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We report the first data for the concentrations of Ba and 226 Ra in rivers draining into the Black Sea. For Ba, concentrations are in the range 215–405 nM L -1 ; for 226 Ra, concentrations are in the range 0.4–4 Bq m -3 . These are dissolved concentrations and do not represent additional inputs due to desorption from fluvial particles. We also report new data for the concentrations of Ba and 226 Ra in Black Sea surface waters from 1991 to 1995. Earlier studies documented significant reductions in the concentrations of these elements in Black Sea surface waters for the periods 1957–1988 (Ra) and 1975–1988 (Ba). During the period 1988–89 to 1995, Ba concentrations followed the earlier trend and decreased by an additional 35%. On the other hand, 226 Ra concentrations reversed the trend and increased by about 60% from 1988 to 1995. The reason for the recent divergence in behavior of these two elements is unknown.

Wetland peat deposits can be excellent archives of past changes in the depositional and ecological conditions under which they formed. Part of the story of these changes can be obtained from analysis of the palynomorphs preserved in these deposits; however, a more meaningful reconstruction can be achieved by combining organic petrography with palynology. An example of this approach is presented from a study of three sites in the northern Everglades of Florida. The primary purpose of this study was to test these combined methods for their utility in distinguishing between long-term, natural successions and recent, anthropogenically-derived disruptions of the ecosystem (such as diversions of drainage and introductions of contaminants from human sources). The combined palyno-petrographic method (supported by 137 Cs and 210 Pb dating) was able to establish with some certainty the first appearance of contaminant-driven plant types (such as cattails and duckweeds) into the region, with the organic petrography being more accurate in determining the actual presence of invading plants at a particular site and the palynologic analysis providing a broader picture of regional changes in the ecosystem (such as the direction of the plant invasion). The distribution of petrographic pyrite in these freshwater peats provided an additional new piece of evidence for contamination from the agricultural lands and also supported other evidence relative to the direction of contaminant infusion into the region. Additionally, the occurrence of significant quantities of pyrite at all three sites suggests that, in fact, none of these sites can be said to be totally unaffected by anthropogenic contamination. Longer hydroperiods were generally indicated by both methods for all sites before the early 1900s, confirming persistently wetter conditions in the Everglades prior to manmade changes in drainage. More specific paleohydrologic conditions could also be elucidated by this combined method, with all three sites displaying significant differences in hydrologic histories. Additionally, one of the sites is hypothesized to have been, until recently, in the center of a surface–water flow pathway, as indicated by the continuous presence at this site of deeper water aquatics (such as water lilies) and the smaller number of zones containing algal-derived carbonates and/or other petrographic indicators of oxidation or bioturbation.

137Cs and excess 210 Pb have been found in rock varnish samples from several localities in the desert regions of the western United States. As 137 Cs was absent in the environment prior to the atmospheric testing of nuclear weapons, and as 210 Pb has a half life of only 22 years, these isotopes must have been added to this slow growing patina during the last half of the present century, demonstrating that, at least in part, the elements constituting varnish are supplied from the atmosphere. The discovery opens a window into the mechanism by which this mysterious substance forms. Not only radioisotopes but also anthropogenic metals can be harnessed to assess the ratio of rate of accumulation of a given element on varnish to its delivery rate from the atmosphere. Based on measurements reported here, for 210 Pb this ratio is about 0.1 and for 137 Cs about 0.01.

Submarine groundwater discharge, estimated from a 228Ra inventory across the upper Atlantic Ocean, provides a flux of 2–4×1013 m3 yr−1, equivalent to 80–160% of the influx from rivers into the Atlantic Ocean.

The fate of dissolved material delivered to the coastal ocean depends on its reactivity and the rate at which it is mixed offshore. To measure the rate of exchange of coastal waters, we employ two short-lived radium isotopes,223Ra and224Ra. Along the coast of South Carolina, shore-perpendicular profiles of223Ra and224Ra in surface waters show consistent gradients which may be modeled to yield eddy diffusion coefficients of 350–540 m2s−1. Coupling the exchange rate with offshore concentration gradients yields estimates of offshore fluxes of dissolved materials. For systems in steady state, the offshore fluxes must be balanced by new inputs from rivers, groundwater, sewers or other sources. Two tracers that show promise in evaluating groundwater input are barium and226Ra. These tracers have high relative concentrations in the fluids and low-reactivity in the coastal ocean. Applying the eddy diffusion coefficients to the offshore gradient of226Ra concentration provides an estimate of the offshore flux of226Ra. Measuring the concentrations of226Ra in subsurface fluids provides an estimate of the fluid flux necessary to provide the226Ra. These estimates indicate that the volume of groundwater required to support these fluxes is of the order of 40% of the surface water flow.

We report unambiguous signals of subsurface fluid input on the continental shelf 40–80 km from shore in water depths of 20–45 m. The site of this input is near the continental shelf break off the South Carolina coast between 32°–33°N and 78.5°–79.5°W. Repeated transects over this area reveal enrichments of 226Ra, 228Ra, and Ba by factors of 3–4 above ambient ocean concentrations. The short-lived 223Ra and 224Ra are enriched by over an order of magnitude. The Ra and Ba enrichments are not caused by up welling of deep water as no such 226Ra or Ba concentrations occur in the upper 2000 m of the Atlantic Ocean. Sediments in the region are largely relic sand and do not offer a viable source for 226Ra, 228Ra, and Ba enrichments. We conclude that the source is the discharge of submarine fluids enriched in 226Ra, 228Ra, and Ba. The salinity of water in the Ra-Ba-enriched zone is diluted by 0.2–0.6‰ relative to water at the same depth farther offshore. Pronounced salinity and density anomalies characterize one site of active discharge. Estimates of 226Ra and Ba concentrations in the fluids and the replacement time of water in the enriched zone suggest the fluid flux is similar to the summertime flow of major rivers in the area. The region of Ra-Ba enrichment is also characterized by a strong fluorescence signal, indicating high concentrations of chlorophyll in the water. It is probable that the leaking fluids supply nutrients that stimulate productivity in the bottom waters which are within the photic zone. Because the fluorescence signal is restricted to depths greater than 20 m, it is unlikely that the signal could be recognized by satellite or aircraft sensors.

Dissolved Ba concentrations in the inner shelf waters of the South Atlantic Bight (SAB) are two- to three-fold enriched in Ba when compared to Atlantic surface water. We determined that river input could account for about 25% of the observed enrichment. However, salty groundwaters are enriched in Ba by as much as an order of magnitude over calculated river water endmembers. The resultant input of Ba to the coastal environment, via enriched groundwater, is the likely source for the observed Ba enrichment on the inner shelf. The flux of Ba from the inner shelf to the ocean is 70 × 103 moles per day. The flux of Ba reaching the inner shelf from salty groundwaters ranges from 25 to 100 × 103 moles of Ba per day. Groundwater input of Ba appears to be much more important than river input for the Southeastern coast of the US.

