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Linkages between submarine groundwater systems and the environment
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Linkages between submarine groundwater systems and the environment Jing Zhang and Ajit K Mandal
Submarine groundwater discharge (SGD) has been recognized as an important source of freshwater discharge into the ocean. Different approaches are used to estimate the SGD magnitude. Principal methods include, first, direct measurement using manual or automated seepage meters, second, chemical tracers, and third, hydrogeologic modeling. Submarine groundwater studies reveal that SGD provides important fluxes of nutrients, carbon, and trace metals to coastal waters that have the potential to impact the chemical budget of coastal water. Apart from the amounts of nutrients (N, P, and Si)/carbon entering the coastal ocean through SGD, it is also important to evaluate the potential effect of SGD on the Redfield ratio, because this ratio determines which nutrient is limiting phytoplankton growth. To pursue detailed SGD studies, an independent discipline would facilitate more progress. Address Earth and Environmental System, Graduate School of Science and Engineering, University of Toyama, Toyama 930-8555, Japan Corresponding author: Zhang, Jing ([email protected])
Current Opinion in Environmental Sustainability 2012, 4:219–226 This review comes from a themed issue on Carbon and nitrogen cycles Edited by Chen-Tung Arthur Chen and Dennis Peter Swaney Received 16 February 2012; Accepted 26 March 2012 Available online 20th April 2012 1877-3435/$ – see front matter # 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cosust.2012.03.006
Introduction Submarine groundwater discharge (SGD) is the direct flow of groundwater into the ocean. Like surface water, groundwater flows down a gradient, and SGD occurs wherever a coastal aquifer is connected to the sea. The term SGD appeared in the literature during the early 1970s [1] and even earlier [2,3]. Annually, about 2400 km3 of freshwater discharges into the world’s seas and oceans in the form of SGD, with 1500 km3 contributed by the continents and 900 km3 by the world’s islands [4]. SGD has been recognized as an important source of freshwater discharge into the ocean and a valuable component of the hydrological cycle. While SGD is a part of the groundwater system, it has an impact on the coastal water www.sciencedirect.com
ecosystem. Hydrogeologists are used to working on land, while marine scientists focus on coastal environments. Hydrogeological studies have gained attention for quantifying the movement of fresh groundwater or examining saltwater intrusion into freshwater aquifers. Modeling methods are also primarily employed by hydrogeologists in quantifying SGD. However, the participation of hydrologists in SGD studies has been less than what it should have been [5], although some hydrologists have published many results on SGD [6,7]. Burnett et al. [8] reviewed the topic of SGD in detail and summarized the findings of various research projects across the globe. The coauthors of this article are from a variety of disciplines, mainly from oceanography and marine departments with good representation of professionals of different backgrounds, but hydrologists or hydrogeologists are still not particularly well represented in the field of SGD [5]. While hydrologists and marine scientists focus on coastal environments, both groups approach the problem of SGD literally from opposite directions, and so have not always been working with the same concepts and definitions of the phenomenon [9]. For the true depth of study that the science of SGD deserves, an independent discipline is required; such a focus on groundwater processes could lead to a more complete picture of SGD. SGD is defined as terrestrially derived freshwater and recirculated seawater, which includes discharge associated with saltwater intrusion along a coastline, tidal pumping, and wave setup; all of these discharges flow out from the seabed into the coastal water through the underlying sediments [10]. The terrestrially derived SGD ranges from 6 to 10% of the surface waters discharging into the ocean [11], and recirculated seawater due only to density effects and mixing may constitute up to 70% of the SGD [12,13]. Furthermore, the recirculated seawater due to wave setup and tides may contribute up to 96% of the SGD [14]. SGD occurs not only near the shore but also a large distance from the shore. Many independent studies of SGD have been performed along the east coast of the United States, northern Italian coasts, and the coasts of Japan and South Korea [6]. SGD has been neglected scientifically because of the difficulty in finding and measuring these features [4]. The location of current SGD studies and volume (or flux) of SGD on the global scale is presented in Figure 1 and Table 1. However, ecological effects in estuaries and the coastal Current Opinion in Environmental Sustainability 2012, 4:219–226
220 Carbon and nitrogen cycles
Figure 1
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Current Opinion in Environmental Sustainability
Global studies and locations of SGD.
zone may depend on water quality, which is influenced by SGD. For example, biological zonation associated with groundwater discharge was identified in Biscayne Bay/Florida Bay (Table 1). Possible relationships between SGD and sea grass distributions were reported from the coastal zone near Perth, Australia, and in the coastal northeastern Gulf of Mexico. However, it is also clear that wide areas of the world have little to no SGD assessments at all. A few studies have been done on the
west coast of the USA and Hawaii. There are limited quantitative data from South America, Africa, India, and China, although indications of groundwater discharge were reported for India [15] (Figure 1). A few studies have been performed on volcanic and coral Islands. Some SGD studies showed large amounts of freshwater flowing into coastal water. Data on SGD with respect to trace metals are few as shown in Figure 1.
Table 1 SGD discharge rates and measurement methods. No.
