Estimating submarine groundwater discharge around Isola La Cura, northern Venice Lagoon (Italy), by using the radium quartet

The four naturally-occurring radium isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra) were used to estimate the submarine groundwater discharge (SGD) in the Isola La Cura marsh area in the northern Venice Lagoon (Italy). By determining the radium contributors to the study area (river, coastal ocean and sediments) the radium excess in the lagoon water was quantified through a mass balance model. This radium excess is attributed to a submarine groundwater discharge source and represents the most important input of radium. Possible endmembers were considered from analysis of groundwater samples (subtidal and marsh piezometers, marsh wells and seepage meters) that were enriched in Ra by one to two orders of magnitude relative to surface waters. In particular, a permeable layer at 80 cm depth in the surrounding marsh is considered to be representative of the most likely SGD source, although similar radium activities were measured in other subtidal porewater samples collected in the Isola La Cura area. The estimated SGD flux to the study area ranged from 1 · 10⁹ to 6 · 10⁹ L·d^− 1, the same order of magnitude as the overall riverine input to the lagoon (3 · 10⁹ L·d^− 1). A major fraction of this SGD flux is likely recirculated seawater, as evidenced by the endmember salinity. The water residence time of 2 days was estimated by both using the shortest-lived radium isotope and estimating the volume of water exchanged between the lagoon and the open sea during a tidal cycle (tidal prism approach). This SGD flux could be used to estimate the input of other chemical species (metals, nutrients, etc.) via SGD which might affect the Venice Lagoon ecosystem.     

Submarine groundwater discharge to Great South Bay, NY, estimated using Ra isotopes

There is increasing evidence that submarine groundwater discharge (SGD) in many areas represents a major source of dissolved chemical constituents to the coastal ocean. In Great South Bay, NY, previous studies have shown that the discharge of nutrients with SGD may cause harmful algal blooms. This study estimates SGD to Great South Bay during August 2006 by performing a mass balance for each of the dissolved Ra isotopes (²²⁴Ra, ²²³Ra, ²²⁸Ra, ²²⁶Ra). The budget indicates a major unknown source (between 30 and 60% of the total input) of Ra to the bay. This imbalance can be resolved by a flux of Ra-enriched groundwater on the order of 3.5–4.5 × 10⁹ L d^− 1, depending on the Ra isotope. The Ra-estimated SGD rates compare well with those previously estimated by models of flow that decreases exponentially away from shore. Compared to previous reports of fresh groundwater discharge to the bay, the Ra-estimated discharge must comprise approximately 90% recirculated seawater. The good agreement between Ra- and model-estimated flow rates indicates that the primary SGD endmember may be best sampled at shallow depths in the sediments a short distance bayward of the low tide line.     

Submarine groundwater discharge and nutrient addition to the coastal zone and coral reefs of leeward Hawai'i

Multiple tracers of groundwater input (salinity, Si, ²²³Ra, ²²⁴Ra, and ²²⁶Ra) were used together to determine the magnitude, character (meteoric versus seawater), and nutrient contribution associated with submarine groundwater discharge across the leeward shores of the Hawai'ian Islands Maui, Moloka'i, and Hawai'i. Tracer abundances were elevated in the unconfined coastal aquifer and the nearshore zone, decreasing to low levels offshore, indicative of groundwater discharge (near-fresh, brackish, or saline) at all locations. At several sites, we detected evidence of fresh and saline SGD occurring simultaneously. Conservative estimates of SGD fluxes ranged widely, from 0.02–0.65 m³ m^− 2 d^− 1at the various sites. Groundwater nutrient fluxes of 0.04–40 mmol N m^− 2 d^− 1 and 0.01–1.6 mmol P m^− 2 d^− 1 represent a major source of new nutrients to coastal ecosystems along these coasts. Nutrient additions were typically greatest at locations with a substantial meteoric component in groundwater, but the recirculation of seawater through the aquifer may provide a means of transferring terrestrially-derived nutrients to the coastal zone at several sites.     

