OS51D-1274
Submarine Groundwater Discharge and Coastal Water Quality on the Kona Coast: The Land Use Connection
For several decades, the Kona, or western, coast of the island of Hawai'i (Hawai'i, USA) has been recognized as a region of exceptionally high submarine groundwater discharge (SGD). Maintaining good water quality on the Kona coast is important for the local coral reef ecosystems and tourism-based economy. However, rapid development in the recent past and planned development in the near future may pose a threat to coastal waters. In this study, we quantified SGD-related fluxes of freshwater, nutrients and trace metals into the coastal ocean at 12 sites on the Kona coast. Radium-224 activity, silica concentration, and salinity were used as groundwater tracers, and a mass-balance approach was used to estimate fluxes. The relation between fresh groundwater quality and land use was also investigated. Fresh SGD was pervasive along the Kona coast, occurring to a measurable extent at 11 of 12 study sites. However, the volume percent of fresh groundwater at coastal ocean sites varied considerably, from 0-47%, indicating that SGD affects some sites much more than others. Inverse, linear relationships between salinity and concentrations of nitrate+nitrite, phosphate, and silica in the coastal ocean indicated conservative behavior and suggested that nutrients are diluted or advected away from shore faster than they can be used biologically. Neither the population density nor the percentage of urbanized, agricultural, forested or bare land in the vicinity of the study sites influenced groundwater nutrient concentrations; however, sites closest to golf courses had significantly higher concentrations of nitrate+nitrite. Relations between land use and trace metal concentrations in groundwater were also investigated.
OS51D-1275 INVITED
Geochemical Profile of Groundwater Discharge Across the Beachface of Long Bay, South Carolina
Groundwater discharge from land to sea provides a significant, yet often overlooked pathway for delivery of
dissolved nutrients and contaminants to nearshore waters. Recent research is focused on refining
groundwater and associated dissolved chemical species flux estimates, geological controls on flow pathways,
and geological contributions to the chemistry of the pore water. In Long Bay, South Carolina, extensive
seismic imagery provides many clues to control of the geologic framework morphology of the nearshore area.
In addition, recent mapping of the electrical structure of the upper few meters of marine nearshore sediment
compared with nearby Chirp subbottom profiles provide indications of the relationships between framework
geology and submarine groundwater discharge. Continuous electrical resistivity profiles also suggest that
mixing between ocean water and upland-derived fresh water within shallow aquifers occurs to a large extent
within the surf zone and shoreface.
At present, work is underway to estimate shallow groundwater seepage rates of nutrients and metals across
the beach face into the adjacent surf zone. A beach perpendicular transect of 1 meter deep wells was
installed from the swash zone to approximately 100 meters inland and sampled over two complete tidal
cycles. The groundwater salinity gradient ranged from approximately 30 to 2 ppt. All wells showed suboxic
conditions and maxima in dissolved nitrogen, phosphorus and iron at mid-range salinity. The latter
corresponded to high dissolved organic content, indicating a maximum in decomposition and a concomitant
release of nutrients into the groundwater. Measurements of radon and radium isotopes will be used to
estimate the net movement of groundwater seaward from the beach. These groundwater discharge
estimates will be applied to measured concentrations of N, P and metals in an effort to derive fluxes.
Preliminary results from Rn activity measurements indicate groundwater fluxes in the range of 1-3 cm/day.
These values are typical of the South Atlantic Bight and are on the order of riverine discharge.