Location
1 2 3 4 5 6 7 8 9 10
Great South Bay Ubatuba, Brazil South-Eastern Sicily Geographe Bay Rhode Island’s Waquoit Bay Yellow Sea Kahana Bay, Hawaii Tampa Bay Venice Lagoon
11 12 13 14 15 16 17 18 19 20 21 22
Northeast Gulf of Mexico East China Sea Eilat, Israel Lesina Lagoon Gulf of lion South Atlantic Bight Bay of Bengal Toyama Bay, Japan Rishiri Island, Japan Perth, Australia Sagami Bay, Japan Biscayne Bay, Florida
Discharge rate 6
3
1
4.5 10 m day 3 105 m3 day 1 3.4–10.4 102 m3 day 1 2.2 105 m3 day 1 1.7 10 2 m3 m2 day 1 6.9 104 m3 day 1 2.7–18.4 m3 day 1 9.0 104 m3 day 1 1.6–10.3 104 m3 day 1 4.3 2.5 107 m3 day 1 4.0 1.7 107 m3 day 1 3.1 103 m3 day 1 0.2–1.0 109 m3 day 1 0.06–0.26 m3 m 2 day 1 9.0–10.5 105 m3 day 1 2.9–11 106 m3 day 1 3 107 m3 day 1 5.5 108 m3 day 1 3.9 102 m3 day 1 2.5 105 m3 day 1 4.8 104 m3 day 1 3.1–5.0 102 m3 day 1 1–3 cm day 1
Current Opinion in Environmental Sustainability 2012, 4:219–226
Method 223
Ra Rn Seepage meter Hydraulic assessment (Darcy’s Law) Residence time models 226Ra and 228Ra 226 Ra 223,224,226,228 Ra Seepage meter 226 Ra 223 Ra 224 Ra Continuous Radon Model 222,223,224,226 Ra 223 Ra and 224Ra 224 Ra 226 Ra and 228Ra 226 Ra 3 He/3H Toyama University SGD-Flux Chamber Water balance method Salinity Heat flow chemical composition Seepage meter 222
Reference Beck et al. [49] Povinec et al. [50] Povinec et al. [51] Varma et al. [52] Stachelhaus et al. [53] Michael et al. [54] Kim et al. [29] Garrison et al. [55] Swarzenski et al. [56] Ferrarin et al. [57] Lambert and Burnett [58] Gu et al. [59] Shellenbarger et al. [60] Rapaglia et al. [61] Ollivier et al. [62] Moore [21] Basu et al. [63] Zhang et al. [35] Hide [64] Johannes et al. [65] Tsunogai et al. [66] Kohout et al. [67]
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Linkages between submarine groundwater systems and the environment Zhang and Mandal 221
Methods used for measuring SGD SGD has been measured at several sites. These studies have successfully employed natural radioactive elements such as 222Ra and other radium isotopes (223Ra, 224Ra, 226 Ra, and 228Ra), water isotopes of 18O and D and dissolved gases such as CH4 as tracers, and seepage meters. Some of the studies adopting these methods to calculate discharge rate are listed in Table 1. Until the late 1990s, quantitative estimation of SGD was impossible, and the impact of SGD on marine coastal processes was unknown. Since 2000, a number of sites across the globe have been selected for detailed studies of the SGD phenomenon. The following sections describe methods that have been used to estimate SGD by direct measurements and indirect analysis, including seepage meters, hydrologic models (Darcy’s law, water budget, and hydrograph separation), and tracers. For precise evaluation of SGD flux, seepage measurement in conjunction with hydrological analysis of groundwater is needed [16]. Direct measurements — seepage meters
Seepage meters are the most commonly used devices for direct measurement of seepage flux, and they have been used in numerous studies of SGD in coastal water [17– 19]. The basic concept of the seepage meter is to cover and isolate seepage water with a chamber open at the base and measure the change in the volume of water contained in a bag attached to the chamber over a measured time interval. Using manual seepage meters is very laborintensive. Therefore, various types of automated seepage meters have been developed to record time-series changes in SGD. Seepage chambers are fitted with different instruments for continually measuring the rate of water flow through the outlet. Direct measurement is sometimes contrary to the results of the modeled-SGD [16]. Indirect analysis — hydrogeologic modeling
Modeling methods are primarily employed by hydrogeologists, whose work focuses on the land side of SGD. There are four major modeling methods (Darcy’s law, water budget, hydrograph separation, and numerical modeling) used to quantify SGD. Hydrogeologic modeling has typically been used to estimate the advecting terrestrial component of SGD. In the water balance approach, the estimate of SGD is based on the water balance equation at the basin scale. This method only provides the fresh groundwater component of SGD as the total volume of groundwater discharging to the sea during the selected time period. Thus, the results of the water balance method are not comparable with locally measured values or with results of numerical models, which provide the distribution of SGD over the discharge area. Mulligan and Charette [20] compared investigative methods including direct measurement via seepage meters, hydrogeologic estimation using Darcy’s law, and tracer-based estimates using radon and radium www.sciencedirect.com
isotopes. They concluded that the hydrogeologic estimate and radon and radium techniques are complimentary for estimating different components of total SGD. Geochemical tracers for SGD research
One approach to local-scale to regional-scale estimation of groundwater input into the ocean uses naturally occurring geochemical tracers. An advantage of groundwater tracers is that they present an integrated signal as they enter the marine water column through various pathways in the aquifer. Natural geochemical tracers have been applied in two ways to evaluate groundwater discharge rates into the ocean. The first is the use of enriched geochemical tracers in the groundwater relative to the seawater. The concentration of a solute in the receiving water body is attributed to inputs of that component derived only from groundwater [21,22,23]. A second approach is the use of vertical profiles of the geochemical compositions in sediment pore waters under the assumption that their distribution can be described by a vertical, one-dimensional advection-diffusion model [24]. However, this is usually limited to the case of homogeneous media. 222
Rn is a radioactive noble gas produced by the radioactive decay of 226Ra. Both are members of the natural 238 U decay chain. 226Ra occurs in all rocks, soil, and sediments within a wide range of concentrations. Radon enters the groundwater and is transported with it through the aquifer. Over the past few years, several studies used natural radium isotopes and 222Rn to assess groundwater discharge into the ocean [22,25,26,27]. A groundwater tracer should be greatly enriched in the discharging groundwater relative to coastal marine waters. Radium isotopes are enriched in groundwater relative to surface waters, especially where saltwater comes into contact with surfaces. Hwang et al. [28] developed a geochemical model for local-scale estimate of SGD. If the system under study is in a steady state, then radium additions are balanced by losses. Additions include radium fluxes from sediment, river, and groundwater; losses are due to mixing and, in the case of 223Ra and 224Ra, radioactive decay. Using a mass balance approach on a large scale with the long-lived isotopes 226Ra and 228Ra, Kim et al. [29] determined that SGD-derived silicate fluxes to the Yellow Sea were on the same order of magnitude as the Si flux from the Yangtze River, the fifth-largest river in the world. Another new approach is based on an in situ analysis of radon decay products in seawater using gamma-ray spectrometer techniques. This has been recognized as a powerful tool for analysis of gamma-ray emitters in seabed sediments as well as for continuous analysis of gamma-ray emitters (137Cs, 40K, 238U, and 232Th decay products) in seawater [30]. Current Opinion in Environmental Sustainability 2012, 4:219–226
222 Carbon and nitrogen cycles
Methane (CH4) is another geochemical tracer that can be used to detect SGD. Both 222Rn and CH4 were used to evaluate SGD in studies performed in a coastal area of the northeastern Gulf of Mexico [22]. Several other natural radioactive (3H, 14C, U isotopes, among others) and stable (2H, 3He, 4He, 13C, 15N, 18O, 87Sr/88Sr, among others) isotopes and some anthropogenic atmospheric gases (e.g. CFCs) have been used for investigating SGD, tracing water masses, and calculating the age of groundwater.
