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Chemistry of Submarine Groundwater Discharge in Kalogria Bay, Messinia-Greece

A. Pavlidou, I. Hatzianestis, Ch. Zeri, E. Rouselaki

Institute of Oceanography, Hellenic Centre for Marine Research (HCMR) 46.7 Km Athens-Sounio Av., Anavyssos, 19013, Greece, aleka@ath.hcmr.gr

Abstract Concentrations of inorganic nutrients (nitrate, nitrite, phosphate, silicate and ammonium), trace metals (Cd, Cu, Ni, Fe, Pb), Total Organic Carbon (TOC), Dissolved Oxygen (DO) and organic pollutants (pesticides and insecticides, or-ganochlorines, hydrocarbons, etc) were determined in samples taken from Kalogria Bay submarine spring and the adjacent marine environment (SW Aegean Sea), in order to present, for the first time, the chemical characteristics of Submarine Groundwater Discharge (SGD) in Kalogria Bay and to study the effect of the SGD on the marine ecosystem. We also used estimations of the mean monthly spring discharge, in order to quantify the release of chemical constituents via the subma-rine discharge system to the marine environment. The results show that the loads of chemical constituents released by the SGD in the marine environment of Kalogria Bay do not impact the functioning of the marine ecosystem. All the chemical con-stituents measured, were well below the criteria set by the Directive 98/83/EC of 3rd of November 1998 on the quality of water intended for human consumption.

1 Introduction

Submarine Groundwater Discharge (SGD) into the sea is an integral part of the global hydrological cycle. The chemical load associated with SGD has been rec-ognized to have a significant impact on coastal marine ecosystems. It is notewor-thy that even a small net flux of submarine groundwater can deliver a compara-tively large flux of nutrients to the sea (Beck et al 2007; Stieglitz 2005; Johannes 1985). Consequently, SGD can potentially contribute to pollution of the marine environment as it is enriched in nutrients, metals and organic pollutants, depend-ing on the anthropogenic activities that impact on the groundwater and play im-portant role as a pathway for the cycling of chemical constituents (Pavlidou 2003; Boehm et al 2006; Gallardo and Marui 2006; Moore 2006).

In most parts of the world the economic development of coastal regions is lead-ing to a series of problems that highlight the urgent need of using the water of submarine springs in order to provide fresh water for human needs.

In Kalogria Bay, in SW Aegean Sea, Greece, four single point submarine springs of varying discharge rates have been observed. The only permanent SGD emanates at 25-26 m depth. It is noteworthy that the outflowing water creates at

N. Lambrakis et al. (Eds.), Advances in the Research of Aquatic Environment, Vol. 2 DOI 10.1007/978-3-642-24076-8, © Springer-Verlag Berlin Heidelberg 2011

230 A. Pavlidou et al.

the sea- surface a gyre with variable diameter, from 25 to 60 m, visible from long distance. The potential use of the SGD water for drinking purposes motivated a multi-disciplinary study of the spring, from July 2009 to May 2010. The main goals of this study are:

to present the chemical data for the Kalogria Bay SGD, in SW Aegean Sea, to study the effect of the SGD to the marine ecosystem close to the SGD dis-

charge and to make preliminary estimates of the chemical loads via the submarine dis-

charge system in order to quantify, the release of chemical groundwater sub-stances to the marine environment of Kalogria Bay in SW Aegean Sea.