Office of Global Programs, NOAA, Silver Spring, Maryland Abstract. Two transient plumes of freshened water in the South Atlantic Bight off the coast of South Carolina were measured and mapped during a continental shelf cruise in July 1994. Salinity in the plumes was depressed 1-2 parts per thousand compared to surrounding shelf waters. One plume was at least 160 kIn long, 20 kIn wide, and 5.7 m deep. The other was 75 km long, 25 kIn wide and 5-7 m deep. Both plumes disintegrated almost completely in 6 days due to mixing or advection away from the sampling area. Measurement and analysis of short-lived Ra isotopes and Si indicated that the plumes were not caused by river discharge but were rather due to an intense rain event a few days prior to the sampling cruise. With only salinity measurements we would have incorrectly concluded that the freshened water was a result of river plumes having entered the shelf. The rain was associated with the system that spawned tropical storm Alberto. We estimate that 220-330 mm of rain fell on the coastal ocean immediately before the cruise. Accompanying the rain were pesticides and nutrients. We estimate that the flux of nitrate from the rain was twice the riverine flux of nitrate to the study area in the month preceding the cruise. Plumes of freshened water in the coastal ocean may be derived from river runoff or heavy offshore rain events. Large rivers may cause ocean freshening at scales of over 1000 km (Atkinson and Wallace, 1975; Ortner et ai., 1995; Moore et ai., 1986). Distinguishing the source of the fresh water after the fact is difficult. Here we employ Si and Ra, tracers previously used to distinguish between Amazon-derived water and rainfall in the tropical Atlantic, to identify rainfall as the agent of a freshened plume off the coast of South Carolina. The Estuary-Shelf-Ocean Exchange experiment (ESOE) was conducted to establish techniques to determine the rate of cross-shelf exchange of estuarine and nearshore waters with waters of the middle and outer continental shelf. The experiment was conducted in the South Atlantic Bight (SAB), a misnamed region off the southeast U.S. coast between Cape Hatteras and Cape Canaveral, July 6-17, 1994, during a prolonged period of vertical shelf stratification. ESOE was designed to use the distributions of 223Ra (t 112 = 11.4d) and 224Ra (t 112 = 3.66d) as an index of the exchange time. Five shore-perpendicular transects between the Cape Fear River (CF) and the Savannah River (SV) were sampled from the coastline to the edge of the shelf. Two transects, Myrtle Beach (MB) and Charleston (CH), were resampled two more times, and the Winyah Bay transect (WB) was sampled three more times to determine if the tracer distributions were stable on a 2- week timescale. Figure 1 illustrates these transects and gives the station locations for the first pass. No single river dominates freshwater input to the South Atlantic Bight (SAB). The Savannah, Santee, and Pee Dee are the largest rivers; each discharged 100-500 m3 S.I during the

The Ganges-Brahmaputra is among the world's three largest river systems in terms of sediment load, but, until now, no high-resolution seismic data have been obtained to document the nature of the sediment deposit seaward of the rivers' mouths. The other two (Amazon, Huanghe) discharge into energetic coastal environments and form subaqueous deltas with characteristic clinoform stratigraphy. High-resolution seismic reflection profiles of the Bengal shelf reveal similar stratigraphy: topset beds dip gently (0.036 °) and diverge offshore; more steeply dipping foreset beds (0.190 °) converge farther seaward; and relatively thin, gently dipping bottomset beds (0.022 °) extend across the outer shelf, overlying an erosional surface presumed to be of Late Pleistocene age. Sediment accumulation rates are highest in the foreset region (≥ 5 cm/year) and reduced in the bottomset region (

The annual flux of sediment from the Ganges-Brahmaputra River is among the highest in the world. The desorption of226Ra and Ba from this sediment produces a major point source of226Ra and Ba input to the ocean. Highest226Ra and Ba fluxes are expected to occur during high river flow (June-September) when most of the sediment is discharged. Surprisingly, during low discharge of the Ganges-Brahmaputra River, fluxes of226Ra and Ba to the northern Bay of Bengal are comparable to expected river-derived fluxes during peak river discharge. A large non-riverine source of Ra and Ba is required to explain the high fluxes during low river discharge. I suggest that this source is submarine discharge of groundwater containing high concentrations of226Ra and Ba. Total annual fluxes of226Ra and Ba to the ocean from this system are significantly greater than estimated previously.

Particle-scavenged 234 Th and 210 Pb can be used to trace the fate of particulate matter reaching the deep-sea floor. We used this technique to demonstrate rapid ingestion of particles arriving at the sea floor (at a depth of 4,100 m) by mobile epibenthic holothuroids (Abyssocucumis abyssorum and Oneirophanta mutabilis). Excess 234 Th and 210 Pb activities of sediment trap material, detrital aggregates from the sea floor, and animal gut contents all were similar while activities of surface sediments (top 0-5 mm) were considerably lower. A simple calculation using the excess 210 Pb concentration of two potential food sources, sediment trap material and surface sediments, shows that ∼91% of gut content material of A. abyssorum must have come from material similar to that found in the sediment trap cups. By setting the sediment trap material as age 0, apparent ages of 100 d, and 12-13 d were estimated for the aggregates, surface sediments, and A. abyssorum guts, respectively. The population of A. abyssorum at this site could potentially process 0.2-4% of the vertical mass flux (m -2 d -1 ) during the high particle flux period between June and October 1994. The percentage of vertical flux processed could be substantially greater when all of the mobile epibenthic megafauna species are considered.

(224)Ra and (223)Ra are adopted as tracers to qualify submarine groundwater discharge (SGD) in Tolo Harbor, a highly urbanized embayment in Hong Kong. Based on the sampling data, a two-layered radium mass balance model is used to estimate lateral SGD and bottom SGD. Total SGD is estimated to be 1.2-3.0 cm d(-1), including lateral SGD of 5.7-7.9 cm d(-1) and bottom SGD of 0.3-2.0 cm d(-1). Fresh SGD is estimated to be (2.1-5.5) × 10(5)m(3)d(-1). Nutrient fluxes (mold(-1)) from SGD are estimated to be (3-7.4) × 10(4) (dissolved inorganic nitrogen), (2.4-6.2) × 10(2) (dissolved inorganic phosphate) and (6.5-16) × 10(4) (dissolved silicate). Primary productivity is estimated to be (1.5-15) × 10(6)gCd(-1), 2-53% of which is supported by SGD-induced phosphate fluxes. The study indicates that SGD is a significant source of nutrients to coastal waters and may cause an obvious increase of primary production. These findings must be considered in future coastal ecological management.