Nutrient transport via SGD into the coastal ocean and its impact SGD is an important source of nutrients, trace elements and contaminants for the coastal ocean in many parts of the world [17]. SGD has received increased attention during recent decades because of its significant environmental consequences in coastal water. Groundwater nutrient concentrations are relatively higher than in seawater, and even small discharges of groundwater may make large contributions to coastal nutrient budgets [11], especially in oligotrophic areas with few other external nutrient sources [31]. Nitrate and phosphate concentrations in groundwater are variable and depend on inputs, soil and aquifer type, aquifer permeability, groundwater recharge rate, and climate [32]. Nitrate and phosphate concentrations in coastal groundwater are often much higher than those in river water [33]. Primary sources for increasing nutrient loads are wastewater disposal, application of fertilizers, and atmospheric deposition [34]. Continued residential and agricultural development of near-shore areas is leading to increased inputs of nitrate and phosphate from fertilizers and wastewater to the groundwater, and some of these nutrients are released into coastal surface waters [30]. Fluxes of nutrients to coastal waters may promote the bloom of phytoplankton [35] and trigger algae blooms, some of which have harmful impact on the local ecosystem and on the economy of coastal zones [36]. For example, Valiela et al. [37] reported that SGD contributes excessive nitrogen loading to salt marshes, thus impacting the ecology of coastal waters. It is important to evaluate the potential effect of SGD on the ratio of nitrate and phosphate in coastal waters, since this ratio determines which nutrient is limiting phytoplankton growth [38]. On a global scale, the average molar ratio is 18:1 [39] and thus closely matches the requirements of phytoplankton (N:P = 16:1, which is the ‘Redfield ratio’). Groundwater DIN/DIP ratios in contaminated aquifers are typically much higher than 18 because P in groundwater is more efficiently immobilized than N [40]. Nutrient inputs through SGD may drive N-limited coastal systems toward Plimitation [41]. Geochemical processes occurring at the interface between fresh and saline groundwaters of the so-called Current Opinion in Environmental Sustainability 2012, 4:219–226
‘subterranean estuary’ have a significant effect on the SGD composition of the coastal ocean [33]. In the subterranean estuary, biogeochemical reactions in these environments are strongly influenced by the flow rates and redox characteristics of the freshwater and seawater [42], desorption–sorption processes, and microbially driven diagenesis. SGD from oxic subterranean estuaries is expected to have high N/P ratios due to nearly conservative transport of N and almost complete removal of P [42]. The efficiency of the removal of N and P in the subterranean estuary may vary seasonally. Denitrification rates are very sensitive to temperature changes [43] and are expected to be lowest in winter. P removal may be more efficient due to the generally more oxic sediment conditions. As a result, N/P ratios in SGD are expected to be higher in winter than in summer. This was observed to be the case for the N/P ratios of SGD and the receiving surface waters of Buzzards Bay in Massachusetts [17]. Charette and Sholkovitz [44] reported on a large accumulation of iron oxides at the fresh-saline interface in Waquoit Bay. These iron oxide sands could act as a geochemical barrier by retaining and accumulating certain dissolved species carried to the subterranean estuary by groundwater and/or seawater. An accurate assessment of the importance of SGD on nutrients on a global scale is needed. In particular, regions consisting of developing countries with increasing population densities and high rates of fertilizer use (e.g. Asia and South America) are of interest, since, analogous to rivers [45], these are the regions where inputs of nutrients to groundwater are expected to increase most.
Global estimates of nutrients/carbon fluxes The ecological importance of nutrient (N, P, and Si) inputs via SGD to areas near shore waters is well recognized, although their concentrations appear to vary considerably (Table 2). For example, N and P input from SGD into Port Royal Sound were 6700 mmol m 2 day 1 and 500 mmol m 2 day 1, respectively [46], whereas N and P input into Guam’s Tumon Bay were 1340 mmol m 2 day 1 and 12 mmol m 2 day 1, respectively. Despite the regional variation in concentrations, nitrate (NO3 ), phosphate (PO4 ) and silica (Si) concentrations in most of the SGD studies varied between 0.93 and 3.5 mg/ L, 0.012 and 0.33 mg/L, and 1.03 and 22 mg/L, respectively. Annually, about 2400 km3 of freshwater discharges into the world’s seas and oceans in the form of SGD [4]. Using a simple calculation (i.e. the product of this average flow rate and the above concentrations), the annual loading of major nutrients via SGD to global flux ranged from 0.5 10 3 to 1.9 10 3 PgN year 1, 0.009 10 3 to 0.25 10 3 PgP year 1 and 1.15 10 3 to 24.69 10 3 PgSi year 1, respectively. Chen et al. [47] estimated that global riverine flux of nitrogen and phosphorus was www.sciencedirect.com
Linkages between submarine groundwater systems and the environment Zhang and Mandal 223
Table 2 DIN, DIP, and DSi fluxes from various coasts. No. A B C D E F
No. G H I J
Locations Bangdu Bay, Korea Eilat, Israel North Intet, S. Carolina Spencer Beach, Hawaii Kamiloloa, Hawaii Pangasinan, Philippines
Locations Tampa Bay, Florida Southern Brazil Yellow Sea Toyama Bay, Japan
DIN (mmol N m
2
day 1)
21.4 2.9–10 2.42 3.3–4.4 0.7–1.3 0.9–4.4
0.16 0.02–2.0 0.91 0.11–0.15 0.16-0.30 0.08–0.2
2
day 1)
DSi – – – – – –
DIN (mol day 1)
DIP (mol day 1)
DSi (mol day 1)
1.3–9.0 105 2.42 106 5.7 105 5.25 105
0.6–4.5 104 5.2 105 7.3 103 1.5 103
0.4–2.9 105 5.92 106 4.7 105 1.13 106
0.028 PgN year 1 and 0.0018 PgP year 1, respectively. Comparing with the global nutrient flux of SGD, the magnitude of nitrogen and phosphorus through SGD was 2–7% and 0.5–13% of riverine flux. The SGD at a global scale estimated to be from 0.01% to 10% of river discharge [6]. However, it indicated that on a global scale concentrations of nutrients are higher in SGD than river water. Therefore, SGD is one of the important sources of nutrients in local/regional coastal water. The flux of carbon from the terrestrial biosphere to the oceans takes place though river transport. A global riverine transfer of C has been estimated to be 0.74 PgC year 1 [47]. Najjar [48] estimated that SGD contributes carbon to the coastal ocean at a rate of 30% of the riverine flux. Therefore, a crude estimate of the global flux of carbon via SGD is 0.23 PgC year 1.
Overview of current SGD research and global urgency of future studies Many professional organizations and scientific bodies involved in SGD research projects, include the Intergovernmental Oceanographic Commission (IOC), the International Hydrological Program (IHP), the Scientific Committee on Oceanic Research (SCOR), which established its working group WG112 on SGD, LandOcean Interactions in the Coastal Zone (LOICZ), and the International Association of Physical Sciences of the Oceans (IAPSO). An initiative on SGD characterization was developed by the International Atomic Energy Agency (IAEA) and UNESCO in 2000 as a five-year plan to assess methodologies and the importance of SGD for coastal zone management. SGD also has been taken up as one of the priority research issues of the GEOTRACES program (GEOTRACES Science Plan). Although many previous and current SGD studies have been carried out worldwide, knowledge of the significance and impact on the global oceans is still limited. This is because SGD fluxes are strongly controlled by many local and regional conditions, with the exception www.sciencedirect.com
DIP (mmol P m
Reference Hwang et al. [68] Shellenbarger et al. [60] Krest et al. [69] Street et al. [70] Street et al. [70] Senal et al. [71]
Reference Swarzenski et al. [56] Niencheski et al. [72] Waska and Kim [73] Hatta et al. [74]
of the influences on the ocean circulation and CO2 fixation via SGD. SGD can influence the ocean circulation structure and stratification of the ocean because it is an important route of freshwater and heat transport, especially in high latitudes. The temperature of the groundwater is warm during the winter and has small changes annually. In polar areas, the higher temperature of SGD relative to the riverine water leads to increases in sea surface temperatures and freshening of the surface seawater’s salinity, when SGD flows into the ocean from late autumn to winter. This inhibits ice formation and interferes with the seawater subduction. Therefore, a quantitative understanding of the SGD variation in the polar regions, including details of heat/freshwater transport, is required, in particular as an important factor affecting the world ocean circulation in the future. Moreover, SGD is also an important carbon supply route to the ocean. Groundwater ranks in third place in terms of freshwater volume on the earth’s surface, following ice and glaciers as reservoirs. Groundwater also plays an important role in providing temporary CO2 storage from the atmosphere to the ocean because of its relatively long residence time. In the global carbon balance, CO2 dissolves into groundwater through the process of weathering (0.4 PgC year 1), an amount equal to about 20% of the total CO2 increase after the industrial revolution (1.9 PgC year 1); this quantity is then carried to the ocean by SGD and river water. However, the buffering capacity of groundwater is lower than that of seawater. In the future, reduction of CO2 storage in groundwater will be a concern because of the expected changes in our groundwater due to anthropogenic acidification and many major changes in the terrestrial environment. Our knowledge of SGD is still insufficient, and so further research is urgently needed. SGD is one of the factors that affect important coastal marine environments and material circulation. Our better understanding of SGD Current Opinion in Environmental Sustainability 2012, 4:219–226
224 Carbon and nitrogen cycles
will help to clarify the global environmental changes due to human activity and climate change.