http://www.coastal.edu/cmws/
OS51D-1276
Pleistocene Submarine Groundwater Discharge Along the Atlantic Continental Shelf, New England: The Role of Ice Sheets
Well bores have long shown that relatively fresh groundwater exists far offshore beneath the Atlantic continental shelves of North and South America. This freshwater was emplaced during Pleistocene sea-level low stands when the shelf was exposed to meteoric recharge and parts were over-run by the Laurentide Ice Sheet. Details of the emplacement mechanism of the fresh water remain poorly understood. At issue is whether the recharge occurred locally through meteoric seepage or was a more regional process driven by ice sheets. These two mechanisms predict very different distributions of water beneath the shelf, and identifying which dominated would help predict whether freshwater is widespread beneath the shelf or isolated in discrete pockets. Here we present results from high-resolution paleohydrologic models of groundwater flow and solute transport beneath the continental shelf from New Jersey to Maine over the last 2 million years. Our analyses show that freshwater emplacement beneath New England's continental shelf was dominated by the presence of the Laurentide Ice Sheet, which the models show increased recharge rates by four times relative to modern levels. Our analyses further suggest that the presence of fresh to brackish water more than 100 km offshore New Jersey was facilitated by discharge from submarine springs along the Baltimore and Hudson Canyons, with outflow rates close to 1 cm/yr during the last glacial maximum (LGM). We estimate that the volume of emplaced Pleistocene continental shelf freshwater (< 1 ppt) to be 1200 km3 in New England, 4300 km3 along the North and South American shelf, and 106,000 km3 along passive margins globally. This water represents a potentially valuable, albeit non-renewable resource for coastal megacities.
OS51D-1277
Insignificant Groundwater Discharge to Lakes in a Large Fractured Rock Watershed
Understanding the rate and pattern of fresh groundwater discharge to lakes and rivers is critical for watershed budgets and for protecting the ecological integrity of lake and river ecosystems. A 900 km2 study watershed contains a river and over 3000 lakes and wetlands, mostly underlain by exposed crystalline bedrock or a thin veneer of coarse-grained sediments. The objective of this study is to constrain the rate and pattern of groundwater discharge at the watershed-scale. Groundwater discharge points were identified by conducting detailed transects of the river and lakes using temperature, conductivity and radon-222 tracers. Surface water samples from representative lakes were analyzed for (delta)2H, (delta)18O, radon- 222 and chloride for three consecutive summers. Radon and chloride concentrations are used in a steady- state advective model to determine groundwater fluxes to the representative lakes. The detailed transects identified minor and highly localized groundwater discharge locations to the river and lakes even in regions underlain by potentially significant geological structures or exposed bedrock fractures. Stable isotope, temperature and conductivity data identified only one subsidiary stream with significant groundwater discharge. The steady-state model indicates that the groundwater flux to lakes is generally less than 0.1 percent of the total input. This integrated thermal, chemical, isotopic and hydraulic dataset indicates that the rate of groundwater discharge to lakes in this crystalline bedrock watershed is not significant and that discharge is localized but not focused at exposed geological structures or bedrock fractures. This conclusion implies that in the watershed groundwater and surface water is largely decoupled, which has significant ecological and water management implications.
OS51D-1278
Tracing groundwater input into Lake Vanda, Wright Valley, Antarctica using major ions, stable isotopes and noble gas
The McMurdo Dry Valleys (MDV), Antarctica, is the largest ice-free region on Antarctica. Lake Vanda, located in central Wright Valley, is the deepest lake among the MDV lakes. It has a relatively fresh water layer above 50 m with a hypersaline calcium-chloride brine below (50-72 m). The Onyx River is the only stream input into Lake Vanda. It flows westward from the coastal Lower Wright Glacier and discharges into Lake Vanda. Suggested by the published literature and this study, there has been and may still be groundwater input into Lake Vanda. Stable isotopes, major ions, and noble gas data from this study coupled with previously published data indicate that the bottom waters of Lake Vanda have had significant contributions from a deep groundwater system. The dissolved gas of the bottom waters of Lake Vanda display solubility concentrations rather than the Ar-enriched dissolved gas seen in the Taylor Valley lakes (such as Lake Bonney). The isotopic data indicate that the bottom calcium-chloride-brine of Lake Vanda has undergone very little evaporation. The calcium-chloride chemistry of the groundwater that discharges into Lake Vanda most likely results from the chemical weathering and dissolution of cryogenic evaporites (antarcticite and gypsum) within the glacial sediments of Wright Valley. The high calcium concentrations of the brine have caused gypsum to precipitate on the lake bottom. Our work also supports previous physical and chemical observations suggesting that the upper portion actively circulates and the hypersaline bottom layer does not. The helium and calcium chloride values are concentrated at the bottom, with a very narrow transition layer between it and the above fresh water. If the freshwater layer did not actively circulate, then diffusion over time would have caused the helium and calcium chloride to slowly permeate upwards through the water column.