This study reports results of analysis of sediment cores collected from the Patos Lagoon estuary. This estuary receives materials from land runoff into Patos and Mirim lagoons and from exchange with the adjacent South Atlantic Ocean through a narrow inlet. Sediment from these sources is mostly natural, but additional contributions associated with source/activities related to the port of Rio Grande. The aim of this study was to estimate rates of accumulation of the sediments and to assess the significance of metal inputs from these activities relative to natural inputs. Our results indicate an average sedimentation rate of 0.3 cm/yr and that the transport of suspended solids from offshore sources into the estuary was enhanced after the deepening of the channel and construction of the breakwaters (in the early 1900). Results for metal accumulation in these sediments suggest that there have been only minor enrichments which can be attributed to anthropogenic sources.

The fluxes of 226Ra (half-life = 1600 years) and 228Ra (half-life = 5.7 years) from the North Inlet salt marsh to the sea are much larger than can be supported by decay of their Th parents in the surface marsh sediments. These fluxes are sustained almost entirely by groundwater flow through the marsh. An average groundwater flow of approximately 10 cm3 cm−2 day−1 is indicated if the groundwater activities we have measured are representative. The fluxes of 223Ra (half-life = 11.4 day) and 224Ra (half-life = 3.6 day) are factors of 22, and ten more than those expected from the flux of 226Ra. Groundwater also sustains most of the flux of the short-lived isotopes. The measured Ra activity ratio pattern in the marsh creeks matches the groundwater signature but is distinct from the pattern of the parent thorium isotopes in the sediment. We present a model to explain the anomalous distribution pattern of these isotopes. Despite their large throughput, the inventories of desorbable 226Ra and 228Ra in the top 15 cm sediment layer are very low. Nevertheless, the activities of 226Ra and 228Ra in the porewaters are large, indicating a low distribution coefficient (∼10) for radium and a short retention time (∼10 days) in the surface sediment layer. We surmise that groundwater flow may be a significant source of radium isotopes in the waters of shallow estuaries and coastal margins. This source must be recognized while considering mass balance of any tracer, be it radium, nutrients, other metals, or δ18O.

In recently formed marine barite (BaSO 4) separated from two equatorial Pacific cores 226Ra activity exceeds that of its parent 230Th by at least an order of magnitude, indicating radium uptake during barite precipitation. The decay of 226Ra with depth, in these samples, is exponential implying that no radium exchange with porewaters occurs after burial. Thus, barite behaves as a closed system below the sediment mixed layer. The absence of 226Ra exs activity in barite samples older than ∼8000 years, the lack of any detectable 228Th, 228Ra, and 224Ra in any of the barite samples analyzed, and the unaltered crystal size and habit with depth in the sediment, further support this conclusion. Hence, assuming that oceanic Ra/Ba ratio has not changed throughout the Holocene, the decay of unsupported 226Ra permits dating of marine barite and estimating sedimentation rates. Holocene sedimentation rates calculated using the decay of 226Ra exs in barite are consistent with 14C and δ 18O derived sedimentation rates of 2–3 cm/kyr for the same cores. The method could be extendible to carbonate-poor Holocene sediments.

We describe a nuclear detector system for measuring low activities of 223Ra and 224Ra in natural waters based on an original design of Giffin et al. (1963). Samples are obtained by adsorbing 223Ra and 224Ra onto a column of MnO2 coated fiber (Mn fiber). The short-lived Rn daughters of 223Ra and 224Ra which recoil from the Mn fiber are swept into a scintillation detector where alpha decays of Rn and Po occur. Signals from the detector are sent to a delayed coincidence circuit, which discriminates decays of the 224Ra daughters, 220Rn and 216Po, from decays of the 223Ra daughters, 219Rn and 215Po. The system is calibrated using 232Th and 227Ac standards with daughters in equilibrium adsorbed on Mn fiber. Results of samples from Tampa Bay, Florida, and the Atchafalaya and Mississippi Rivers mixing zones are reported. The method is extendible to measurements of 227Ac, 231Pa, 228Th, and 228Ra.

A comprehensive study of a variety of radionuclide tracers has been coupled with other geochemical investigations and with sedimentary and physical oceanographic measurements to elucidate processes and their characteristic time scales at the mouth of the Amazon River. This two-year field study on the Amazon continental shelf involved four cruises designed to provide information during different stages of the river hydrograph. Although the cruises were coordinated with river stage, other physical variables including spring-neap tidal stages, the flow of the North Brazil Current and trade-wind stress caused important effects on the shelf environment. Partitioning of uranium among dissolved, colloidal and particulate phases was investigated during AmasSeds. A detailed examination of uranium water-column behavior during low river discharge found that most (89%) of the uranium near the Amazon River mouth was associated with the particulate phase and that most (92%) of the riverine dissolved-phase uranium was in the colloidal size fraction (0.001–0.4 μm). A non-conservative uranium/salinity distribution was observed for dissolved uranium, indicating large-scale uranium removal from surface waters with salinities less than 20 ppt. Colloidal uranium was non-conservative across the entire salinity regime, exhibiting removal of colloidal uranium from waters with salinities less than 12 ppt and a significant input at higher salinities. A short-lived particle-reactive tracer,234Th, was used to evaluate the rates of particle scavenging on the shelf. Suspended-sediment concentrations respond to each turn of the tide, thus limiting the time available for equilibrium to be established between the particles and the tracers. Experiments demonstrated that on the Amazon shelf the partitioning and distribution of trace elements are governed by particle dynamics (particle residence times < sorption times). The high suspended load, including fluid muds, retards the incorporation of adsorbed234Th into the seabed. Once scavenged,234Th remains part of the suspended-sediment and fluid-mud inventory for periods of at least 4–8 weeks. Another particle-reactive tracer,210Pb, was used to evaluate the potential supply of reactive metals from offshore waters to the shelf. As open-ocean waters move into the Amazon mixing zone, in response to the estuarine-like circulation, they lose210Ph through scavenging processes associated with delta formation. This oceanic input of210Pb dominates other inputs to the Amazon shelf system. Based on210Pb analyses from more than 40 ☐ and kasten cores, the flux of water moving shoreward and depositing210Pb in the sediments was calculated to be on the order of 6 × 10161y−1 ∼10 times the riverine flux from the Amazon. The distribution of210Pb in the sediments suggests that if particle-reactive species (such as certain trace metals) are released in dissolved form on the shelf, they will be scavenged quickly in this turbid environment, with the largest inventories occurring in the foreset beds (although the highest concentrations occur in the bottomset beds). The large landward flow of water indicates that if particle-reactive species are released in the western equatorial Atlantic via aerosol transport or other mechanisms, there is a good chance that a sizeable portion will be buried in the Amazon delta. Fluxes of radium isotopes,226Ra,228Ra and224Ra, from the bottom sediments were used to evaluate sediment resuspension across the shelf. The average flux of226Ra from the Amazon shelf balanced the annual desorption of226Ra from river-derived sediments; however, departures between the226Ra flux and sediment necessary to support the flux occurred for different sampling periods. During falling and low discharge, less sediment entered the system than was required to support the sedimentary desorption226Ra flux. During rising and high discharge, more sediment entered than was necessary to sustain the226Ra flux. Considerable recycling of particles between the seabed and water column was required to wash away most of the desorbable226Ra from the sediment. To support the sedimentary228Ra flux, 3.4 × 1016 g of sediment must be resuspended each year. Such mixing would resuspend the top 35 em of sediment over the entire shelf each year, or mix the surface layer (SL, the region of uniform sedimentary210Pb depth profiles) to a depth of 90 cm annually. Such mixing would turn over the entire SL every 1–2 years. Desorption of sedimentary radium isotopes must be accompanied by the desorption of other exchangeable species including phosphate, cesium and cadmium. Correlations among phosphate and radium isotopes, infer that phosphate is being released from bottom sediments as the sediments are resuspended into the water column. As was the case for226Ra, bottom sediment must be repeatedly suspended into the water column to release all of the exchangeable phosphate. The desorption of224Ra from shelf sediments provides a tracer of the Amazon plume into the Atlantic. In July and September 1989, the ship crossed the Demerara retroflection 380 km from the Amazon shelf and excess224Ra activities up to 8 dpm 100 kg−1 were measured. It was concluded that these waters were 80 em s−1 would be required to advect the water this distance in