Seasonal changes in submarine groundwater discharge to coastal salt ponds estimated using Ra and Ra as tracers

Submarine groundwater discharge (SGD) to coastal southern Rhode Island was estimated from measurements of the naturally-occurring radioisotopes ²²⁶Ra (t_1/2 = 1600 y) and ²²⁸Ra (t_1/2 = 5.75 y). Surface water and porewater samples were collected quarterly in Winnapaug, Quonochontaug, Ninigret, Green Hill, and Pt. Judith–Potter Ponds, as well as nearly monthly in the surface water of Rhode Island Sound, from January 2002 to August 2003; additional porewater samples were collected in August 2005. Surface water activities ranged from 12–83 dpm 100 L^− 1 (60 dpm = 1 Bq) and 21–256 dpm 100 L^− 1 for ²²⁶Ra and ²²⁸Ra, respectively. Porewater ²²⁶Ra activities ranged from 16–736 dpm 100 L^− 1 (2002–2003) and 95–815 dpm 100 L^− 1 (2005), while porewater ²²⁸Ra activities ranged from 23–1265 dpm 100 L^− 1. Combining these data with a simple box model provided average ²²⁶Ra-based submarine groundwater fluxes ranging from 11–159 L m^− 2 d^− 1 and average ²²⁸Ra-derived fluxes of 15–259 L m^− 2 d^− 1. Seasonal changes in Ra-derived SGD were apparent in all ponds as well as between ponds, with SGD values of 30–472 L m^− 2 d^− 1 (Winnapaug Pond), 6–20 L m^− 2 d^− 1 (Quonochontaug Pond), 36–273 L m^− 2 d^− 1 (Ninigret Pond), 29–76 L m^− 2 d^− 1 (Green Hill Pond), and 19–83 L m^− 2 d^− 1 (Pt. Judith–Potter Pond). These Ra-derived fluxes are up to two orders of magnitude higher than results predicted by a numerical model of groundwater flow, estimates of aquifer recharge for the study period, and values published in previous Ra-based SGD studies in Rhode Island. This disparity may result from differences in the type of flow (recirculated seawater versus fresh groundwater) determined using each technique, as well as variability in porewater Ra activity.     

Radium tracing of submarine groundwater discharge (SGD) and associated nutrient fluxes in a highly-permeable bed coastal zone, Korea

In order to estimate submarine groundwater discharge (SGD) and SGD-driven nutrient fluxes, we measured the concentrations of nutrients, ²²⁴Ra, and ²²⁶Ra in seawater, river water, and coastal groundwater of Yeongil Bay (in the southeastern coast of Korea) in August 2004 and February 2005. The bottom sediments over the shallow areas of this bay are composed mainly of coarse sands. Large excess concentrations of ²²⁴Ra, ²²⁶Ra, and Si supplied from SGD were observed in August 2004, while these excess concentrations were not apparent in February 2005. Based on the mass balance for ²²⁴Ra, ²²⁶Ra, and Si, which showed conservative mixing behavior in seawater, SGD was estimated to be approximately 6 × 10⁶ m³ day^− 1 (seepage rate = 0.2 m day^− 1) in shallow areas (< 9 m water depth) in August 2004, which is much higher than the SGD level typically found in other coastal regions worldwide. During the summer period, SGD-driven nutrients in this bay contributed approximately 98%, 12%, and 76% of the total inputs for dissolved inorganic nitrogen (DIN), phosphorus (DIP), and silicate (DSi), respectively. Our study implies that the ecosystem in this highly permeable bed coastal zone is influenced strongly by SGD during summer, while such influences are negligible in winter.     

