Nitrate from aquifers and influences on carbon cycling in marine ecosystems

A 3-dimensional regional hydrodynamic and eutrophication model was set up for the Ho Bay, Graadyb Estuary. The model was based on MIKE 3 FM, DHI's flexible mesh model system, extended with the ECOLab biogeochemical process module. The model was calibrated using level- and current data from a campaign in 2004, and the eutriphication model calibrated using Ribe County's monitoring data from 2001. The influx of nitrate bearing groundwater was estimated during WP 5, mainly covering the NE coast of Ho Bay. Simulations with and without influx of groundwater were made for the spring 2001 period. Comparison of the results showed that the influence of the nitrate bearing groundwater is relatively small, at most 0.5% on selected water quality parameters. The minor effect is explained by the relatively small contribution to the total nutrient input to the estuary and the relatively efficient tidal flushing of the estuary.

A 3D groundwater model was set up for the Sj?lborg Site based on the geological model developed in WP1. The groundwater model was calibrated automatically using the shuffled complex evolution algorithm (SCE) against observed hydraulic heads in wells installed as part of the NAME-project. A 3D transport model was developed based on the flow rates simulated in the flow model. The denitrification process was modelled using different redox zonations and degradation rates. Using this model estimates of the nitrate load to the sea within the model area were obtained. To upscale the results from the site to the entire catchments of Ho Bay areas similar to the site where there are no surface waters and groundwater will discharge directly to the sea were determined by analysis of topography. By multiplying these areas with the actual recharge, an estimate of the groundwater discharge to the bay can be calculated. These results have been used as input to the regional marine model developed in Workpackage 6.

UNDER WATER MULTI ELECTRODE PROFILING, so called UMEP, was used to produce a map of the underground resistivity at the sea bottom. Seven arrays were used and filtered to reach a data density of around one sounding every 5 metre. First results show apparent resistivities measured for seven depths of investigation levels. Results show that the longer electrode spacing ( 100m) shows resistive anomalies along the coast. The inversion process was a 1D inversion made by an automatic routine. The water depth and resistivity are known, the number of layers is 3 and the resistivity of the deeper layer is less than 20ohm. It has been shown that the resistivity of the clay as well as the resistivity of the salt-water saturated sand is less than this value. The results can be represented on inversed profiles. The anomalies identified on the apparent resistive profiles are still present on the inversion results and correspond to outflows of fresh water. The results could also be presented in term of depth map of the interface between the resistive ( fresh water saturated sand) and conductive layers ( tertiary clay or salt water saturated sand).

Current reactive transport models for surface sediments have been specifically designed for (impermeable) muddy environments. They are one-dimensional (1D), focused on diffusive transport and only implement mass conservation. In sandy sediments however, adjective processes take over, either induced by bottom topography, groundwater discharge or biological activity (bio-irrigation). These adjective flows have a clear 3D signature, and as a result, reactive transport models for permeable sediments require a multi-dimensional approach, which combines both momentum (pore water flow) and mass conservation (reactive transport). In the NAME project, we have developed a suite of 1D/2D/3D reactive transport models for permeable sediments. Process models for fluid flow and reactive transport in highly- permeable near-shore environments were improved and/or newly developed. To this end, we used multi-purpose modelling software that allows fast multidimensional model prototyping, i.e. the Chemical Engineering Module of FEMLAB® (www.comsol.com) in combination with MATLAB® (www.mathworks.com). Taking advantage of the unprecedented data management and visualisation capabilities built-in, this approach allows a far more economical and efficient way to develop reactive transport models, unreachable by conventional self-coded software. The model code of the 1D/2D/3D reactive transport models that were developed is made available to the scientific community and environmental agencies.

Various environments exist in the coastal zones and these comprise highly interrelated processes in terms of exchange and circulation of water and chemical substances. The monitoring and management of these environments involves cooperation between different partners and integration of monitoring methods. An integrated monitoring practice focussing on the coastal zone is the main output of this result. The main purpose is to bring the different scientific disciplines together and focus on the interaction between the environments in a comprehensive way. The integrated monitoring practice includes an overall strategy for the monitoring. Focus should be on nitrate-containing groundwater and the interaction with the marine environment. The subjects are followed up by examples and a toolbox describing the most useful methods.

Nitrogen cycling was evaluated in a coastal setting, at the interface of a nitrate containing aquifer and a coastal marine environment. Methods and procedures were developed and evaluated to quantify the discharge of freshwater and of nitrate into the marine environment and the biogeochemical processes that may modify the nitrate concentration. Various models were developed and tested to quantitatively describe the physical and chemical processes at the freshwater/seawater interface. The overall results indicate that there is a potential for significant discharge of groundwater nitrate into the marine environment. However, depending on the local biogeochemical conditions, a major part of the nitrate becomes reduced to free nitrogen during passage of the shore-face.

Exploitable results

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