Natural 222 Rn and 220 Rn indicate the impact of the Water–Sediment Regulation Scheme (WSRS) on submarine groundwater discharge in the Yellow River estuary, China Xu Bochao a,b,c , Xia Dong c , William C. Burnett d , Natasha T. Dimova e , Wang Houjie f , Zhang Longjun c , Gao Maosheng g , Jiang Xueyan a,b , Yu Zhigang a,b,⇑ a Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China b College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China c Key Laboratory of Marine Environment and Ecology, Ministry of Education, Ocean University of China, Qingdao 266100, China d Department of Earth, Ocean and Atmospheric Sciences, Florida State University, Tallahassee, FL 32306, USA e Department of Geological Sciences, University of Alabama, Tuscaloosa, AL 35406, USA f College of Marine Geosciences, Ocean University of China, Qingdao 266100, China g Qingdao Institute of Marine Geology, Qingdao 266071, China article info Article history: Available online 7 October 2014 Editorial handling by M. Kersten abstract Submarine groundwater discharge (SGD) in estuaries brings important influences to coastal ecosystems. In this study, we observed significant SGD in the Yellow River estuary, including a fresh component, dur- ing the Water–Sediment Regulation Scheme (WSRS) period. We used the 222 Rn and 220 Rn isotope pair to locate sites of significant SGD within the study area. Three apparent SGD locations were found during a non-WSRS period, one of which became much more pronounced, according to the remarkably elevated radon levels, during the WSRS. Increased river discharge (from 245 m 3 s 1 to 3560 m 3 s 1 ) and the ele- vated river water level (from 11 m to 13 m) during the WSRS led to a higher hydraulic head, enhancing groundwater discharge in the estuary. Our results suggest that high river discharge (>3000 m 3 s 1 ) might be necessary for elevated fresh submarine groundwater discharging (FSGD). Vertical profiles of salinity, DO and turbidity anomalies along the benthic boundary layer also indicated significant FSGD in the estu- ary during the WSRS. Nutrient concentrations had positive correlations with 222 Rn during a 24-h obser- vation, which indicates that SGD is a dominant nutrient pathway in this area. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Submarine groundwater discharge (SGD) has become recog- nized as an important contributor to coastal ocean geochemical budgets (Burnett et al., 2006; McCoy and Corbett, 2009; Rodellas et al., 2014; Porubsky et al., 2014). Not only because of the elevated concentrations of nutrients and other constituents (e.g., carbon, trace metals) in groundwaters due to various water–sediment interactions in underground aquifers, but also because of compara- ble water fluxes relative to river discharge in some cases (Moore, 1996; Swarzenski et al., 2007; Wu et al., 2013). Under a broad def- inition (Burnett et al., 2003; Moore, 2010), SGD does not simply refer to terrestrial/fresh groundwater discharging (FSGD) to coastal waters, but also includes oceanic recirculating saline waters (RSGD) with scale lengths of at least meters. Radon (Rn), a naturally occurring radioactive noble gas, has been widely used to quantify SGD fluxes (Cable et al., 1996; Burnett and Dulaiova, 2006; Smith et al., 2008; Wen et al., 2014). The very large enrichment of radon in groundwater over surface waters (typically 1000-fold or greater), its unreactive nature, and short half-life make radon an excellent tracer to identify areas of significant groundwater discharge (Burnett and Dulaiova, 2006). 222 Rn (half-life 3.82 d) and 220 Rn (half-life 55.6 s) are the two most abundant natural radon isotopes. The longer-lived radon isotope 222 Rn has been more extensively applied to study SGD process. In contrast, the short-lived 220 Rn is less useful generally because its short half-life makes it very difficult to measure. Some recent stud- ies used 220 Rn combined with 222 Rn and other gases (e.g., CO 2 , CH 4 ) as a proxy for other environmental assessment, such as soil gas transport (Giammanco et al., 2007; Huxol et al., 2013). Due to its very short half-life, 220 Rn migrates over a much shorter distance than 222 Rn after being released into the environment. Therefore, http://dx.doi.org/10.1016/j.apgeochem.2014.09.018 0883-2927/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: 238# Songling Road, Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China. Tel.: +86 532 66781006; fax: +86 532 66782799. E-mail address: zhigangyu@ouc.edu.cn (Z. Yu). Applied Geochemistry 51 (2014) 79–85 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem