FREEZE - Submarine FREshwater dischargEs: characteriZation and Evaluation
study on their impact on the Algarve coastal ecosystem;
PTDC/MAR/102030/2008
3rd REPORT on the water springs at Olhos d’Água, Algarve, Portugal – Hydrological
year May 2010–April 2011.
Helena Amaral, Judite Fernandes, Augusto Costa – Groundwater Unit of LNEG.
September 2012
1. INTRODUCTION AND METHODOLOGY
During the hydrological year of May 2010 to April 2011, six field campaigns were undertaken at
Olhos d’Água beach, and adjacent smaller beaches, to identify, locate and monitor the several
groundwater springs, as part of the overall Submarine Groundwater Discharge (SGD) study
focused by the project FREEZE. The six field campaigns took place in 2010 [May (spring), August
(summer) and November (autumn)] and in 2011 [February (winter), March and April (spring)],
hence covering the four seasons during one hydrological year. All fieldwork was carried out
during low tide (most of the times, the lowest tide in that month) to allow access to the springs,
which are mainly found in the intertidal zone of the beach, and are therefore inaccessible during
high tide.
According to field observations [by Pedro Terrinha], the springs occur within the karst
limestone of Miocene age, which is overlaid by the Pleistocene red sands, where no groundwater
spring was found in the study area; no discharge was found to occur in the contact between the
two geological units either.
Detection of the springs was firstly eye-spotted: in the sand and in the rock the water
pops-out, whereas in the sea it is (sometimes) possible to identify a SGD by a different seawater
transparency and texture. To verify if the water discharge in the intertidal zone corresponded to
freshwater, basic field parameters were measured in situ, such as electrical conductivity and
dissolved oxygen, which alone were found to be efficient freshwater indicators. The
physicochemical parameters (temperature, electrical conductivity (EC), total dissolved solids,
pH, dissolved oxygen, redox potential, salinity and pressure) were measured in situ by using a
multi-parameter electrode (Model HI9828, Hanna Int.).
The locations of the sampled springs were obtained by GPS measurements, with a typical
error of 3 m (GPS numbering can be seen in Fig. 1A). Several GPS measurements were done
over the year to be sure that the springs were the same. Most springs had its discharge at the
same location, and were easily recognized in the field (see Fig. 1B). The springs ‘Beach’, ‘Salty’
and ‘Sand’ were not always found at exactly the same location, but some meters apart in relation
to a previous field campaign - this was due to sand cover (beach dynamics). Besides the
geographical proximity, the physicochemical parameters allowed to confirm the spring, which
are depicted in Fig. 1B.
The waters bubbling out of the sand required special sampling, which was done by
placing the smaller aperture of a half PET bottle into the exit point of the discharging water (see
photo 7). The target water was then collected with the least ‘seawater-contamination’ possible.
The bubbling springs were ‘Water’ and ‘Fuzzy’, and in one field campaign, ‘Bubble’ (Fig. 1B and
Photos 1B, 3 and 6).
Radon was measured in some springs, for all campaigns except May 2010. Alkalinity,
nitrate and nitrite were determined in situ in the last two field campaigns (March and April
2011), as well as the flow rate Q of the spring ‘Olheiro’, which was determined using a current
meter.
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Figure 1: A – Location of the sampled waters (identified by the GPS number), each color corresponds to a date
of fieldwork (see Table A.1. in Appendix for relation between GPS number and spring properties). B –
Interpretation of the several GPS locations to a single spring based on proximity and characteristic of the
sampled waters; colors correspond to mean electrical conductivities [blue=less mineralized, orange=most
mineralized]; ‘S.’ is for Spring.
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A
B
Photo 1: ‘Olheiro’ spring, bubbling out from the sand, November 2010 (A) and flowing out from a fracture,
April 2011 (B). In November, the spring discharging point was covered due to sand accumulation. In April,
there was no sand and the water was flowing out from within the Miocene limestone.
Photo 2: ‘Jet’ spring, February 2011.
Photo 3: ‘Fuzzy’ spring, February 2011.
Photo 4: ‘Beach’ spring, March 2011.
Photo 5: ‘Rocks’ spring, March 2011.
Photo 6: ‘Water’ spring, May 2010.
Photo 7: ‘Sand’ spring, May 2010.
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2. RESULTS
Table 1 depicts the physicochemical parameters measured in situ in all springs over one
hydrological year. Some springs were not measured in all field campaigns because they were not
detected, mainly due to being covered by sand hence its discharge location was not found. The
table also shows the Radon (222Rn) concentrations (measured in the laboratory, and discussed
further in the manuscript), as well as the alkalinity, nitrate and nitrite concentrations, and the
‘Olheiro’ flow rate.
