海底地下水排泄研究回顾与进展

海底地下水排泄是全球水循环的重要组成部分,是近岸、滩涂和河口地区典型而重要的海水—地下水相互作用过程。作为全球水循环的重要组成部分,海底地下水排泄是海洋中水和各种化学物质的重要来源之一,同时也是各种污染物从陆地向海洋输送的一个重要而隐蔽的通道。综述了海底地下水排泄(SGD)的研究现状与进展,对海底地下水排泄的过程、研究方法、研究分布以及环境意义等方面进行了详细介绍,指出了目前研究存在的不足和需要努力的方向,从而为我国开展大规模海底地下水排泄研究提供了一定的思路。     

海底地下水排放:重要的海岸带陆海相互作用过程

沿海地区海底地下水排放在北美和欧洲等发达地区受到了越来越多的重视,被认为是一个重要的海岸带陆海相互作用过程。但这一过程在我国尚未引起足够认识,有关研究极少见。海底地下水排放的研究历史不长,只是近十多年才有了快速的发展,有了越来越多的定量研究成果。其研究方法主要有水文计算法、现场实测法和地球化学示踪法,各种方法之间的对比实验是目前的热点问题。沿海地下水排放具有重要的环境意义,它可以是陆地营养物质和污染物质的一个重要排放通道,可以对海岸带环境产生一定影响。我国沿海地区应该加强有关的研究工作,为海岸带环境管理作出贡献。     

Assessment of Submarine Groundwater Discharge (SGD) as a Source of Dissolved Radium and Nutrients to Moorea (French Polynesia) Coastal Waters

Previous work has documented large fluxes of freshwater and nutrients from submarine groundwater discharge (SGD) into the coastal waters of a few volcanic oceanic islands. However, on the majority of such islands, including Moorea (French Polynesia), SGD has not been studied. In this study, we used radium (Ra) isotopes and salinity to investigate SGD and associated nutrient inputs at five coastal sites and Paopao Bay on the north shore of Moorea. Ra activities were highest in coastal groundwater, intermediate in coastal ocean surface water, and lowest in offshore surface water, indicating that high-Ra groundwater was discharging into the coastal ocean. On average, groundwater nitrate and nitrite (N + N), phosphate, ammonium, and silica concentrations were 12, 21, 29, and 33 times greater, respectively, than those in coastal ocean surface water, suggesting that groundwater discharge could be an important source of nutrients to the coastal ocean. Ra and salinity mass balances indicated that most or all SGD at these sites was saline and likely originated from a deeper, unsampled layer of Ra-enriched recirculated seawater. This high-salinity SGD may be less affected by terrestrial nutrient sources, such as fertilizer, sewage, and animal waste, compared to meteoric groundwater; however, nutrient-salinity trends indicate it may still have much higher concentrations of nitrate and phosphate than coastal receiving waters. Coastal ocean nutrient concentrations were virtually identical to those measured offshore, suggesting that nutrient subsidies from SGD are efficiently utilized.     

Trace element cycling in a subterranean estuary: Part 2. Geochemistry of the pore water

