Henry J. Bokuniewicz

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.

Groundwater seepage into Great South Bay, New York

Great South Bay (New York) is a large lagoon on the northeast coast of the United States. The flow of groundwater across the floor of Great South Bay has been reported to account for as much as 2/3 of the total freshwater inflow. In situ measurements of this seepage flow have been made along four offshore transects in the Bay. These measurements show that the flow rate decreases rapidly offshore; within 30 m of the shoreline, the submarine outflow rates were typically 40 l (day m 2 ) −1 and decreased to less than 10 l (day m 2 ) −1 at a distance of 100 m from shore. The Bay floor at the study locations was sand or silty sand with vertical intrinsic permeabilities ranging from 14 to 78 darcys. The flow rate across the Bay floor may be described by an exponentially decreasing function. The flow distribution may, therefore, be specified with two parameters—the flow value at the shoreline, A , and a ‘decay’ constant, c , that governs the rate of decrease of the flow offshore. The calculated total flows along the four transects were 2·1 × 10 3 , 1·1 × 10 3 , 8·5 × 10 3 and 3·9 × 10 3 l (day m) −1 . Between 40% and 98% of this flow enters the Bay within 100 m from shore. The total flow of groundwater across the Bay floor was calculated to be about 2 × 10 8 l day −1 or 10–20% of the total freshwater inflow.

Analytical Descriptions of Subaqueous Groundwater Seepage

Direct measurements of groundwater seepage show the importance of subaqueous discharges as sources of fresh water and of dissolved chemicals to lakes and the coastal ocean. The rate of seepage decreases rapidly offshore; an analytical solution was developed that describes the discharge as Ki(In(coth πxk/4l))/k where i is the hydraulic gradient, K is the vertical hydraulic conductivity,l is the aquifer thickness, x is the distance from the shoreline, and k2 is K divided by the horizontal hydraulic conductivity. In addition to variations due to the inhomogeneities in the aquifer, seepage into the coastal ocean involves some recirculation of the salt water. In Great South Bay, New York measured fluxes were as great as 150 I m−2 d−1. The discharge near the shore was typically 50 I m−2 d−1, decreasing to 30 1 m−2 d−1 at a distance of 100 m offshore. Secondary convection due to an unstable density structure at the sediment-water interface may also be superimposed on the seepage distribution. Fingers of salt should be capable of carrying marine water many decimenters downward against the fresh groundwater advection. As a result, care must be exercised in interpreting direct measures of seepage flux to recognize the contribution of recirculated seawater.

From and migration of sand waves in a large estuary, Long Island Sound

Abstract Sand waves occur in eastern Long Island Sound with heights up to 4 m and lengths to 100 m. The waves do not form if either more than 10% mud or 12% coarse sand is present in the sediment. Mud suppresses wave formation by increasing the cohesion of the sediment. Sand-wave shape is independent of the water depth, d , provided the sand-wave height, H , is smaller than 0.86 d 1.19 . Both symmetric and asymmetric wave forms are present. Observation of the migration of sand waves by repeated bathymetric surveys indicates a net sand flux greater than 0.01 cm 3 cm −1 sec −1 in the direction faced by the steep slopes of the waves (i.e. westward, into the Sound). Under this sand flux, waves more than 30 cm high will not be measurably altered by a reversal of the semidiurnal tidal current.

