Nutrient inputs to a Lagoon through submarine groundwater discharge: The case of Laoye Lagoon, Hainan, China Tao Ji a , Jinzhou Du a, ⁎, Willard S. Moore b , Guosen Zhang a , Ni Su a , Jing Zhang a a State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China b Department of Earth and Ocean Sciences, University of South Carolina, Columbia, SC 29208, USA abstract article info Article history: Received 24 April 2012 Received in revised form 12 November 2012 Accepted 20 November 2012 Available online 29 November 2012 Keywords: Radium isotopes Laoye Lagoon Submarine groundwater discharge Flushing time Water age Water budget Nutrients Submarine groundwater discharge (SGD) with inputs of nutrients in certain regions may play a significant role in controlling water quality in the coastal regions. In this paper, we have determined four naturally oc- curring radium isotope ( 223 Ra, 224 Ra, 226 Ra and 228 Ra) activities and nutrient concentrations in surface water, coastal groundwater and river water in the mixing zone of Laoye Lagoon to estimate the fluxes of SGD by sev- eral models. The activities of the four radium isotopes of ground water were considerably greater than those in surface water samples. Using a 224 Ra/ 228 Ra activity ratio (AR) model, we estimated the average lagoon water age to be 3.2 days, which was comparable with the flushing time of 4.0 days. Based on the excess ra- dium isotopes and the water age of the lagoon, the estimated fluxes of SGD (in 10 6 m 3 /d) ranged from 2.64 to 5.32 with an average of 4.11. Moreover, we used Si balance to evaluate the flux of SGD (4.8 × 10 6 m 3 /d) which was close to the result calculated by radium. The SGD-derived nutrient fluxes (in mol/d) were DIN = 1.7×10 5 , PO 4 3- =5.2×10 2 , and SiO 3 =5.3×10 4 . Furthermore, we applied the biogeochemical budget ap- proach using SiO 3 as a tracer to evaluate the impact of SGD. The differences between the results estimated by radium and SiO 3 may indicate different pathways for the input of nutrients. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Historically, submarine groundwater discharge (SGD) has been ig- nored as a source of nutrients to the coast. However, during the past two decades SGD has become recognized as a source of nutrients by many scientists (Burnett et al., 2001; Johannes, 1980; Moore, 1999; Simmons, 1992). Groundwater may be enriched in nutrients and heavy metals by natural causes and by the residential and agricultural development of near-shore areas, which lead to increased inputs of N and P from fertilizer and wastewater (Burnett et al., 2003; Johannes, 1980). Due to high nutrient concentrations in SGD, even a small SGD flux can transport a large flux of nutrients and other materials (Burnett et al., 2006; Corbett et al., 1999; Simmons, 1992; Slomp and Van, 2004). These nutrients released to estuarine and coastal sur- face waters by SGD may be of considerable importance for the ecolo- gy of some regions (Valiela et al., 1990); whereas, in other regions the nutrients from SGD are suggested to be the primary reason for water eutrophication (Paerl, 1997; Valiela et al., 2002). For example in Masan Bay, Korea, the fluxes of Si and P from SGD were 2–3 fold higher than that from stream water (Lee et al., 2009). As almost half of the population in the world is now living in the coastal zone and the ecosystems of these areas are relatively vulnerable (Jickells, 1998), it is very important to focus research on SGD. For example, in the industrial coastal area of Zhejiang Province, Eastern China, a large amount of industrial wastewater, irrigation water, and sewage infiltrates into groundwater, raising the possibility that the nutrients in the polluted water may be discharged into the sea through SGD (Zhang, 2010). These industrial processes are aug- mented by mariculture, which causes N and P enrichment in local seawater. Being the fundamental elements for the growth of algae, excessive amounts of these nutrients may lead to coastal eutrophica- tion, and even to harmful algae blooms. Such red tidal out-brakes have occurred nearly every year since the 1980s; and, this trend is becoming more severe with time in this coastal area (Hwang et al., 2005). Although SGD is considered as one of the significant vectors that input nutrients, heavy metals, and other materials to the estuary and coast regions (Burnett et al., 1990, 2001; Corbett et al., 1999; Johannes, 1980; Moore, 1996, 1999; Valiela et al., 1990), work to quan- tify the sources of these materials are not adequate. The flux of SGD has been demonstrated by many chemical tracers including 222 Rn (Burnett and Dulaiova, 2003; Cable et al., 1996), methane (Corbett et al., 1999), barium (Moore, 1997) as well as radi- um isotopes (Moore, 1996, 2003). Among these methods, estimating SGD by Ra isotopes is one of the most efficient (Moore, 1996). There are four naturally occurring radium isotopes, i.e., 223 Ra (T 1/2 = 11.4 days), 224 Ra (T 1/2 = 3.6 days), 226 Ra (T 1/2 = 1600 years), and 228 Ra (T 1/2 = 5.7 years). Because of the large variation in the rates of generation and decay, these four isotopes can be used to study the Journal of Marine Systems 111–112 (2013) 253–262 ⁎ Corresponding author. Tel.: +86 21 62232761; fax: +86 21 62546441. E-mail address: jzdu@sklec.ecnu.edu.cn (J. Du). 0924-7963/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmarsys.2012.11.007 Contents lists available at SciVerse ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys