Background.Acid mine drainage (AMD) is a major environmental impact associated with the mining industry. Elevated acidic conditions resulting from the discharge of AMD into the surrounding environment can cause heavy metals to dissolve and transport through water streams and accumulate in the aquatic environment, posing a risk to the health of living organisms. There have been several novel approaches in the remediation of AMD involving passive treatment techniques. The constructed treatment wetland approach is a passive remediation option that has proven to be a cost effective and long-lasting solution in abating toxic pollutant concentrations.Objectives.The present study investigates the applicability of water hyacinth (Eichhornia crassipes), a tropical aquatic plant with reported heavy metal hyper-accumulation in microcosm floating wetland treatment systems designed to remediate AMD with copper (Cu) and cadmium (Cd) concentrations exceeding threshold limits.Methods.Twelve water hyacinth samples were prepared with varying concentrations of Cu (1 mg/L, 2 mg/L, 4 mg/L) and Cd (0.005 mg/L, 0.01 mg/L, 0.02 mg/L). Water samples of 5 ml each were collected from each sample at 24-hour intervals for analysis with an atomic absorption spectrometer.Results.Plant growth varied according to Cu and Cd concentrations and no plants survived for more than 14 days. There was a significant discrepancy in the rate at which the Cd concentrations abated. The rate of reduction was rapid for higher concentrations and after 24 hours a substantial reduction was achieved. There was a reduction in Cu concentration after the first 24-hour period, and after the next 24-hour period the concentrations were again elevated in the samples at initial concentrations of 2 mg/L and A4 mg/L. 4 mg/L Cu concentration was shown to be toxic to the plants, as they had low accumulations and rapid dying was evident.Conclusions.Water hyacinth has the capability to reduce both Cu and Cd concentrations, except at an initial concentration of 4 mg/L of Cu, which was toxic to the plants.Competing Interests.The authors declare no competing financial interests.

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Water-retaining earth structures often experience cyclic hydraulic flow patterns, which are believed to alter the soil fabric. Gap-graded soils are identified to be internally unstable. These soils exhibit a change in their hydraulic conductivity when the critical hydraulic gradient is exceeded. The changes beyond the critical hydraulic gradient, however, are yet to be understood. This research study examines the unsteady nature of clogging and unclogging process in saturated gap-graded soils subjected to hydraulic pulses. A set of 60 min flow (suffusion) tests was conducted using a positive displacement pump to subject a cylindrical specimen to controlled hydraulic flow pulses. The global hydraulic gradient across the specimen was continuously monitored. The grain-size distribution of the soil was re-evaluated after the test. The hydraulic shear stresses induced by the flow pulses were estimated numerically and normalised over the maximum value. Similar trends of the hydraulic conductivity and the applied flow rates do not hold when the duration of the flow pulse is increased. A simple procedure based on statistical inference is proposed to visualise the temporal variation of the clogging and unclogging process subjected to the frequency and the phase shift parameters of the flow pulse.

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