Sergio Bea

CHEPROO: A Fortran 90 object-oriented module to solve chemical processes in Earth Science models

Accurate prediction of contaminant migration in surface and ground water bodies, including interaction with aquifer and hyporheic zone materials requires reactive transport modeling. The increasing complexity and the procedure-oriented type of programming often used in reactive transport hinder codes reuse and transportability. We present a Fortran 90 module using object-oriented concepts that simulates complex hydrobiogeochemical processes (CHEPROO, CHEmical PRocesses Object-Oriented). CHEPROO consists of a general structure with two classes. The Nodal Chemistry class accounts for the description of local chemistry and geochemical state variables. As such, it provides many functions related to basic operations (evaporation, mixing, etc.) and can easily grow on this direction (extreme dry conditions, biochemical state variables, etc.). The Chemical System class includes kinetic and thermodynamic models that describe reactions between and within phases. As such, it can grow in the direction of increasingly complex chemical systems (solid solutions, microorganisms as individual phases, etc.), without loss in the handling of simple problems. These two classes are overlaid by CHEPROO, a general structure designed for interaction with other codes. CHEPROO can be used as a geochemical tool for the modeling of complex processes such as biodegradation or evaporation at high salinities. However, many functions CHEPROO are devoted to coupling a broad range of chemical processes to other phenomena (flow, transport, mechanical). We have shown that reactive transport (based on either DSA or SIA approaches) could be easily implemented into existing conservative transport code with a minimal number of changes.

Geochemical and environmental controls on the genesis of soluble efflorescent salts in Coastal Mine Tailings Deposits: a discussion based on reactive transport modeling.

Water-soluble efflorescent salts often form on tailings in hyperarid climates. Their high solubility together with the high risk of human exposure to heavy metals such as Cu, Ni, Zn, etc., makes this occurrence a serious environmental problem. Understanding their formation (genesis) is therefore key to designing prevention and remediation strategies. A significant amount of these efflorescences has been described on the coastal area of Chañaral (Chile). There, highly soluble salts such as halite (NaCl) and eriochalcite (CuCl(2).2H(2)O) form on 4km(2) of marine shore tailings. Natural occurrence of eriochalcite is rare: its formation requires extreme environmental and geochemical conditions such as high evaporation rate and low relative air humidity, and continuous Cl and Cu supply from groundwater, etc. Its formation was examined by means of reactive transport modeling. A scenario is proposed involving sea water and subsequently a mixture of sea water/freshwater in the groundwater composition in the formation of these efflorescences. The strong competition from other halides (i.e. halite and silvite (KCl)) for the Cl may inhibit the precipitation of eriochalcite. Therefore, the Cl/Na ratio trend >1 is a key parameter in its formation. Cation-exchange between Na(+) and other major ions such as K(+), Ca(2+), Mg(2+) and Cu(2+) in the clay fraction of tailings is proposed to account for realistic Cl/Na ratios. With regard to preventing the formation of eriochalcite, a capillary barrier on the tailings surface is proposed as a suitable alternative. Its efficiency as a barrier is also tested by means of reactive transport models.

Modeling of concentrated aqueous solutions: Efficient implementation of Pitzer equations in geochemical and reactive transport models

Modeling concentrated solutions demands the use of ion-interaction models such as Pitzer equations, which involve a large number of operations. Implementation of these models in large reactive transport simulations significantly increases the computation time with respect to traditional activity coefficient models. CPU time depends on the efficiency of (1) the Pitzer algorithm itself, and (2) the speciation algorithm. We present an implementation of the Pitzer model that improves traditional implementations by using a compact matrix approach. This facilitates programming and computation of derivatives. The use of analytic derivatives allows the use of Newton-Raphson algorithms, which converge quickly. The approach is implemented in an object-oriented programming (OOP) scheme by creating an entity that represents the thermodynamic behavior of both dilute and concentrated solutions. This entity is readily linked to any geochemical or reactive transport codes. We show that the code is robust, in that its implementation improves the convergence in a broad range of geochemical calculations, and efficient, in that its CPU time compares favorably with other codes.

Simulation of remediation alternatives for a 137Cs contaminated soil

Summary We analyze remediation alternatives for a soil contaminated with 137Cs, which sorbs strongly to clay aggregates where water flux is negligible. The mobile portion of the soil (macropores) retains little water and cesium. Some of the remediation alternatives involve infiltration of seawater enriched with KCl, to promote mobilization of Cs through exchange with K. Therefore, a fully coupled reactive transport model is used to test these alternatives. We conclude that flushing is a viable alternative, provided that some recommendations, derived from the modelling exercise are followed. These include high rate periodic infiltration and draining, as well as performing infiltration from independent cells to limit the effect of preferential flowpaths.

Evaluation of the Potential for Dissolved Oxygen Ingress into Deep Sedimentary Basins during a Glaciation Event

Geochemical conditions in intracratonic sedimentary basins are currently reducing, even at relatively shallow depths. However, during glaciation-deglaciation events, glacial meltwater production may result in enhanced recharge (Bea et al., 2011; and Bea et al., 2016) potentially having high concentrations of dissolved oxygen (O2). In this study, the reactive transport code Par-MIN3P-THCm was used to perform an informed, illustrative set of simulations assessing the depth of penetration of low salinity, O2-rich, subglacial recharge. Simulation results indicate that the large-scale basin hydrostratigraphy, in combination with the presence of dense brines at depth, results in low groundwater velocities during glacial meltwater infiltration, restricting the vertical ingress of dilute recharge waters. Furthermore, several geochemical attenuation mechanisms exist for O2, which is consumed by reactions with reduced mineral phases and solid organic matter (SOM). The modeling showed that effective oxidative mineral dissolution rates and SOM oxidation rates between 5 × 10−15 and 6 × 10−13 mol dm−3 bulk s−1 were sufficient to restrict the depth of O2 ingress to less than 200 m. These effective rates are low and thus conservative, in comparison to rates reported in the literature. Additional simulations with more realistic, yet still conservative, parameters reaffirm the limited ability for O2 to penetrate into sedimentary basin rocks during a glaciation-deglaciation event.