Table 1: Sampled springs and seawater during six field campaigns undertaken in 2010 and 2011 - see Fig. 1B
for spring location. (Empty cells mean no measurement was done.)
The groundwater discharge occurs in a relatively diffuse way along the sampled ~400 m
coastline (Fig. 1), where the location (and flow) of discharge seemed to be more tide-dependent
rather than season-dependent. In general, the most freshwater springs were located in areas
where the local relief is protruding (see Fig. 1B), whereas the waters showing a greater mixing
with seawater were found in more sandy areas.
Based on the mean EC values, and indistinctively of season, the springs can be classified
into three main groups (Fig. 1B):
A) A predominantly freshwater group with mean EC up to ~5’550 µS/cm (springs
‘Beach’, ‘Rocks’, ‘Sand’, ‘Fuzzy’, ‘Jet’ and ‘Olheiro’ – blue and green dots)
B) A freshwater-seawater mix group with mean EC up to ~14’300 µS/cm (springs
‘Bubble’, ‘Water’, ‘Rock’, ‘Soup’ and ‘Salty’ – yellow and orange dots)
C) Seawater with EC up to 55’000 µS/cm.
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The springs, ‘Olheiro’, ‘Water’, ‘Bubble’ and ‘Jet’ were the easiest springs to locate. The easiest
spring to sample was ‘Olheiro’, because there was no contact with seawater and most of the
times it flowed out directly from the rock, thus being the sample having the least air contact.
2.1
MAJOR PARAMETERS
Seawater has a distinct composition with very high salinity (up to 34 %), very high
electrical conductivity (up to 55 mS/cm), basic pH (8) and dissolved O2 concentrations typical of
waters in equilibrium with the atmosphere, i.e. ~10.5 mg/L (see Fig. 2). Seawater showed the
highest water temperature variation, as expected due to its exposure to the atmosphere.
The springs mainly represent freshwater. Compared to seawater, the springs showed
much lower electrical conductivity values, ranging from ~2’300 to 10’000 µS/cm (some springs
on some seasons go up to 14’000 µS/cm), and lower dissolved oxygen mainly between 5-6.6
mg/L; the very high oxygen values of March 2011 show inappropriate sampling due to air
contamination, or a badly calibrated oxygen probe. All springs showed a pH close to neutrality.
The mean temperature of all springs was around 19 °C, as well as that of seawater. The
springs ‘Rocks’ (n=3), ‘Water’ (n=6) and ‘Olheiro’ (n=6) showed very low temperature variation,
thus showing less seasonal response. Such water temperature stability reflects the mean air
temperature of the area and implies longer groundwater residence times or less mixing than the
adjacent springs, which show greater temperature oscillation throughout the hydrological year.
The EC of the springs showed mixing between freshwater and saltwater, in which
mixture processes most probably relate to ‘intertidal’ seawater intrusion.
Figure 2: Physicochemical parameters measured in situ for the identified springs (see Fig. 1B for locations).
Nitrite and ammonium were not detected and nitrate was found in all sampled springs
(April 2011) at concentrations ranging between 20-32 mg/L. The alkalinity of the springs
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ranged values typical of groundwater (245-310 mg/L), decreasing from March to April 2011.
That of seawater was also typical for such waters (135 mg/L).
2.2
TIME SERIES
The springs ‘Sand’, Olheiro’, ‘Water’ and ‘Bubble’ could be sampled in all field campaigns,
as opposed to the other springs that were discontinuously monitored. Those springs were used
to verify the variation of the main physicochemical parameters during one hydrological year
(Fig. 3). The water temperature was higher in the summer campaign except for the ‘Water’
spring that showed its highest temperature in November 2010; ‘Sand’ was the spring that
responded the most to the seasonality of air temperature variation (Fig. 3A). The ‘Water and
‘Bubble’ springs showed mean EC values typical of mixing between freshwater and seawater,
whereas the springs ‘Sand’ and ‘Olheiro’ showed a more freshwater signal. ‘Sand’ was also the
spring that showed a more constant EC (Fig. 3C) hence, its freshwater component was
predominant. The higher EC values were measured in the summer, which agrees with the fact
that in that season groundwater is less diluted because of less rainwater recharge. An exception
is ‘Sand’, which saw a decrease in EC. In the transition between winter and spring of 2011, all
springs increased in EC, except ‘Water’. Since this is the period of highest discharge, lower EC
values were expected (due to greater groundwater recharge, thus dilution). However, that was
not observed and could be due to another water component/source feeding those springs. The
pH of the springs steadily decreased to values lower than 7 in March 2011, increasing again in
April 2011. Hence, the waters discharging in March seem to have a lower residence time, with a
stronger meteoric imprint (Fig. 3B).
