Radium mass-balance in Jamaica Bay, NY: Evidence for a substantial flux of submarine groundwater (original) (raw)
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Ra and Rn isotopes as natural tracers of submarine groundwater discharge in Tampa Bay, Florida
Marine Chemistry, 2007
A suite of naturally occurring radionuclides in the U/Th decay series ( 222 Rn, 223,224,226,228 Ra) were studied during wet and dry conditions in Tampa Bay, Florida, to evaluate their utility as groundwater discharge tracers, both within the bay proper and within the Alafia River/estuarya prominent free-flowing river that empties into the bay. In Tampa Bay, almost 30% of the combined riverine inputs still remain ungauged. Consequently, groundwater/surface water (hyporheic) exchange in the discharging coastal rivers, as well as submarine groundwater discharge (SGD) within the bay, are still unresolved components of this system's water and material budgets.
Salt marsh submarine groundwater discharge as traced by radium isotopes
Marine Chemistry, 2003
Submarine groundwater discharge (SGD) generally occurs through permeable sediments where the hydraulic head of an aquifer is above sea level, and often includes a recirculated seawater component. In order to determine SGD to the Great Sippewissett Marsh, West Falmouth, MA, we measured the activities of four radium isotopes ( 226 Ra, 228 Ra, 223 Ra, 224 Ra) at the marsh inlet in July 1999 and 2001 and compared our data with Ra activities measured at the same location in 1983 and 1985. A radium-based approach for estimating SGD to the marsh yielded a summer average of 3900 m 3 day À 1 . This flux was in good agreement with independent estimates from the literature, which ranged from 600 to 23,000 m 3 day À 1 . Radium activities of the long-lived isotopes ( 226 Ra, 228 Ra) were on average more than two times higher in 1999 than during the three other time periods. These results suggest that drought conditions leading to enhanced seawater -sediment interactions may be an important mechanism in delivering certain dissolved substances to coastal waters. D
Nutrient and Radium Fluxes from Submarine Groundwater Discharge to Port Royal Sound, South Carolina
Aquatic Geochemistry, 2000
Water exchange between the coastal ocean and underlying aquifers provides a newlyrecognized source of materials to the ocean. The flux of materials into the ocean from this process is termed submarine groundwater discharge (SGD). Both surficial and semi-confined aquifers contribute to SGD. Here we use 226 Ra and 228 Ra to quantify fluxes of SGD to Port Royal Sound, South Carolina, and to separate fluxes from the Upper Floridan (UFA) and surficial aquifers. Higher activity ratios of 228 Ra/ 226 Ra in the surficial aquifer make this separation possible. We estimate total SGD fluxes of about 100 m 3 s −1 with about 80% being derived from the surficial aquifer. The SGD flux provides about 1.8 × 10 6 mol d −1 of NH 4 with almost 90% from the surficial aquifer. Because of strong differences in the concentration of PO 4 within the UFA, PO 4 fluxes are less certain. Using the UFA wells with low PO 4 concentrations yields a flux of 1.2 × 10 5 mol d −1 ; using wells with high concentrations yields a flux of 2.0 × 10 5 mol d −1 . In the first case virtually all of the PO 4 flux is from the surficial aquifer; in the second case, 40% is from the UFA.
Marine Chemistry, 2010
River discharges are usually gauged at sites farther upstream than estuarine tidal reaches. As a result, global estimates of river water and nutrient fluxes to the ocean are likely underestimated as they often neglect groundwater discharge occurring in estuaries downstream of river gauging stations. We used radon and radium isotopes as tracers of groundwater discharge into the Sebastian River Estuary, a gaining stream in Florida, USA. We developed a spatially-distributed mass balance model that accounts for radon sources and sinks in waters above and below the estuarine pycnocline. Radium isotopes ( 224 Ra, 223 Ra, and 226 Ra) were not enriched in groundwater relative to surface water and thus had limited usefulness as tracers at this specific site. The detection of fresh groundwater just beneath the sediment:water interface overlain by brackish bottom water implies that fresh groundwater dominates over saline groundwater in this salt wedge estuary. Lateral groundwater inputs from sandy banks into waters above the estuarine pycnocline were about 6-fold higher than inputs into waters below the pycnocline. Groundwater discharge rates into the surface layer of the estuary estimated from a radon mass balance ranged from 5 to 18 m 3 /s (or 18 to 62 cm/day if uniformly distributed throughout the entire estuary area). The fluxes into the bottom layer ranged from 0.8 to 1.1 m 3 /s (or 2.8 to 3.9 cm/day). These groundwater inputs augmented river discharges gauged upstream of the estuary tidal reaches by about 260% during the dry period and 135% during the wet period. As nutrient and other dissolved species are often highly enriched in groundwaters, groundwater probably controls surface water quality in this and other Florida estuaries.
Submarine groundwater discharge to Great South Bay, NY, estimated using Ra isotopes
Marine Chemistry, 2008
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 ( 224 Ra, 223 Ra, 228 Ra, 226 Ra). 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 × 10 9 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.
