『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