Submarine groundwater discharge: A large, previously unrecognized source of dissolved iron to the South Atlantic Ocean

This paper reports the initial results of a study of groundwater and coastal waters of southern Brazil adjacent to a 240 km barrier spit separating the Patos Lagoon, the largest coastal lagoon in South America, from the South Atlantic Ocean. The objective of this research is to assess the chemical alteration of freshwater and freshwater–seawater mixtures advecting through coastal permeable sands, and the influence of the submarine discharge of these fluids (SGD) on the chemistry of coastal waters. Here we focus on dissolved iron in this system and use radium isotopic tracers to quantify SGD and cross-shelf fluxes. Iron concentrations in groundwaters vary between 0.6 and 180 μM. The influence of the submarine discharge of these fluids into the surf zone produces dissolved Fe concentrations as high as several micromolar in coastal surface waters. The offshore gradient of dissolved Fe, coupled with results for Ra isotopes, is used to quantify the SGD flux of dissolved Fe from this coastline. We estimate the SGD flux to be 2 × 10⁶ mol day^− 1 and the cross-shelf flux to be 3.2 × 10⁵ mol day^− 1. This latter flux is equal to about 10% of the soluble atmospheric Fe flux to the entire South Atlantic Ocean. We speculate on the importance of this previously unrecognized iron input to regional ocean production and on the potential significance of this source to understanding variations in glacial–interglacial ocean production.     

Radium mass-balance in Jamaica Bay, NY: Evidence for a substantial flux of submarine groundwater

A mass balance for the naturally-occurring radium isotopes (²²⁴Ra, ²²³Ra, ²²⁸Ra, and ²²⁶Ra) in Jamaica Bay, NY, was conducted by directly estimating the individual Ra contributions of wastewater discharge, diffusion from fine-grained subtidal sediments, water percolation through marshes, desorption from resuspended particles, and water exchange at the inlet. The mass balance revealed a major unknown source term accounting for 19–71% of the total Ra input, which could only be resolved by invoking a source from submarine groundwater. Shallow (< 2 m depth) groundwater from permeable sediments in Jamaica Bay was brackish and enriched in Ra relative to surface bay waters by over two orders of magnitude. To balance Ra fluxes, a submarine groundwater input of 0.8 × 10⁹–9.0 × 10⁹ L d^− 1 was required. This flux was similar for all four isotopes, with individual estimates varying by less than a factor of 2. Our calculated groundwater flux was 6- to 70-fold higher than the fresh groundwater discharge to the bay estimated by hydrological methods, but closely matched direct flow rates measured with seepage meters. This suggests that a substantial portion of the discharge consisted of recirculated seawater. The magnitude of submarine groundwater discharge varied seasonally, in the order: summer > autumn > spring. Chemical analyses suggest that the recirculated seawater component of submarine groundwater delivers as much dissolved nitrogen to the bay as the fresh groundwater flux.     

TIMS measurements of Ra and Ra in the Gulf of Lion, an attempt to quantify submarine groundwater discharge

Submarine groundwater discharge (SGD) is now recognized as an important pathway for water and chemical species fluxes to the coastal ocean. In order to determinate SGD to the Gulf of Lion (France), we measured the activities of ²²⁶Ra and ²²⁸Ra by thermal ionization mass spectrometry (TIMS) in coastal waters and in the deep aquifer waters of the Rhone deltaic plain after pre-concentration of radium by MnO₂. Compared to conventional counting techniques, TIMS requires lower quantities of water for the analyses, and leads to higher analytical precision. Radium isotopes were thus measured on 0.25–2 L water samples containing as little as 20 fg of ²²⁶Ra and 0.2–0.4 fg of ²²⁸Ra with precision equal to 2%. We demonstrate that coastal surface waters samples are enriched in ²²⁶Ra and ²²⁸Ra compared to the samples further offshore. The high precision radium measurements display a small but significant ²²⁶Ra and ²²⁸Ra enrichment within a strip of circa 30 km from the coast. Radium activities decrease beyond this region, entrained in the northern current along the shelf break or controlled by eddy diffusion. The radium excess in the first 30 km cannot be accounted for by the river nor by the early diagenesis. The primary source of the radium enrichment must therefore be ascribed to the discharge of submarine groundwater. Using a mass-balance model, we estimated the advective fluxes of ²²⁶Ra and ²²⁸Ra through SGD to be 5.2 × 10¹⁰ and 21 × 10¹⁰ dpm/d respectively. The ²²⁶Ra activities measured in the groundwater from the Rhone deltaic plain aquifer are comparable to those from other coastal groundwater studies throughout the world. By contrast, ²²⁸Ra activities are higher by up to one order of magnitude. Taking those groundwater radium activities as typical of the submarine groundwater end-member, a minimum volume of 0.24–4.5 × 10¹⁰ l/d is required to support the excess radium isotopes on the inner shelf. This has to be compared with the average rivers water runoff of 15.4 × 10¹⁰ l/d during the study period (1.6 to 29% of the river flow).     

