Tolo Harbour is naturally eutrophic with frequent algal blooms in the past. Water quality of the Harbour has been improved in past 20 years but the phytoplankton density is still high compared to other waters in Hong Kong. This study aimed to quantify submarine groundwater discharge (SGD) to Tolo Harbour and to show the important role of SGD in terms of nutrient loading with ecological effect. SGD was determined by one of the natural geochemical tracers, 226Ra. Mass balance of 226Ra was applied to quantify SGD and SGD was estimated to be 8.28 × 106 m3 d-1. A large portion of the SGD was recirculated seawater while the freshwater component of SGD, or submarine fresh groundwater discharge (SFGD), was only a small portion of that. SFGD was estimated to be 2.31 ×105 m3 d-1 using water balance. The flushing time of Tolo Harbour was estimated to be 77.9 d by using SFGD as one of the fresh water input components. Different nutrients including NH3—N, NO2—N, NO3—N, PO43-—P, SiO2—Si and dissolved inorganic nitrogen (DIN) were applied to nutrient mass balance analyses. The mass balance is based on non-point sources which include aerosol deposition (dry deposition), rainfall (wet deposition), diffusion from sediment, river, SGD and tidal exchange. Removal of nutrients was determined once all the non-point sources were estimated. SGD is the major source which contributes more than 88 % for different nutrients. Other sources contribute less than 10 % individually. The total nutrient loads of DIN, PO43-–P and SiO2—Si are 8.33 ×105, 1.03 ×104 and 1.28 ×106 mol d-1 respectively. The removals of DIN, PO43-–P and SiO2—Si are 6.87 ×105, 5.10 × 103 and 1.28 ×106 mol d-1 respectively. Assume the removal of nutrients was only due to consumption of phytoplankton, primary productivity was estimated to be 1.16g C m-2 d-1. Limitations of this study were about the determination of atmospheric depositions and diffusion from sediment. However, the contribution of nutrient loading from these sources was insignificant compared with SGD. Even if SGD is replaced by SFGD for the nutrient loading estimation, it is still the major contributor among all other non-point sources. In spite of the limitations, the important role of SGD in terms of nutrient loading is evident. Groundwater quality should be monitored and controlled as it has significant ecological impact to the Harbour. / published_or_final_version / Earth Sciences / Master / Master of Philosophy

Thesis (Ph. D.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), September 2003. / Includes bibliographical references.

published_or_final_version / Applied Geosciences / Master / Master of Science

[Truncated abstract] It has become widely apparent throughout the world that the discharge of nitrogen contaminated groundwater is reducing surface water quality of marine coastal waters, and is subsequently contributing to the decline of benthic habitats such as seagrasses. A process-based understanding of submarine groundwater discharge (SGD) has lagged behind these impacts, and this lack of understanding is addressed by this thesis. This thesis, of the spatial and temporal complexity of SGD, has uncovered and answered questions regarding the sources, fate and transport of SGD in a complex coastal discharge environment. Radium isotope techniques, groundwater biogeochemical investigations and HAMSOM surface water modelling have identified the magnitude, transport and fate of SGD in Cockburn Sound, a semienclosed embayment in Western Australia. A temporal periodicity that encompassed end-of-winter, early-summer, late-summer and mid-winter regimes of hydrology and oceanography, was employed in field studies that spanned the years 2003, 2004 and 2005. ... The fate of the groundwater in the semi-enclosed embayment was investigated using knowledge of surface water currents. Localised regions of high groundwater influence were identified in the surface waters of the embayment, and through the application of a 3-dimensional hydrodynamic model (HAMSOM) it was discovered that, despite similar total volume residence times, variation in the surface flow regime resulted in very different fates for groundwater discharged to the embayment. For three of the four investigated seasonal regimes, groundwater discharged at the shoreline was shown to be rapidly exported out of the embayment (within approximately 1-3 days). During mid-winter very different wind and current regimes existed, resulting in the lateral transport of shoreline groundwater across the embayment, presenting potential for nutrient recirculation within the system for longer time periods (10+ days). Lateral transport of groundwater during mid-winter from the limestone region of the coastline, may contribute to peaks in phytoplankton biomass that have been reported to occur at this time. The investigations into spatial, temporal and biogeochemical dynamics of SGD provided for further dissertation of the processes that affect these dynamics, at a scale that was relevant to marine embayments, coastal aquifers and the coastal ecosystem. It is hoped that this thesis will contribute to a better understanding of the inputs, dynamics and impacts of SGD on coastal ecosystems and lead to improved management strategies for coastal zones.

M.S.

Thesis (Ph. D.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), September 2003. / Includes bibliographical references. / Groundwater discharge into estuaries and the coastal ocean is an important mechanism for the transport of dissolved chemical species to coastal waters. Because many dissolved species are present in groundwater in concentrations that are orders of magnitude higher than typical river concentrations, groundwater-borne nutrients and pollutants can have a substantial impact on the chemistry and biology of estuaries and the coastal ocean. However, direct fluxes of groundwater into the coastal ocean (submarine groundwater discharge, or SGD) can be difficult to quantify. Geochemical tracers of groundwater discharge can reflect the cumulative SGD flux from numerous small, widely dispersed, and perhaps ephemeral sources such as springs, seeps, and diffuse discharge. The natural radiocarbon content (A14C) of dissolved inorganic carbon (DIC) was developed as a tracer of fresh, terrestrially driven fluxes from confined aquifers. This A14C method was tested during five sampling periods from November 1999 to April 2002 in two small estuaries in southeastern North Carolina. In coastal North Carolina, fresh water artesian discharge is characterized by a low A14C signature acquired from the carbonate aquifer rock. Mixing models were used to evaluate the inputs from potential sources of DIC-A'4C to each estuary, including seawater, springs, fresh water stream inputs, and salt marsh respiration DIC additions. These calculations showed that artesian discharge dominated the total fresh water input to these estuaries during nearly all sampling periods. / (cont.) These new A14C-based SGD estimates were compared with groundwater flux estimates derived from radium isotopes and from radon-222. It is clear that these tracers reflect different components of the total SGD. The fluxes of low-A14C and of 222Rn were dominated by artesian discharge. Estuarine 226Ra showed strong artesian influence, but also reflected the salt water SGD processes that controlled the other three radium isotopes. The flux of 228Ra seemed to reflect seepage from the terrestrial surficial aquifer as well as salt water recirculation through estuarine sediments. The fluxes of 224Ra and 223Ra were dominated by salt water recirculation through salt marsh sediments. This multi-tracer approach provides a comprehensive assessment of the various components contributing to the total SGD. / by Carolyn M. Gramling. / Ph.D.