Chloride variation in surface layers of colliery spoil heaps

Abstract

An investigation of chloride Ion levels In coarse colliery discards was undertaken because modem British Coal practice is to restore discard tips and lagoon embankments at an early stage in construction, commonly before much leaching has taken place. High chloride ion concentrations result in physiological drought and the failure of vegetation cover. Sites at two collieries were studied using grid and traverse (depth) sampling procedures. Results Indicate downward leaching of chloride ions into the embankment during Winter months and an increase in chloride levels towards the surface of the discard during dry, summer months. Salt hotspots occur on embankment surfaces during Summer months. These high chloride/sulphate concentrations represent the desiccation of seepages, most of which are related to embankment construction inadequacies, rather than to lagoons incorporated into the structure. Chlorides within the discard originate from formation waters intimately associated with Coal Measures sequences. In the eastern coalfields in particular, high chloride ion concentrations in the coal and waste rock from deep underground excavations are not removed by the coal washing processes. Consequently, they are retained in fresh colliery discards. Seasonal movements of chloride ions are associated with an increase In (negative) suction pressures within the near-surface layers of a colliery embankment. Suction pressures were monitored experimentally in two experimental tips and in the field at a third colliery site (Bilsthorpe Colliery). On an annual basis, suction pressures become operative early in April, reversing to residual negative or small positive pore pressures in mid-September. The leaching of chlorides from discard embankments is a function of the drainage characteristics of the materials and in clayey discards leaching to low levels is shown to take 5 to 7 years. Hotspots do not decrease significantly. The results of the present investigation can be applied to current embankment restoration schemes. In particular, the sowing of an embankment during the Autumn window, when electrolytes move downwards into the structure, would enable young vegetation to become established before being subjected to the higher Summer chloride concentrations. Hotspots require individual field drainage treatment.