Radium and radon radioisotopes in regional groundwater, intertidal groundwater, and seawater in the Adelaide Coastal Waters Study area: Implications for the evaluation of submarine groundwater discharge

The input of groundwater-borne nutrients to Adelaide's (South Australia) coastal zone is not well known but could contribute to the ongoing decline of seagrass in the area. As a component of the Adelaide Coastal Waters Study (ACWS), the potential for using the radium quartet (²²³Ra, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra) and ²²²Rn to evaluate submarine groundwater discharge (SGD) was evaluated. Potential isotopic signatures for SGD were assessed by sampling groundwater from three regional aquifers potentially contributing SGD to the ACWS area. In addition, intertidal groundwater was sampled at two sand beach sites. In general, the regional groundwaters were enriched in long-lived Ra isotopes (²²⁶Ra and ²²⁸Ra) and in ²²²Rn relative to intertidal groundwater. Radium activity (but not ²²²Rn activity) was positively correlated to salinity in groundwater from one of the regional aquifers and in intertidal groundwater. Radium isotope ratios (²²³Ra/²²⁶Ra, ²²⁴Ra/²²⁶Ra and ²²⁸Ra/²²⁶Ra) were less variable than individual Ra isotope activities within potential SGD sources. Recirculated seawater (estimated from the intertidal groundwater samples with seawater-like salinities) also had distinctly higher Ra isotope ratios than the regional groundwaters. The activities for all radioisotopes were relatively low in seawater. The activity of the short-lived ²²³Ra and ²²⁴Ra were highest at the shoreline and declined exponentially with distance offshore. In contrast, ²²⁸Ra and ²²⁶Ra activities had a weak linear declining trend with distance offshore. Rn-222 activity was at or near background in all seawater samples. The pattern of enrichment in short-lived Ra isotopes and the lack of ²²²Rn in seawater suggest that seawater recirculation is the main contributor to SGD in the ACWS area. Preliminary modeling of the offshore flux of ²²⁸Ra and ²²⁶Ra suggest that the SGD flux to the ACWS area ranges between 0.2 and 3 · 10^− 3 m³ (m of shoreline)^− 1 s^− 1.     

Submarine groundwater discharge of nutrients to the ocean along a coastal lagoon barrier, Southern Brazil

The Patos–Mirim Lagoon system along the southern coast of Brazil is linked to the coastal ocean by a narrow mouth and by groundwater transport through a Holocene barrier. Although other groundwater systems are apparently active in this region, the hydraulic head of the lagoon, the largest in South America, drives groundwater transport to the coast. Water levels in wells placed in the barrier respond to changing water level in the lagoon. The wells also provide a measure of the nutrient concentrations of groundwater flowing toward the ocean. Additionally, temporary well points were used to obtain nutrient samples in groundwater on the beach face of the barrier. These samples revealed a subterranean freshwater–seawater mixing zone over a ca. 240 km shoreline. Previously published results of radium isotopic analyses of groundwater and of surface water from cross-shelf transects were used to estimate a water flux of submarine groundwater discharge (SGD) to nearshore surface waters of 8.5 × 10⁷ m³/day. Using this SGD and the nutrient concentrations in different compartments, nutrient fluxes between groundwater and surface water were estimated. Fluxes were computed using both average and median reservoir (i.e. groundwater and surface water) nutrient concentrations. The SGD total dissolved inorganic nitrogen, phosphate and silicate fluxes (2.42, 0.52, 5.92 × 10⁶ mol day^− 1, respectively) may represent as much as 55% (total N) to 10% (Si) of the nutrient fluxes to the adjacent shelf environment. Assuming nitrogen limitation, SGD may be capable of supporting a production rate of ca. 3000 g C m² year^− 1in the nearshore surf zone in this region.     