A
B
C
Figure 3: Temperature (A), pH (B) and electrical conductivity (C) in one hydrological year (6 field campaigns),
of the springs: ‘Bubble’, ‘Water’, ‘Sand’ and ‘Olheiro’.
To understand the relationship between the springs, and how the different parameters
vary seasonally, correlation coefficients were determined. Correlations were determined for two
cases: case 1) all springs simultaneously, for a distinct time of sampling and indicated parameter
(Fig. 4), and case 2) by comparing the springs individually, indistinctively of field campaign and
for one parameter at a time (Fig. 5).
Case 1: Good positive and (mainly) good negative correlations existed between the measured
parameters, including Radon (Fig. 4). The last field campaign (April 2011) was the only showing
very good correlations between all parameters, and in general, the winter-spring campaigns of
2011, showed good correlations. The first field campaign showed the least good correlations
(only good between temperature and pH). The correlation between any of two parameters
varied during the year, i.e. it was good positive, good negative or non-existent. Examples were
pH-Temperature, pH-EC, pH-Radon. Temperature-EC and Temperature-Radon were either non6
correlated or negatively correlated. The pair EC-Radon was either non-correlated or positively
correlated. The relation Temperature-EC was always negatively correlated, indicating an
apparent seasonal independence.
Figure 4: Plot of the correlation coefficients between temperature (T), pH, electrical conductivity (EC) and
radon, in one hydrological year (six field campaigns), at the springs: ‘Bubble’, ‘Water’, ‘Sand’ and ‘Olheiro’. The
table shows the values: the yellow cells correspond to good (<-0.6) negative correlations and the green cells
to good (>0.6) positive correlations.
Case 2: The hydrogeological connection between the springs and their seasonal response was
assessed using temperature, EC, pH and Radon (Fig. 5). The EC was only good correlated
(positively) between the springs ‘Bubble’ and ‘Water’. The pH was well correlated between all
springs, except in the pair ‘Water’-‘Olheiro’. The temperature and Radon were either well
correlated or not. Despite the proximity of the springs ‘Bubble’ and ‘Water’, there was no good
correlation between them for temperature and Radon. This seems to imply that different
processes (e.g., mixing) may affect the two springs, at least partly. The springs that correlated
best with each other were the pairs ‘Bubble’-‘Sand’, ‘Bubble’-‘Olheiro’, and ‘Sand’-‘Olheiro’,
hence, apparently indicating a similar hydrodynamic behavior. The pairs that worse correlated
were ‘Water’-‘Olheiro’ and ‘Water’-‘Sand’. Since the pair ‘Bubble’-‘Water’ only partially
correlated, it is understandable that one of the springs (‘Bubble’) correlates well with the less
mineralized waters ‘Olheiro’ and ‘Sand’, whereas ‘Water’ does not. Maybe this is due to a greater
seawater influence? However, it is difficult to explain why the springs ‘Bubble’ and ‘Water’ do
not correlate better, since they are only a few meters apart.
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Figure 5: Plot of the correlation coefficients between temperature (T), pH, electrical conductivity (EC) and
Radon measured within one hydrological year (six field campaigns) in four springs: ‘Bubble’, ‘Water’, ‘Sand’
and ‘Olheiro’. The light blue bar is indicative of good correlations (positive, >0.6).
SECOND-BASED TIME SERIES
To understand how chemically stable were the freshwater springs, second-based time
series of several minutes were taken at the springs ‘Olheiro’, ‘Salty’ and ‘Beach’.

‘Olheiro’ (May 2010)
In the ‘Olheiro’ spring, the water flows out from the rock through a fracture. This is the easiest
recognizable freshwater spring found in the study area. In May 2010, the effect of the rising tide
on the level of the discharging water was observed. It could be seen that as the seawater level
rose, so did the height of the ‘Olheiro’ discharging point, changing from a horizontal fracture to a
vertical one, a few centimetres above. At that time, the discharge flow rate was not measured,
but (visually) it remained unchanged during the sea level rise, until the tide reached the spring,
covering it.
Figure 6 shows the second-based evolution of the monitored parameters during about
135 minutes. Before the seawater reached the spring, all parameters remained virtually
constant, being the Oxy-Redox Potential (ORP) the parameter with the maximum variation of
3.5% observed. Figure 7 shows all parameters before seawater reached the ‘Olheiro’ spring.
