Willard S. Moore

The subterranean estuary: a reaction zone of ground water and sea water

Mixing between meteoric water and sea water produces brackish to saline water in many coastal aquifers. In this mixing zone, chemical reactions of the salty water with aquifer solids modify the composition of the water; much as riverine particles and suspended sediments modify the composition of surface estuarine waters. To emphasize the importance of mixing and chemical reaction in these coastal aquifers, I call them subterranean estuaries. Geochemical studies within subterranean estuaries have preceded studies that attempt to integrate the effect of these systems on the coastal ocean. The mixing zone between fresh ground water and sea water has long been recognized as an important site of carbonate diagenesis and possibly dolomite formation. Biologists have likewise recognized that terrestrial inputs of nutrients to the coastal ocean may occur through subterranean processes. Further evidence of the existence and importance of subterranean estuaries comes from the distribution of chemical tracers in the coastal ocean. These tracers originate within coastal aquifers through chemical reactions of the saline water with aquifer solids. They reach the coastal ocean as the surface and subterranean systems exchange fluids. Exchange between the subterranean estuary and the coastal ocean may be quantified by the tracer distribution in the coastal ocean. Examples from the east and Gulf coasts of the U.S., as well as the Bay of Bengal, will be used to evaluate the importance of these unseen estuaries in supplying not only chemical tracers, but also nutrients, to coastal waters. Anthropogenic effects on subterranean estuaries are causing significant change to these systems. Ground water mining, sea level rise, and channel dredging impact these systems directly. The effects of these changes are only beginning to be realized in this vital component of the coastal ecosystem.

Groundwater and pore water inputs to the coastal zone

Both terrestrial and marine forces drive underground fluid flows in the coastal zone. Hydraulic gradients on land result in groundwater seepage near shore and may contribute to flows further out on the shelf from confined aquifers. Marine processes such as tidal pumping and current-induced pressure gradients may induce interfacial fluid flow anywhere on the shelf where permeable sediments are present. The terrestrial and oceanic forces overlap spatially so measured fluid advection through coastal sediments may be a result of composite forcing. We thus define “submarine groundwater discharge” (SGD) as any and all flow of water on continental margins from the seabed to the coastal ocean, regardless of fluid composition or driving force. SGD is typically characterized by low specific flow rates that make detection and quantification difficult. However, because such flows occur over very large areas, the total flux is significant. Discharging fluids, whether derived from land or composed of re-circulated seawater, will react with sediment components. These reactions may increase substantially the concentrations of nutrients, carbon, and metals in the fluids. These fluids are thus a source of biogeochemically important constituents to the coastal ocean. Terrestrially-derived fluids represent a pathway for new material fluxes to the coastal zone. This may result in diffuse pollution in areas where contaminated groundwaters occur. This paper presents an historical context of SGD studies, defines the process in a form that is consistent with our current understanding of the driving forces as well as our assessment techniques, and reviews the estimated global fluxes and biogeochemical implications. We conclude that to fully characterize marine geochemical budgets, one must give due consideration to SGD. New methodologies, technologies, and modeling approaches are required to discriminate among the various forces that drive SGD and to evaluate these fluxes more precisely.

Major ion chemistry of the Ganga-Brahmaputra river system: Weathering processes and fluxes to the Bay of Bengal