OS51D-1279
Significant Groundwater Discharge of Nutrients to Western Long Island Sound Inferred From Radioisotope, Nutrient and Organic Geochemical Tracers
Western Long Island Sound suffers from seasonal oxygen depletion due to both nutrient loading in this heavily populated region as well as restricted circulation of the Sound. The role played by groundwater in delivering nutrients to the Sound is not well understood, which served as motivation for the sampling we initiated in May, 2008. Work was carried out in both Manhasset Bay, a portion of which is sewered, and Northport Harbor, which is largely unsewered. There is clear evidence of discharge of groundwater to each embayment, as reflected in surface-water Rn-222 time series, seepage meter and high-resolution piezometer transects installed perpendicular to shore). Seepage rates were as high as 32 cm/day and modulated by the tide. Initial data reveal variable groundwater total DIN concentrations, spanning similar concentration ranges (as high as 500 uM), in the sewered and unsewered locations. Concentrations of organic geochemical tracers of sewage (including caffeine and imidacloprid) are high in samples with high nutrient concentrations and also span comparable ranges in sewered and unsewered locations. A preliminary interpretation of these results would suggest that most of the nutrient flux from groundwater is from wastewater in both the sewered and unsewered settings (rather than from fertilizer application, atmospheric deposition, etc.), implying that the sewering is not very effective. If this result is verified with additional sampling this fall, it would suggest that wastewater-influenced groundwater discharge is indeed a prominent source of nutrients to western Long Island Sound which in turn contributes to eutrophication and oxygen depletion.
OS51D-1280 INVITED
A Geochemical and Geophysical Examination of Submarine Groundwater Discharge and Associated Nutrient Loading Estimates into Lynch Cove, Hood Canal, WA
Geochemical tracer data (i.e., 222Rn and four naturally occurring Ra isotopes), electromagnetic (EM) seepage meter results, and high-resolution, stationary electrical resistivity images were used to examine the bi-directional (i.e., submarine groundwater discharge and recharge) exchange of a coastal aquifer with sea water. Our study site for these experiments was Lynch Cove, the terminus of Hood Canal, WA, where fjord- like conditions dramatically limit water column circulation that can lead to recurring summer-time hypoxic events. In such a system a precise nutrient budget may be particularly sensitive to groundwater-derived nutrient loading. Shore-perpendicular time-series subsurface resistivity profiles show clear, decimeter-scale tidal modulation of the coastal aquifer in response to large, regional hydraulic gradients, hydrologically- transmissive glacial terrain, and large (4-5m) tidal amplitudes. A 5-day 222Rn time-series shows a strong inverse covariance between 222Rn activities (0.5 - 29 dpm L-1) and water level fluctuations, and provides compelling evidence for tidally-modulated exchange of groundwater across the sediment / water interface. Mean Rn-derived submarine groundwater discharge (SGD) rates of 85±84 cm d-1 agree closely in the timing and magnitude with EM seepage meter results that showed discharge during low tide and recharge during the high tide events. To evaluate the importance of fresh versus saline SGD, Rn-derived SGD rates (as a proxy of total SGD) were compared to excess 226Ra-derived SGD rates (as a proxy for the saline contribution of SGD).
OS51D-1281
Submarine Groundwater Discharge and Nutrient Additions to the Puerto Morelos Coast of the Yucatan Peninsula
Coral reef ecosystems along the coast of the Caribbean are currently threatened by anthropogenic impacts such as sedimentation, nutrient loading, and rapidly expanding coastal development. Submarine groundwater discharge (SGD) can contribute to large nutrient inputs in coastal ecosystems, particularly in regions such as the Yucatan Peninsula where surficial runoff is minimal, the geology is dominated by karst dissolution features, and terrestrial output occurs primarily from SGD. SGD inputs were assessed using radioisotopes, and related freshwater, nutrient, and metal fluxes were measured in the vicinity of coral reefs at Puerto Morelos along the Caribbean coast of the Yucatan Peninsula. High 223Ra and 224Ra activity in the coastal zone suggests large SGD inputs to the coastal waters off Puerto Morelos, Mexico. Concentrations and fluxes of nutrients associated with SGD will be reported.