Editorial handling by M. Kersten a b s t r a c t Salinization in groundwater systems can induce water-rock interaction, including the release of naturallyoccurring trace elements of health significance such as radium (Ra), with possible implications for the usability of water resources in addition to the increase of dissolved solids (TDS) concentrations. In general, radium mobility is limited by chemical removal mechanisms including adsorption onto clays and/or Mn and Fe oxides, exchange processes, and coprecipitation with secondary barite. In order to examine the effect of aquifer salinity gradients on the distribution of naturally-occurring Ra in fresh to saline groundwater and the relationship to water-rock interaction and Ra removal mechanisms, two contrasting systems were investigated: the shallow unconfined coastal aquifer in Agadir (southwestern Morocco) and the confined Cretaceous (Cape Fear) and Pliocene (Yorktown) aquifers of the Atlantic Coastal Plain (North Carolina, USA). Geochemical and isotopic indicators of salinity sources (e.g. cation ratios, d 18 O, d 2 H, Br À /Cl À , d 34 S-SO 2À 4 ; d 18 O-SO 2À 4 ) were used to identify the relative contributions of seawater and other saline waters and subsequent geochemical modification by waterrock interaction. Radium activities ( 224 Ra, 226 Ra, 228 Ra), radon-222, alkaline earth metal (Mg, Ca, Sr, Ba) concentrations and ratios, and 87 Sr/ 86 Sr ratios were analyzed to identify water-rock interaction processes affecting alkaline earth metals including Ra. The Morocco coastal aquifer is generally oxic, exhibits a range of salinity and water types (Cl À 163-2120 mg/L, median 932 mg/L), and exhibits Ca/Na ratios above the seawater value, typical of monovalent-divalent cation exchange (base-exchange reactions) in coastal aquifers. In contrast, the Atlantic Coastal Plain aquifers are anoxic, sulfate-reducing, cover a wider salinity range (Cl À 5-9890 mg/L, median 800 mg/L) representing a transition between Na-HCO À 3 and Na-Cl À waters, and exhibit Ca/Na ratios below that of modern seawater typical of reverse base-exchange reactions. Possible salinity sources in the Morocco coastal aquifer include seawater intrusion, Mesozoic evaporites, other natural saline waters, and/or wastewater, whereas the Atlantic Coastal Plain is primarily affected by old seawater present in the aquifer system. Radium activities are generally low and vary significantly within each aquifer, for example 226 Ra ranges from 1.8-27.7 mBq/L in the Morocco coastal aquifer (median 8.1 mBq/L) and 1.9-214 mBq/L in the Atlantic Coastal Plain (median 18.9 mBq/L). The highest Ra activities were observed in the most saline wells sampled in the Atlantic Coastal Plain. At total dissolved solids (TDS) concentrations above an apparent threshold of 5000mg/L,radiumactivitiesincreaseinagenerallylinearfashionwithsalinityintheAtlanticCoastalPlain,broadlycomparabletopreviousstudiesindicatingathresholdrangeof5000 mg/L, radium activities increase in a generally linear fashion with salinity in the Atlantic Coastal Plain, broadly comparable to previous studies indicating a threshold range of 5000mg/L,radiumactivitiesincreaseinagenerallylinearfashionwithsalinityintheAtlanticCoastalPlain,broadlycomparabletopreviousstudiesindicatingathresholdrangeof3000-10,000 mg/L. At lower TDS concentrations, water-rock interaction processes that vary with local aquifer conditions appear to control Ra distribution rather than merely salinity. In the Morocco coastal aquifer, adsorption of Ra and coprecipitation with secondary barite are apparently favorable to control Ra levels in groundwater. Radium removal in the anoxic Atlantic Coastal Plain aquifers appears to be associated with adsorption and/or exchange processes, with the additional possibility of barite precipitation in the Cape Fear aquifer indicated by barite saturation. Overall, the locally-variable factors that can control Ra sources and mobility in fresh to brackish groundwater at near-neutral pH include variation in solid-phase radioactivity, redox state affecting adsorption sites, availability of competing divalent cations, and barite saturation.
Sources and fluxes of submarine groundwater discharge delineated by radium isotopes
Biogeochemistry, 2000
This paper reports the results derived from radium isotopes of a submarine groundwater discharge (SGD) intercomparison in the northeast Gulf of Mexico. Radium isotope samples were collected from seepage meters, piezometers, and surface and deep ocean waters. Samples collected within the near-shore SGD experimental area were highly enriched in all four radium isotopes; offshore samples were selectively enriched. Samples collected from seepage meters were about a factor of 2-3 higher in radium activity compared to the overlying waters. Samples from piezometers, which sampled 1-4 meters below the sea bed were 1-2 orders of magnitude higher in radium isotopes than surface waters. The two long-lived Ra isotopes, 228 Ra and 226 Ra, provide convincing evidence that there are two sources of SGD to the study area: shallow seepage from the surficial aquifer and input from a deeper aquifer. A three end-member mixing model can describe the Ra distribution in these samples.
Seasonal distribution and flux of radium isotopes on the southeastern U.S. continental shelf
Journal of Geophysical Research, 2007
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 226 Ra and 228 Ra (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 226 Ra and 228 Ra 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.
Tunnelling and Underground Space Technology, 2007
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 time scale of weeks to months, seasonal patterns of radium isotope distribution may be used to indicate changes in SGD through the year. Here I report the seasonal distributions of 226 Ra measured in surface waters of the continental shelf of southern USA. The study area encompassed most of the South Atlantic Bight. Activities of 226 Ra were highest off the coast of Georgia. In summer, these high activities extended throughout the study area; but during the spring and winter they decreased markedly off the coast of South Carolina. The primary source of excess 226 Ra (that is activities in excess of open ocean values) is SGD. Because the activities of 226 Ra in SGD vary little with season, the lower excess activities off South Carolina imply lower rates of SGD during the spring and winter.