Submarine groundwater discharge (SGD) is an important source of dissolved elements to the ocean, yet little is known regarding the chemical reactions that control their flux from sandy coastal aquifers. The net flux of elements from SGD to the coastal ocean is dependent on biogeochemical reactions in the groundwater-seawater mixing zone, recently termed the “subterranean estuary.” This paper is the second in a two part series on the biogeochemistry of the Waquoit Bay coastal aquifer/subterranean estuary. The first paper addressed the biogeochemistry of Fe, Mn, P, Ba, U, and Th from the perspective of the sediment composition of cores Charette et al. [Charette, M.A., Sholkovitz, E.R., Hansell, C.M., 2005. Trace element cycling in a subterranean estuary: Part 1. Geochemistry of the permeable sediments. Geochim. Cosmochim. Acta, 69, 2095-2109]. This paper uses pore water data from the subterranean estuary, along with Bay surface water data, to establish a more detailed view into the estuarine chemistry and the chemical diagenesis of Fe, Mn, U, Ba and Sr in coastal aquifers. Nine high-resolution pore water (groundwater) profiles were collected from the head of the Bay during July 2002. There were non-conservative additions of both Ba and Sr in the salinity transition zone of the subterranean estuary. However, the extent of Sr release was significantly less than that of its alkaline earth neighbor Ba. Pore water Ba concentrations approached 3000 nM compared with 25-50 nM in the surface waters of the Bay; the pore water Sr-salinity distribution suggests a 26% elevation in the amount of Sr added to the subterranean estuary. The release of dissolved Ba to the mixing zone of surface estuaries is frequently attributed to an ion-exchange process whereby seawater cations react with Ba from river suspended clay mineral particles at low to intermediate salinity. Results presented here suggest that reductive dissolution of Mn oxides, in conjunction with changes in salinity, may also be an important process in maintaining high concentrations of Ba in the pore water of subterranean estuaries. In contrast, pore water U was significantly depleted in the subterranean estuary, a result of SGD-driven circulation of seawater through reducing permeable sediments. This finding is supported by surface water concentrations of U in the Bay, which were significantly depleted in U compared with adjacent coastal waters. Using a global estimate of SGD, we calculate U removal in subterranean estuaries at 20 × 10⁶ mol U y⁻¹, which is the same order of magnitude as the other major U sinks for the ocean. Our results suggest a need to revisit and reevaluate the oceanic budgets for elements that are likely influenced by SGD-associated processes.     

镭同位素示踪隆教湾的海底地下水排泄

福建省漳州市隆教湾海水中镭同位素的研究,目的是评价海底地下水排泄量。在2007年6月的航次中,垂直于岸线的9km剖面上布置15个站位,每个站位用潜水泵采集表层海水样60L于塑料桶中。水样运回实验室后,立即用装有锰纤维的PVC管以虹吸的方式富集水样中的镭同位素,水通过PVC管的流速小于300ml/min。224Ra活度用连续射气法测定,测完224Ra后密封7d以上,然后用直接射气法测定226Ra活度。224Ra和226Ra活度都呈现自岸向海逐渐降低的规律,表明扩散控制镭同位素的分布,由224Ra获得68.83km2d-1的扩散系数,同时226Ra形成-0.963dpm100l-1km-1的活度梯度。用扩散系数和活度梯度建立的226Ra的离岸通量为6.62×1011dpmkm-2d-1,这个通量一定是得到SGD输入的镭支持,从而获得隆教湾的海底地下水排泄量是3.03×109m3km-2d-1。该排泄量包括陆源地下淡水排泄量和再循环海水排泄量,绝大部分可能是再循环海水,有待进一步研究。     

国内外地下水入海通量研究现状与趋势

地下水向海排泄是海陆相互作用的重要过程,它可能是陆地污染物质和营养盐排放入海的重要通道,会对近岸的海域环境产生一定的影响。它已在国外受到相当多的重视,而我国的相关研究还很少见。文中介绍了该领域的三种主要方法:原位测量方法、模型技术和示踪技术,且探讨了其发展方向及趋势。     

用镭同位素评价海水滞留时间及海底地下水排泄

海底地下水排泄(submarine groundwater discharge, SGD)难以直接测量, 镭同位素和氡-222等天然示踪剂使得间接评价SGD通量成为可能.为了评价五缘湾的水体滞留时间和SGD通量, 实测了湾内海水、湾外海水和地下水中224Ra和226Ra的活度, 利用224Ra和226Ra半衰期的差异, 采用224Ra与226Ra的活度比值计算湾内水团的年龄和平均滞留时间, 利用224Ra和226Ra的质量平衡模型计算SGD通量.五缘湾13个站位的水团年龄在0.6~2.4 d之间, 湾顶水团年龄相对较大, 平均海水滞留时间1.4 d.地下水输入五缘湾的224Ra和226Ra通量分别为5.17×106 Bq/d和5.28×106 Bq/d, 将该通量用地下水端元的活度转换成为SGD通量分别是0.21 m3/m2/d(224Ra平衡模型)和0.23 m3/m2/d(226Ra平衡模型), 两种模型的结果较接近, 其平均值0.22 m3/m2/d可作为五缘湾的海底地下水排泄通量.      

