Carlos Ayora

Modeling of non-isothermal multi-component reactive transport in field scale porous media flow systems

Abstract A general 2-D finite element multi-component reactive transport code, TRANQUI, was developed, using a sequential iteration approach (SIA). It is well suited to deal with complex real-world thermo-hydro-geochemical problems for single-phase variably water saturated porous media flow systems. The model considers a wide range of hydrological and thermodynamic as well as chemical processes such as aqueous complexation, acid-base, redox, mineral dissolution/precipitation, gas dissolution/ex-solution, ion exchange and adsorption via surface complexation. Under unsaturated conditions only water flow is considered, although gas pressures are allowed to vary in space in a depth-dependent manner specified by the user. In addition to the fully iterative sequential approach (SIA), a sequential non-iterative approach (SNIA), in which transport and chemistry are de-coupled, was implemented and tested. The accuracy and numerical performance of SIA and SNIA have been compared using several test cases. The accuracy of SNIA depends on space and time discretization as well as on the nature of the chemical reactions. The capability of the code to model a real case study in the field is illustrated by its application to the modeling of the hydrochemical evolution of the Llobregat Delta aquitard in northeastern Spain over the last 3500 years during when fresh-water flow from a lower aquifer displaced the native saline aquitard waters. Manzano and Custodio carried out a reactive transport model of this case study by using the PHREEQM code and considering water flow, aqueous complexation, cation exchange and calcite dissolution. Their results compare favorably well with measured porewater chemical data, except for some of the cations. Our code is not only able to reproduce the results of previous numerical models, but leads to computed concentrations which are closer to measured data mainly because our model takes into consideration redox processes in addition to the processes mentioned above. A number of sensitivity runs were performed with TRANQUI in order to analyze the effect of errors and uncertainties on cation selectivities.

On the behavior of approaches to simulate reactive transport

Two families of approaches exist to simulate reactive transport in groundwater: The Direct Substitution Approach (DSA), based on Newton-Raphson and the Picard or Sequential Iteration Approach (SIA). We applied basic versions of both methods to several test cases and compared both computational demands and quality of the solution for varying grid size. Results showed that the behavior of the two approaches is sensitive to both grid size and chemistry. As a general rule, the DSA is more robust than the SIA, in the sense that its convergence is less sensitive to time step size (any approach will converge given a sufficiently small time step). Moreover, the DSA leads to a better simulation of sharp fronts, which can only be reproduced with fine grids after many iterations when the SIA is used. As a consequence, the DSA runs faster than SIA in chemically difficult cases (i.e., highly non-linear and/or very retarded), because the SIA may require very small time steps to converge. On the other hand, the size of the system of equations is much larger for the DSA than for the SIA, so that its CPU time and memory requirements tend to be less favorable with increasing grid size. As a result, the SIA may become faster than the DSA for very large, chemically simple problems. The use of an iterative linear solver for the DSA makes its CPU time less sensitive to grid size.

Removal of cadmium, copper, nickel, cobalt and mercury from water by Apatite II™: Column experiments

Apatite II™, a biogenic hydroxyapatite, was evaluated as a reactive material for heavy metal (Cd, Cu, Co, Ni and Hg) removal in passive treatments. Apatite II™ reacts with acid water by releasing phosphates that increase the pH up to 6.5-7.5, complexing and inducing metals to precipitate as metal phosphates. The evolution of the solution concentration of calcium, phosphate and metals together with SEM-EDS and XRD examinations were used to identify the retention mechanisms. SEM observation shows low-crystalline precipitate layers composed of P, O and M. Only in the case of Hg and Co were small amounts of crystalline phases detected. Solubility data values were used to predict the measured column experiment values and to support the removal process based on the dissolution of hydroxyapatite, the formation of metal-phosphate species in solution and the precipitation of metal phosphate. Cd(5)(PO(4))(3)OH(s), Cu(2)(PO(4))OH(s), Ni(3)(PO(4))(2)(s), Co(3)(PO(4))(2)8H(2)O(s) and Hg(3)(PO(4))(2)(s) are proposed as the possible mineral phases responsible for the removal processes. The results of the column experiments show that Apatite II™ is a suitable filling for permeable reactive barriers.

