A survey of the frequency, duration and oxygen content of surface and sub-surface water in rehabilitated mined areas at Huntley Mine

T. Burgess; S. Collins; G.E.St.J. Hardy; I.J. Colquhoun; J.A. McComb

Surface ponding of water is evident in many recently (0—3 years old) rehabilitated minepits; this occurs at the bottom of the pit but also in riplines on the upper slopes of the pit: The presence of surface and subs urface ponds has the potential to affect plant growth and survival. The aims of this project were to monitor the duration and frequency of ponding, and to monitor oxygen levels to assess the potential for waterlogging to occur. Observations at the Huntly mine found that ponding is highly variable within and between rehabilitated minepits. Level floats attached to data loggers in six rehabilitated minepits were used to determine the duration and frequency of surface ponding. Between September and May 1997 depressions held water between 5 and 25% of the time. The average duration of ponding each week was highly correlated to the weekly rainfall. We believe that surface ponding can be caused by three circumstances: (i) clay collecting in depressions, (ii) low infiltration capacity of a sub-surface clay layer or (iii) a surface expression of subsurface accumulation of water. Seven minepits with obvious areas of ponding were selected for monitoring of the depth of sub-surface ponding and oxygen content of the water. Piezometers were placed in 78 ponds at depths of 30, 60 and up to 90 cm. There were five types of surface ponds and profiles identified; (a) fast draining ponds with little impediment to water movement through the soil, (b) slowly draining ponds with some impediment to water movement through the soil, (c) ponds that drained into a sub-surface pond that prevented water movement, (d) ponds that were actually drainage sumps formed in low lying areas of the pits and (e) ponds overlying a sub-surface water channel. About 30% of the ponds were of type (c) and (d), and were associated with subsurface water that had a low oxygen concentration. Thus, the potential exists in early vegetation rehabilitation for surface ponding to be associated with sub-surface ponding or waterlogging. The low oxygen content of sub-surface water would have detrimental effects on plant growth. Eucalyptus marginata is the main tree species used to revegetate the minepits However, the collars of E. marginata seedlings are very susceptible to infection by Phytôphthora cinnamomi and surface ponding provides ideal conditions for infection. Consequently, the combination of surface ponds and sub-surface waterlogging would be expected to result in increased disease severity.

A survey of the frequency, duration and oxygen content of surface and sub-surface water in rehabilitated mined areas at Huntley Mine

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