Massive chemical reactions are not expected when injecting CO(2) in siliceous sandstone reservoirs, but their performance can be challenged by small-scale reactions and other processes affecting their transport properties. We have conducted a core flooding test with a quartzarenite plug of Lower Cretaceous age representative of the secondary reservoir of the Hontomín test site. The sample, confined at high pressure, was successively injected with DIW and CO(2)-saturated DIW for 49 days while monitoring geophysical, chemical, and hydrodynamic parameters. The plug experienced little change, without evidence of secondary carbonation. However, permeability increased by a factor of 4 (0.022-0.085 mD), and the V(P)/V(S) ratio, whose change is related with microcracking, rose from ~1.68 to ~1.8. Porosity also increased (7.33-8.1%) from the beginning to the end of the experiment. Fluid/rock reactions were modeled with PHREEQC-2, and they are dominated by the dissolution of Mg-calcite. Mass balances show that ~4% of the initial carbonate was consumed. The results suggest that mineral dissolution and microcracking may have acted in a synergistic way at the beginning of the acidic flooding. However, dissolution processes concentrated in pore throats can better explain the permeability enhancement observed over longer periods of time.

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The present research is concerned with some numerical developments and practical application of a physically based numerical model FreshWaterSheds that incorporates a finite element solution to the steady/transient problems of the joint ground/surface fresh/salt water flows in inland and coastal regulated watersheds. The proposed model considers surface and groundwater interactions to be 2-D horizontally distributed and depth-averaged through a diffusive wave approach. Infiltration rates, overland flows and evapotranspiration processes are considered by diffuse discharge from surface water, unsaturated subsoil and groundwater table. New improvements also allow for the management of surface water flow control through the capacity of diversion on flooding zones of catchment areas, as well as on river beds, spillways and outflow operations of floodgates in weirs and dams of reservoirs. Practical application regards the flooding hazard of Aznalcóllar toxic spillages. This flooding disaster was caused by the sequential ruptures of the dikes of two mining residual reservoirs of a pyrite mine, releasing about 10•10 6 m 3 of contaminated wastewater and mining sludge onto the Guadiamar River. The numerical model was adapted to the wastewater and sludge properties of both sudden spillages, as well as to the river bed, the flooded zones and the underneath alluvial aquifer. The model simulation and calibration were made during the date of this hydrological hazard to the likely discharges and dual hydrograph produced by the sudden twofold failure of both reservoirs.

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