Significantly deficient 210Po/210Pb activity ratios (0.14±0.14) are measured in the hydrothermal vent fluids collected from Juan de Fuca Ridge. Chimney deposits generally showed 1200 dpm g−1 of excess 210Po. If the observed 210Po-210Pb disequilibrium in vent fluids is a result of 210Po removal by the precipitates, residence time of polonium with respect to its irreversible removal from hot fluids is estimated to be of the order of a few minutes. Alternatively, if the disequilibrium is a result of an insufficient growth of 210Po from in-situ210Pb following heating, the residence time of hot fluids within the hydrothermal system can be shown to be less than 30 days, which is considerably lower than previously reported values.

Spencer Gulf in South Australia is an inverse estuary, where an excess of evaporation over precipitation and the absence of major rivers lead to a well defined horizontal salinity gradient from 36‰ near its entrance to about 46‰ at its head. The excess of salt is exported annually from the gulf as a gravity current moving along the sea floor and across the shelf. Water balance is maintained by an inflow of less saline shelf water at the surface. The inverted salinity gradient which persists throughout the year can be used to advantage in the study of processes which govern the cycling of radionuclides of the uranium and thorium decay series in the nearshore marine environment. Radionuclides investigated include 234U, 238U226Ra and 228Ra. The results obtained indicate that uranium isotopes in Spencer Gulf have a typical marine signature, but that uranium is being removed from the water column at a rate of about 0.2 μg cm−2 y−1, based on conservation of salt. The 234U/238U ratio of uranium in the predominantly carbonate sediments of Spencer Gulf are consistent with uranium input from seawater, and the measured accumulation rate of authigenic uranium in the sediments supports the model removal rate, indicating th at Spencer Gulf is a net sink for uranium. A systematic increase in 228Ra/226Ra with salinity, coupled with a general increase in 226Ra above that predicted from evaporation of open ocean sea water demonstrate that both radium isotopes enter the gulf. In order to maintain the excess radium in the water column, diffusive fluxes of 0.3 dpm cm−2 y−1 and 0.05 dpm cm−2 y−1 for 228Ra and 226Ra, respectively, from the seafloor would be required. The standing crop of 232Th in the surface mixed layer of the sediment as a potential source for 228Ra via decay and recoil processes is sufficient to support the excess 228Ra in the water column, but there appears to be insufficient 230Th in the sediment to generate the required 226Ra flux, suggesting an external source for some of the excess 226Ra, such as periodic surface runoff after rainfall events and/or submarine discharge of groundwater from granitic basement rocks.

Radium isotope distributions on the continental shelf adjacent to the Amazon River are a complex function of salinity. The river and seawater endmembers for each radium isotope are low. Data from intermediate salinities plot as a highly scattered region above the conservative mixing line connecting the river and ocean endmembers. Spatial and temporal variability in sediment mixing and water residence times cause the scatter in the radium isotope data. We have used the distributions of 226 Ra, 228 Ra, and 224 Ra collected during four cruises representing different conditions on the shelf to construct inventories of radium due to nonconservative additions from sediments. Using estimates of water residence times, we calculate fluxes of these radium isotopes from the Amazon system. The amount of fresh river sediment entering the system each year is adequate to support the nonconservative flux of 226 Ra from the shelf sediments; however, the ultimate release of 226 Ra from the sediments requires weeks to months. To support the nonconservative flux of 228 Ra from this system, sediments of the 1–2 m thick surface mixed layer must be resuspended every two years. The resulting flux supports 15% of the 228 Ra entering the surface Atlantic Ocean. Release of 224 Ra from suspended sediments, primarily fluid mud, is more than adequate to support the inventory of 224 Ra on the Amazon shelf. Comparing the AmasSeds low discharge cruise (November 1991) to a low discharge sampling in December 1982 revealed no significant changes in the 226 Ra and 228 Ra distributions.

Ages in the range 1–15 yr for chimneys deposited from hydrothermal vents from the Endeavour Segment of the Juan de Fuca Ridge are estimated based on the ingrowth of228Th from228Ra. Cleaning samples by digestion in an acid bath confirms that the mineral barite is the phase being dated. A detailed examination of sample ALV 2463 R7 collected from the Clam Bed Site yielded a vertical growth rate of 5.8 cm/yr and a radial growth rate of 2.6 cm/yr. This investigation revealed that this chimney did not grow in a regular layer by layer accretion. Therefore, an average of ages obtained from several barite crystals a few centimeters apart (

Tolo Harbour, located in the northeastern part of Hong Kong's New Territories, China, has a high frequency of algal blooms and red tides. An attempt was made to first quantify the submarine groundwater discharge (SGD) into Tolo Harbour using (226)Ra, and then to estimate the nutrient fluxes into the Harbour by this pathway. The total SGD was estimated to be 8.28×10(6) m(3) d(-1), while the fresh submarine groundwater discharge (FSGD) was estimated to be 2.31×10(5) m(3) d(-1). This showed that a large amount of SGD was contributed by recirculated seawater rather than fresh groundwater in the Harbour. Using the SGD and groundwater nutrient information around Tolo Harbour, the nutrient loading through SGD was estimated to be 1.1×10(6) mold(-1) for DIN, 1.4×10(4) mold(-1) for PO(4)(3-)-P and 1.4×10(6) mold(-1) for SiO(2)-Si, which was much more significant than its counterpart through the river discharge. Despite uncertainties in the estimation, the nutrient loading to Tolo Harbour by SGD is clearly significant. Thus, the current efforts for management of red tides in Tolo Harbour have to be reviewed and control of groundwater contamination is obviously required.