Coincident application of a mass balance of radium and a hydrodynamic model for the seasonal quantification of groundwater flux into the Venice Lagoon, Italy

A mass balance of the naturally occurring short-lived radium isotopes (^223,224Ra) in the Venice Lagoon was conducted by an integrated approach combining the directly estimated individual Ra contributions and hydrodynamic model results. Hydrodynamic data allows for the calculation of the Ra mass balance in sub-sections of the Venice Lagoon (boxes), which are characterized by physically homogeneous properties, instead of investigating the entire lagoon. Utilizing this method, both the seasonal and the spatial variability of the submarine groundwater discharge in the Venice Lagoon have been estimated. Between 14–83 × 10⁹ L d^− 1 of water were calculated to flow across the sediment–sea interface, corresponding to 5–28 times the mean annual river input. The submarine groundwater discharge estimates were correlated with the residence time calculation to better understand spatial and seasonal variation.     

Evaluation of the flushing rates of Apalachicola Bay, Florida via natural geochemical tracers

We used naturally occurring radium isotopes as tracers of water exchange in Apalachicola Bay, a shallow coastal-plain estuary in northwestern Florida. The bay receives fresh water and radium from the Apalachicola River, and mixes with Gulf of Mexico waters through four inlets. We deployed moored buoys with attached Mn-fibers at several stations throughout the estuary during two summer and two winter periods. After deployment for at least one tidal cycle we measured the ratio of the two short-lived radium isotopes ²²³Ra (half-life = 11 d) and ²²⁴Ra (3.6 d) to estimate “radium ages” of the water in the bay.During our four seasonal deployments the river discharge ranged from 338 to 1016 m³ s^− 1. According to our calculations the water turnover time in the bay during these samplings ranged from 6 to 12 days. Age contours in the bay showed that winds and tides as well as river discharge influence the water movement and the residence time of freshwater in the bay. We also calculated the mean age of river water in the bay which was between 5 to 9 days during the studied periods. We suggest that this approach can be used to quantify transport processes of dissolved substances in the bay. For example, soluble nutrient or pollutant transport rates from a point source could be examined. We conclude that the radium age technique is well suited for flushing rate calculations in river dominated shallow estuaries.     

Estimation of submarine groundwater discharge in Plover Cove, Tolo Harbour, Hong Kong by Rn

Algal blooms in Tolo Harbour, Hong Kong have received much attention and submarine groundwater discharge is speculated to be a significant pathway carrying nutrients into the constricted estuary. Plover Cove, a small cove in the Harbour, was selected for SGD analysis using ²²²Rn budget. The volumetric SGD rates are estimated to be about 8000 m³/day for neap tide and about 17,000 m³/day for spring tide. Result of nutrient analysis of the porewater indicates that the nutrient loading through this pathway is speculated to be crucial for eutrophication in Tolo Harbour. Current practice for the management of algal blooms in Hong Kong, in which nutrient loading through SGD was ignored, has to be reviewed and the control measures of groundwater contamination are obviously required.     

