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Coastal Groundwater Flow and Associated Nutrient Transport into the Sea

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Yusuke UCHIYAMAABSTRACT1. INTRODUCTION2. NUMERICAL PROCEDURE3. COMPUTATION 1: FORMATION OF LOCAL CIRCULATION CELL4. COMPUTATION 2: NUTRIENT FLUXES INTO THE SEA BY SGWD5. CONCLUSIONACKNOWLEDGMENTSREFERENCESCoastal Groundwater Flow and Associated Nutrient Transport into the Sea Yusuke UCHIYAMA Hasaki Oceanographical Research Station, Port and Harbour Research Institute, Ministry of Transport 3-1-1 Nagase, Yokosuka 239-0826, Japan ABSTRACT A numerical model is developed to examine coastal groundwater processes in a sandy beach, including the effects of tidal fluctuations, saltwater intrusion into an aquifer, and dynamics in unsaturated layers. The simulation results indicate: 1) ‘Local circulation’ is clearly formed in the aquifer near the shoreline owing mainly to tidal oscillations. 2) The circulation induces a landward bending profile of salinity, wherein saline seawater is present. 3) As land-derived freshwater discharge increases, the circulation cell decreases in size and finally almost disappears. Subsequently, the model is linked with the nutrient data sampled in the aquifer of the sandy beach, Hasaki, Japan, for estimating nutrient transport into the sea. The nutrient flux via groundwater seepage is considered to have a minor component of marine nutrient budget in the surf zone; river discharge dominates at this beach. Key Words: groundwater, local circulation cell, numerical simulation, and nutrient flux 1. INTRODUCTION Groundwater flow in beaches and associated material transport plays a significant role in sediment transport mechanisms at the foreshore, coastline stability, the design of most coastal structures, and marine ecosystems of coastal flora and fauna. Since it is generally impossible to directly measure complicated velocity fields in coastal aquifers, a numerical model describing the groundwater dynamics is strongly expected to be of great advantage to the prediction of detailed groundwater flow. During the ebb tide the beach water table may be decoupled from the ocean, resulting in the formation of a seepage face (i.e., the beach face between the exit point of the water table and the shoreline) where groundwater outcrops on the intertidal profile [e.g., Nielsen, 1990; Turner, 1993]. The groundwater flow field in the intertidal zone is important for evaluating material transport due to submarine groundwater discharge (SGWD) as shown in Figure 1 [e.g., Johannes, 1980]. Since unsaturated layers are attributed to the pressure potential distribution near the groundwater table, including the intertidal zone, the numerical model should be based on the Richards equation for saturated-unsaturated flow [Richards, 1931]. In addition, density effects induced by intrusion of saline seawater are not negligible in analyzing flow in a coastal aquifer. Pinder and Cooper [1970] demonstrated the movement of the saltwater front in confined coastal aquifers by using a numerical model. Their model used a steady groundwater flow equation and a time-dependant advection-dispersion equation for salinity. Segol et al. [1975] employed this model for a computation of steady flow in unconfined aquifers. Others incorporated Richards equation for unsteady flow [e.g., Kohno et al., 1983]. Although the previous works have treated transient flow related to surface recharge and drainage in coastal aquifers, unsteady flow due to the fluctuating tides and waves has not been simulated using this framework. Li et al. [1997a] recently presented a Boundary Element Method model solving the Laplace equation for saturated flow in an unconfined coastal aquifer. They incorporated the capillary effects derived from the approximate solution of the one-dimensional Richards equation [Parlange and Brutsaert, 1987] into the model and demonstrated beach water table fluctuations due to tides and waves [Li et al., 1997a, 1997b]. In their model, however, spatial distribution of hydraulic conductivity could not be included. Additionally, they neglected the effect of density-driven flow caused by the presence of salt water. Coastal marine ecosystems are affected by dissolved nutrient inputs from circulating offshore water, river runoff, and groundwater seepage. The regional nutrient budget also includes atmospheric deposition, fertilizer application, wastewater treatment plant discharge, livestock waste, and decomposition of organic matter in sediment. Among these sources, the effects of groundwater on marine environments are not as well known as those of river water and offshore water. Groundwater containing inorganic nutrients can have a significant influence on coastal ecosystems, especially when nutrient concentrations are high or the relative contribution of submarine groundwater discharge (SGWD) is large. Due to nutrient leaching from surface-applied fertilizers, groundwater usually has a higher concentration of inorganic nutrients than does seawater. Therefore, even low rates of SGWD often needs to be accounted for in the nutrient budget for a coastal ecosystem [e.g., Rölke et al., 1998]. Fresh groundwater flows out to the sea through a narrow seepage face as shown in Figure 1. Thevelocity field in a coastal aquifer appears to be complicated by the influence of the saltwater wedge and sea-level variances. Previous studies have mostly not taken into account the spatially variable flow-field. Since it is extremely difficult to measure the flow in the aquifer directly, piezometers are often used to obtain data on the head field from which the flow patterns can be inferred [e.g., Neilsen and Dunn, 1998]. On the other hand, the Darcy’s Law and Ghyben-Herzberg relation have been used to calculate the velocity approximately [Raghunath, 1982; McLachlan and Illenberger, 1986]. Previous estimates of SGWD have generally yielded worldwide figures in the range 0.01 to 10% of total discharge into the sea [e.g., Church, 1996; Johannes, 1980]. Nevertheless, Moore [1996] and Church [1996] inferred from 226Ra measurements in coastal waters that SGWD into the South Atlantic Bight might amount to a flux equivalent to 40% of the total flow entering the sea from adjacent rivers. It is unsure whether the figure of 40% is precise, however, the uncomplicated calculations might lead to misestimates of SGWD as discussed by Younger [1996]. In the study reported below, a numerical model is developed to examine the coastal groundwater flows in sandy beaches, based on the Richards


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