15. Moore WS: High fluxes of radium and barium from the mouth of the Ganges–Brahmaputra River during low river discharge suggest a large groundwater source. Earth Planet Sci Lett 1997, 150:141-150.
Acknowledgements
16. Peng TR, Chen CTA, Wang CH, Zhang J, Lin YJ: Assessment of terrestrial factors controlling the submarine groundwater discharge in water shortage and highly deformed island of Taiwan, western Pacific Ocean. J Oceanogr 2008, 64:323-337.
The authors are grateful to the editor, C.T. Arthur Chen, for his critical comments and valuable suggestions to improve this paper. We also thank Jan ten Have for his help during the submission process and M.S. Nahar for assistance during the revision process. This work was supported by the Ministry of Education, Science, Sports and Culture, Japan, through the Grant in Aid 22403001, and by the ‘4000 m environmental project’ of the University of Toyama.
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34. Valiela I, Collins G, Kremer J, Lajtha K, Geist M, Seely B, Brawley J, Sham CH: Nitrogen loading from coastal watersheds to receiving estuaries: new method and application. Ecol Appl 1997, 7:358-380. 35. Zhang J, Satake H: The chemical characteristics of submarine groundwater seepage in Toyama Bay, Central Japan. In Land and Marine Hydrogeology. Edited by Taniguchi M et al.: Elsevier; 2003:45-60. 36. LaRoche J, Nuzzi R, Waters R, Wyman K, Falkowski PG, Wallace DWR: Brown tide blooms in Long Island’s coastal waters linked to interannual variability in groundwater flow. Glob Change Biol 1997, 3:397-410. 37. Valiela I, Bowen JL, Kroeger KD: Assessment of models for estimation of land-derived nitrogen loads to shallow estuaries. Appl Geochem 2002, 17:935-953. 38. Howarth RW: Nutrient limitation of net primary production in marine ecosystems. Ann Rev Ecol 1988, 19:89-110. The author discussed about nutrient limitation and its impact on coastal water. 39. Meybeck M: Carbon, nitrogen and phosphorus transport by world rivers. Am J Sci 1982, 282:401-450. 40. Lapointe B, Oconnell JD, Garrett GS: Nutrient coupling between on-site sewage disposal systems, groundwaters and nearshore surface waters of the Florida Keys. Biogeochemistry 1990, 10:289-307. 41. Weiskel PK, Howes BL: Differential transport of sewagederived Nitrogen and Phosphorus through a coastal watershed. Environ Sci Technol 1992, 26:352-360.
52. Varma S, Turner J, Underschultz J: Estimation of submarine groundwater discharge into Geographe Bay, Western Australia. J Geochem Explor 2009, 101:107. 53. Stachelhaus SL, Moran SB, Kelly RP: An evaluation of the efficacy of radium isotopes as tracers of submarine groundwater discharge to southern Rhode Island’s coastal ponds. Mar Chem 2012, 130–131:49-61. 54. Michael HA, Charette MA, Harvey CF: Patterns and variability of groundwater flow and radium activity at the coast: a case study from Waquoit Bay, Massachusetts. Mar Chem 2011, 127:100-114. 55. Garrison GH, Glenn CR, McMurtry GM: Measurement of submarine groundwater discharge in Kahana Bay, Oahu, Hawaii. Limnol Oceanogr 2003, 48:920-928. 56. Swarzenski PW, Reich C, Kroeger KD, Baskaran M: Ra and Rn isotopes as natural tracers of submarine groundwater discharge in Tampa Bay, Florida. Mar Chem 2007, 104:69-84. 57. Ferrarin C, Rapaglia J, Zaggia L, Umgiesser G, Zuppi GM: Coincident application of a mass balance of radium and a hydrodynamic model for the seasonal quantification of groundwater flux into the Venice Lagoon, Italy. Mar Chem 2008, 112:179-188. 58. Lambert MJ, Burnett WC: Submarine groundwater discharge estimates at a Florida coastal site based on continuous radon measurements. Biogeochemistry 2003, 66:55-73.
42. Slomp CP, Cappellen PV: Nutrient inputs to the coastal ocean through submarine groundwater discharge: controls and potential impact. J Hydrol 2004, 295:64-86.
59. Gu H, Moore WS, Zhang L, Du J, Zhang J: Using radium isotopes to estimate the residence time and the contribution of submarine groundwater discharge (SGD) in the Changjiang effluent plume, East China Sea. Cont Shelf Res 2012, 35:95-107.
43. Holtan-Hartwig L, Dorsch P, Bakken LR: Low temperature control of soil denitrifying communities: kinetics of N2O production and reduction. Soil Biol Biochem 2002, 34:17971806.
60. Shellenbarger GC, Monismith SG, Genin A, Paytan A: The importance of submarine groundwater discharge to the near shore nutrient supply in the Gulf of Aqaba (Israel). Limnol Oceanogr 2006, 51:1876-1886.
44. Charette MA, Sholkovitz ER: Oxidative precipitation of groundwater-derived ferrous iron in the subterranean estuary of a coastal bay. Geophys Res Lett 2002, 29:1444 http:// dx.doi.org/10.1029/2001GL014512.
61. Rapaglia J, Koukoulas S, Zaggia L, Lichter M, Manfe´ G, Vafeidis AT: Quantification of submarine groundwater discharge and optimal radium sampling distribution in the Lesina Lagoon, Italy. J Marine Syst 2012, 91:11-19.
