The management and control of water overlows from the settling ponds to coastal waters in PT. X

Abstract

The settling pond is a reservoir that collects excess water from the mining area that carries solid material. It allows suspended materials to settle first before disposal, preventing pollution in the surrounding environments. In the research area, the settling pond dimension, however, cannot accommodate incoming water discharge during heavy rainfall. The overflows then directly enter the coastal waters, and without the settling or sedimentation process, they contaminate the coasts with solid material. This research was designed to examine the dimensions of the settling pond and identify erosion hazard level, along with the existing countermeasures. The mine drainage system was calculated through five analyses of rainfall data. (1) The maximum rainfall obtained from the recurrence interval analysis using the Gumbel method was 156.046 mm/day. (2) The intensity of rainfall was 28.73 mm/hour. (3) The areas of the catchment CA I, CA II, and CA III were 0.43 km2, 0.11 km2, and 0.43 km2, respectively. (4) The runoff discharges in CA I, CA II, and CA III were 2.36 m3/s, 0.79 m3/s, and 2.66 m3/s. (5) The total discharge of runoff water from the three catchments was 2.36 m3/s. From this discharge data and the settling rate of 0.000997 m/s, the settling pond area and volume were identified, i.e., 5839.5 m2 and 23,520 m3 with a depth of 4 m. This volume is higher than the actual size of the settling pond in the field. Aside from the inadequate capacity of the settling pond, the overflows are caused by the fast rate of erosion. In this case, the excess water flows directly into the settling pond without going through open channels, accelerating the water flow. The rate of erosion (A), calculated with the USLE equation, reached averagely (A) 540.5646 tons/ha/year and, therefore, fell into the category of severe or Class V (B). By replanting trees on the slope (revegetation) as countermeasures, the average rate of erosion (A) can be reduced to 172.98 tons/ha/year, which downgrades the erosion hazard level to Class III or moderate (S). Revegetation must be done immediately so that the erosion rate becomes even smaller.