless, both U concentrations and AR234/238 can differ due to mixing between seawater
and fresh water sources, such as subterranean discharges. The U
concentration and activity ratio can be used as a quantitative
tracer of fresh water and sea water mixing processes (Zielinski
et al., 1997) with potential applicability to coastal zones
(Baskaran et al., 2009).
Another motivation for specifically studying U is that there has
been a growing interest in monitoring various radionuclides that
can affect directly human health (Suksi et al., 2006). There are few
studies of this kind in the area of the Bay of Cadiz, located in
southwestern Spain. To meet these objectives, we have conducted a
preliminary feasibility study of water from seawater monitoring
locations along, river stations and groundwater locations. U isotopes and concentrations have been determined herein by the
technique of inductively coupled plasma mass spectrometry (ICPMS) (Halicz et al., 2000; Ketterer et al., 2000).
2. Materials & methods
As a feasibility study, a preliminary set of 12 water samples were
collected in July 2010; Fig. 1 depicts the study site and the sampling
locations; GPS coordinates are given in Table 1. Samples 1e7
represent coastal seawater; Samples 8e10 were obtained from the
mouth of the Guadalete River, and likely represent seawater-SGD
mixtures. Samples 11 and 12 consisted of groundwater obtained
from residential wells. Fig. 1 also depicts the hydrogeology of the
study site; hatched areas correspond to aquifers (SIAS, 2013).
Sample 11 (Cantarranas) was obtained from the El Puerto de Santa
María aquifer, and Sample 12 (Chiclana) is located within the
Puerto Real-Conil aquifer.
Seawater and groundwater samples were obtained in 500 mL
polyethylene containers at depths of 1 m below the surface. All
samples were filtered through 0.45 mm nitrocellulose filters
immediately after returning to the laboratory. After filtration, each
sample was acidified with 3 mL of 16 M HNO3. 45 mL aliquots were
used for 234U/238U activity ratio determinations; these were mixed
with 5 mL of 16 M HNO3 in order to enhance the recovery of U from
the water matrix. Under these conditions, the UTEVA resin removes
Table 1
Locations of the sample stations.
Sample number
1
2
3
4
5
6
7
10
9
8
11
12
Sample id.
RA-1
RA-2
RA-3
RA-4
RA-5
RA-6
RA-7
RG-1
RG-2
RG-3
Cantarranas
Chiclana
Position
Latitude ( N)
Longitude ( W)
36 25,6640
36 24,0140
36 21,0680
36 35,8570
36 36,4540
36 42,7500
36 40,8300
36 34,6720
36 35,2180
36 35,8270
36 37,3590
36 24,3910
6 15,2340
6 14,3370
6 11,4980
6 17,1940
6 18,2780
6 26,8090
6 25,8520
6 14,4270
6 13,9220
6 13,3510
6 14,5040
6 05,3840
70
J.F. Rodrigo et al. / Journal of Environmental Radioactivity 130 (2014) 68e71
Table 2
AR234/238 and U concentrations for each of the sampling stations.
Sample id
AR234/238a
RA-1
RA-2
RA-3
RA-3(Dc)
RA-4
RA-5
RA-6
RA-7
RG-1
RG-2
RG-3
RG-3(Dc)
Cantarranas
Chiclana
1.143
1.148
1.149
1.158
1.151
1.149
1.155
1.135
1.167
1.181
1.215
1.216
1.336
1.154
0.013
0.013
0.010
0.003
0.007
0.011
0.011
0.015
0.008
0.010
0.013
0.009
0.030
0.018
U (mg/L)b
3.34
3.36
3.37
3.39
3.38
3.39
3.52
3.45
3.23
2.91
2.17
2.28
1.22
1.10
a
The accepted AR234/238 value for seawater is 1.148 0.002.
The one-s uncertainties in U concentrations are conservatively estimated as
0.05 mg/L. The detection limit for determination of U in water by this procedure
(after 100 dilution) is about 0.02 mg/L.
c
The letter D means duplicate sample.
b
the saline matrix and pre-concentrates U with recoveries of >80%.