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Figure 6: Measured parameters in the ‘Olheiro’ spring during 135 minutes in May 2010. All parameters
remained virtually the same within 3.5%. The tide rise increases the value of all measured parameters,
except that of temperature.
Figure 7: Measured parameters in the ‘Olheiro’ spring before the arrival of seawater. All parameters varied
within 0.05% (Pressure) and 3.5% (Oxi-Redox Potential, ORP), but most parameters by 2.5%, i.e. they
remained basically constant over the sampled timeframe. The steady decrease in the EC, Total Dissolved
Solids and salinity and increase in the ORP, are most probably related to the stabilization of the multiparameter sensor rather than to any variation of the water composition.

‘Rocks’ (August 2010)
The spring ‘Rocks’ was observed for the first time in the August campaign of 2010, as during
May 2010 that area was covered with sand and no superficial expression of it was detected. The
point of discharge was unclear and water was sampled in a narrow water line with a constant
flow. Figure 8 depicts the measured parameters during about 9 minutes (measuring every 5
seconds). The measured parameters varied by a minimum of 0.02% (Pressure) and a maximum
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of 1.05% (Salinity). Thus, the water composition underwent negligible change over the
measured 9 minutes period.
Figure 8: Variation of the field parameters over a 9 minutes observation set, at the spring ‘Rocks’ in the Olhos
d’Água beach. Despite the steady increase in the EC, Total Dissolved Solids, Salinity and Pressure and
decrease in the Oxi-Redox Potential, all parameters varied by a maximum of about 1%, i.e. they were constant
over the monitored period.

‘Beach Spring’ (March 2011)
The spring ‘Beach’ showed different discharge locations (within metric distance), due to sand
cover. Figure 9 depicts the measured parameters during about 5 minutes (measuring every 5
seconds). The measured parameters varied by a minimum of 0.04% (Pressure) and a maximum
of 3.9% (Salinity). Thus, the water composition underwent negligible change over the measured
5 minutes period.
Figure 9: Variation of the field parameters over a 5 minutes observation set, at the ‘Beach’ spring in the Olhos
d’Água beach.
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3. RADON
The radioactive noble gas 222-Radon (222Rn) can be used to qualitatively trace the groundwater
residence time (of up to 10 days) and to differentiate between Rn-free seawater and (possible)
Rn-bearing groundwater, ultimately allowing determining groundwater flow discharge into
coastal areas. The effective residence time of 222Rn in coastal waters is usually short (hours to
days) due to a fast decay (t1/2=3.84 d) and (atmospheric) losses (also due to mixing) [1].
222Rn was measured in most springs, but not in all field campaigns. Each sample was
collected in previously rinsed glass bottles (2*120 mL bottles/sample) avoiding headspace and
sealed with airtight screw caps. All samples were measured 24 hours after sampling using the
RD-200 and RDU-200 devices (EDA Instruments Inc.), at LNEG. Samples were purged with
atmospheric air for 3 minutes and were degassed twice or three times to guarantee maximum
radon extraction. Tap water was used as a blank sample, and all collector-cell’s backgrounds
(i.e., cell without sample and under vacuum) were measured before being used. As no
atmospheric radon on site was measured, a full calibration of the data was not possible.
However, it was assumed that the atmosphere is virtually 222Rn-free, hence, the 222Rn
concentrations measured in the water samples were assumed to be solely those aqueous
concentrations and no atmospheric Radon contamination happened.
It is important to note that the Radon measurements had very high errors. The errors
were obtained after analysing several samples of the spring ‘Olheiro’ collected at the same time:
in November 2010, the analysis of ten samples had error of 37%, and in February of 2011, the
analysis of six samples had an error of 31%. The Radon values obtained for the other springs
were obtained from a single sample at a certain field day. Such high errors require a more
modern method for the analysis of Radon to be used. In fact, with the applied analytical method,
Radon is surely lost to the atmosphere hence the reported concentrations are probably lower
than real values.