During the past century, anthropogenic forces changed the composition of water in coastal aquifers. These changes were brought about by excessive mining of coastal groundwater, dredging and pier construction, wetland draining, and hard surface expansion. These forces caused an increase of salinity in the subterranean estuary, which led to a series of biogeochemical reactions. These changes continue today.

An examination of suspended sediment dispersal in the coastal ocean off the Ganges-Brahmaputra river system using shipboard and satellite data reveal distinct patterns of sediment distribution and sediment motion which help explain the dynamics of the world's largest sediment dispersal system. During the low-discharge period, suspended sediment concentration distribution is mainly a function of tidal energy and the sediments are primarily transported as a suspension population. Satellite and sea-bed data indicate that transport of river sediments is largely restricted to an area landward of about the 20-isobath during the low-discharge period. A simplistic estimate of residual transport and its separation into tidal and non-tidal components indicate that non-tidal transport contributes about 47% of the total transport and has transport directions mostly southwestward during the northeast monsoon. The tidal residual transport, which contributes about 53% of the total transport, however, does not show any particular direction for most of the stations, although some landward stations show shoreward transport. Although few water-column measurements were made during the high-discharge period, data from this and previous studies suggest that the dominant transport direction is to the southwest for most of the year.

[1] Storms can cause significant groundwater flow in coastal settings, but prior studies of the effects of storms on groundwater flow and transport have largely focused on very large storms and used salinity as a tracer. We have little information about the effects of smaller storms on coastal flow and how storm-induced variability affects key tidal wetlands like salt marshes, which may remain saline throughout a storm. Here we show that even the distant passage of a moderate storm can strongly increase groundwater flow and transport in salt marsh ecosystems and adjacent barrier islands. Groundwater monitoring and radium isotope tracer analyses revealed significant influx of saline creek water into the confined aquifer below the marsh platform, driven by storm surge. This pulse of fluids reached depths exceeding 5 m, and surge-enhanced tides propagated through the aquifer to affect flow in the upland >100 m from the creek bank. Groundwater discharge from the marsh varied significantly prior to the storm, doubling during inundating tides compared to a period of noninundating neap tides. Storm surge then caused groundwater discharge to decline ∼50% compared to similar inundating tides. Ra- and nutrient-poor creek water that entered the confined aquifer below the marsh was quickly enriched in nutrients and carbon, even on 12 h tidal cycles, so that nutrient discharge was likely proportional to groundwater discharge. Storm-related flow could also drive significant contaminant discharge from developed coastlines. The enhanced transport and variability observed here likely affected hundreds of kilometers of the coastline impacted by the storm.

The Ganges-Brahmaputra river system supplies more dissolved uranium to the ocean than any other system in the world (S arin et al., 1990; S ackett et al. 1973). However, there have been no investigations to determine whether riverine supplies of uranium are altered by geochemical reactions in the river-ocean mixing zone. In this study, uranium and salinity data were collected in the Ganges-Brahmaputra mixing zone during a period of low river discharge. The uranium distribution with salinity shows that in waters The behavior of uranium in the Ganges-Brahmaputra is in sharp contrast to its in teh Amazon mixing zone where MCKee et at. (1978) found uranium activities significantly higher than predicted from conservative micxing. The contrasting behaviours for uranium in theses systems are due to the different locatons where mixing between river and seawater occurs. For the Amzon, mixing takes place on the continental shelf whereas for the Ganges-Brahamputra, mixing occurs within shoreline sedimentary environments. The physiochimical processes controlling uranium removal to sediment deposits in the Amazon are partly known. We discuss mechanisms which may be removing uranium to suspended and mangrove sediments in the Ganges-Brahmaputra.

The Ganges-Brahmaputra river system is ranked fourth among world rivers as a source of freshwater to the oceans and is believed to supply more sediment to the ocean than any other; 1.5 × 1012 kg/yr (Milliman and Meade, 1983). Barium and226Ra are typically enriched in waters where sediment-laden rivers enter the ocean. As such, the Ganges-Brahmaputra is likely to produce globally significant barium and226Ra fluxes to the ocean. Water samples for barium and226Ra were collected within four major channels of the Ganges-Brahmaputra mixing zone during a period of low sediment and freshwater discharge. The data suggest that, in addition to suspended sediments supplied directly from rivers, river sediments deposited during high discharge in mangroves and on islands are desorbing barium and226Ra to seawater. The release of barium and226Ra from these sediment deposits is out-of-phase with the direct supply of sediments from the rivers. Estimates of the annual fluxes of barium and226Ra from the Ganges-Brahmaputra mixing zone were also derived. The fluxes of barium and226Ra are 5.3 × 108 mol barium/yr and 9.5 × 1014 dpm radium/yr. The first silicate and phosphate mixing profiles for this system are reported.

Radium isotopes provide unique and important information concerning water exchange in a salt marsh-tidal creek system. Seasonal radium data collected over five tidal cycles from a creek draining a South Carolina salt marsh, radium and thorium data from the adjacent marsh sediments and interstitial water radium data from the drainage basin are modeled to yield residence times of water in the upper 10 cm of the marsh sediments ranging from less than one to twenty-six hours. Water residence times derived from chambers which directly measure the flux of Ra isotopes to the creek waters agree well with these calculated interstitial water turnover times. Dissolved radium activities in the tidal creek were greater during the summer than during other times of the year. We suspect that seasonal changes in the storage of organic carbon and rates of bioturbation lead to net reduction of radium carrier phases in the marsh sediments during the summer.

Radium isotopes provide a means of identifying the source of freshened waters in the ocean and determining the time elapsed since these waters were in the estuary. We present evidence that during April, waters from the Amazon mixing zone pass within 50 km of the mouth of the Orinoco River. These Amazon waters are characterized by a lower 228Ra/226Ra activity ratio (AR) than are waters from the Orinoco at similar salinities. During autumn, the increased discharge of the Orinoco displaces the freshened Amazon waters seaward, yet the two can be distinguished clearly. Within the Caribbean Sea, waters of Orinoco origin carry a characteristic radium signature including excess activities of 224Ra. This isotope may be used to estimate the time elapsed since the waters were removed from contact with sediments. Current speeds based on 224Ra dating ranged from 15 to 33 cm/s during April. The radium isotopes also provide an assessment of sediment mixing in the estuary. During low discharge (April), considerable mixing of older sediment by physical or biological processes or dredging maintained high activities of 228Ra in the estuary and produced the highest 228Ra/226Ra AR's yet measured in any estuary. During high discharge (September), a large fraction of the 228Ra was derived from desorption from fresh sediment rather than mixing of older sediments. Activities of 224Ra were high in the estuary during both high and low discharge, indicating that considerable mixing of recently introduced sediment must occur during each period. The high 224Ra/228Ra AR's measured in September imply that the same sediments were being resuspended during high discharge, and that there was little exposure of older sediments which had been stored in the estuary for months to years. During April, 224Ra and 228Ra activities in the water were about equal, indicating that most of the sediment being resuspended had been stored in the estuary long enough to reestablish radioactive equilibrium in the 232Th decay series (i.e., 20 years).