OS51D-1282
Seasonal Short-Lived Radium Activity in the Venice Lagoon: The Role of Residence Time
Radium is considered to be an excellent tracer of submarine groundwater discharge (SGD) and, therefore, has been used in many studies of this process in the past decade. Comprehensive surveys of excess 223,224Ra activity were completed in the surface waters of the Venice Lagoon over 6 seasons in order to quantify seasonal variation of SGD into the lagoon. The mass balance of radium found that SGD was 5-26 times greater than total river discharge (35.5 m3 s-1), and that total SGD could differ by almost an order of magnitude pending season. Several possible parameters, which may cause the seasonal variation, were tested. These included precipitation events, average tidal elevation, average tidal excursion, wind speed and direction, yet none provided a satisfactory explanation for the difference. Residence time based on a hydrodynamic model, however, was very strongly correlated with the observed variation. When the average residence time in the lagoon was low (5 days) the SGD was calculated to be 930 m3 s-1 and when the average residence time was high (9 days) the SGD was quantified as 160 m3 s-1. Radioactive decay is already accounted for in the mass balance model and therefore this correlation must be explained by another process. The Venice Lagoon is characterized by low residence time during periods of spring tides and bora or northerly winds, both of which create exceptionally strong currents in the Venice Lagoon. The currents as well as the large tidal excursion which occurs at spring tides drive a recirculation of seawater through the surface sediments, which greatly increases short-lived Ra activity in the surface waters. This evidence suggests, therefore, that short-lived Ra mass balance studies, which are based on a single survey, may under or overestimate the mean annual SGD pending the hydrodynamics of the investigated location.
OS51D-1283
Detection of Groundwater Discharge in a Subtropical Estuarine System using CDOM fluorescence measurements
The exchange between groundwater discharge and the overlying surface waters has become an increasingly important focus of study due to the potential impacts that contaminants or nutrients within groundwater have on chemical budgets of surface water ecosystems, particularly in the coastal zone. This paper seeks to demonstrate a means of detecting groundwater discharge from the Floridan Aquifer to a subtropical estuary, Tampa Bay, Florida, using fluorescence measurements. This novel approach utilizes multi-spectral characterization of Colored Dissolved Organic Matter (CDOM) as a means of fingerprinting groundwater from overlying surface waters. Currently, Radon-222 is widely used as an indicator of groundwater discharge in the ocean. Although advances have been made with in situ detection of radionuclides, measurements have limited capabilities for autonomous collection and there exists no means of remotely sensed detection from space. CDOM fluorescence (FDOM), however, offers a sensitive technique to discern sources of organic matter using wavelength ratios to discriminate groundwater from overlying surface waters. CDOM is an ideal proxy for groundwater discharge, as measurement is source specific, inexpensive, can be collected real-time from moored locations, and is capable of being measured from space in aquatic environments. Utilizing fluorescence measurements offers a powerful new tool for ascertaining groundwater discharge over large temporal and spatial scales.
OS51D-1284
Using Fiber Optic Distributed Temperature Sensing (DTS) to Assess Groundwater-Lake Exchange in an Acid Mine Lake in Eastern Germany
Groundwater flows through contaminated mine sites are a major concern in many parts of the world. In this study, a variety of instrumentation was used to locate and quantify groundwater inflows into an acid lake on an abandoned mine site in Brandenburg, Germany. While previously-installed piezometers and seepage meters had identified several points of groundwater influx into the lake, such techniques are spatially limited to the point of installation. To address this limitation, a fiber-optic distributed temperature sensor (DTS) was deployed across the lake bottom and in vertical profilers to confirm and expand the previously generated data sets. Fiber-optic DTS, a relatively new technology, provides the opportunity to measure temperature on very high spatial and temporal scales using Raman spectra scattering of pulsed light within a glass fiber. A 1000 meter cable was deployed spatially along the sediment-water interface to identify spatially scattered areas of groundwater inflow, while two high-resolution probes (which return temperature readings every 2.4 vertical cm) were installed vertically near existing seepage meters. Preliminary analysis of the vertical deployments showed substantial groundwater upwelling, confirming the results of previous seepage meter measurements which showed significant vertical flux into the lake. Ongoing analysis of the lateral deployment is expected to identify areas in which there are anomalies in the diurnal temperature cycle at the lake bottom; such anomalies may indicate groundwater influx into the lake. These areas will be used to locate future seepage meter and piezometer installations.
