CHANGES IN THE HYDROLOGICAL PROPERTIES OF THE SUBSURFACE FLOW TREATMENT SYSTEM DUE TO CHEMICAL REACTIONS

The application of constructed subsurface-flow wetlands for treatment of wastewater from abandoned mines is recently increasing in Korea. In some cases, various materials such as the crushed limestone, wood, or mushroom composites are employed as the substrates in the bed media. For optimal planning, design and operation of constructed wetlands, chemical and hydrological properties of the media should be well understood. Efficiency of the subsurface-flow wetlands drops with time, as the hydraulic conductivity of the wetland soil decreases significantly due to the chemical reactions with the wastewater, eventually leading to overland flow or flooding. An experimental program was undertaken to investigate the changes in the hydrological properties of the substrates in a constructed wetland for mine drainage. Constant-head permeability tests were performed for hydraulic conductivity measurement, using 4-foot long 5-inch diameter columns at an abandoned mining site. Columns were installed near the tunnel entrance, and wastewater flowing out of the tunnel was used for the tests. Flows were directed upward and downward inside each column, composed of alternating sections of crushed limestone and mushroom composite. The amount of head loss occurring as the water flows through each substrate was measured at preset intervals. It was observed that the flow rate through the column was substantially lowered with time. mainly due to significant amount of sludge within the media. Reduction in the hydraulic conductivities of substrate materials with time was taken into account in the design of the subsurface flow system, using a numerical analysis program incorporating finite element method. Chemical properties, including the pH, Eh, TDS, dissolved oxygen, and the major element contents, of outflow from the column were also measured to monitor the efficiency of the substrates on the water quality. The effect of micro-organisms such as the sulfate reducing bacteria in the mushroom composite was represented by the variation of water quality of outflows.