Advances in the Research of Aquatic Environment || Chemistry of Submarine Groundwater Discharge in Kalogria Bay, Messinia-Greece
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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, firstname.lastname@example.org
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.
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
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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).
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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 (20.0511.05 mol/L; maxi-mum silicate value: 34.3 mol/L in May 2010), low values of soluble reactive phosphate (0.100.02 mol/L) and relatively high dissolved inorganic nitrogen (DIN) (7.08 4.30 mol/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
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farming uses. Low phosphate concentrations (
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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 g/L; Cu: 0.445 0.325 g/L; Ni: 0.219 0.107 g/L; Pb: 0.191
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0.146 0.146 g/L; Zn: 3.90 1.21 g/L. Fe was measured only during October 2009 and found 1.10 g/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.
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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 g/L for Cd; 0.477 0.293 g/L for Cu; 0.254 0.071 g/L for Ni; 0.148 0.079 g/L for Pb and 4.14 1.45 g/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 mol/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 mol/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).
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3.4 Organic Pollutants
Pesticides and insecticides: The concentrations of these compounds were below the detection limit (0.02 g/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 (
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Chemistry of Submarine Groundwater Discharge in Kalogria Bay, Messinia-Greece1 Introduction2 Materials and methods3 Results and Discussion3.1 Dissolved Oxygen and Nutrients3.2 Trace Metals3.3 Organic Carbon3.4 Organic Pollutants