Dynamics of the Coastal Zone

Earth’s coastline has evolved for many thousands ofyears, experiencing changes to habitat, coastal dynam-ics and the supply of sediment from the continentalinterior. Relative sea level has risen in some areas, butfallen elsewhere. There is an acknowledged range innatural variability within a given region of the globalcoastal zone, within a context of longer-term geologicalprocesses.Many of the regional controls on sea level involvelong-term geological processes (e.g., subsidence, iso-stasy), and have a profound influence on controllingshort-term dynamics. As sea levels fluctuate, the mor-phology of a coastal zone will further evolve, changingthe boundary conditions of other coastal processes: cir-culation, waves, tides and the storage of sediment onflood plains.Human development of coastal regions has modifiedpristine coastlines around the globe, by deforestation,cultivation, changes in habitat, urbanisation, agriculturalimpoundment and upstream changes to river flow.Humans can also influence changes in relative sea levelat the local scale. For example, removal of groundwaterand hydrocarbons from subterranean reservoirs maycause subsidence in nearby areas, with a concomitantrise in relative sea level. Our concern in LOICZ is notjust in the magnitude of change, but also in the recentand accelerated rate of change. Our interests extendto whether alterations on the local level can cumula-tively give rise to coastal zone changes of global signifi-cance.Climate warming may also contribute significantlyto sea level fluctuations. Predictions by the InternationalPanel on Climate Change (IPCC) suggest that sea levelis rising globally (15 to 95 cm by 2100) as a result of therecent warming of the ocean and the melting of ice caps(Houghton et al. 2001). As sea levels rise, coastal desta-bilisation may occur due to accelerated beach erosion,trapping of river sediment on flood plains and increas-ing water residence during floods. The predicted IPCCclimate-warming scenario will undoubtedly impactsome regions more than others. The Siberian coast isexperiencing a reduction in offshore sea-ice cover, witha associated increase in ocean fetch, leading to highersea levels during the open-water summer and accelera-tion of coastal erosion. Recent studies also suggest thattropical and temperate coastal environments are expe-riencing stormier conditions (i.e., increased numbersand severity of hurricanes). Will local storm surges mag-nify the impact of a global sea-level rise, increasing risksto humans and their infrastructure? Are there negativefeedbacks to engineering options for the protection ofcoastal settlements?Perhaps the largest impact on coastal stability is dueto modification to the global flux of sediment to thecoastal zone. Changes in global hydrology have modi-fied the timing and intensity of floods, and thereforethe effective discharge available for sediment transport.Climate shifts have varied the contributions from melt-water (snow, ice), altered the intensity of rainfall,changed drainage basin water-storage capacity, and al-tered precipitation and evaporation rates. Human influ-ences have also greatly modified downstream flow. Overhalf of the world’s rivers have seen stream-flow modi-fication through the construction of large reservoirs.These and other rivers have also been impacted by wa-ter withdrawal for agriculture, industry and settlements.Our understanding of the importance of submarinegroundwater discharge in the coastal zone and of itsprocesses has improved markedly in recent years; asignificant impetus has been given to this understand-ing by the LOICZ-associated SCOR Working Group 112.The outcomes of its work are summarised in this chap-ter.Human migration to the coastal zone and consequentland-use changes have also greatly impacted the stabil-ity of our coastal areas. Human impacts on the coastalzone ranges from massive (e.g., reduction in wetlands,urbanisation) to non-existent (e.g., many polar coast-lines). This chapter synthesises how climate shifts andhumans can affect and have affected our coasts on a glo-bal scale.

Groundwater seepage along a Barrier Island

Resistivity and water level measurements were made on a barrier island on the south shore of Long Island, New York to examine the distribution of fresh groundwater and the potential for recirculation of saline groundwater. The depth to the base of the freshwater lens was overpredicted by calculations of the static-equilibrium depth to a sharp interface apparently because of the sensitivity of the calculation to the low water-table elevations which are in turn sensitive to variations in sea level because of the existence of a transition zone between fresh and saline groundwater. Mixing and recirculation of saline groundwater at the base of the lens produced a transition zone up to 9.65 m thick. Measurements also support model forecasts of a mean bay level several centimeters above sea level, augmented by atmospheric forcing and wave setup. A time lag of about 8 hours between the response of the ocean level to longshore winds and the corresponding response of the bay level can result in a difference in elevation between the bay and the ocean that is up to four times that produced by other agents such as Stokes transport and density differences. In the presence of differential hydraulic head, bay and ocean water may be exchanged via groundwater flow between the base of the freshwater lens under the barrier beach and a deeper clay layer.

The typological approach to submarine groundwater discharge (SGD).

Coastal zone managers need to factor submarine groundwater discharge (SGD) in their integration. SGD provides a pathway for the transfer of freshwater, and its dissolved chemical burden, from the land to the coastal ocean. SGD reduces salinities and provides nutrients to specialized coastal habitats. It also can be a pollutant source, often undetected, causing eutrophication and triggering nuisance algal blooms. Despite its importance, SGD remains somewhat of a mystery in most places because it is usually unseen and difficult to measure. SGD has been directly measured at only about a hundred sites worldwide. A typology generated by the Land–Ocean Interaction in the Coastal Zone (LOICZ) Project is one of the few tools globally available to coastal resource managers for identifying areas in their jurisdiction where SGD may be a confounding process. (LOICZ is a core project of the International Geosphere/Biosphere Programme.) Of the hundreds of globally distributed parameters in the LOICZ typology, a SGD subset of potentially relevant parameters may be culled. A quantitative combination of the relevant hydrological parameters can serve as a proxy for the SGD conditions not directly measured. Web-LOICZ View, geospatial software then provides an automated approach to clustering these data into groups of locations that have similar characteristics. It permits selection of variables, of the number of clusters desired, and of the clustering criteria, and provides means of testing predictive results against independent variables. Information on the occurrence of a variety of SGD indicators can then be incorporated into regional clustering analysis. With such tools, coastal managers can focus attention on the most likely sites of SGD in their jurisdiction and design the necessary measurement and modeling programs needed for integrated management.