2 Materials and methods

Samples were taken during 8 surveys (July, September, October, November and December 2009 and January, February, March and May 2010), at the site of the selected SGD, at water depth of 25 m, 20 m and 10 m and at the sea surface. Sam-ples were also taken at different sites in the marine environment, located at differ-ent distances from the SGD (ST01:790 m; ST02: 580 m; ST03:1180 m; ST04: 330 m; ST05: 1490 m; and M2: 2500 m from the SGD) (Fig. 1). Station M2 was used as reference station as it is monitored since 2006, in the framework of a monitor-ing program of HCMR. Measurements of trace metals (Cd, Cu, Ni, Fe, Pb), inor-ganic nutrients (nitrate, nitrite, phosphate, silicate and ammonium), TOC, Dis-solved Oxygen (DO) and organic pollutants (insecticides, organochlorines, hydrocarbons, phthalates) were performed. DO measurements were performed immediately after the sampling using the Winkler method modified by Carpenter (Carpenter 1965). Nutrient analysis was performed at the certified by ISO 17025 biogeochemical laboratories of HCMR using standard methods. Ammonium was measured with a UV-VIS Perkin-Elmer 25 Lamda spectrophotometer (Korroleff 1970). Nitrate, nitrite, silicate and phosphate concentrations were measured with a BRAN+LUEBBE III nutrient autoanalyzer using standard methods (Murphy and Rilley 1962; Mullin and Rilley 1955; Strickland and Parsons 1977). TOC analysis was carried out following the method described by Cauwet (1994), using an auto-matic analyzer (Shimadzu TOC-5000). Dissolved trace metals were determined following the method described by Riley and Taylor (1968) as modified by King-ston et al (1978) by graphite furnace AAS, using a Perkin-Elmer 4100, HGA 700. Organic pollutants were determined by gas chromatography – mass spectrometry and gas chromatography – ECD after extraction of the water samples collected in clean glass bottles with n-hexane.

In order to quantify the SGD-derived flux of chemical constituents from land to the marine environment, we used measurements of dissolved constituents at the outflow of the spring and calculation of SGD mass flux based on radionuclides as tracers (Tsabaris et al 2011, in this book).

Chemistry of Submarine Groundwater Discharge in Kalogria Bay, Messinia-Greece 231

Fig. 1. Study area location (a) and sampling stations in Kalogria Bay (b).

3 Results and Discussion

3.1 Dissolved Oxygen and Nutrients

Salinity values measured in all the samples taken from the submarine discharge (25m depth) for chemical analyses, varied from 14 to 24, indicating that the ema-nated groundwater is strongly influenced by seawater. Only in November 2009 the samples taken from the SGD had salinity 36, indicating that in this case seawater was rather sampled instead of freshwater. During this survey, the divers could hardly reach the SGD without the danger of abrupt ascending, because of the high discharge of the spring. Figure 2a-f shows the monthly concentrations of DO and nutrients in the SGD at 25 m, at 20 m and 10 m under sea surface and at the sea surface. Temporal variation is related to the flow rate variation of the SGD. Most nitrate and silicate concentrations in SGD are higher than those measured at the sea surface and in the adjacent marine area. Relatively low nutrient concentrations were recorded at the SGD during November 2009, coinciding with the high salin-ity of the samples. The brackish SGD collected at 25 m depth contained relatively high concentrations of silicate during all the surveys (20.05±11.05 μmol/L; maxi-mum silicate value: 34.3 μmol/L in May 2010), low values of soluble reactive phosphate (0.10±0.02 μmol/L) and relatively high dissolved inorganic nitrogen (DIN) (7.08 ± 4.30 μmol/L; maximum nitrate concentration: 13.8 in October 2009); on average, in the samples taken at 25 m during all the surveys, nitrate comprised 95% of the nitrogen. The nitrate concentrations in SGD may be associ-ated with large pools of nitrate and high rates of remineralization in upland soils. Human activities also alter N concentrations in groundwater, mainly ammonium and nitrite concentrations. In this region (near Stoupa), the nitrates found in groundwater could originate from fertilizers and manure from agricultural and

232 A. Pavlidou et al.

farming uses. Low phosphate concentrations (<0.14 μmol/L) were recorded at all depths in the SGD, as at the sea surface, indicating underground absorption me-chanisms of phosphorus. During the sampling period the mean water discharge of the spring was 319 ± 165 l/s (Tsabaris et al 2011, in this book).

