Understanding the fate of nitrates in hard-rock aquifers through an integrated isotopic, experimental and modelling approach

Nitrate pollution of groundwater through non-point sources of agricultural origin is one of the major threats for groundwater worldwide. The heterogeneous structure of hard-rock aquifers comprised of a) a weathered layer (the saprolite or regolith), overlying b) a fissured layer below which fissures density decreases with depth and, c) occurrence of tectonic fractures is crucial regarding the transfer and fate of nitrates. This has been revealed through geochemical studies conducted at different catchments of Brittany (Western France): a 27000 km² area of hard rock aquifers characterized by intensive agricultural activities. The difference of migration rate of both water and nitrates through the aquifer layer as well as occurrence of biogeochemical processes contribute to induce high spatial variability of nitrate concentration. Age determination through CFC analysis shows that groundwater residence time increases with depth but is homogenous for each aquifer compartment at regional scale. Similarities between sulphate and nitrogen cycles in groundwater imply a particular relevance of sulphate isotopes pattern to improve the understanding of nitrate behaviour, but here again the results are homogenous at regional scale. The shallow nitrates contaminated groundwater is typical of the highly weathered zone of the aquifer (saprolite or regolith). In this compartment, δ³⁴S-SO4, δ¹⁸O-SO4 and δ¹⁵N-NO3 show that sulphate and nitrate concentration are the result of a multiple mixing between atmospheric deposition, fertilizers application and soil cycling. Autotrophic and heterotrophic denitrifications have been evidenced in groundwater. Actually, heterotrophic process is restricted at shallow depth, mainly in bottomland (wetlands), whereas autotrophic process occurs within the fissured part of the aquifer provided that sulphide minerals are present. The two processes can be distinguished on the basis of stables isotopes of sulphates. Sustainability of denitrification and a better knowledge of its chemical impacts (leaching of sulphates and undesirable or toxics compounds) are crucial questions regarding groundwater management for drinking water purpose and implementation of European Groundwater Directive. Biogeochemical laboratory batch and 1D column experiments of denitrification were conducted and allowed establishing kinetic constants. Results, namely evolution of aqueous speciation of nitrogen and leached compounds, were simulated by taking into account the thermodynamic processes (free enthalpy of the redox reactions) and the biological phenomena (bacterial growth). But denitrification is not the only process inducing a variability of chemical composition. A NO3-free older groundwater was identified at deeper depths which is easily discriminated from present-day denitrified groundwater on the basis of the δ³⁴S and δ¹⁸O(SO4). Through mixing, this old component can contribute to significantly increase salinity of pumped groundwater.