Nitrate enrichment of coastal waters under study

Eutrophication of the coastal marine environment is often attributed to the nutrients leaching from agricultural fields. Most of the nutrients presumably reach the sea via rivers and streams. However, nitrate also enters the sea through groundwater discharge from the shore face at many locations along the European coast.

Climate Change and Environment

Groundwater discharge is an important factor for determining the chemistry of the coastal ocean. This is mainly due to the fact that groundwater often contains dissolved nitrogen in various forms such as nitrate, from human sources. The underground flow of these chemicals into the ocean leads to the over-enrichment (eutrophication) of many coastal bays and waterways. The net flux of groundwater nitrate into the marine environment has been the subject of investigation and research through the NAME project - Nitrate from Aquifers and influence on the carbon cycling in Marine Ecosystems. It has been demonstrated that ground water containing nitrate enters the marine environment through a biologically active layer at the sea bottom. Data has been collected and modelling tools have been developed. This was done in order to provide an overview and a better understanding of the pattern of biogeochemical processes and ground water flow in the coastal zone. The geophysicists in the project developed the UMEP method, Under Water Multi Electrode Profiling, which is a fast method to measure the penetration zones of fresh water along the coastline. The method was used to produce a map of the underground resistivity at the sea bottom. Resistivity measurements are obtained by passing an electrical current into the ground using a pair of electrodes and measuring the resulting potential gradient within the subsurface using a second electrode pair. Increasing the spacing between the current/potential electrodes is referred as resistivity sounding and it is used to increase the depth of investigation. The results show apparent resistivities measured for seven depths of investigation levels. Furthermore the longer electrode spacing shows resistive anomalies along the coast. The anomalies identified on the resistive profiles obtained are still present after the inversion process, made by an automatic routine, corresponding to outflows of fresh water. Results may be presented in term of a depth map of the interface between the fresh water saturated sand and the tertiary clay or salt water saturated sand. Further research and development will exploit the existing outcomes leading to a better understanding of the pattern of biogeochemical processes and ground water flow in the coastal zone.