Formation of natural gypsum megacrystals in Naica, Mexico

Exploration in the Naica mine (Chihuahua, Mexico) recently unveiled several caves containing giant, faceted, and transparent single crystals of gypsum (CaSO4•2H2O) as long as 11 m. These large crystals form at very low supersaturation. The problem is to explain how proper geochemical conditions can be sustained for a long time without large fluctuations that would trigger substantial nucleation. Fluid inclusion analyses show that the crystals grew from low-salinity solutions at a temperature of ∼54 °C, slightly below the one at which the solubility of anhydrite equals that of gypsum. Sulfur and oxygen isotopic compositions of gypsum crystals are compatible with growth from solutions resulting from dissolution of anhydrite previously precipitated during late hydrothermal mineralization, suggesting that these megacrystals formed by a self-feeding mechanism driven by a solution-mediated, anhydrite-gypsum phase transition. Nucleation kinetics calculations based on laboratory data show that this mechanism can account for the formation of these giant crystals, yet only when operating within the very narrow range of temperature identified by our fluid inclusion study. These singular conditions create a mineral wonderland, a site of scientific interest, and an extraordinary phenomenon worthy of preservation.

X-ray microanalysis of fluid inclusions and its application to the geochemical modeling of evaporite basins

Direct access to brines trapped in halite is possible by freezing and breaking halite crystals on a cooling stage inside a scanning electron microscope. Quantitative analyses of major solutes (Na, Mg, K, Ca, Cl and SO4) in natural brines were obtained by means of X-ray microanalysis of the frozen fluid inclusions using frozen droplets of brines with known composition as standards. Precision errors were estimated to be lower than 10%. and detection limits vary from below 0.02 mol/L for Ca, K, and SO4 to 0.55 mol/L for Na. Analyses of several inclusions of sizes larger than 20 μm were performed for each cross section of halite. The composition of primary fluid inclusions in halites from a drill core in the Messinian basin of Lorca, SE Spain, were used to constrain the parameters of a geochemical model of the evolution of the basin. A computer program to simulate evaporation paths was developed using thermodynamic equilibrium and mass balance as constraints. The halite sequence was interpreted as resulting from the evaporation of initial seawater in a hydrologically open basin, with seawater replacing the evaporated water and a small fraction of basinal brine leaking out. In the middle of the sampled sequence, the basin was closed to the sea and only continental water flowed into the basin, reworking part of the halite deposited in the margins. In contrast with these results, only very poor information would have been obtained from the mineralogical record alone, consisting mainly of halite and minor gypsum.

The chemical and hydrological evolution of an ancient potash-forming evaporite basin as constrained by mineral sequence, fluid inclusion composition, and numerical simulation

The chemical evolution of the brine in a potash evaporite basin has been investigated by X-ray microanalysis of frozen primary inclusions trapped in halite. A Computer program based on thermodynamic equilibrium and mass balance principles has been used to simulate evaporation paths. The comparison between the results of calculations, the observed mineralogy and mineral sequence, and the solute content in fluid inclusions has placed constraints on the hydrological evolution of the basin. The upper Eocene basin of Navarra, southern Pyrenees, Spain, began as a marine basin, evolving from a moderate to a high degree of restriction, depositing first a basai anhydrite horizon, and then a thick sequence of massive halite. An additional inflow of CaCl2 in the basin during seawater evaporation is proposed as the process responsible for the sulfate depletion required for sylvite instead of Mg-sulfates to form. Mixing of seawater with continental waters, bacterial sulfate reduction and “in situ” dolomitization are discarded. The basin subsequently closed to the sea and evolved with decreasing volume. Alternating bands of clays-halite-sylvite and then clays-halite-carnallite were deposited under the influence of seasonal continental recharge. Before reaching total desiccation the residual brine was diluted by continental water. The basin then evolved under an endoreic regime, where continental recharge and the recycling of previously-formed halite led to deposition of alternating beds of clays and halite.

Formation of diclofenac and sulfamethoxazole reversible transformation products in aquifer material under denitrifying conditions: batch experiments.