The impact of groundwater discharges on mercury partitioning, speciation and bioavailability to mussels in a coastal zone

The Mussel Watch program conducted along the French coasts for the last 20 years indicates that the highest mercury concentrations in the soft tissue of the blue mussel (Mytilus edulis) occur in animals from the eastern part of Seine Bay on the south coast of the English Channel, the “Pays de Caux”. This region is characterized by the presence of intertidal and submarine groundwater discharges, and no particular mercury effluent has been reported in its vicinity. Two groundwater emergence systems in the karstic coastal zone of the Pays de Caux (Etretat and Yport with slow and fast water percolation pathways respectively) were seasonally sampled to study mercury distribution, partitioning and speciation in water. Samples were also collected in the freshwater–seawater mixing zones in order to compare mercury concentrations and speciation between these “subterranean” or “groundwater” estuaries and the adjacent macrotidal Seine estuary, characterized by a high turbidity zone (HTZ). The mercury concentrations in the soft tissue of mussels from the same areas were monitored at the same time.The means of the “dissolved” (< 0.45 μm) mercury concentrations (HgT_D) in the groundwater springs were 0.99 ± 0.15 ng l^− 1 (n = 18) and 0.44 ± 0.17 ng l^− 1 (n = 17) at Etretat and Yport respectively. High HgT_D concentrations were associated with strong runoff over short water pathways during storm periods, while low concentrations were associated with long groundwater pathways. Mean particulate mercury concentrations were 0.22 ± 0.05 ng mg^− 1 (n = 16) and 0.16 ± 0.10 ng mg^− 1 (n = 17) at Etretat and Yport respectively, and decreased with increasing particle concentration probably as a result of dilution by particles from soil erosion. Groundwater mercury speciation was characterized by high reactive-to-total mercury ratios in the dissolved phase (HgR_D/HgT_D: 44–95%), and very low total monomethylmercury concentrations (MMHg < 8 pg l^− 1). The HgT_D distributions in the Yport and Etretat mixing zones were similar (overall mean concentration of 0.73 ± 0.21 ng l^− 1, n = 43), but higher than those measured in the adjacent industrialized Seine estuary (mean: 0.31 ± 0.11 ng l^− 1, n = 67). In the coastal waters along the Pays de Caux dissolved monomethylmercury (MMHg_D) concentrations varied from 9.5 to 13.5 pg l^− 1 (2 to 8% of the HgT_D). Comparable levels were measured in the Seine estuary (range: 12.2– 21.1 pg l⁻¹; 6–12% of the HgT_D). These groundwater karstic estuaries seem to be mostly characterized by the higher HgT_D and HgR_D concentrations than in the adjacent HTZ Seine estuary. While the HTZ of the Seine estuary acts as a dissolved mercury removal system, the low turbid mixing zone of the Pays de Caux receives the dissolved mercury inputs from the groundwater seepage with an apparent Hg transfer from the particulate phase to the “dissolved” phase (< 0.45 μm). In parallel, the soft tissue of mussels collected near the groundwater discharges, at Etretat and Yport, exhibited significantly higher values than those found in the mussel from the mouth of the Seine estuary. We observe that this difference mimics the differences found in the mercury distribution in the water, and argue that the dissolved phase of the groundwater estuaries and coastal particles are significant sources of bioavailable mercury for mussels.     

Short-lived radium isotopes in the Hawaiian margin: Evidence for large fluid fluxes through the Puna Ridge