OS51D-1285
Use of Pore water Rn and Ra Profiles to Evaluate the Nature of Flow through Permeable Coastal Sands in Huntington Beach, Southern California
Submarine Groundwater Discharge (SGD), as commonly defined, can represent (1) water recharged above sea level, or (2) water that is circulated locally through permeable sediments by pressure gradients generated by flow over rough topography, by wave activity, and by physical pumping of irrigating macrofauna. Under some circumstances, rapid increases in water column density over time could also drive episodes of pore fluid circulation. Budgets for radium isotopes in the water column have been used to evaluate SGD in a number of recent studies, and used to calculate fluxes of nutrients due to SGD. However, because the scale distances for nutrients may differ from those of the Ra isotopes, it is important to constrain whether SGD calculated from near-shore water column Ra budgets represents local circulation of overlying water through sediments, or regional flow driven by recharge above sea level. This also can define whether nutrient fluxes are driven by re-mineralization of biogenic material formed in the overlying water, or by transport from adjacent land areas. We have measured profiles of Rn-222 and Ra isotopes (223,224, 228) in pore waters of permeable sediments offshore from Huntington Beach on multiple occasions, working at the shoreline and at water depths of 5 to 15 m. By also determining the rate at which these isotopes emanate from solid phases and the adsorption constant for Ra on solid phases, we can evaluate the nature of SGD circulation in this system. Results indicate that nearly all of the SGD is due to local recirculation of overlying water, with macrofaunal irrigation probably driving most of the flow. Ra-228 profiles, coupled with water column budgets, can be used to put constraints on regional vertical flow.
OS51D-1286
Monitoring Submarine Groundwater Fluxes Using a Novel Pressure Transducer-Based Instrument
We introduce a novel instrument for characterizing fluxes in the shallow subsurface and discuss its application to high-resolution, long-term monitoring of groundwater-surface water exchanges in Waquoit Bay, an estuary on Cape Cod, USA. The instrument incorporates three sensors: a differential pressure sensor, a thermistor, and a conductivity sensor. It is deployed on the seafloor to measure the pressure difference between the base of the water column and porewater at a depth of 1 meter into the sediment, as well as the electrical conductivity and temperature of these two waters. In this configuration, it is able to resolve vertical head gradients with magnitudes smaller than 0.1%. Once programmed, the instrument functions autonomously over deployment periods exceeding six months, locally logging data at user-specified intervals, with the additional capability of direct wireless transmission to shore. Here we elaborate on the estimation of vertical seafloor fluxes using the collected pressure, conductivity and temperature data and starting from the pressure form of Darcy's law, which accommodates the variable density conditions in coastal aquifers. We also analyze the sources and magnitudes of errors in these flux estimates based on laboratory and field experiments. Finally, we present recent field data collected by our instruments in Waquoit Bay and compare them to observations of submarine groundwater discharge in this bay attained using other types of seepage meters.
OS51D-1287
Numerical models of fresh and saline groundwater flow at the beach calibrated with geophysical, hydrologic and salinity data.
We present a multidisciplinary approach to characterize submarine groundwater discharge in Waquoit Bay, Cape Cod, USA. We focus on the fluctuations in fresh and saline discharge over tidal cycles and spring-neap cycles of tidal magnitude. Geophysical (electrical resistivity), chemical (salinity and basic chemical parameters) and hydrological data (head measurements) were monitored during complete tidal cycles. The data set was used to calibrate a density dependent flow model. With this model, we analyze discharge patterns, investigate how these patterns depend on tidal forcing and hydraulic parameters, and finally consider how the complex pattern of flow at the beach may affect the complex geochemistry of submarine groundwater discharge.