45. Rabouille C, Mackenxie FT, Ver LM: Influence of the human perturbation on carbon, nitrogen, and oxygen biogeochemical cycles in the global coastal ocean. Geochim Cosmochim Acta 2001, 65:3615-3641. 46. Crotwell AM, Moore WS: Nutrient and radium fluxes from submarine groundwater discharge to Port Royal Sound, South Carolina. Aquat Geochem 2003, 9:191-208. 47. Chen CTA, Liu KK, MacDonald R: Continental margin exchanges. In Ocean Biogeochemistry: A JGOFS Synthesis. Edited by Fasham MJR. Springer; 2003:53-97. The authors discuss about the role of the carbon cycle and nutrients on global scale. 48. Najjar R, Butman DE, Cai W-J, Friedrichs MAM, Kroeger KD, Mannino A, Raymond PA, Salisbury J, Vandemark DC, Vlahose P: Carbon budget for the continental shelf of the Eastern United States: a preliminary synthesis. Ocean Carbon Biogeochem 2010, 3. 49. Beck AJ, Rapaglia JP, Cochran JK, Bokuniewicz HJ, Yang S: Submarine groundwater discharge to Great South Bay, NY, estimated using Ra isotopes. Mar Chem 2008, 109:279-291. 50. Povinec PP, Bokuniewicz H, Burnett WC, Cable J, Charette M, Comanducci JF, Kontar EA, Moore WS, Oberdorfer JA, Oliveira JD et al.: Isotope tracing of submarine groundwater discharge offshore Ubatuba, Brazil: results of the IAEA–UNESCO SGD project. J Environ Radioact 2008, 99:1596-1610. 51. Povinec PP, Aggarwal PK, Aureli A, Burnett WC, Kontar EA, Kulkarni KM, Moore WS, Rajar R, Taniguchi M, Comanducci JF et al.: Characterisation of submarine groundwater discharge offshore south-eastern Sicily. J Environ Radioact 2006, 89:81-101. www.sciencedirect.com
62. Ollivier P, Claude C, Radakovitch O, Hamelin B: TIMS measurements of 226Ra and 228Ra in the Gulf of Lion, an attempt to quantify submarine groundwater discharge. Mar Chem 2008, 108:337-354. 63. Basu AR, Jacobsen SB, Poreda RJ, Dowling CB, Aggarwal PK: Large groundwater strontium flux to the oceans from the Bengal Basin and the marine strontium isotope record. Science 2001, 293:1470-1473. 64. Hide K: Report on the Geology and Hydrology of Rishiri Island, Japan. The Hokkaido Farmland Development Section; 1961:. (in Japanese with English translation). 65. Johannes RE, Hearn CJ: The effect of subsurface groundwater discharge on nutrient and salinity regimes in a coastal lagoon off Perth, West Australia. Estuar Coast Shelf Sci 1985, 21:789-800. 66. Tsunogai U, Ishibashi J, Wakita H, Gamo T, Masuzawa T, Nakatsuka T, Nojiri Y, Nakamura T: Fresh water seepage and pore water recycling on the seafloor: Sagami Trough subduction zone, Japan. Earth Planet Sci Lett 1996, 138:157-168. 67. Kohout FA, Kolipinski MC: Biological zonation related to groundwater discharge along the shore of Biscayne Bay, Miami, Florida. In Estuaries. Edited by Lauff G. Washington, DC: AAAS; 1967. (Am. Assoc. Advancement of Science Publication No. 83). 68. Hwang DW, Lee WY, Kim G: Large submarine groundwater discharge and Benthic eutrophication in Bangdu Bay on volcanic Jeju Island, Korea. Limnol Oceanogr 2005, 50:1393-1403. Current Opinion in Environmental Sustainability 2012, 4:219–226
226 Carbon and nitrogen cycles
69. Krest JM, Moore WS, Gardner LR, Morris JT: Marsh nutrient export supplied by groundwater discharge: evidence from radium measurements. Glob Biogeochem Cycles 2000, 14:167-176. 70. Street JH, Knee KL, Grossman EE, Paytan A: Submarine groundwater discharge and nutrient addition to the coastal zone and coral reefs of leeward Hawaii. Mar Chem 2008, 109:355-376. 71. Senal MIS, Jacinto GS, Diego-McGlone MLS, Siringan F, Zamora P, Soria L, Cardenas MB, Villanoy C, Cabrera O: Nutrient inputs from submarine groundwater discharge on the Santiago reef flat, Bolinao, Northwestern Philippines. Mar Pollut Bull 2011, 63:95-200.
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72. Niencheski LFH, Windom HL, Moore WS, Jahnke RA: Submarine groundwater discharge of nutrients to the ocean along a coastal lagoon barrier, Southern Brazil. Mar Chem 2007, 106:546-561. 73. Waska H, Kim G: Submarine groundwater discharge (SGD) as a main nutrient source for benthic and water-column primary production in a large intertidal environment of the Yellow Sea. J Sea Res 2011, 65:103-113. 74. Hatta M, Zhang J, Hiroshi S, Ishizaka J, Nakaguchi Y: Water mass structure and fresh water fluxes (riverine and SGD’S) into Toyama Bay. Chikyukagaku (Geochemistry, Japanese with English abstract) 2005, 39:157-164.
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Linkages between submarine groundwater systems and the environment Jing Zhang and Ajit K Mandal
Submarine groundwater discharge (SGD) has been recognized as an important source of freshwater discharge into the ocean. Different approaches are used to estimate the SGD magnitude. Principal methods include, first, direct measurement using manual or automated seepage meters, second, chemical tracers, and third, hydrogeologic modeling. Submarine groundwater studies reveal that SGD provides important fluxes of nutrients, carbon, and trace metals to coastal waters that have the potential to impact the chemical budget of coastal water. Apart from the amounts of nutrients (N, P, and Si)/carbon entering the coastal ocean through SGD, it is also important to evaluate the potential effect of SGD on the Redfield ratio, because this ratio determines which nutrient is limiting phytoplankton growth. To pursue detailed SGD studies, an independent discipline would facilitate more progress. Address Earth and Environmental System, Graduate School of Science and Engineering, University of Toyama, Toyama 930-8555, Japan Corresponding author: Zhang, Jing ([email protected])
Current Opinion in Environmental Sustainability 2012, 4:219–226 This review comes from a themed issue on Carbon and nitrogen cycles Edited by Chen-Tung Arthur Chen and Dennis Peter Swaney Received 16 February 2012; Accepted 26 March 2012 Available online 20th April 2012 1877-3435/$ – see front matter # 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cosust.2012.03.006
Introduction Submarine groundwater discharge (SGD) is the direct flow of groundwater into the ocean. Like surface water, groundwater flows down a gradient, and SGD occurs wherever a coastal aquifer is connected to the sea. The term SGD appeared in the literature during the early 1970s [1] and even earlier [2,3]. Annually, about 2400 km3 of freshwater discharges into the world’s seas and oceans in the form of SGD, with 1500 km3 contributed by the continents and 900 km3 by the world’s islands [4]. SGD has been recognized as an important source of freshwater discharge into the ocean and a valuable component of the hydrological cycle. While SGD is a part of the groundwater system, it has an impact on the coastal water www.sciencedirect.com
ecosystem. Hydrogeologists are used to working on land, while marine scientists focus on coastal environments. Hydrogeological studies have gained attention for quantifying the movement of fresh groundwater or examining saltwater intrusion into freshwater aquifers. Modeling methods are also primarily employed by hydrogeologists in quantifying SGD. However, the participation of hydrologists in SGD studies has been less than what it should have been [5], although some hydrologists have published many results on SGD [6,7]. Burnett et al. [8] reviewed the topic of SGD in detail and summarized the findings of various research projects across the globe. The coauthors of this article are from a variety of disciplines, mainly from oceanography and marine departments with good representation of professionals of different backgrounds, but hydrologists or hydrogeologists are still not particularly well represented in the field of SGD [5]. While hydrologists and marine scientists focus on coastal environments, both groups approach the problem of SGD literally from opposite directions, and so have not always been working with the same concepts and definitions of the phenomenon [9]. For the true depth of study that the science of SGD deserves, an independent discipline is required; such a focus on groundwater processes could lead to a more complete picture of SGD. SGD is defined as terrestrially derived freshwater and recirculated seawater, which includes discharge associated with saltwater intrusion along a coastline, tidal pumping, and wave setup; all of these discharges flow out from the seabed into the coastal water through the underlying sediments [10]. The terrestrially derived SGD ranges from 6 to 10% of the surface waters discharging into the ocean [11], and recirculated seawater due only to density effects and mixing may constitute up to 70% of the SGD [12,13]. Furthermore, the recirculated seawater due to wave setup and tides may contribute up to 96% of the SGD [14]. SGD occurs not only near the shore but also a large distance from the shore. Many independent studies of SGD have been performed along the east coast of the United States, northern Italian coasts, and the coasts of Japan and South Korea [6]. SGD has been neglected scientifically because of the difficulty in finding and measuring these features [4]. The location of current SGD studies and volume (or flux) of SGD on the global scale is presented in Figure 1 and Table 1. However, ecological effects in estuaries and the coastal Current Opinion in Environmental Sustainability 2012, 4:219–226
220 Carbon and nitrogen cycles
Figure 1
90˚
60˚ 15
10
14
7
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21 A
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8 22
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Discharge rate Origin Discharge rate & nutrient
4
2 H
Trace metal Nutrient
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Global studies and locations of SGD.