The exchange of groundwater between land and sea is a major component of the hydrological cycle. This exchange, called submarine groundwater discharge (SGD), is comprised of terrestrial water mixed with sea water that has infiltrated coastal aquifers. The composition of SGD differs from that predicted by simple mixing because biogeochemical reactions in the aquifer modify its chemistry. To emphasize the importance of mixing and chemical reaction, these coastal aquifers are called subterranean estuaries. Geologists recognize this mixing zone as a site of carbonate diagenesis and dolomite formation. Biologists have recognized that terrestrial inputs of nutrients to the coastal ocean may occur through subterranean processes. Further evidence of SGD comes from the distribution of chemical tracers in the coastal ocean. These tracers originate within coastal aquifers and reach the ocean through SGD. Tracer studies reveal that SGD provides globally important fluxes of nutrients, carbon, and metals to coastal waters.

[1] New data of fluxes of radium isotopes and concentrations of radium in coastal groundwater enable an estimation of fluxes of submarine groundwater discharge (SGD) to a large section of the South Atlantic Bight (SAB). The new fluxes are considerably greater than SGD fluxes to this region estimated earlier. The annual average total SGD fluxes, which represent a range of salinity, are 3 times greater than the river fluxes. The highest fluxes of radium and SGD occur in the summer off the coast of Georgia. When scaled per kilometer of shoreline, the SGD flux is the same for the coastlines of Onslow Bay, NC (140 km), a section of the South Atlantic Bight from Onslow Bay, NC, to Cresent Beach, Florida, United States (600 km), and the entire Atlantic Ocean (75,000 km). The fact that each independent estimate is based on different methods and assumptions gives confidence to the results. These SGD fluxes are not restricted to the shoreline but occur throughout the continental shelf. They are important for supplying not only radium but also nutrients, carbon, metals, and freshwater to these coastal waters. Citation: Moore, W. S. (2010), A reevaluation of submarine groundwater discharge along the southeastern coast of North America, Global Biogeochem. Cycles, 24, GB4005, doi:10.1029/2009GB003747.

Water column distributions of226Ra were determined at stations in the Sea of Marmara and the Black Sea as part of the 1988 Joint U.S.—Turkish Black Sea Expedition. Black Sea surface water226Ra concentrations were a factor of three to four lower than measurements made 20 years earlier. The most likely cause is increased removal of226Ra and Ba [35] due to increased surface biological activity; a secondary effect is decreased fluvial discharge and related dimunition of inputs by desorption from fluvial suspended sediments. The amount of226Ra missing from the surface waters of the Black Sea over this period is accounted for in the high-porosity surficial “fluff” sediment layer. Throughout the Black Sea, depth profiles of226Ra exhibited pronounced maxima of approximately 25 dpm/100 L at aboutσθ = 16.2–16.3, in the vicinity of a bacterial maximum, but slightly shallower than the total dissolved Mn and Fe maxima (σθ = 16.4–16.5) reported by Lewis and Landing [38]. While the226Ra maximum may, in part, be linked to the cycling of Mn and Fe oxyhydroxides near theO2H2S interface, its distribution appears to be more plausibly explained as a result of the microbial breakdown of particulate organic matter and the subsequent release and partial dissolution of associated barite in this region. A simple steady-state two-☐ model has been used to obtain a semiquantitative understanding of the behavior of226Ra in the Black Sea. By incorporating reasonable estimates for the input and removal of226Ra in the Black Sea, an excellent agreement between predicted and observed (1988)226Ra concentrations was achieved. The model suggests that the dominant variables controlling the distribution of226Ra in the Black Sea are riverine input and cycling with Ba.

We discuss observations of hydrothermal activity from sites along the Juan de Fuca Ridge and report the ages and radium isotope ratios of samples from eight sites collected on a series of 11 Alvin dives in September 1988. This was a pilot program to assess whether uranium and thorium series isotopes could provide age information on the duration of active venting. The ages are based on the ingrowth of 228Th from 228Ra in both active and inactive chimneys and chimney talus composed of barite, anhydrite, chalcopyrite, wurtzite, and pyrite. Samples were obtained from (from north to south) the Fairycastle Site (northern Endeavour Ridge), the Endeavour Hydrothermal Site, and the southern Juan de Fuca Sites (named vent 3, vent 1, and Plume areas). Samples from active chimneys (20) yielded 228Th/228Ra ages in the range 0.2 to 1.2 years. We recorded no ages in the range 1.2 to 12 years for inactive chimneys or rubble. Our tentative conclusions are that the individual chimneys form rapidly and that the presently active chimney structures are

The 226 Ra/Ba ratio in particles collected in sediment traps decreases with depth. Fluxes of Ba and Al increase with depth. These observations cannot be explained by simple vertical processes. Instead they require a component to the deep flux which originates at the ocean margins. We suggest that the deep traps are enriched in Ba through the advection of barite originally precipitated in biologically productive regions of the ocean margin. This source is less important for 226 Ra because some is lost by radioactive decay between the precipitation of barite on the margin and outward transport. The boundary flux is an important source of material to the deep sea. From 226 Ra fluxes in the upper 1000 m of the water column, we estimate that the residence time of 226 Ra in the upper ocean is 400 years. The removal of 226 Ra in the particle flux is approximately balanced by the upwelling of waters enriched in 226 Ra.

Dissolved Cu, Cd, and Zn concentrations in continental shelf waters of the Gulf of Cadiz are much higher than reported for other coastal areas. Direct observations of the metal enrichments in this region were made in March–April 1986 and October 1986; indirect observations (in the Alboran Sea) suggest similar enrichments occurred in June 1982 and September 1980, and it is probable that these enrichments are a persistent feature of the region. Zinc concentrations varied by more than two orders of magnitude between Atlantic water entering the Strait of Gibraltar (

THE nearly landlocked waters of the Black Sea support a valuable fishery, but are also particularly vulnerable to anthropogenic disturbance. Here we use dissolved barium and radium-226 as tracers, to investigate the biogeochemical health of the sea. Both elements are brought to surface waters by vertical mixing of deeper, enriched waters1,2, and by rivers3–8; these inputs should ordinarily be balanced by outflow of surface waters at the Bosporus, and by biologically mediated removal of 226Ra-bearing barite. We show, however, that surface-water inventories have been substantially depleted over the past few decades: recent (1988–89) barium concentrations were 1.6 times lower than in 19759; and recent 226Ra concentrations were 3.5 times lower than in 195810 and 196711. These observations suggest that steady-state cycling of these elements has been perturbed by increased primary productivity, presumably fuelled by nutrients from industry and agricultural runoff, and to a lesser extent by decreased fluvial sediment loads owing to extensive impoundment of rivers in the region.