Fig. 2. (a-f) Concentrations of DO and nutrients together with SGD flow rate at the depth of the SGD (25m), 10m, 20m, and sea surface, for the sampling period July 2009 – May 2010 in Ka-logria Bay (SW Aegean Sea).

The calculations of nutrient fluxes showed that the submarine spring releases to the marine environment 21890 ± 15090 mol/month of silicates, 7830 ± 5804 mol/month of nitrates, 456 ± 773 mol/month of ammonium and 94.8 ± 64.6 mol/month of phosphates. The variation of nutrient fluxes during the surveys is shown in Figure 3. Mean integrated nutrient concentrations of the water column in the marine ecosystem of Kalogria Bay; correspond to an oligotrophic environ-ment, even close to the SGD. At the stations ST04 and ST01 nitrate and silicate concentrations were an order of magnitude lower than those measured in the close SGD, indicating that the delivery of nutrients during submarine groundwater dis-charge does not impact the functioning of the marine ecosystem. N: P ratio at the

Chemistry of Submarine Groundwater Discharge in Kalogria Bay, Messinia-Greece 233

SGD was higher than the theoretical values for phytoplankton growth, whereas in the adjacent marine area was lower, indicating the nitrogen as the limiting factor for phytoplankton growth.

Fig. 3. Variation of nutrient fluxes at 25 m of SGD for the sampling period July 2009 - May 2010 in Kalogria Bay (SW Aegean Sea).

Fig. 4. Silicate and Nitrate relationship with salinity at the SGD (25m, 20, 10, and sea-surface) during all the samplings.

Nutrient and salinity correlation diagrams showed good negative correlation for nitrate and silicate (Fig. 4) but no significant correlation for ammonium, nitrate and phosphate. This picture implies that the groundwater discharge carry some amounts of nitrates and silicates, but these are rapidly mixed with the oligotrophic Mediterranean waters without posing any problems in the adjacent marine ecosys-tem (about 7-8 times decrease of nitrate and silicate concentrations at seawater sa-linities). Nitrate, ammonium and phosphate concentrations measured at the SGD were well below the limits set by the Directive 98/83/EC of 3rd of November 1998 on the quality of water intended for human consumption.

3.2 Trace Metals

The mean average values of dissolved Cd, Cu, Ni, Fe and Pb at the brackish SGD collected at 25m depth of the seawater column for all the samplings were: Cd: 0.004 ± 0.001 μg/L; Cu: 0.445 ± 0.325 μg/L; Ni: 0.219 ± 0.107 μg/L; Pb: 0.191 ±

234 A. Pavlidou et al.

0.146 ± 0.146 μg/L; Zn: 3.90 ± 1.21 μg/L. Fe was measured only during October 2009 and found 1.10 μg/L. Our calculations of metal fluxes via SGD to the ma-rine ecosystem resulted the following amounts for each metal: 4.3 ± 3.8 g Cd /month, 310.2 ± 129 g Cu /month, 2242 ± 1501 g Ni /month, 4163 ± 2764 g Zn /month and 131.9 ± 26.31 g Pb/month. The monthly variations of the concentra-tions of Cd, Cu, Ni and Pb in the SGD at 25m under sea, at 20m, 10m and finally at the sea surface are shown in Fig. 5.

Fig. 5. (a-e) Trace metal concentrations at the depth of the SGD (25m), 10m, 20, and sea surface, for the sampling period October 2009 – May 2010. (f) Ni relationship with salinity.

Chemistry of Submarine Groundwater Discharge in Kalogria Bay, Messinia-Greece 235

Most metal concentrations in SGD are lower than those measured at the sea sur-face and the adjacent marine area. Cd, Cu, and Pb did not show trends relative to salinity indicating that the SGD is not a source of these metals for the marine envi-ronment of Kalogria Bay. Only Ni showed significant positive correlation with sa-linity (R2: 0.51) which further indicates that there is no enrichment in metals from the SGD. It seems that biogeochemical processes along the groundwater flowpath may impact metal concentrations at the SGD.

