Andersen,M.S., Baron,L., Gudbjerg,J., Gregersen,J., Chapellier,D., Jakobsen,R. and Postma,D.(2007): Discharge of nitrate-containing groundwater into a coastal marine environment. Journal of Hydrology, 336, 98-114.

『沿岸海洋環境への硝酸塩を含む地下水の流量』


Abstract
 The discharge of nitrate-containing fresh groundwater from a sandy coastal aquifer and into the adjacent shallow marine environment was investigated near Esbjerg at the northern end of the Wadden Sea in Denmark. The geological structure of the coastal site was determined from drilling data and geoelectrical methods such as multi-electrode profiling (MEP). The distribution of shoreface sediment containing freshwater was mapped using MEP, underwater multi-electrode profiling (UMEP) and the electrical conductivity measured on pore waters. Freshwater discharge fluxes were measured using seepage meters and estimated from observed head gradients and measured hydraulic conductivities in the aquifer sediments. The nitrate distribution in the coastal sediments was obtained from water samples. A groundwater flow model was established to quantify the groundwater flow and travel times, and the distribution of freshwater discharge along the coastline. Results show that on the watershed scale the fresh groundwater discharge is controlled by sand filled buried channels that connect the aquifer with the shoreface sediments. The freshwater discharge along the coastline is at this scale probably best estimated by the groundwater flow model. However, at a more detailed scale the distribution of freshwater discharge in the intertidal zone is controlled by small scale geological heterogeneity and models are unable to predict what happens at a small scale on the beach and offshore. For that purpose UMEP, seepage meters, pore water sampling and local hydraulic gradients are more useful. These measurements indicate that the freshwater discharge occurs in distinct zones and that the highest discharge is near the high tide line, decreasing rapidly in a seaward direction. Nitrate is abundant in the shallow groundwater of the coast aquifer and is also present in the discharging fresh groundwater at certain patches along the coast. However, on average very little nitrate is observed in the freshwater discharging at the coast. The maximum travel time of groundwater through the aquifer until it discharges at the coast is 100 years and since nitrate leaching from soils has only taken place during the last 40-50 years, part of the fresh groundwater discharging at the coast must be free of nitrate.

Keywords: Submarine ground water discharge; Geophysics; Nitrate; Coastal zone; Modeling』

Introduction
Materials and methods
 Geophysical surveying
 Pore water sampling and hydrogeological surveys
 Numerical modeling of groundwater flow, travel times and nitrate leaching
Results
 Distribution of fresh- and saline groundwater at the coast mapped by geophysics
 Pore water resistivity of the intertidal zone sediments
 Hydraulic conductivity
 Hydraulic head distribution
 Submarine fresh groundwater discharge (SFGD)
 Distribution of nitrate in the discharging groundwater
Discussion
 Hydrogeology of the coastal aquifer
 Spatial distribution of submarine fresh groundwater
 Fluxes and distribution of freshwater discharge
 Nitrate transport to the marine zone
Conclusions
Acknowledgements
Appendix A. Supplementary data
References


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