Discharge of nitrate-containing groundwater into a coastal marine environment

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 coastal 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.