Uranium was pre-concentrated from the HNO3-fortified water
samples using lab-fabricated 100 mg UTEVA resin columns
(Ketterer et al., 2000). After passing the water sample through the
UTEVA column, each column was rinsed with three successive 1 mL
portions of 2 M HNO3. Thereafter, U was eluted with the following
sequence: 1 mL H2O, 1 mL 0.05 M ammonium oxalate, and 1 mL
H2O. Uranium activity ratios are determined experimentally by
measuring the atom ratio 234U/235U; this atom ratio is related to the
234 238
U/ U
atom
ratio
by
the
constant
value
of
(235U/238U ¼ 0.0072527). The 234U/238U atom ratio is converted into
an activity ratio using the specific activities of 234U and 238U; at
secular equilibrium, the 234U/238U atom ratio has a value of
0.00005472 0.00000011 (Chen et al., 1986). A solution of U
prepared from modern coral, having the same activity ratio value as
seawater, was used to control and correct for mass discrimination
in the ICP-MS measurements.
U concentrations were determined in separate 100-fold diluted
water samples that were spiked with 233U. The 233U solution was
prepared from material obtained from IRMM-057 (Geel, Belgium).
The 233U tracer solution was calibrated contemporaneously with
the concentration measurements by reverse isotope dilution vs. a
commercially obtained 238U solution (CPI Scientific, USA).
Along with the 12 individual samples, two samples were prepared and analyzed in duplicate (RA-3 and RG-3). Three blanks
(composed of deionized water) were prepared and analyzed along
with the samples. The analyses of these blanks reveal no significant
contribution in either the activity ratio preparations or in the
diluted samples used for concentration measurements, although
background signal contributions were nevertheless subtracted.
The U activity ratios were obtained with a Thermo X Series II
quadrupole ICP-MS equipped with an APEX sample introduction
system and PFA Teflon nebulizer. The Thermo quadrupole ICPMS
typically exhibits very stable mass discrimination behavior, which
is advantageous in this analysis.
The U concentration results were obtained using a VG Axiom
sector-field ICP-MS, also equipped with an APEX sample introduction system and PFA Teflon nebulizer. The one-s uncertainties
in U concentrations are conservatively estimated as 0.05 mg/L,
and the detection limit for determination of U in water by this
procedure (after 100x dilution) is about 0.02 mg/L. Due to a temporary maintenance issue for the quadrupole ICPMS, the sectorfield ICPMS was used for these analyses. It is noted that it is
easier to collect both sets of data on the same instrument, and if
appropriate amounts of a pre-calibrated 233U spike are added to the
sample before separation, both concentrations and activity ratios
can be collected in the same mass spectrometric run, both provided
similar ion-count rates at sample-uptake rates of 0.3e0.4 mL/min.
The electrostatic scanning mode with 10 ms dwell time was used to
switch rapidly between the center of the flat-top region of each
mass spectral peak to acquire 3e5 sequential integrations of
w1 min each from each sample solution.
3. Results and discussion
The distribution of U has been shown to be affected by coastal
groundwater or SGD in estuaries (Charette and Sholkovitz, 2006;
Swarzenski and Baskaran, 2007; Windom and Niencheski, 2003).
As U concentrations are often lower in coastal aquifers than in
seawater, measurement of both U concentrations and AR234/238
provides a sensitive indicator of coastal groundwater or SGD influence in estuaries.
The activity ratio results indicate that most of the values are very
close to the expected ratios for seawater (Table 2) although the data
set spanned from 1.135 0.015 to 1.336 0.030. For the samples
within analytical error range of seawater (Fig. 2), the U concentrations are also found to agree well with the expected seawater
Fig. 2. AR234/238 schematic representation for each of the samples.
J.F. Rodrigo et al. / Journal of Environmental Radioactivity 130 (2014) 68e71
71
methods in order to decrease the uncertainties in the measured U
activity ratios. A broader-scope sampling of additional sources of
potentially contributing groundwater supplies is also warranted.
Acknowledgments
This research has been supported by the Spanish Department of
Science and Technology through the project “Determination of
scavenging rates and sedimentation velocities using reactiveparticle radionuclides in coastal waters; application to pollutants
dispersion” (Ref.: CTM2009-14321-C02-02).
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Fig. 3. Mixing plot of AR234/238 vs.1/[U].
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