zone may depend on water quality, which is influenced by SGD. For example, biological zonation associated with groundwater discharge was identified in Biscayne Bay/Florida Bay (Table 1). Possible relationships between SGD and sea grass distributions were reported from the coastal zone near Perth, Australia, and in the coastal northeastern Gulf of Mexico. However, it is also clear that wide areas of the world have little to no SGD assessments at all. A few studies have been done on the
west coast of the USA and Hawaii. There are limited quantitative data from South America, Africa, India, and China, although indications of groundwater discharge were reported for India [15] (Figure 1). A few studies have been performed on volcanic and coral Islands. Some SGD studies showed large amounts of freshwater flowing into coastal water. Data on SGD with respect to trace metals are few as shown in Figure 1.
Table 1 SGD discharge rates and measurement methods. No.
Location
1 2 3 4 5 6 7 8 9 10
Great South Bay Ubatuba, Brazil South-Eastern Sicily Geographe Bay Rhode Island’s Waquoit Bay Yellow Sea Kahana Bay, Hawaii Tampa Bay Venice Lagoon
11 12 13 14 15 16 17 18 19 20 21 22
Northeast Gulf of Mexico East China Sea Eilat, Israel Lesina Lagoon Gulf of lion South Atlantic Bight Bay of Bengal Toyama Bay, Japan Rishiri Island, Japan Perth, Australia Sagami Bay, Japan Biscayne Bay, Florida
Discharge rate 6
3
1
4.5 10 m day 3 105 m3 day 1 3.4–10.4 102 m3 day 1 2.2 105 m3 day 1 1.7 10 2 m3 m2 day 1 6.9 104 m3 day 1 2.7–18.4 m3 day 1 9.0 104 m3 day 1 1.6–10.3 104 m3 day 1 4.3 2.5 107 m3 day 1 4.0 1.7 107 m3 day 1 3.1 103 m3 day 1 0.2–1.0 109 m3 day 1 0.06–0.26 m3 m 2 day 1 9.0–10.5 105 m3 day 1 2.9–11 106 m3 day 1 3 107 m3 day 1 5.5 108 m3 day 1 3.9 102 m3 day 1 2.5 105 m3 day 1 4.8 104 m3 day 1 3.1–5.0 102 m3 day 1 1–3 cm day 1
Current Opinion in Environmental Sustainability 2012, 4:219–226
Method 223
Ra Rn Seepage meter Hydraulic assessment (Darcy’s Law) Residence time models 226Ra and 228Ra 226 Ra 223,224,226,228 Ra Seepage meter 226 Ra 223 Ra 224 Ra Continuous Radon Model 222,223,224,226 Ra 223 Ra and 224Ra 224 Ra 226 Ra and 228Ra 226 Ra 3 He/3H Toyama University SGD-Flux Chamber Water balance method Salinity Heat flow chemical composition Seepage meter 222
Reference Beck et al. [49] Povinec et al. [50] Povinec et al. [51] Varma et al. [52] Stachelhaus et al. [53] Michael et al. [54] Kim et al. [29] Garrison et al. [55] Swarzenski et al. [56] Ferrarin et al. [57] Lambert and Burnett [58] Gu et al. [59] Shellenbarger et al. [60] Rapaglia et al. [61] Ollivier et al. [62] Moore [21] Basu et al. [63] Zhang et al. [35] Hide [64] Johannes et al. [65] Tsunogai et al. [66] Kohout et al. [67]
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Linkages between submarine groundwater systems and the environment Zhang and Mandal 221
Methods used for measuring SGD SGD has been measured at several sites. These studies have successfully employed natural radioactive elements such as 222Ra and other radium isotopes (223Ra, 224Ra, 226 Ra, and 228Ra), water isotopes of 18O and D and dissolved gases such as CH4 as tracers, and seepage meters. Some of the studies adopting these methods to calculate discharge rate are listed in Table 1. Until the late 1990s, quantitative estimation of SGD was impossible, and the impact of SGD on marine coastal processes was unknown. Since 2000, a number of sites across the globe have been selected for detailed studies of the SGD phenomenon. The following sections describe methods that have been used to estimate SGD by direct measurements and indirect analysis, including seepage meters, hydrologic models (Darcy’s law, water budget, and hydrograph separation), and tracers. For precise evaluation of SGD flux, seepage measurement in conjunction with hydrological analysis of groundwater is needed [16]. Direct measurements — seepage meters
Seepage meters are the most commonly used devices for direct measurement of seepage flux, and they have been used in numerous studies of SGD in coastal water [17– 19]. The basic concept of the seepage meter is to cover and isolate seepage water with a chamber open at the base and measure the change in the volume of water contained in a bag attached to the chamber over a measured time interval. Using manual seepage meters is very laborintensive. Therefore, various types of automated seepage meters have been developed to record time-series changes in SGD. Seepage chambers are fitted with different instruments for continually measuring the rate of water flow through the outlet. Direct measurement is sometimes contrary to the results of the modeled-SGD [16]. Indirect analysis — hydrogeologic modeling
Modeling methods are primarily employed by hydrogeologists, whose work focuses on the land side of SGD. There are four major modeling methods (Darcy’s law, water budget, hydrograph separation, and numerical modeling) used to quantify SGD. Hydrogeologic modeling has typically been used to estimate the advecting terrestrial component of SGD. In the water balance approach, the estimate of SGD is based on the water balance equation at the basin scale. This method only provides the fresh groundwater component of SGD as the total volume of groundwater discharging to the sea during the selected time period. Thus, the results of the water balance method are not comparable with locally measured values or with results of numerical models, which provide the distribution of SGD over the discharge area. Mulligan and Charette [20] compared investigative methods including direct measurement via seepage meters, hydrogeologic estimation using Darcy’s law, and tracer-based estimates using radon and radium www.sciencedirect.com
isotopes. They concluded that the hydrogeologic estimate and radon and radium techniques are complimentary for estimating different components of total SGD. Geochemical tracers for SGD research
One approach to local-scale to regional-scale estimation of groundwater input into the ocean uses naturally occurring geochemical tracers. An advantage of groundwater tracers is that they present an integrated signal as they enter the marine water column through various pathways in the aquifer. Natural geochemical tracers have been applied in two ways to evaluate groundwater discharge rates into the ocean. The first is the use of enriched geochemical tracers in the groundwater relative to the seawater. The concentration of a solute in the receiving water body is attributed to inputs of that component derived only from groundwater [21,22,23]. A second approach is the use of vertical profiles of the geochemical compositions in sediment pore waters under the assumption that their distribution can be described by a vertical, one-dimensional advection-diffusion model [24]. However, this is usually limited to the case of homogeneous media. 222