We report a statistically significant negative correlation between the concentration (and the deposition rate) of the cosmogenic radionuclide 10Be and δ18O (of Globigerinoides ruber) in the sediments deposited in the anoxic Orca Basin (OB) of the Gulf of Mexico during the past ∼20 ka. Possible causes for this finding consecutively include: (1) enrichment of 10Be in the soils/easily erodable crustal material which were discharged into the OB during the Late Wisconsin Interglacial; (2) introduction of significant amounts of 10Be into the oceans due to the melting of continental ice sheets resulting in the enrichment of the radionuclide in the sediments deposited during that time; and (3) generation of a focussing effect resulting in the 10Be enrichment of the OB sediment.

The measurement of short-lived Ra-223 often involves a second measurement for supported activities, which represents Ac-227 in the sample. Here we exploit this fact, presenting a set of 284 values on the oceanic distribution of Ac-227, which was collected when analyzing water samples for short-lived radium isotopes by the radium delayed coincidence counting system. The present work compiles Ac-227 data from coastal regions all over the northern hemisphere, including values from ground water, from estuaries and lagoons, and from marine end-members. Deep-sea samples from a continental slope off Puerto Rico and from an active vent site near Hawaii complete the overview of Ac-227 near its potential sources.The average Ac-227 activities of nearshore marine end-members range from 0.4 dpm m(-3) at the Gulf of Mexico to 3.0 dpm m(-3) in the coastal waters of the Korean Strait. In analogy to Ra-228, we find the extension of adjacent shelf regions to play a substantial role for Ac-227 activities, although less pronounced than for radium, due to its weaker shelf source. Based on previously published values, we calculate an open ocean Ac-227 inventory of 1.35 * 10(18) dpm Ac-227(ex) in the ocean, which corresponds to 37 moles, or 8.4 kg. This implies a flux of 127 dpm m(-2) y(-1) from the deep-sea floor. For the shelf regions, we obtain a global inventory of Ac-227 of 4.5 * 10(15) dpm, which cannot be converted directly into a flux value, as the regional loss term of Ac-227 to the open ocean would have to be included.Ac has so far been considered to behave similarly to Ra in the marine environment, with the exception of a strong Ac source in the deep-sea due to Pa-231(ex). Here, we present evidence of geochemical differences between Ac, which is retained in a warm vent system, and Ra, which is readily released [Moore, W.S., Ussler, W. and Paull, C.K., 2008-this issue. Short-lived radium isotopes in the Hawaiian margin: Evidence for large fluid fluxes through the Puna Ridge. Marine Chemistry]. Another potential mechanism of producing deviations in Ac-227/Ra-228 and daughter isotope ratios from the expected production value of lithogenic material is observed at reducing environments, where enrichment in uranium may occur. The presented data here may serve as a reference for including Ac-227 in circulation models, and the overview provides values for some end-members that contribute to the global Ac distribution. (c) 2007 Elsevier B.V. All rights reserved.

Results of groundwater and seawater analyses for radioactive (3H, 222Rn, 223Ra, 224Ra, 226Ra, and 228Ra) and stable (D and 18O) isotopes are presented together with in situ spatial mapping and time series 222Rn measurements in seawater, direct seepage measurements using manual and automated seepage meters, pore water investigations using different tracers and piezometric techniques, and geoelectric surveys probing the coast. This study represents first time that such a new complex arsenal of radioactive and non-radioactive tracer techniques and geophysical methods have been used for simultaneous submarine groundwater discharge (SGD) investigations. Large fluctuations of SGD fluxes were observed at sites situated only a few meters apart (from 0 cm d(-1) to 360 cm d(-1); the unit represents cm3/cm2/day), as well as during a few hours (from 0 cm d(-1) to 110 cm d(-1)), strongly depending on the tidal fluctuations. The average SGD flux estimated from continuous 222Rn measurements is 17+/-10 cm d(-1). Integrated coastal SGD flux estimated for the Ubatuba coast using radium isotopes is about 7x10(3) m3 d(-1) per km of the coast. The isotopic composition (deltaD and delta18O) of submarine waters was characterised by significant variability and heavy isotope enrichment, indicating that the contribution of groundwater in submarine waters varied from a small percentage to 20%. However, this contribution with increasing offshore distance became negligible. Automated seepage meters and time series measurements of 222Rn activity concentration showed a negative correlation between the SGD rates and tidal stage. This is likely caused by sea level changes as tidal effects induce variations of hydraulic gradients. The geoelectric probing and piezometric measurements contributed to better understanding of the spatial distribution of different water masses present along the coast. The radium isotope data showed scattered distributions with offshore distance, which imply that seawater in a complex coast with many small bays and islands was influenced by local currents and groundwater/seawater mixing. This has also been confirmed by a relatively short residence time of 1-2 weeks for water within 25 km offshore, as obtained by short-lived radium isotopes. The irregular distribution of SGD seen at Ubatuba is a characteristic of fractured rock aquifers, fed by coastal groundwater and recirculated seawater with small admixtures of groundwater, which is of potential environmental concern and has implications on the management of freshwater resources in the region.

Publisher Summary A number of U- and Th-series isotopes have become popular tools for quantifying submarine groundwater discharge (SGD). These isotopic techniques enable large-scale estimates of various components of SGD, allowing detailed studies on the processes involved. Radium isotopes have proven to be useful tracers of total SGD in many environments on both small- and large scales. The existence of four naturally occurring radium isotopes makes Ra particularly useful for quantifying multiple sources of SGD. The utility of 222 Rn as a tracer of SGD has been demonstrated in a wide range of environments from coastal embayments to the coastal ocean. The approach for quantifying SGD using 222 Rn is similar to radium ( 226 Ra), except for a few key differences. The 222 Rn signature of SGD is best observed near the source because of its short half-life. In contrast, 226 Ra has the ability to integrate the SGD signal over much wider spatial scales. Finally, of the intercomparison studies conducted till date, there appears to be no systematic difference in the two techniques based on the coastal hydrogeologic setting, though a wider range of estimates has been typically observed in areas with fractured crystalline rock aquifers and where springs have been a major conduit for SGD. More recently, the geochemical budget for uranium has been shown to be impacted by SGD. As uranium is enriched in seawater relative to most coastal groundwater, the observed depletion of uranium in estuarine and coastal waters may provide an estimate of the seawater component of SGD that recharges coastal aquifers, or submarine groundwater recharge (SGR). Additionally, new isotope techniques for in situ study are under development including underwater g-spectrometry and continuous radon monitoring. These techniques may be more useful for location of SGD discharges rather than estimation of their magnitudes.