Tracing submarine groundwater discharge flux in Tolo Harbor,Hong Kong (China)

Submarine groundwater discharge (SGD) is an important pathway for groundwater and associated chemicals to discharge to the sea. Groundwater levels monitored along a transect perpendicular to the shoreline are used to calculate SGD flux from the nearshore aquifer to Tolo Harbor, Hong Kong (China). The calculated SGD flux—recharge/discharge measured with Darcy’s Law methods—agrees well with estimates based on geo-tracer techniques and seepage meter in Tolo Harbor during previous studies. The estimated freshwater SGD is 1.69–2.0 m²/d at the study site and 0.3?±?0.04 cm/d for the whole of Tolo Harbor, which is comparable to the river discharge (0.25?±?0.07 cm/d) and precipitation (0.45?±?0.15 cm/d). The tide-driven SGD in the intertidal zone is 13.98–17.59 m²/d at the study site and 2.42?±?0.56 cm/d for the whole of Tolo Harbor. The SGD occurring in the subtidal zone and the bottom of Tolo Harbor is 3.12?±?4.63 cm/d. Fresh SGD accounts for ~5% of the total SGD, while the rest (~95%) is contributed by saline SGD driven by various forces. About 96% of the tide-driven SGD in the intertidal zone occurs in the ebbing tide period because the head difference between the groundwater level and sea level is great during this period. Tide-driven SGD in the spring tide is ~1.2 times that during neap tide. The tidal fluctuation amplitude and tide-driven SGD in the intertidal zone are positively correlated to each other; thus, a spring neap variation of the tide-driven SGD is observed.     

用氡-222评价五缘湾的地下水输入

海底地下水排泄(SGD)近年来成为陆-海相互作用的研究热点,地球化学示踪方法是其主要研究手段,尝试用天然示踪剂氡-222评价厦门五缘湾的SGD。为了评价五缘湾SGD的入海通量及其变化,对五缘湾海水中²²²Rn和²²⁶Ra活度、大气中²²²Rn活度、风速、水温和水深进行了连续2 d的测量,对沉积物进行了培养实验用以获得其²²²Rn扩散通量和孔隙水中²²²Rn活度。基于海水中²²²Rn通量的质量平衡,对实测的海水中²²²Rn活度实施了母体支持、涨落潮影响、大气逃逸损失、沉积物扩散输入、混合损失的校正,保守估计SGD输入的²²²Rn通量在0~126.7 Bq/(m²·h)范围内变化,对海水中²²²Rn的平均贡献达54%。以井水和孔隙水中²²²Rn的加权平均值作为SGD端元的代表,获得SGD的输入速率为0~29.3 cm/d,平均输入速率9.3 cm/d。SGD输入速率的动态变化基本围绕12 h的周期波动,是对本海域正规半日潮的具体响应。假设SGD以平均速率在五缘湾海底输入,则五缘湾海底的SGD输入量为1.86×10⁵ m³/d。以陆源地下淡水占SGD输入量的10%考虑,五缘湾的陆源地下淡水输入量约为1.86×10⁴ m³/d。     

人类活动影响下的龙口海岸带海底地下水排泄通量研究

在沿海地区,以²²³Ra和²²⁴Ra为示踪剂建立的镭质量平衡模型已广泛应用于海底地下水排泄量(SGD)的研究中,然而目前国内外关于在人类活动复杂影响较大情况下的SGD研究却极为少见。本文对比研究了在有防渗墙(A区)和填海造陆(B区)两种不同人为因素影响下的龙口海岸带水体表现年龄、海底地下水排泄量及其携带的氮磷营养盐通量。结果表明,A区平均水体表现年龄为14.26 d,B区平均水体表现年龄为10.64 d。此外,B区沿岸地下水以及近岸海水中的Ra活度均普遍高于A区,而盐度低于A区。在SGD方面,A区的SGD速率为1.26~1.60 cm·d^-1,B区为1.43~1.82 cm·d^-1,考虑SGD在评估方法上存在一定的误差,因此两个区域的SGD速率相差不大。但与我国其他自然海域相比,这两个区域的SGD速率均处于较低水平。此外,B区的氮磷营养盐浓度普遍高于A区,而且由SGD驱动的氮磷营养盐通量不同,地下水输入的不平衡的营养盐极易改变龙口海域的营养盐结构,对海洋生态环境产生不利影响,这也进一步证实SGD在沿海生态环境以及水体污染治理中的重要地位。     