We measured significant activities of short-lived radium isotopes, ²²³Ra (half-life = 11 days) and ²²⁴Ra (half-life = 3.7 days), around the margins of the Hawaiian Islands to water depths of 3500 m. These measurements suggest fluid inputs from the basalt to the surrounding ocean. In general ²²³Ra activities were considerably greater than ²²⁴Ra in spite of the expected higher production rate of ²²⁴Ra activity in basalt. The ²²³Ra was not supported by dissolved ²²⁷Ac. The highest enrichments of ²²³Ra were measured over the Puna Ridge (2100 m depth) east of Hawaii. Here ²²³Ra activities reached 19 dpm/m³, similar to activities measured near sites of active submarine groundwater discharge in the South Atlantic Bight. To explain the high activities of ²²³Ra unaccompanied by ²²⁴Ra, we postulate that thermally-driven circulation of seawater through the Puna Ridge deposits ²³¹Pa on basalt surfaces. With time the ²³¹Pa produces ²²⁷Ac and ²²³Ra; and ²²³Ra desorbs into the circulating fluids. These fluids then transport ²²³Ra into the overlying ocean. Based on the inventory of ²²³Ra above the Puna Ridge, we estimate the flow of fluids through the ridge to be on the order of 20–60 cm³ cm^− 2 day^− 1. In less than 1000 years the incoming seawater could provide enough ²³¹Pa to basalt surfaces to balance the inventory of ²²³Ra above the ridge if only 8% of the ²²³Ra was transported to the overlying water. These observations on the flanks of a volcanically-active ocean island have significant implications for quantifying fluid fluxes from the flanks of the mid-ocean ridge system. By mapping ²²³Ra inventories in the ocean above ridge flanks and measuring the activity of ²²³Ra in the emerging fluids, the fluid flux can be obtained.     

Factors controlling excess radium in the Nakdong River estuary,Korea: submarine groundwater discharge versus desorption from riverine particles

The activities of naturally occurring radium isotopes (²²⁶Ra and ²²⁸Ra) in estuarine water were measured downstream of the dam constructed in the Nakdong River, Korea. The sampling of surface waters for radium, silicate, and suspended solid (SS) analyses was conducted at 18 stations during three periods (July 1997, April 1998, and June 1999). In general, radium activities exceeded the value expected from the mixing of two freshwater and seawater endmembers. We characterized the responses of Ra and Si according to three different conditions: (1) when the freshwater discharge and the water level of the dam relative to the sea level at low tide were lower (April 1998), the excess Ra and Si contents were lower in the estuary; (2) when the fresh water discharge was larger following heavy precipitation (July 1997), both excess Ra and Si contents were higher in the estuary with conservative mixing of Si; and (3) when the water level of the dam relative to the sea level at low tide was highest under low freshwater discharge (June 1999), high excess Ra but low Si levels were observed. The occurrence of high Ra activity in June 1999 was likely due to the large submarine brackish groundwater discharge downstream of the estuary. Since brackish groundwater in general contains high concentrations of nutrients, Ba, Ra, etc, our result suggests an important role for the submarine groundwater discharge on the biogeochemistry of estuarine/coastal waters, especially when the water level of the dam (hydraulic head) is high.     

Water mass ages of coastal ponds estimated using Ra and Ra as tracers

Measurements of the naturally occurring radioisotopes ²²³Ra (t_1/2 = 11.4 days) and ²²⁴Ra (t_1/2 = 3.66 days) in southern Rhode Island salt ponds were combined with a simple model to obtain independent estimates of the age of these coastal waters. Surface water and porewater samples were collected quarterly in Winnapaug, Quonochontaug, Ninigret, Green Hill, and Pt. Judith-Potter Ponds, as well as nearly monthly in the surface water of Rhode Island Sound, beginning January 2002 through August 2003. Surface water activities ranged from 1–78 dpm 100 L^− 1 and 5–885 dpm 100 L^− 1 for ²²³Ra and ²²⁴Ra, respectively. Porewater radium activities ranged from 3 to 715 dpm 100 L^− 1 for ²²³Ra, and 57–4926 dpm 100 L^− 1 for ²²⁴Ra. Results indicate seasonally varying water mass ages for Ninigret (5–12 days), Winnapaug (2–6 days) and Pt. Judith-Potter Ponds (1–9 days) and, in contrast, relatively constant ages for Green Hill (5–7 days) and Quonochontaug Ponds (3–6 days).     

