Département Ressources aquatiques et eau potable

An index aquifer vulnerability study from western Canada (left) compared with a new logistic regression map of these vulnerability index values on the online GAP platform (right). The red colour shows areas with the highest vulnerability. The green areas are less vulnerable or adequately protected from surface contamination.

Electron transfer to ferric iron in (oxyhydr-)oxides (hereafter iron oxides) is a critical step in many processes that are central to the biogeochemical cycling of elements and to pollutant dynamics. Understanding these processes requires analytical approaches that allow for characterizing the reactivity of iron oxides toward reduction under controlled thermodynamic boundary conditions. Here, we used mediated electrochemical reduction (MER) to follow changes in iron oxide reduction extents and rates during abiotic ferrous iron-induced transformation of six-line ferrihydrite. Transformation experiments (10 mM ferrihydrite-Fe^III) were conducted over a range of solution conditions (pH_trans = 6.50 to 7.50 at 5 mM Fe²⁺ and for pH_trans = 7.00 also at 1 mM Fe²⁺) that resulted in the transformation of ferrihydrite into thermodynamically more stable goethite or magnetite. The changes in iron oxide mineralogy during the transformations were quantified using X-ray diffraction analysis. MER measurements on iron oxide suspension aliquots collected during the transformations were performed over a range of pH_MER at constant applied reduction potential. The extents and rates of iron oxide reduction in MER decreased with decreasing reaction driving force resulting from both increasing pH_MER and increasing transformation of ferrihydrite into thermodynamically more stable iron oxides. We show that the decreases in iron oxide reduction extents and rates during ferrihydrite transformations can be linked to the concurrent changes in iron oxide mineralogy.

Relatively small fluctuations in the surface energy balance and evapotranspiration in semiarid and arid regions can be indicative of significant changes to ecosystem health. Therefore, it is imperative to have approaches for monitoring surface fluxes in these regions. The remote sensing-based two-source energy balance (TSEB) model is a suitable method for flux estimation over sparsely vegetated semiarid and arid landscapes since it explicitly considers surface energy flux contributions from soil and vegetation. However, previous studies indicate that TSEB generally underestimates sensible heat flux (H) and hence overestimates latent heat flux (LE) or evapotranspiration for these regions unless soil resistance coefficients are modified based on additional ground information. In this study, TSEB is applied over semiarid and arid regions on three continents using the original soil resistance formulation with modified coefficients and a recently developed physically based soil resistance formulation. Model sensitivity analysis demonstrates the high sensitivity of TSEB with original soil resistance formulation to soil resistance coefficients, while TSEB with the new soil resistance formulation has relatively low sensitivity to uncertainties in all coefficients. The performance of TSEB using different soil resistance formulations is evaluated by comparing modeled H against eddy covariance measurements in six semiarid and arid study sites and ranking the error statistics. Our results indicate that incorporating the new soil resistance formulation into TSEB would enhance its utility in flux estimation over heterogeneous landscapes by obviating its reliance on semiempirical coefficients and thus provide more robust fluxes over sparsely vegetated regions without model calibration and/or parameter tuning.