Rn is a radioactive noble gas produced by the radioactive decay of 226Ra. Both are members of the natural 238 U decay chain. 226Ra occurs in all rocks, soil, and sediments within a wide range of concentrations. Radon enters the groundwater and is transported with it through the aquifer. Over the past few years, several studies used natural radium isotopes and 222Rn to assess groundwater discharge into the ocean [22,25,26,27]. A groundwater tracer should be greatly enriched in the discharging groundwater relative to coastal marine waters. Radium isotopes are enriched in groundwater relative to surface waters, especially where saltwater comes into contact with surfaces. Hwang et al. [28] developed a geochemical model for local-scale estimate of SGD. If the system under study is in a steady state, then radium additions are balanced by losses. Additions include radium fluxes from sediment, river, and groundwater; losses are due to mixing and, in the case of 223Ra and 224Ra, radioactive decay. Using a mass balance approach on a large scale with the long-lived isotopes 226Ra and 228Ra, Kim et al. [29] determined that SGD-derived silicate fluxes to the Yellow Sea were on the same order of magnitude as the Si flux from the Yangtze River, the fifth-largest river in the world. Another new approach is based on an in situ analysis of radon decay products in seawater using gamma-ray spectrometer techniques. This has been recognized as a powerful tool for analysis of gamma-ray emitters in seabed sediments as well as for continuous analysis of gamma-ray emitters (137Cs, 40K, 238U, and 232Th decay products) in seawater [30]. Current Opinion in Environmental Sustainability 2012, 4:219–226
222 Carbon and nitrogen cycles
Methane (CH4) is another geochemical tracer that can be used to detect SGD. Both 222Rn and CH4 were used to evaluate SGD in studies performed in a coastal area of the northeastern Gulf of Mexico [22]. Several other natural radioactive (3H, 14C, U isotopes, among others) and stable (2H, 3He, 4He, 13C, 15N, 18O, 87Sr/88Sr, among others) isotopes and some anthropogenic atmospheric gases (e.g. CFCs) have been used for investigating SGD, tracing water masses, and calculating the age of groundwater.
Nutrient transport via SGD into the coastal ocean and its impact SGD is an important source of nutrients, trace elements and contaminants for the coastal ocean in many parts of the world [17]. SGD has received increased attention during recent decades because of its significant environmental consequences in coastal water. Groundwater nutrient concentrations are relatively higher than in seawater, and even small discharges of groundwater may make large contributions to coastal nutrient budgets [11], especially in oligotrophic areas with few other external nutrient sources [31]. Nitrate and phosphate concentrations in groundwater are variable and depend on inputs, soil and aquifer type, aquifer permeability, groundwater recharge rate, and climate [32]. Nitrate and phosphate concentrations in coastal groundwater are often much higher than those in river water [33]. Primary sources for increasing nutrient loads are wastewater disposal, application of fertilizers, and atmospheric deposition [34]. Continued residential and agricultural development of near-shore areas is leading to increased inputs of nitrate and phosphate from fertilizers and wastewater to the groundwater, and some of these nutrients are released into coastal surface waters [30]. Fluxes of nutrients to coastal waters may promote the bloom of phytoplankton [35] and trigger algae blooms, some of which have harmful impact on the local ecosystem and on the economy of coastal zones [36]. For example, Valiela et al. [37] reported that SGD contributes excessive nitrogen loading to salt marshes, thus impacting the ecology of coastal waters. It is important to evaluate the potential effect of SGD on the ratio of nitrate and phosphate in coastal waters, since this ratio determines which nutrient is limiting phytoplankton growth [38]. On a global scale, the average molar ratio is 18:1 [39] and thus closely matches the requirements of phytoplankton (N:P = 16:1, which is the ‘Redfield ratio’). Groundwater DIN/DIP ratios in contaminated aquifers are typically much higher than 18 because P in groundwater is more efficiently immobilized than N [40]. Nutrient inputs through SGD may drive N-limited coastal systems toward Plimitation [41]. Geochemical processes occurring at the interface between fresh and saline groundwaters of the so-called Current Opinion in Environmental Sustainability 2012, 4:219–226
‘subterranean estuary’ have a significant effect on the SGD composition of the coastal ocean [33]. In the subterranean estuary, biogeochemical reactions in these environments are strongly influenced by the flow rates and redox characteristics of the freshwater and seawater [42], desorption–sorption processes, and microbially driven diagenesis. SGD from oxic subterranean estuaries is expected to have high N/P ratios due to nearly conservative transport of N and almost complete removal of P [42]. The efficiency of the removal of N and P in the subterranean estuary may vary seasonally. Denitrification rates are very sensitive to temperature changes [43] and are expected to be lowest in winter. P removal may be more efficient due to the generally more oxic sediment conditions. As a result, N/P ratios in SGD are expected to be higher in winter than in summer. This was observed to be the case for the N/P ratios of SGD and the receiving surface waters of Buzzards Bay in Massachusetts [17]. Charette and Sholkovitz [44] reported on a large accumulation of iron oxides at the fresh-saline interface in Waquoit Bay. These iron oxide sands could act as a geochemical barrier by retaining and accumulating certain dissolved species carried to the subterranean estuary by groundwater and/or seawater. An accurate assessment of the importance of SGD on nutrients on a global scale is needed. In particular, regions consisting of developing countries with increasing population densities and high rates of fertilizer use (e.g. Asia and South America) are of interest, since, analogous to rivers [45], these are the regions where inputs of nutrients to groundwater are expected to increase most.
