GNOM · Groundwater-derived Nutrient and Organic Matter: alteration during transit through coastal sediment

Final Activity and Management Report Summary - GNOM (Groundwater-derived Nutrient and Organic Matter: alteration during transit through coastal sediment)

Coastal zones are the most productive and resource-rich areas on the planet. Nowadays approximately 40 % of the world's population live in coastal areas and the United Nations predict that by 2025 it will be 75 %. Such a growth in population will maintain the increasing pressure on our coastal resources, many of which are already experiencing significant stress-related conditions and decline. Environmental pressure, such as agricultural, industrial, and residential development, leading to the modification of those vital coastal ecosystems need to be predicted on the global scale. To do so, all relevant sources of nutrients, organic materials as well as synthetic pollutant should be assessed specifically in shallow coastal aquifers. Large uncertainties regarding subsurface nutrient and organic loading exist because shallow groundwater nutrient and organic matter concentrations may be altered substantially during transit through coastal sediments. The introduction of groundwater into the coastal subsurface zone hence promotes biogeochemical processes capable of reducing the groundwater derived nutrient and organic load to the coastal ocean. Estimating groundwater nutrient and organic loading rates is commonly done using data from points of discharge into coastal waters. Here, we used a different method, kinetic analyses with flow-through reactors, to study the ability of sediment processes to reduce or transform groundwater-derived nutrient loading. Two costal marshes, with similar geology and vegetation, were selected. The main difference between the two sites was their human pressure: a residential area showed high impact and protected area was chosen as pristine reference site. We were able to demonstrate that sediment exposed for several decades to groundwater with human impact was less efficient in the degradation of dissolved organic matter compared to the reference site. As a consequence the role of coastal marsh as filtering zones will become less and less efficient with increasing pollutant loads. The ultimate consequence is that the nearby coastal waters will receive higher organic carbon inputs. In a follow-up study we verified the concept on lake sediments with contrasting sedimentation of organic matter, and nutrient concentration. Indeed, recent studies showed that substantial amounts of methane are released from run-of-the-river reservoirs in Switzerland. The use of PTR reveals that high concentration of methane and nitrous oxide are produce in laboratory conditions, when mimicking special in situ condition. In low nitrate concentration, high concentration of methane is released, while in high nitrate concentration, high concentration of N2O is released. The two studied lakes sediment are sources of greenhouse gas (CH4 or N2O). The fluxes depend on the quality of the overlying water studied, that is high or low nitrate concentration in this study case.

Data: CORDIS, © European Union