The Ganga-Brahmaputra, one of the worlds largest river systems, is first in terms of sediment transport and fourth in terms of water discharge. A detailed and systematic study of the major ion chemistry of these rivers and their tributaries, as well as the clay mineral composition of the bed sediments has been conducted. The chemistry of the highland rivers (upper reaches of the Ganga, the Yamuna, the Brahmaputra, the Gandak and the Ghaghra) are all dominated by carbonate weathering; (Ca + Mg) and HCO3 account for about 80% of the cations and anions. In the lowland rivers (the Chambal, the Betwa and the Ken), HCO3 excess over (Ca + Mg) and a relatively high contribution of (Na + K) to the total cations indicate that silicate weathering and/or contributions from alkaline/saline soils and groundwaters could be important sources of major ions to these waters. The chemistry of the Ganga and the Yamuna in the lower reaches is by and large dictated by the chemistry of their tributaries and their mixing proportions. Illite is the dominant clay mineral (about 80%) in the bedload sediments of the highland rivers. Kaolinite and chlorite together constitute the remaining 20% of the clays. In the Chambal, Betwa and Ken, smectite accounts for about 80% of the clays. This difference in the clay mineral composition of the bed sediments is a reflection of the differences in the geology of their drainage basins. The highland rivers weather acidic rocks, whereas the others flow initially through basic effusives. The Ganga-Brahmaputra river system transports about 130 million tons of dissolved salts to the Bay of Bengal, which is nearly 3% of the global river flux to the oceans. The chemical denudation rates for the Ganga and the Brahmaputra basins are about 72 and 105 tons· km−· yr−1, respectively, which are factors of 2 to 3 higher than the global average. The high denudation rate, particularly in the Brahmaputra, is attributable to high relief and heavy rainfall.

Measurement of 223Ra and 224Ra in coastal waters using a delayed coincidence counter

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.

High fluxes of radium and barium from the mouth of the Ganges-Brahmaputra River during low river discharge suggest a large groundwater source

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.

Marsh nutrient export supplied by groundwater discharge: Evidence from radium measurements

We use 228 Ra and 226 Ra to determine the mass balance of dissolved inorganic nitrogen (DIN) and dissolved reactive phosphorus (DRP) in the North Inlet salt marsh -estuarine system. While this system has only minor freshwater inputs of nutrients or radium, it is an extremely productive ecosystem. In addition, there are significant exports of these dissolved species to the coastal ocean. Saline groundwater in this estuarine system contains nutrient and radium concentrations more than an order of magnitude greater than surface waters. Using a radium mass balance, we estimate the groundwater discharge necessary to support the export of radium to the coastal ocean and the corresponding flux of nutrients from the groundwater. From these calculations, we show that the underlying aquifer supplies nutrients sufficient to support the net primary productivity of the salt marsh ecosystem and to account for the known export of nutrients from the marsh. We conclude that the major nutrient source to the North Inlet, South Carolina, salt marsh is the saline aquifer lying just beneath the surface of the marsh. Furthermore, extrapolation of the nutrient export to include other South Carolina marshes suggests that nutrient fluxes from salt marshes to the coastal ocean rival riverine nutrient fluxes for the region.

Using the radium quartet for evaluating groundwater input and water exchange in salt marshes

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.

Radium isotope measurements using germanium detectors

Abstract Recent advances in sampling and counting techniques have provided the means of measuring 226Ra, 228Ra and 224Ra at low activities in natural waters. Samples are preconcentrated in the field by adsorbing radium on a fibre coated with manganese oxides. Absolute activities and activity ratios are measured using germanium detectors supplemented in some cases by alpha scintillation measurements of 222Rn. This paper describes tests and results obtained using a Ge(Li) crystal and an intrinsic germanium crystal with a 1 cm diameter well. The well detector has an efficiency two to three greater than the flat Ge(Li) system and thus has a considerably higher sensitivity. Results from ground waters and estuarine waters are presented which demonstrate the usefulness of the germanium detectors in studies of radium in natural waters.

Sampling 228Ra in the deep ocean

Abstract A sampler using manganese impregnated acrylic fibers inside a 30-1. Niskin bottle is used to concentrate radium isotopes from 500 to 1500 1. of seawater near the sea floor. Seawater contact is minimized during descent by expendable closures held in place with a soluble link and during ascent by the regular Niskin bottle closures. A single sampler will concentrate radium equivalent to 1000 to 2000 1. of seawater during a 2-h soak.

The flux of barium to the coastal waters of the southeastern USA: the importance of submarine groundwater discharge

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.