Soil-aquifer processes have proven to work as a natural treatment for the attenuation of numerous contaminants during artificial recharge of groundwater. Nowadays, significant scientific effort is being devoted to understanding the fate of pharmaceuticals in subsurface environments, and to verify if such semipersistent organic micropollutants could also be efficiently removed from water. In this context we carried out a series of batch experiments involving aquifer material, selected drugs (initial concentration of 1 μg/L and 1 mg/L), and denitrifying conditions. Diclofenac and sulfamethoxazole exhibited an unreported and peculiar behavior. Their concentrations consistently dropped in the middle of the tests but recovered toward the end, which suggest a complex effect of denitrifying conditions on aromatic amines. The transformation products Nitro-Diclofenac and 4-Nitro-Sulfamethoxazole were detected in the biotic experiments, while nitrite was present in the water. Their concentrations developed almost opposite to those of their respective parent compounds. We conjecture that this temporal and reversible effect of denitrifying conditions on the studied aromatic amines could have significant environmental implications, and could explain at least partially the wide range of removals in subsurface environments reported in literature for DCF and SMX, as well as some apparent discrepancies on SMX behavior.

Sequential extraction and DXRD applicability to poorly crystalline Fe- and Al-phase characterization from an acid mine water passive remediation system

Abstract Iron and Al precipitates play very important hydrochemical and environmental roles in aquatic environments affected by acid mine drainage. Despite their great importance, reliable characterization of these precipitates is problematic due to the high proportion of amorphous or poorly ordered mineral phases comprising these precipitates and because of their coexistence with intermediate to highly crystalline phases. To facilitate and improve the characterization of poorly ordered Fe and Al phases, a coupled differential X-ray diffraction (DXRD) and sequential extraction (SE) study was performed on a set of samples from an acid mine water passive treatment system. The results of these techniques indicate the presence of schwertmannite and goethite in the upper 5 cm of the passive treatment reactive material. Furthermore, a progressive decrease of the SO42- adsorbed to the schwertmannite surface is suggested by one of the SE steps. The presence of hydrobasaluminite and amorphous Al(OH)3 is suggested on the basis of SE and thermodynamic modeling analysis. These techniques also allow a quantitative estimation of the proportion of each mineral present. As a result, a complete study of the distribution of each mineral throughout the reactive material profile and the role of each phase in removing metals from the mine water can be obtained. This information is useful, not only to improve the reactive material design, but also to understand the natural processes taking place in aquatic systems affected by mining.

Fate of β-blockers in aquifer material under nitrate reducing conditions: Batch experiments

The fate of the three environmentally relevant β-blockers atenolol, metoprolol and propranolol has been studied in batch experiments involving aquifer material and nitrate reducing conditions. Results from the about 90 d long tests indicate that abiotic processes, most likely sorption, jointly with biotransformation to atenololic acid were responsible for the 65% overall removal observed for atenolol. Zero order kinetics, typical of enzyme-limited reactions, controlled the transformation of this beta blocker to its corresponding carboxylic acid. The mass balance evidences that no mineralization of atenolol occurs in the biotic experiment and that atenololic acid is more stable than its parent compound under the studied conditions. This finding stresses the importance of considering atenololic acid as target compound in the environmental studies on the fate of atenolol. For metoprolol and propranolol the results from the experiment suggest a slower sorption to be the dominant removal process, which led to final decreases in concentrations of 25-30% and 40-45%, respectively. Overall, the removals observed in the experiments suggest that subsurface processes potentially constitute an alternative water treatment for the target beta-blockers, when compared to the removals reported for conventional wastewater treatment plants.

Microcosm experiments to control anaerobic redox conditions when studying the fate of organic micropollutants in aquifer material.

The natural processes occurring in subsurface environments have proven to effectively remove a number of organic pollutants from water. The predominant redox conditions revealed to be one of the controlling factors. However, in the case of organic micropollutants the knowledge on this potential redox-dependent behavior is still limited. Motivated by managed aquifer recharge practices microcosm experiments involving aquifer material, settings potentially feasible in field applications, and organic micropollutants at environmental concentrations were carried out. Different anaerobic redox conditions were promoted and sustained in each set of microcosms by adding adequate quantities of electron donors and acceptors. Whereas denitrification and sulfate-reducing conditions are easily achieved and maintained, Fe- and Mn-reduction are strongly constrained by the slower dissolution of the solid phases commonly present in aquifers. The thorough description and numerical modeling of the evolution of the experiments, including major and trace solutes and dissolution/precipitation of solid phases, have been proven necessary to the understanding of the processes and closing the mass balance. As an example of micropollutant results, the ubiquitous beta-blocker atenolol is completely removed in the experiments, the removal occurring faster under more advanced redox conditions. This suggests that aquifers constitute a potentially efficient alternative water treatment for atenolol, especially if adequate redox conditions are promoted during recharge and long enough residence times are ensured.