Evaluating dual-domain models for upscaling multicomponent reactive transport in mine waste rock.

Reactive transport models have proven abilities to simulate the quantity and quality of drainage from mine waste rock. Tracer experiments indicate the presence of fast and slow flow regimes in many heterogeneous waste-rock piles. Although multidomain models have been developed specifically for systems with such distinctive hydrodynamics, there have been limited applications of multidomain reactive transport models to simulate composite drainage chemistries from waste-rock piles to date. This work evaluated the ability of dual-domain multicomponent reactive transport models (DDMRTMs) to reproduce breakthrough curves of conservative (chloride) and reactive (molybdenum) solutes observed at a well-characterized experimental waste-rock pile at the Antamina Mine, Peru. We found that the DDMRTM simulations quantitatively matched eight-year-long records of conservative transport through the waste-rock pile when parameterized mainly with field-measured properties obtained from the site and limited calibration. The DDMRTM model also provided a reasonable match to field observations of the reactive solute. The limited calibrated parameters are physically realistic, corroborating the ability of these multidomain models to reproduce the complex reactive-transport processes governing polluted rock drainage from large-scale waste-rock piles.

Reactive transport modelling to investigate multi-scale waste rock weathering processes.

Prediction of drainage quantity and quality is critical to reduce the environmental risks associated with weathering mine waste rock. Reactive transport models can be effective tools to understand and disentangle the processes underlying waste-rock weathering and drainage, but their validity and applicability can be impaired by poor parametrization and the non-uniqueness conundrum. Here, a process-based multicomponent reactive transport model is presented to interpret and quantify the processes affecting drainage quantity and quality from 15 waste- rock experiments from the Antamina mine, Peru. The deployed uniform flow formulation and consistent set of geochemical rate equations could be calibrated almost exclusively with measured bulk waste-rock properties in experiments ranging from 2 kg to 6500 tons in size. The quantitative agreement between simulated dynamics and the observed drainage records, for systems with a variety of rock lithologies and over a wide range of pH, supports the proposed selection of processes. The controls of important physicochemical processes and feedbacks such as secondary mineral precipitation, surface passivation, oxygen limitations, were confirmed through sensitivity analyses. Our work shows that reactive transport models with a consistent formulation and evidence-based parametrization can be used to explain waste-rock drainage dynamics across laboratory to field scales.

Scale dependence of effective geochemical rates in weathering mine waste rock.

Hydrogeochemical models for the prediction of drainage quality from full-scale mine waste-rock piles are often parameterized using data from small-scale laboratory or field experiments of short duration. Yet, many model parameters and processes (e.g., sulfide-oxidation rates) vary strongly with the spatiotemporal dimensions of the experiment: the "upscaling" of prediction models remains a critical challenge for mine-waste management worldwide. Here, we investigate scale dependence in laboratory and field experiments that spanned orders-of-magnitude in size (i.e. 2 kg to 100,000 kg) at the Antamina mine in Peru. Normalized drainage mass loading rates systematically decreased with increasing scale, irrespective of waste-rock type. A process-based reactive-transport model was used to simulate observed rates and reproduce the geochemical composition of drainage across scales. Long-term trends in drainage quality could be quantitatively reproduced when the model was parameterized with mostly scale- and experiment-specific measured bulk properties or literature values, leaving geochemical rate coefficients the sole calibrated model parameters. Analysis of these fitted parameters revealed that the scale dependence of geochemical rates was largely explained by reactive mineral surface area. This work demonstrates that practical drainage quality predictions for full-scale waste-rock piles can be established from readily available bulk parameters determined at multiple scales.

Assessment of the Sulfamethoxazole mobility in natural soils and of the risk of contamination of water resources at the catchment scale.

Sulfamethoxazole (SMX) is one of the antibiotics most commonly detected in aquatic and terrestrial environments and is still widely used, especially in low income countries. SMX is assumed to be highly mobile in soils due to its intrinsic molecular properties. Ten soils with contrasting properties and representative of the catchment soil types and land uses were collected throughout the watershed, which undergoes very rapid urban development. SMX displacement experiments were carried out in repacked columns of the 10 soils to explore SMX reactive transfer (mobility and reactivity) in order to assess the contamination risk of water resources in the context of the Bolivian Altiplano. Relevant sorption processes were identified by modelling (HYDRUS-1D) considering different sorption concepts. SMX mobility was best simulated when considering irreversible sorption as well as instantaneous and rate-limited reversible sorption, depending on the soil type. SMX mobility appeared lower in soils located upstream of the watershed (organic and acidic soils - Regosol) in relation with a higher adsorption capacity compared to the soils located downstream (lower organic carbon content - Cambisol). By combining soil column experiments and soil profiles description, this study suggests that SMX can be classified as a moderately to highly mobile compound in the studied watershed, depending principally on soil properties such as pH and OC. Potential risks of surface and groundwater pollution by SMX were thus identified in the lower part of the studied catchment, threatening Lake Titicaca water quality.