Tidal pumping drives nutrient and dissolved organic matter dynamics in a Gulf of Mexico subterranean estuary

We hypothesize that nutrient cycling in a Gulf of Mexico subterranean estuary (STE) is fueled by oxygen and labile organic matter supplied by tidal pumping of seawater into the coastal aquifer. We estimate nutrient production rates using the standard estuarine model and a non-steady-state box model, separate nutrient fluxes associated with fresh and saline submarine groundwater discharge (SGD), and estimate offshore fluxes from radium isotope distributions. The results indicate a large variability in nutrient concentrations over tidal and seasonal time scales. At high tide, nutrient concentrations in shallow beach groundwater were low as a result of dilution caused by seawater recirculation. During ebb tide, the concentrations increased until they reached a maximum just before the next high tide. The dominant form of nitrogen was dissolved organic nitrogen (DON) in freshwater, nitrate in brackish waters, and ammonium in saline waters. Dissolved organic carbon (DOC) production was two-fold higher in the summer than in the winter, while nitrate and DON production were one order of magnitude higher. Oxic remineralization and denitrification most likely explain these patterns. Even though fresh SGD accounted for only ∼5% of total volumetric additions, it was an important pathway of nutrients as a result of biogeochemical inputs in the mixing zone. Fresh SGD transported ∼25% of DOC and ∼50% of total dissolved nitrogen inputs into the coastal ocean, with the remainder associated with a one-dimensional vertical seawater exchange process. While SGD volumetric inputs are similar seasonally, changes in the biogeochemical conditions of this coastal plain STE led to higher summertime SGD nutrient fluxes (40% higher for DOC and 60% higher for nitrogen in the summer compared to the winter). We suggest that coastal primary production and nutrient dynamics in the STE are linked.     

Characterising the distribution of near-shore submarine groundwater discharge along a coastline using 222Rn and electrical conductivity

The spatial and temporal distribution of near-shore fresh submarine groundwater discharge (SGD) was characterised from the coastal aquifers of the Willunga Basin, South Australia, an extensive aquifer system that supports an important viticultural region. Measurements of electrical conductivity (EC) and ²²²Rn (radon) activity were collected at 19 sites along the coastline during the Southern Hemisphere spring (2011) and summer (2013). At each site, samples were collected from the surf zone as well asporewater from beach sediment in the intertidal zone. Surf-zone radon activity ranged from <5 to 70mBq L^–1, and intertidal porewater radon ranged over two orders of magnitude (220–36 940 mBq L^–1) along the Willunga Basin coastline during both surveys. Overall, surf-zone and porewater EC was lower in the spring 2011 survey than in the summer 2013 survey. Porewater EC was similar to that of coastal water at most sites along the coastline, except at three sites where porewater EC was found to be lower than coastal water during both surveys, and three sites where evaporated seawater was observed in the summer survey. Based on the patterns in radon and EC along the coastline, two sites of localised fresh SGD were identified, in addition to a groundwater spring that is known to discharge to the coast. The results indicate that near-shore fresh SGD occurs as localised seeps rather than diffuse seepage along the entire coastline. The apparent absence of groundwater discharge at most locations is also consistent with current evidence suggesting that extensive groundwater pumping within the basin has resulted in seawater intrusion across much of the coastline. These observations also suggest that previous studies are likely to have over-estimated SGD rates from the Willunga Basin because they assumed that SGD occurred along the entire coastline.     

中国近海海洋锋和锋面预报研究进展

海洋锋是水文要素特性不同的2个水团之间的狭窄过渡带,是一种重要的中尺度海洋现象,对渔业、军事和海洋环境保护等许多领域有重要影响,已经成为近年来物理海洋学以及海洋交叉学科中的重要课题之一。中国近海由于众多环流、水团和涡旋在此交汇,海洋锋现象十分显著,近年来,我国学者对中国近海的海洋锋产生机制和变化规律取得了一定的进展。首先从实测水文资料分析、遥感资料分析和数值模拟3个方面分别回顾了黄海、东海、南海北部等中国近海海域海洋锋的研究进展。对于海洋锋的预报研究,我国起步较晚,但目前对近海海洋锋预报的需求十分迫切。回顾总结了国外海洋锋预报的进展,集中介绍了黑潮锋、冰岛—法罗群岛锋、湾流锋和墨西哥湾锋面涡旋的预报方法和现状,最后对如何开展中国近海海洋锋预报提出思考和展望。     