Global estimates of nutrients/carbon fluxes The ecological importance of nutrient (N, P, and Si) inputs via SGD to areas near shore waters is well recognized, although their concentrations appear to vary considerably (Table 2). For example, N and P input from SGD into Port Royal Sound were 6700 mmol m 2 day 1 and 500 mmol m 2 day 1, respectively [46], whereas N and P input into Guam’s Tumon Bay were 1340 mmol m 2 day 1 and 12 mmol m 2 day 1, respectively. Despite the regional variation in concentrations, nitrate (NO3 ), phosphate (PO4 ) and silica (Si) concentrations in most of the SGD studies varied between 0.93 and 3.5 mg/ L, 0.012 and 0.33 mg/L, and 1.03 and 22 mg/L, respectively. Annually, about 2400 km3 of freshwater discharges into the world’s seas and oceans in the form of SGD [4]. Using a simple calculation (i.e. the product of this average flow rate and the above concentrations), the annual loading of major nutrients via SGD to global flux ranged from 0.5 10 3 to 1.9 10 3 PgN year 1, 0.009 10 3 to 0.25 10 3 PgP year 1 and 1.15 10 3 to 24.69 10 3 PgSi year 1, respectively. Chen et al. [47] estimated that global riverine flux of nitrogen and phosphorus was www.sciencedirect.com
Linkages between submarine groundwater systems and the environment Zhang and Mandal 223
Table 2 DIN, DIP, and DSi fluxes from various coasts. No. A B C D E F
No. G H I J
Locations Bangdu Bay, Korea Eilat, Israel North Intet, S. Carolina Spencer Beach, Hawaii Kamiloloa, Hawaii Pangasinan, Philippines
Locations Tampa Bay, Florida Southern Brazil Yellow Sea Toyama Bay, Japan
DIN (mmol N m
2
day 1)
21.4 2.9–10 2.42 3.3–4.4 0.7–1.3 0.9–4.4
0.16 0.02–2.0 0.91 0.11–0.15 0.16-0.30 0.08–0.2
2
day 1)
DSi – – – – – –
DIN (mol day 1)
DIP (mol day 1)
DSi (mol day 1)
1.3–9.0 105 2.42 106 5.7 105 5.25 105
0.6–4.5 104 5.2 105 7.3 103 1.5 103
0.4–2.9 105 5.92 106 4.7 105 1.13 106
0.028 PgN year 1 and 0.0018 PgP year 1, respectively. Comparing with the global nutrient flux of SGD, the magnitude of nitrogen and phosphorus through SGD was 2–7% and 0.5–13% of riverine flux. The SGD at a global scale estimated to be from 0.01% to 10% of river discharge [6]. However, it indicated that on a global scale concentrations of nutrients are higher in SGD than river water. Therefore, SGD is one of the important sources of nutrients in local/regional coastal water. The flux of carbon from the terrestrial biosphere to the oceans takes place though river transport. A global riverine transfer of C has been estimated to be 0.74 PgC year 1 [47]. Najjar [48] estimated that SGD contributes carbon to the coastal ocean at a rate of 30% of the riverine flux. Therefore, a crude estimate of the global flux of carbon via SGD is 0.23 PgC year 1.
Overview of current SGD research and global urgency of future studies Many professional organizations and scientific bodies involved in SGD research projects, include the Intergovernmental Oceanographic Commission (IOC), the International Hydrological Program (IHP), the Scientific Committee on Oceanic Research (SCOR), which established its working group WG112 on SGD, LandOcean Interactions in the Coastal Zone (LOICZ), and the International Association of Physical Sciences of the Oceans (IAPSO). An initiative on SGD characterization was developed by the International Atomic Energy Agency (IAEA) and UNESCO in 2000 as a five-year plan to assess methodologies and the importance of SGD for coastal zone management. SGD also has been taken up as one of the priority research issues of the GEOTRACES program (GEOTRACES Science Plan). Although many previous and current SGD studies have been carried out worldwide, knowledge of the significance and impact on the global oceans is still limited. This is because SGD fluxes are strongly controlled by many local and regional conditions, with the exception www.sciencedirect.com
DIP (mmol P m
Reference Hwang et al. [68] Shellenbarger et al. [60] Krest et al. [69] Street et al. [70] Street et al. [70] Senal et al. [71]
Reference Swarzenski et al. [56] Niencheski et al. [72] Waska and Kim [73] Hatta et al. [74]
of the influences on the ocean circulation and CO2 fixation via SGD. SGD can influence the ocean circulation structure and stratification of the ocean because it is an important route of freshwater and heat transport, especially in high latitudes. The temperature of the groundwater is warm during the winter and has small changes annually. In polar areas, the higher temperature of SGD relative to the riverine water leads to increases in sea surface temperatures and freshening of the surface seawater’s salinity, when SGD flows into the ocean from late autumn to winter. This inhibits ice formation and interferes with the seawater subduction. Therefore, a quantitative understanding of the SGD variation in the polar regions, including details of heat/freshwater transport, is required, in particular as an important factor affecting the world ocean circulation in the future. Moreover, SGD is also an important carbon supply route to the ocean. Groundwater ranks in third place in terms of freshwater volume on the earth’s surface, following ice and glaciers as reservoirs. Groundwater also plays an important role in providing temporary CO2 storage from the atmosphere to the ocean because of its relatively long residence time. In the global carbon balance, CO2 dissolves into groundwater through the process of weathering (0.4 PgC year 1), an amount equal to about 20% of the total CO2 increase after the industrial revolution (1.9 PgC year 1); this quantity is then carried to the ocean by SGD and river water. However, the buffering capacity of groundwater is lower than that of seawater. In the future, reduction of CO2 storage in groundwater will be a concern because of the expected changes in our groundwater due to anthropogenic acidification and many major changes in the terrestrial environment. Our knowledge of SGD is still insufficient, and so further research is urgently needed. SGD is one of the factors that affect important coastal marine environments and material circulation. Our better understanding of SGD Current Opinion in Environmental Sustainability 2012, 4:219–226
224 Carbon and nitrogen cycles
will help to clarify the global environmental changes due to human activity and climate change.
15. Moore WS: High fluxes of radium and barium from the mouth of the Ganges–Brahmaputra River during low river discharge suggest a large groundwater source. Earth Planet Sci Lett 1997, 150:141-150.
Acknowledgements
16. Peng TR, Chen CTA, Wang CH, Zhang J, Lin YJ: Assessment of terrestrial factors controlling the submarine groundwater discharge in water shortage and highly deformed island of Taiwan, western Pacific Ocean. J Oceanogr 2008, 64:323-337.
The authors are grateful to the editor, C.T. Arthur Chen, for his critical comments and valuable suggestions to improve this paper. We also thank Jan ten Have for his help during the submission process and M.S. Nahar for assistance during the revision process. This work was supported by the Ministry of Education, Science, Sports and Culture, Japan, through the Grant in Aid 22403001, and by the ‘4000 m environmental project’ of the University of Toyama.
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