Radon and radium isotopes as tracers of submarine groundwater discharge – Results from the Ubatuba,Brazil SGD assessment intercomparison

We determined groundwater flow rates shortly after the wet season into an embayment near Ubatuba, Brazil as part of an international intercomparison experiment for submarine groundwater discharge (SGD) assessment techniques. Our estimated rates were determined by the combined use of continuous radon measurements and assessment of radium isotope patterns. The spatial distribution of the short-lived radium isotopes (²²³Ra and ²²⁴Ra) provided the means for independent evaluations of radon losses by mixing and atmospheric evasion. We were thus able to construct a well-constrained mass balance for radon that included a groundwater flux term. Our results showed that the groundwater discharge into this embayment from the fractured crystalline rock aquifer is not steady-state but varies with tidal modulation and rain-induced forcing. Tidally modulated and rain-induced flow rates were comparable during this period. The SGD rates estimated from radon ranged from 1 cm/day to 29 cm/day (cm³/cm² day) with a mean and standard deviation of 13 ± 6 cm/day. These estimates were mostly similar to a dye-dilution automatic seepage meter (15 ± 19 cm/day) and were within the broad ranges estimated by manual and continuous heat seepage meters but lower than indicated by an artificial tracer test performed nearshore.     

基于223Ra和224Ra的桑沟湾海底地下水排放通量

海底地下水排放（SGD）是陆地向海洋输送水量和营养物质的重要通道之一,对沿海物质通量及其生物地球化学循环有重要的影响,对生态环境起着不可忽视的作用。本文运用天然放射性同位素223Ra和224Ra示踪估算了我国北方典型养殖基地桑沟湾的海底地下水排放通量。结果表明,海底地下水样尤其是间隙水中Ra活度[224Ra=（968±31）dpm/（100 L）,223Ra=（31.4±4.9）dpm/（100 L）,n=9]远高于表层海水[224Ra=（38.7±2.0）dpm/（100 L）,223Ra=（1.70±0.50）dpm/（100 L）, n=21]。假设稳态条件下,考虑Ra的各源、汇项,利用Ra平衡模型,估算出桑沟湾SGD排放通量为（0.23~1.03）×107 m3/d。潮周期内的观测结果显示,涨潮时,水力梯度较小,SGD排放变弱,落潮时,水力梯度较大,导致了相对较多的SGD排放。在一个潮周期间,基于223Ra和224Ra得到的SGD排放通量平均为0.39×107 m3/d。潮汐动力下的SGD排放平均占总SGD排放的61%,因此桑沟湾沿岸的地下水排放主要受潮汐动力的影响,并对海水组成及海陆间物质交换有显著贡献。     

Estimating submarine groundwater discharge at a subtropical river estuary along the Beibu Gulf,China

In certain regions,submarine groundwater discharge(SGD) into the ocean plays a significant role in coastal material fluxes and their biogeochemical cycle;therefore,the impact of SGD on the ecosystem cannot be ignored.In this study,SGD was estimated using naturally occurring radium isotopes(~(223)Ra and ~(224)Ra) in a subtropical estuary along the Beibu Gulf,China.The results showed that the Ra activities of submarine groundwater were approximately 10 times higher than those of surface water.By assuming a steady state and using an Ra mass balance model,the SGD flux in May 2018 was estimated to be 5.98×10~6 m~3/d and 3.60×10~6 m~3/d based on ~(224)Ra and ~(223)Ra,respectively.At the same time,the activities of Ra isotopes fluctuated within a tidal cycle;that is,a lower activity was observed at high tide and a higher activity was seen at low tide.Based on these variations,the average tidal pumping fluxes of SGD were 1.15×10~6 m~3/d and 2.44×10~6 m~3/d with ~(224)Ra and ~(223)Ra,respectively.Tidaldriven SGD accounts for 24%-51% of the total SGD.Therefore,tidal pumping is an important driving force of the SGD in the Dafengjiang River(DFJR) Estuary.Furthermore,the SGD of the DFJR Estuary in the coastal zone contributes significantly to the seawater composition of the Beibu Gulf and the material exchange between land and sea.