Submarine groundwater discharge is an important net source of light and middle REEs to coastal waters of the Indian River Lagoon, Florida, USA

Porewater (i.e., groundwater) samples were collected from multi-level piezometers across the freshwater-saltwater seepage face within the Indian River Lagoon subterranean estuary along Florida’s (USA) Atlantic coast for analysis of the rare earth elements (REE). Surface water samples for REE analysis were also collected from the water column of the Indian River Lagoon as well as two local rivers (Eau Gallie River, Crane Creek) that flow into the lagoon within the study area. Concentrations of REEs in porewaters from the subterranean estuary are 10-100 times higher than typical seawater values (e.g., Nd ranges from 217 to 2409 pmol kg⁻¹), with submarine groundwater discharge (SGD) at the freshwater-saltwater seepage face exhibiting the highest REE concentrations. The elevated REE concentrations for SGD at the seepage face are too high to be the result of simple, binary mixing between a seawater end-member and local terrestrial SGD. Instead, the high REE concentrations indicate that geochemical reactions occurring within the subterranean estuary contribute substantially to the REE cycle. A simple mass balance model is used to investigate the cycling of REEs in the Indian River Lagoon and its underlying subterranean estuary. Mass balance modeling reveals that the Indian River Lagoon is approximately at steady-state with respect to the REE fluxes into and out of the lagoon. However, the subterranean estuary is not at steady-state with respect to the REE fluxes. Specifically, the model suggests that the SGD Nd flux, for example, exported from the subterranean estuary to the overlying lagoon waters exceeds the combined input to the subterranean estuary from terrestrial SGD and recirculating marine SGD by, on average, ∼100 mmol day⁻¹. The mass balance model also reveals that the subterranean estuary is a net source of light REEs (LREE) and middle REEs (MREE) to the overlying lagoon waters, but acts as a sink for the heavy REEs (HREE). Geochemical modeling and statistical analysis further suggests that this fractionation occurs, in part, due to the coupling between REE cycling and iron redox cycling within the Indian River Lagoon subterranean estuary. The net SGD flux of Nd to the Indian River Lagoon is ∼7-fold larger than the local effective river flux to these coastal waters. This previously unrecognized source of Nd to the coastal ocean could conceivably be important to the global oceanic Nd budget, and help to resolve the oceanic “Nd paradox” by accounting for a substantial fraction of the hypothesized missing Nd flux to the ocean.     

扬子陆块东南缘早古生代的巨型斜坡带

寒武、奥陶纪，在扬子陆块东南缘，发育—条横垣两千多公里的古台缘斜坡带。本文综合研究了此斜坡带的沉积类型，计有重力滑塌、滑移沉积、重力流沉积（碎屑流、浊流、颗粒流）、等深流沉积、上升流沉积、风暴流沉积以深水瘤状灰岩和生物扰动灰岩。台缘斜坡带沉积序列所反映的大地构造背景为被动大陆边缘，当时华南小洋（或海）盆为—拉伸下沉盆地。     

Influence of sea level rise on iron diagenesis in an east Florida subterranean estuary