Active and inactive hydrothermal chimneys from the Mariana Trough have young ages ranging from 0.5 to 2.5 years. These estimates are based on disequilibrium between228Th and228Ra in barite-rich samples. Mineralogical investigations revealed that the chimneys contain a mixture of barite, galena, sphalerite and chalcopyrite which appear to have formed contemporaneously. Based on radium isotope ratios andRa/Ba ratios, leaching of freshly exposed basaltic rock must have been the source of the barium found in these deposits.

The most comprehensive data set on uranium, thorium, and radium isotopes in the Ganga-Brahmaputra, one of the major river systems of the world, is reported here. The dissolved 238U concentration in these river waters ranges between 0.44 and 8.32 μ/1, and it exhibits a positive correlation with major cations (Na + K + Mg + Ca). The 238 U ∑Cations ratio in waters is very similar to that measured in the suspended sediments, indicating congruent weathering of uranium and major cations. The regional variations observed in the [ 234 U 238 U ] activity ratio are consistent with the lithology of the drainage basins. The lowland tributaries (Chambal, Betwa, Ken, and Son), draining through the igneous and metamorphic rocks of the Deccan Traps and the Vindhyan-Bundelkhand Plateau, have [ 234 U 238 U ] ratio in the range 1.16 to 1.84. This range is significantly higher than the near equilibrium ratio (~1.05) observed in the highland rivers which drain through sedimentary terrains. The dissolved 226Ra concentration ranges between 0.03 and 0.22 dpm/1. The striking feature of the radium isotopes data is the distinct difference in the 228Ra and 226Ra abundances between the highland and lowland rivers. The lowland waters are enriched in 228Ra while the highland waters contain more 226Ra. This difference mainly results from the differences in their weathering regimes. The discharge-weighted mean concentration of dissolved 238U in the Ganga (at Patna) and in the Brahmaputra (at Goalpara) are 1.81 and 0.63 μ/1, respectively. The Ganga-Brahmaputra river system constitutes the major source of dissolved uranium to the Bay of Bengal. These rivers transport annually about 1000 tons of uranium to their estuaries, about 10% of the estimated global supply of dissolved uranium to the oceans via rivers. The transport of uranium by these rivers far exceeds that of the Amazon, although their water discharge is only about 20% of that of the Amazon. The high intensity of weathering of uranium in the Ganga-Brahmapura River system can also be deduced from the [ 232 Th 238 U ] and [ 230 Th 238 U ] activity ratios measured in the suspended sediments. 230Th is enriched by about 19% in the suspended sediments relative to its parent 238U. The flux of excess 230Th supplied to the Bay of Bengal via these river sediments is 980 × 1012 dpm/a, about six times more than its in situ production from seawater in the entire Bay of Bengal.

Natural radionuclides: Tracers for transport processes in the oceanRates of many transport and reaction processes in the ocean can be evaluated with the natural clocks provided by the radioactive decay series of threeprimordial radionuclides present in the earth's crust: 232Th, 238U and 235U (detailed review in Ivanovich and Harmon, 1992). In all three decay series,isotopes of relatively soluble elements like U, Ra and Rn, decay to isotopes of highly particle-reactive elements (Th, Pa, Po, Pb), and vice versa. Theresulting disequilibria in the decay chains can be interpreted as a measure of transport rates of the particulate and liquid phases relative to each other.

[1] Enrichments of radium isotopes in coastal waters have served as indicators of submarine groundwater discharge (SGD). Because coastal waters exchange with the open ocean on a timescale of weeks to months, seasonal patterns of radium isotope distributions may be used to indicate changes in SGD through the year. Here I report the seasonal distributions of four naturally occurring radium isotopes measured throughout the water column of the southeastern U.S. continental shelf, commonly known as the South Atlantic Bight (SAB). The study area extended from Onslow Bay, North Carolina, to Crescent Beach, Florida. Activities of the long-lived isotopes were highest off the coast of Georgia. In the summer, high activities extended throughout the study area, but during spring and winter, they decreased markedly off the coast of SC. The primary source of excess 226Ra and 228Ra (that is activities in excess of open ocean values) is shown to be SGD. Because the activities of these isotopes in SGD differ little with season, the lower excess activities off South Carolina imply lower rates of SGD during the spring and winter. The excess inventories and fluxes of 226Ra and 228Ra provide an estimate of the residence time of water on the shelf. These residence times range from 30 to 60 days with a mean of about 40 days.

A complex approach in characterisation of submarine groundwater discharge (SGD) off south-eastern Sicily comprising applications of radioactive and non-radioactive tracers, direct seepage measurements, geophysical surveys and a numerical modelling is presented. SGD fluxes in the Donnalucata boat basin were estimated by direct seepage measurements to be from 4 to 12Ls(-1), which are comparable with the total SGD flux in the basin of 17Ls(-1) obtained from radon measurements. The integrated SGD flux over the Donnalucata coast estimated on the basis of Ra isotopes was around 60m(3)s(-1) per km of the coast. Spatial variations of SGD were observed in the Donnalucata boat basin, the average (222)Rn activity concentration in seawater varied from approximately 0.1kBqm(-3) to 3.7kBqm(-3) showing an inverse relationship with salinity. The continuous monitoring carried out at the site closest to the coast has revealed an inverse relationship of (222)Rn activity concentration on the tide. The (222)Rn concentrations in seawater varied from 2.3kBqm(-3) during high tides to 4.8kBqm(-3) during low tides, thus confirming an influence of the tide on submarine groundwater discharge. Stable isotopes (delta(2)H and delta(18)O) showed that SGD samples consist up to 50% of groundwater. Geo-electrical measurements showed a spatial variability of the salt/fresh water interface and its complex transformation in the coastal zone. The presented results imply that in the studied Donnalucata site there are at least two different sources of SGD, one superficial, represented by mixed fresh water and seawater, and the second one which originates in a deeper limestone aquifer.

[1] A physical model based on determining the fraction of the tidal prism that returns to the estuary on the next high tide is used to estimate the flushing time of the Okatee River estuary. The return flow factor (b) of 0.81 yields a flushing time of 2 days. A mass balance model of 228Ra and salinity is also used to estimate b. This model yields an average b = 0.79, virtually the same as the physical model. A third model based on the decay of 224Ra relative to 228Ra is used to determine the apparent age of water in the estuary. These ages range from 1.6 to 5 days, with an average of 3.4 days. These three independent estimates are in remarkably close agreement, certainly within the error of each estimate. We use these residence times to develop a mass balance model for the radium isotopes in the Okatee estuary. We consider decay, mixing, sedimentary input, river input, and submarine groundwater discharge (SGD). The major loss term for each isotope is mixing with water in Port Royal Sound; the major input for each isotope is SGD. At steady state these terms must balance. Knowing the water age and the radium isotope composition of groundwater entering the Okatee allows us to estimate an average SGD flux of 1 m3/s. The SGD flux is a factor of 3–4 greater during the summer relative to the winter. This SGD supplies a considerable quantity of nutrients and carbon into the Okatee system.