Mean metal concentrations of the water column at the reference station M2, during the sampling periods were at the same levels as at SGD (0.005± 0.325 μg/L for Cd; 0.477± 0.293 μg/L for Cu; 0.254± 0.071 μg/L for Ni; 0.148± 0.079 μg/L for Pb and 4.14± 1.45 μg/L for Zn), indicating again that SGD is not a source of metals for the area.

3.3 Organic Carbon

TOC concentrations in the marine stations adjacent to SGD (ST1, ST2, ST3, ST4, ST5 and M2) ranged from 58 to 136 μmol/L. These values are considered typical for an oligotrophic coastal marine environment and indicate the absence of an-thropogenic organic carbon sources. At the SGD station, the TOC values were lower than those in the rest of the area and varied between 34 and 87 μmol/L. Fig-ure 6a presents the monthly variations of TOC concentrations at the SGD station. It is noteworthy, that in most cases the lowest values were recorded close to the bottom (25 m) indicating that the groundwater organic carbon load was very low. This is further confirmed by the good positive correlation (R2: 0.60) observed be-tween TOC values and salinity (Fig. 6b).

Based to the estimated monthly water discharge (Tsabaris et al. 2011, in this book), the spring releases to the marine environment are 58815 ± 5804 mol TOC/month. It is clear that the quality of the discharged groundwater regarding the organic carbon content is very good.

Fig. 6. (a) TOC concentrations at the 25, 20 and 10 m of the SGD and at the sea surface and (b) correlation between TOC and salinity values for the SGD waters (July 2009 – May 2010).

236 A. Pavlidou et al.

3.4 Organic Pollutants

Pesticides and insecticides: The concentrations of these compounds were below the detection limit (0.02 μg/L) in all analyzed samples

Organochlorine compounds: Polychorinated biphenyls (PCBs) were detected in all samples but in very low concentrations (0.8-3.2 pg/L). DDTs concentrations were also very low (1.5-4.9 pg/L) while lindane and hexachlorobenzene were un-detectable (<0.1 pg/L). These values are characteristic of unpolluted marine areas. There was no differentiation between the SGD site and the other stations and no correlation between PCBs or DDTs concentrations and salinity was observed.

It was calculated that 978.1± 502.6 μg of PCBs /month and 1837 ± 984 μg of DDTs/month were released to the marine environment via the SGD.

Hydrocarbons: The only hydrocarbons detected were n-alkanes and some low molecular weight PAHs (especially naphthalene and phenanthrene). Their concen-trations were very low and similar to those found in unpolluted aquatic ecosys-tems. No correlation was observed between hydrocarbons and salinity. The lowest values were recorded at water depth of 20 and 25m of SGD site where the lowest values of TOC and salinity were also recorded.

Other organic pollutants: Phthalates were detected in all samples in low concen-trations. These compounds are commonly used as plasticizers and are widespread in all aquatic environments. It is noteworthy that their concentrations around the SGD, although very low, were slightly elevated compared to the reference station M2. This might be an indication that the groundwater contains trace amounts of phthalates.

The concentrations of all the organic pollutants detected were well below any drinking water criteria values, suggesting the good quality of the groundwater for any human use.

4 Conclusions

Measurements of Dissolved Oxygen, inorganic nutrients (nitrate, nitrite, phos-phate, silicate and ammonium), trace metals (Cd, Cu, Ni, Fe, Pb), TOC and or-ganic pollutants at Kalogria SGD and estimation of their fluxes released in the ma-rine environment per month, showed that SGD does not impact the functioning of the marine ecosystem of Kalogia Bay. According to our results, all the chemical constituents measured were well below the criteria set by the Directive 98/83/EC on the quality of water intended for human consumption.

Chemistry of Submarine Groundwater Discharge in Kalogria Bay, Messinia-Greece 237

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