Subterranean estuary occupies the transition zone between hypoxic fresh groundwater and oxic seawater, and between terrestrial and marine sediment deposits. Consequently, we hypothesize, in a subterranean estuary, biogeochemical reactions of Fe respond to submarine groundwater discharge (SGD) and sea level rise. Porewater and sediment samples were collected across a 30-m wide freshwater discharge zone of the Indian River Lagoon (Florida, USA) subterranean estuary, and at a site 250 m offshore. Porewater Fe concentrations range from 0.5 μM at the shoreline and 250 m offshore to about 286 μM at the freshwater-saltwater boundary. Sediment sulfur and porewater sulfide maxima occur in near-surface OC-rich black sediments of marine origin, and dissolved Fe maxima occur in underlying OC-poor orange sediments of terrestrial origin. Freshwater SGD flow rates decrease offshore from around 1 to 0.1 cm/day, while bioirrigation exchange deepens with distance from about 10 cm at the shoreline to about 40 cm at the freshwater-saltwater boundary. DOC concentrations increase from around 75 μM at the shoreline to as much as 700 μM at the freshwater-saltwater boundary as a result of labile marine carbon inputs from marine SGD. This labile DOC reduces Fe-oxides, which in conjunction with slow discharge of SGD at the boundary, allows dissolved Fe to accumulate. Upward advection of fresh SGD carries dissolved Fe from the Fe-oxide reduction zone to the sulfate reduction zone, where dissolved Fe precipitates as Fe-sulfides. Saturation models of Fe-sulfides indicate some fractions of these Fe-sulfides get dissolved near the sediment-water interface, where bioirrigation exchanges oxic surface water. The estimated dissolved Fe flux is approximately 0.84 μM Fe/day per meter of shoreline to lagoon surface waters. Accelerated sea level rise predictions are thus likely to increase the Fe flux to surface waters and local primary productivity, particularly along coastlines where groundwater discharges through sediments.     

Submarine Groundwater Discharge Estimates Using Radium Isotopes and Related Nutrient Inputs into Tauranga Harbour (New Zealand)

Land-based pollutants such as fertilizers and wastewater can infiltrate into aquifers and discharge into surrounding coastal water bodies as submarine groundwater discharge (SGD). Oceanic islands, with a large coast length to land area ratio, may be hot spots of SGD into the global ocean. Although SGD may be a major pathway of dissolved nutrients, carbon and metals to coastal waters, studies have been limited due to the difficulties in measuring this often diffuse process. This study used radium isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra) to investigate SGD and the associated fluxes of nutrients into Tauranga Harbour, New Zealand. We calculated the apparent water mass ages of the harbour to be between ~4.1 and 7.8 days, which was similar to a previous numerical model of ~2–8 days. A ²²⁶Ra mass balance was constructed to quantify SGD fluxes at the harbour scale. A minimum SGD flux rate of 0.53 cm day^?1 was calculated by using the maximum groundwater end-member value from 22 sample sites. However, using the geometric mean from these samples as a representative end-member, a final value of 2.83 cm day^?1 or a flux of 3.09 × 10⁶ m³ day^?1 was calculated. These values were between ~1 and 2.8 times greater than all the major river and creeks discharging into the harbour during the sampling period. Due to the higher observed nutrient concentrations in groundwater, the SGD-derived dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and total dissolved phosphorus (TDP) fluxes were calculated to be 1.07, 0.87 and 0.05 mmol m² day^?1, respectively. These SGD inputs were ~5 times (for nitrogen) and ~8 times (for phosphorus) greater than the input from surrounding rivers and streams. The average N:P ratio in groundwater samples was 36:1 (which was greatly in excess of the Redfield ratio of 16). The harbour water had a N:P ratio of ~17:1. A positive relationship between radium isotopes and N:P ratios in the harbour further supported the hypothesis that SGD can have major implications for primary production, including recurrent algal bloom events which occur in the harbour. We suggest SGD as a major driver of nutrient dynamics in Tauranga Harbour and potentially other similar coastal lagoon systems and estuaries on oceanic islands.     

Nutrient and Radium Fluxes from Submarine Groundwater Discharge to Port Royal Sound, South Carolina

Water exchange between the coastal ocean and underlying aquifers provides a newly-recognized source of materials to the ocean. The flux of materials into the ocean from this process is termed submarine groundwater discharge (SGD). Both surficial and semi-confined aquifers contribute to SGD. Here we use ²²⁶Ra and ²²⁸Ra to quantify fluxes of SGD to Port Royal Sound, South Carolina, and to separate fluxes from the Upper Floridan (UFA) and surficial aquifers. Higher activity ratios of ²²⁸/²²⁶Ra in the surficial aquifer make this separation possible. We estimate total SGD fluxes of about 100 m³ s^-1 with about 80% being derived from the surficial aquifer. The SGD flux provides about1.8 × 10⁶ mol d^-1 of NH4 with almost 90% from the surficial aquifer. Because of strong differences in the concentration of PO4 within the UFA, PO₄ fluxes areless certain. Using the UFA wells with low PO₄ concentrations yields a flux of 1.2 × 10⁵ mol d^-1; using wells with high concentrations yields a flux of 2.0 × 10⁵ mol d^-1. In the first case virtually all of the PO₄ flux is from the surficial aquifer; in the second case, 40% is from the UFA.The UFA in this region has experienced dramatic changes as a result of withdrawals for human use. Prior to these withdrawals, total nutrient fluxes from the UFA may have been even larger. These changes in the UFA and similar coastal aquifers worldwide have the potential to significantly alter a major nutrient source for the coastal ocean.     