Sediment mass balance of a large estuary, Long Island Sound

Abstract Acoustic reflection profiles and bottom sampling are used to measure the volume of sediments accumulated in Long Island Sound. There is present 1.0 × 10 10 m 3 of sediment of which 5.3 × 10 9 m 3 is marine mud and 4.9 × 10 8 m 3 is probably of pre-marine, lacustrine origin. The balance consists of reworked sand derived from glacial drift. The acoustically determined sub-bottom structure of the Sound and available sea level data indicate that the Sound basin was occupied by a large lake for at least 6000 years and has been an arm of the sea since 8000 years b.p. The volume of lacustrine sediment is accounted for by direct riverine input over 6000 years but the volume of marine mud present substantially exceeds the riverine supply over 8000 years. The Sound is shown to act as a trap for sediments originating on the continental shelf.

Isotopic, geophysical and biogeochemical investigation of submarine groundwater discharge: IAEA-UNESCO intercomparison exercise at Mauritius Island

Submarine groundwater discharge (SGD) into a shallow lagoon on the west coast of Mauritius Island (Flic-en-Flac) was investigated using radioactive ((3)H, (222)Rn, (223)Ra, (224)Ra, (226)Ra, (228)Ra) and stable ((2)H, (18)O) isotopes and nutrients. SGD intercomparison exercises were carried out to validate the various approaches used to measure SGD including radium and radon measurements, seepage rate measurements using manual and automated meters, sediment bulk conductivity and salinity surveys. SGD measurements using benthic chambers placed on the floor of the Flic-en-Flac Lagoon showed discharge rates up to 500 cm/day. Large variability in SGD was observed over distances of a few meters, which were attributed to different geomorphological features. Deployments of automated seepage meters captured the spatial and temporal variability of SGD with a mean seepage rate of 10 cm/day. The stable isotopic composition of submarine waters was characterized by significant variability and heavy isotope enrichment and was used to predict the contribution of fresh terrestrially derived groundwater to SGD (range from a few % to almost 100%). The integrated SGD flux, estimated from seepage meters placed parallel to the shoreline, was 35 m(3)/m day, which was in reasonable agreement with results obtained from a hydrologic water balance calculation (26 m(3)/m day). SGD calculated from the radon inventory method using in situ radon measurements were between 5 and 56 m(3)/m per day. Low concentrations of radium isotopes observed in the lagoon water reflected the low abundance of U and Th in the basalt that makes up the island. High SGD rates contribute to high nutrients loading to the lagoon, potentially leading to eutrophication. Each of the applied methods yielded unique information about the character and magnitude of SGD. The results of the intercomparison studies have resulted a better understanding of groundwater-seawater interactions in coastal regions. Such information is an important pre-requisite for the protection and management of coastal freshwater resources.

Sediment transport and deposition in Long Island Sound

Publisher Summary This chapter discusses sediment transport and deposition in Long Island Sound (LIS). The processes of sediment transport and deposition are believed to have operated continuously with little change in rate since the start of the present marine regime in LIS at 8000-yr BP. The integrated flux of riverine sediment supplied to the Sound over the past 8000 years at the present supply rate is nearly equal to the mass of mud, which has accumulated in that time. Characteristic parameters of the LIS sedimentary system are presented, and the characteristic power parameters for bottom processes in the LIS are summarized. Nature of materials forming the shore of Long Island is also described. Sediment transport and bottom stability is analyzed, and evidences are presented to show that new silt-clay-size sediment entering LIS is rapidly incorporated into the surfacial layer of pellets that mantles the mud bottom. The average rate of accumulation of marine mud on the bottom of LIS over the past 8000 years is also calculated.