Claus Kohfahl

The impact of cemented layers and hardpans on oxygen diffusivity in mining waste heaps A field study of the Halsbrücke lead-zinc mine tailings (Germany).

This article reports fibre-optic oxygen measurements on a reactive mine waste heap located in the polymetallic sulphide mine district of Freiberg in south-eastern Germany. The heaped material consists of sulphide-bearing tailings from a processing plant of a lead-zinc mine. Mine waste material was deposited in the water phase after separation of mining ores in a flotation process. The tailing impoundment is partly covered with coarse sand and topsoil. Oxygen profiles were monitored during one year at eleven locations showing different physical and mineralogical compositions. At each location a borehole was drilled where the optic sensors were installed at 2-5 different depths. After installation the oxygen profiles were monitored seven times during one year from 2006-2007 and three to five oxygen profiles at each location were obtained. Oxygen measurements were accompanied by physical, chemical and mineralogical data of the tailing material. Additionally, a detailed mineralogical profile was analysed at a location representative for the central part of the heap, where the cemented layers show lateral continuity. Results showed that cemented layers have a significant influence on natural attenuation of the toxic As and Pb species owing to their capacity of water retention. The measured oxygen profiles are controlled by the zone of active pyrite weathering as well as by the higher water content in the cemented layers which reduces gaseous atmospheric oxygen supply. In contrast, gypsum bearing hardpans detected at three other locations have no detectable influence on oxygen profiles. Furthermore, the grain size distribution was proved to have a major effect on oxygen diffusivity due to its control on the water saturation. Temporal changes of the oxygen profiles were only observed at locations with coarse sediment material indicating also an important advective part of gas flux.

Numerical investigation of coupled density‐driven flow and hydrogeochemical processes below playas

Numerical modeling approaches with varying complexity were explored to investigate coupled groundwater flow and geochemical processes in saline basins. Long-term model simulations of a playa system gain insights into the complex feedback mechanisms between density driven flow and the spatio-temporal patterns of precipitating evaporites and evolving brines. Using a reactive multicomponent transport model approach the simulations reproduced, for the first time in a numerical study, the evaporite precipitation sequences frequently observed in saline basins (“bulls eyes”). Playa-specific flow, evapo-concentration and chemical divides were found to be the primary controls for the location of evaporites formed, and the resulting brine chemistry. Comparative simulations with the computationally far less demanding surrogate single-species transport models showed that these were still able to replicate the major flow patterns obtained by the more complex reactive transport simulations. However, the simulated degree of salinization was clearly lower than in reactive multicomponent transport simulations. For example, in the late stages of the simulations, when the brine becomes halite-saturated, the non-reactive simulation overestimated the solute mass by almost 20%. The simulations highlight the importance of the consideration of reactive transport processes for understanding and quantifying geochemical patterns, concentrations of individual dissolved solutes and evaporite evolution. This article is protected by copyright. All rights reserved.

The impact of hardpans and cemented layers on oxygen diffusivity in mining waste heaps: diffusion experiments and modelling studies.

This study reports column tests and modelling results to assess the impact of hardpans and cemented layers on oxygen supply in mine waste sediments. The analysed sediment samples were obtained from a low-sulphide and low-carbonate polymetallic mine waste tailings impoundment located in the Freiberg mining district in Germany. The three samples were characterised by different degrees and types of cementation. After physical and mineralogical properties of the samples had been determined, breakthrough curves of oxygen were measured in column studies at different degrees of water saturation, and the diffusivities were assessed using a numerical modelling approach. Results demonstrate that cemented layers and hardpans in undisturbed sediments associated with fine-grained material operate as preferential pathways for diffusive gas transport during rewetting, leading to higher oxygen diffusivities compared to disturbed sediments. Under air-dry conditions, the disturbed samples show higher diffusivities than the undisturbed sample, indicating clogging of the porosity by precipitation of secondary minerals such as trivalent Fe oxyhydroxides acting as a barrier and thereby decreasing the diffusivity of the undisturbed sample. In contrast to sediments without cementation, diffusion experiments of sediments with cemented layers used in this study yield similar tortuosities in spite of their different grain size distributions, pointing to the important role of these heterogeneities for gas diffusion.

Algae metabolism and organic carbon in sediments determining arsenic mobilisation in ground- and surface water. A field study in Doñana National Park, Spain.

A study has been performed to explore the origin, spatiotemporal behaviour and mobilisation mechanism of the elevated arsenic (As) concentrations found in ground water and drinking ponds of the Doñana National Park, Southern Spain. At a larger scale, 13 piezometers and surface water samples of about 50 artificial drinking ponds and freshwater lagoons throughout the National Park were collected and analysed for major ions, metals and trace elements. At a smaller scale, 5 locations were equipped with piezometers and groundwater was sampled up to 4 times for ambient parameters, major ions, metals, trace elements and iron (Fe) speciation. As was analysed for inorganic and organic speciation. Undisturbed sediment samples were analysed for physical parameters, mineralogy, geochemistry as well as As species. Sediment analyses yielded total As between 0.1 and 18 mg/kg and are not correlated with As concentration in water. Results of the surface- and groundwater sampling revealed elevated concentration of As up to 302 μg/L within a restricted area of the National Park. Results of groundwater sampling reveals strong correlation of As with Fe(2+) pointing to As mobilisation due to reductive dissolution of hydroferric oxides (HFO) in areas of locally elevated amounts of organic matter within the sediments. High As concentrations in surface water ponds are correlated with elevated alkalinity and pH attributed to algae metabolism, leading to As desorption from HFO. The algae metabolism is responsible for the presence of methylated arsenic species in surface water, in contrast to ground water in which only inorganic As species was found. Temporal variations in surface water and groundwater are also related to changes in pH and alkalinity as a result of enhanced algae metabolism in surface water or related to changes in the redox level in the case of groundwater.

Investigating the Impact of Climate Change on Groundwater Recharge Using a High Precision Meteo Lysimeter in a Dune Belt of the Doñana National Park

Although precise weighing lysimeters are mostly installed for agricultural purposes in crop-producing areas, these instruments are also useful in areas of ecological interest where soil and aquifer recharge-discharge processes need to be better understood. In September 2015 a high precision meteo-lysimeter was installed in a coastal dune of the Donana Natural Reserve to quantify the recharge in dune belts, and to estimate its dependence on regional climate trends. In addition to the lysimeter, also six soil moisture sensors (model CS650) and 2 meteorological stations were installed. The first results show recharge rates of natural precipitation ranging from 19 to 97%, depending on the meteorological conditions and the initial soil moisture. The lysimeter-measured rainfall generally exceeded the measurements of the tipping bucket. Also the contribution of the dew was detected by the lysimeter. A soil water model is currently being set up to simulate recharge, soil water movement, evaporation and percolation, as well as to quantify uncertainty and noise effects.