Stable isotopic and geochemical data for inferring sources of recharge and groundwater flow on the volcanic island of Rishiri,Japan

An isotopic and chemical study was conducted on precipitation, spring water, streams, groundwater wells and submarine groundwater discharge (SGD) to constrain the recharge areas and flow paths of SGD. The isotopic values of precipitation were used to determine the local meteoric water lines (LMWLs) of Rishiri Island. The d-excess values of precipitation showed seasonal variation, with lows of 2.5‰ in the summer and highs of 24.2‰ in the winter. The d-excess values of spring water, streams, groundwater wells and SGD ranged from 12.5‰ to 23.0‰, indicating that the resulting waters were a mix of two seasons of precipitation. The isotopic composition of the groundwater wells sampled along the coast and SGD showed more negative values than that of the spring water sampled along the coast. This indicated that SGD recharged at high altitudes and flowed into the sea. The isotopic and chemical composition of SGD indicated unidirectional flow from land to sea.     

Temporal variability of freshwater and pore water recirculation components of submarine groundwater discharges at Baffin Bay,Texas

Submarine groundwater discharges (SGDs) are an important source of freshwater as well as nutrients and other chemicals to bays and estuaries. SGDs are particularly important for coastal bodies in arid and semi-arid regions that are not fed by perennial streams. The Baffin Bay, TX is a shallow coastal water body that is weakly connected to the Gulf of Mexico and has no major rivers or streams draining into it. A year-long submarine groundwater discharge measurement study was carried out at the Loyola Beach of the Baffin Bay during the months of July 2005–June 2006. A total of 23 synoptic SGD sampling events were carried out with most events collecting SGD data continuously over a period of 24 h at a 1-min temporal resolution using an ultrasonic seepage meter. The median SGD was noted to be 3.83 cm/d with an inter-quartile range (IQR) of 11.36 cm/d. Four sampling events had anomalously high SGD values (~27–48 cm/d) which are hypothesized to be due to the geologic heterogeneity of the sea bed and meteorological effects. Eight of the 23 sampling events had a negative average SGD flux indicating landward flow. The short-term diurnal variability of SGD was comparable or sometimes higher than the longer-term and between-events variability. No long-term trend could be inferred. In the short-term, SGD measurements showed considerable persistence and the effective sample size analysis indicated each sampling event (consisting of over 1,000 samples) yielded only a handful of statistically independent measurements of SGD. The measured SGD values exhibited both negative (hydraulically controlled) and positive (wave set-up controlled) correlations with the bay water levels. Marine controls appeared to be the most significant SGD drivers and are in turn controlled by prevailing aeolian forcings. The salinity of the SGDs were estimated from measured sonic velocities and used in conjunction with the end-member mixing models to estimate fresh (meteoric) and re-circulated pore-water fractions. The freshwater fraction of the SGD was estimated to vary between nearly 4 and 89 % with a median value of 9.96 % and an IQR of 7.16 %. Three events were noted to have abnormally high freshwater fractions (~28, 50 and 84 %) which are likely artifacts caused by bay water freshening from rainfall and plausible thermal expansion. The meteoric and pore-water partitioning was sensitive to the assumed end-member concentrations. This study provides preliminary estimates for SGDs along the South Texas coast line and is useful for calibrating groundwater flow models and understanding the relative importance of terrestrial and marine controls on SGD. However, the heterogeneous nature of the sedimentary geology of the Texas Gulf Coast implies the SGD fluxes are likely to exhibit considerable spatial variation that has not been characterized yet. Therefore, the study provides useful insights for such future data collection and monitoring activities. The measured SGD values at Baffin Bay, TX are comparable to those reported at other parts of the Gulf of Mexico.