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Acid rock drainage (ARD) is a major environmental problem that poses serious ecological risks during and after mining activities. To minimize the ecological risks and to lower remediation costs in the mining industry, ecological risk assessment is highly important in various phases of mining. In this study, a methodology for ecological risk assessment using probability bounds is presented. The methodology is demonstrated with a case study at a mine site. A fugacity-based model was employed to conduct the exposure characterization. Median lethal concentrations from toxicity studies were used to derive predicted no-effect concentrations (PNEC) and to characterize the effect of copper and zinc on the receptors. Probabilistic risk-quotient and overlaps between distributions of exposure and effect were employed to characterize risk. Data and parameter uncertainties in exposure, effect, and risk characterizations were propagated and quantified using a probability bounds approach. The exposure modeling results showed that the predicted concentrations of copper and zinc slightly exceeded the observed concentrations. The results of the effect characterization showed that the derived effect concentrations for copper and zinc are acceptable compared with guideline values. The risk characterization result indicated that a high probability of ecological risk may exist due to metals that are transported into a nearby lake. Moreover, the results showed that the methodology handles uncertainties due to imprecision and randomness in an integrated manner.

Summary The landuse/cover changes in the Ethiopian highlands have significantly increased the variability of runoff and sediment fluxes of the Blue Nile River during the last few decades. The objectives of this study were (i) to understand the long-term variations of runoff and sediment fluxes using statistical models, (ii) to interpret and corroborate the statistical results using a physically-based hydrological model, Soil and Water Assessment Tool (SWAT), and (iii) to validate the interpretation of SWAT results by assessing changes of landuse maps. Firstly, Mann–Kendall and Pettitt tests were used to test the trends of Blue Nile flow (1970–2009) and sediment load (1980–2009) at the outlet of the Upper Blue Nile basin at El Diem station. These tests showed statistically significant increasing trends of annual stream flow, wet season stream flow and sediment load at 5% confidence level. The dry season flow showed a significant decrease in the trend. However, during the same period the annual rainfall over the basin showed no significant trends. The results of the statistical tests were sensitive to the time domain. Secondly, the SWAT model was used to simulate the runoff and sediment fluxes in the early 1970s and at the end of the time series in 2000s in order to interpret the physical causes of the trends and corroborate the statistical results. A comparison of model parameter values between the 1970s and 2000s shows significant change, which could explain catchment response changes over the 28 years of record. Thirdly, a comparison of landuse maps of 1970s against 2000s shows conversion of vegetation cover into agriculture and grass lands over wide areas of the Upper Blue Nile basin. The combined results of the statistical tests, the SWAT model, and landuse change detection are consistent with the hypothesis that landuse change has caused a significant change of runoff and sediment load from the Upper Blue Nile during the last four decades. This is an important finding to inform optimal water resources management in the basin, both upstream in the Ethiopian highlands, and further downstream in the plains of Sudan and Egypt.

This paper presents the results of a study that compares the sediment routing of the Simiyu River using the hydrologic model, Soil and Water Assessment Tool (SWAT) and the 1D hydrodynamic simulation software for Rivers and Estuaries (SOBEK-RE) model. Routing in SWAT is completed using the simplified Bagnold’s equation and in the SOBEK-RE model is undertaken using the Saint Venant equation. The upstream boundary conditions for the routing modules were derived from the subcatchments sediment yields that were estimated by SWAT using the Modified Universal Soil Loss Equation (MUSLE). The sediment loads extrapolated or interpolated from the sediment rating curve for the catchment outlet were used for calibration and validation purposes. The SWAT model predicted an erosion rate of 2.09 Mt/yr. The total sediment load transported to the main outlet of the catchment simulated by the SWAT and SOBEK-RE models was equal to 2.94 and 2.72 Mt/yr, respectively. Thus the models computed a net erosion in the channels of 0.84 Mt/yr (SWAT) and 0.63 Mt/yr (SOBEK-RE). When comparing the results of the models for the different reaches of the main channel and main tributaries, the models showed different results both in magnitude and in sign (erosion/deposition). However, in a situation where data is scarce (such as grain size, channel geometry), the more complex hydrodynamic model does not necessarily lead to more reliable results.

Computer models assist basin-scale decision making by taking into account upstream-downstream interdependencies. The SWAT (hydrological) model code was developed into an OpenMI-compliant version and linked with the SOBEK (hydrodynamic) model to extend SWAT's simulations of basin-scale streamflow and sediment transport. The development of an OpenMI-compliant version of SWAT involved reorganizing the SWAT model code and wrapping it with the OpenMI wrapper utility. The modified SWAT model was linked to the SOBEK model and applied to simulate sediment transport in the Blue Nile River basin. The SWAT model simulated the streamflow and soil erosion in the upstream catchment, while the SOBEK model routed the streamflow and sediment downstream to the basin outlet. Prior to the linking, both the SWAT and SOBEK models were individually calibrated. The results showed that the coupled models simulated the observed hydrodynamics and sediment deposition due to backwater effects, which would not be possible with the SWAT model alone. The developed OpenMI-compliant SWAT model can further be linked to groundwater, climate change, and socioeconomic models to address integrated water resources management needs.

Waterlogging is a challenge to wheat (Triticum durum Desf.) productivity under the rainfed system on Vertisols in the Ethiopian Highlands. However using suitable seedbed types and manipulating planting dates can minimize the effects. A four-year (2000–2003) field experiment was conducted to evaluate three seedbed types broad bed and furrows (BBF) with early planting the traditional ridge and furrows (RF) and its modified version (wide ridge and furrows (WRF)) both under early and late planting in terms of water productivity of wheat. The FAO AquaCrop model was used to estimate crop water requirements. The result indicated that late planting on WRF or RF significantly (p

Acid rock drainage (ARD) is a major environmental problem that poses significant environmental risks during and after mining activities. A new methodology for environmental risk assessment based on probability bounds and a geochemical speciation model (PHREEQC) is presented. The methodology provides conservative and non-conservative ways of estimating risk of heavy metals posed to selected endpoints probabilistically, while propagating data and parameter uncertainties throughout the risk assessment steps. The methodology is demonstrated at a minesite located in British Columbia, Canada. The result of the methodology for the case study minesite shows the fate-and-transport of heavy metals is well simulated in the mine environment. In addition, the results of risk characterization for the case study show that there is risk due to transport of heavy metals into the environment.