Figure 10 depicts the 222Rn concentrations (in Bq/L) obtained during the monitored
hydrological year, on most springs. Seawater showed the lowest Radon concentration (2 Bq/L),
which is relatively high considering that seawater is expected to be 222Rn–free. The Radon
concentrations of springs ranged between 3-14 Bq/L. The spring ‘Water’ is the one with the
most stable Radon content (least standard deviation) whereas ‘Bubble’ is the spring with the
larger variation in Rn-concentration. If a water body contains Radon and piston-flow is assumed,
the older the water the less 222Rn it contains due to its decay. Thus, neglecting loss through
diffusion and/or water mixing, the spring ‘Salty’ seems to be the youngest water in the studied
area, having the least residence time in the aquifer (close to 14 Bq/L), followed by the sample
‘Bubble’ (about 11.5 Bq/L), whereas the springs ‘Beach’ (about 3.5 Bq/L) and ‘Fuzzy’ (0 Bq/L)
could be the oldest waters, i.e. with the longest residence time.
Figure 10: Radon concentrations in all identified springs at Olhos d’Água beach.
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Figure 11 shows the temporal evolution of 222Rn concentrations in four springs. As
mentioned above, the ‘’Water’ spring is the one with the most stable 222Rn concentration,
basically the same throughout the year. The springs ‘Bubble’, ‘Sand’ and ‘Olheiro’ showed lower
222Rn concentrations in the autumn and spring. In the summer, the concentrations of Radon
showed greater variability, apparently reacting to higher air temperature. It is known that the
air concentration of 222Rn can be highly variable, within several orders of magnitude [2]. Such
natural variation may explain the variation in concentration measured within the same spring
and from spring to spring. As seen previously, 222Rn was well correlated with temperature and
EC in the winter and spring of 2011, and with pH in the summer and autumn of 2010 and April
of 2011.
Figure 11: Radon concentrations in the four Springs: ‘Bubble’, ‘Water’, ‘Sand’ and ‘Olheiro’, during one
hydrological year (August 2010-April 2011, five field campaigns).
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COMPARISON WITH LOCAL WELLS
To better understand regional to local groundwater flow and mixing, time series of
physicochemical data of water-quality-monitoring wells was analyzed. Figure 12 shows the
location of those wells, and Figure 13 depicts the time series for EC, pH, chloride and nitrate
concentrations. The data is available from SNIRH (Portuguese National Information System of
Hydric Resources) and ranges from 1995 to 2009. The well 348 was sampled only once, in
2002. The data of 'Olheiro' spring is also plotted to allow a direct comparison with the wells, and
verify if there is any chemical similarity. Although the 'Olheiro' data reports to years after the
wells time-series, it is still sufficiently near time-wise to be used for the purpose of this work.
The EC of 'Olheiro' is about 3 to 4 times higher than that of the monitoring wells (see Fig.
13). The exception is well 348, whose single sampling date showed EC typical of seawater
intrusion. The well 348 is located at the Olhos d'Água beach and is the nearest to the spring
'Olheiro'. In 2010-2011, ‘Olheiro’ showed EC and chloride content that indicated mixing between
fresh groundwater and seawater. Its pH indicated no difference to the wells, and nitrate
concentrations were slightly higher than recommended, and in the range of those in well 48. In
previous years, well 198, the second well nearest to 'Olheiro', showed very high concentrations
of nitrate, indicating probable anthropogenic contamination.
From this data, the water of 'Olheiro' seems to fit well in the regional groundwater
composition, being subject to seawater intrusion all year-round, as indicated by high EC and
chloride concentration. This is expected due to its intertidal location.
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Figure 12: Location of wells used to monitor the water level (P, in blue) and the water quality (Q, in green), as
part of the national groundwater monitoring survey [data available at http://snirh.pt). The numbered wells
(in green) were used to compare with the ‘Olheiro’ spring at Olhos d’Água. There is almost no data available
for well 348, which is located at the Olhos d’Água beach, and is the nearest to the springs.
Figure 13: Time series [wells: 1995-2009; ‘Olheiro’ spring: 2010-2011) of physicochemical data (electrical
conductivity, pH, chloride and nitrate concentrations). The well no. 348 (pink) located at Olhos d’Água beach,
was sampled only once, in 2002.
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REFERENCES
1. Burnett, W. C.; Dulaiova, H., Radon as a tracer of submarine groundwater discharge into a boat basin in
Donnalucata, Sicily. Continental Shelf Research 2006, 26, (7), 862-873.
2. Barbosa, S. M.; Zafrir, H.; Malik, U.; Piatibratova, O., Multiyear to daily radon variability from continuous
monitoring at the Amram tunnel, southern Israel. Geophysical Journal International 2010, 182, (2), 829842.
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APPENDIX
Table A.1: All measured parameters at the several springs in the Olhos d’Água beach and
surrounding areas, including one well (Furo). The different colors relate to the different dates
of sampling. A few samples were excluded from overall analysis because they were not
considered to be representative.
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