Physical and Hydrological Characteristics of Ponds in Reclaimed Upland Landscape Settings and Their Impact on Agriculture Capability

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  • In 1985, a one hectare pond developed in the upland reclaimed landscape at Vesta Mine in an area where extensive ponding had not previously been observed. Because of the thickness of the spoil, about 20 to 25 m, and the proximity to the active pit, a few hundred metres, it was inferred that the pond was perched above the water table. This pond was instrumented in September 1987 to monitor the subsurface water and salinity regime. Observations that continued throughout 1988 and 1989 provide the basis of this report. The closed basin responsible for the existence of pond S195 was formed by construction of a low berm transverse to a long southward draining slope. The resulting drainage basin collects and channels runoff water during spring snow melt into a series of subsidence depressions in the lowest part of the basin. Compaction during placement and grading of the lower subsoil and upper spoil produced a hydraulic barrier with sufficiently high density and low hydraulic conductivity that rapid downward drainage of the ponded water was prevented. The hydraulic conductivity was further reduced by sealing of the upper surface of the spoil as a result of structural collapse of sodic clay in response to wetting. Perched ponds impact the agricultural capability of reclaimed landscapes in three ways. (1) Perched ponds reduce the amount of farmable land within upland reclaimed landscapes and disturb field patterns as compared to upland reclaimed sites without such ponds. To put this in perspective, the area occupied by ponding in upland settings in the reclaimed sites studied is less than half that in unmined sites in the same area. (2) Perched ponds, such as S 195, result in progressive development of saline and potentially sodic soils in the area adjacent to the pond. The saturated or nearly saturated conditions in the soil surrounding the pond result in upward movement of subsurface water, which is lost by evaporation and evapotranspiration, and accumulation of salts in the soil zone over time. (3) Perched ponds result in accelerated groundwater recharge, at least early in their life. On the basis of evidence from our study, however, a single isolated pond does not produce sufficient recharge to cause the water table to approach the surface in areas of thick spoil. The recharge rate at pond S195 during the period of observation was considerably lower than during the early period of ponding. It is not clear whether this diminished recharge rate resulted from a decrease in hydraulic conductivity or from a series of drier than normal years. It appears unlikely that the size of perched ponds will increase significantly over time. It is considered more likely that the size of such ponds is limited by the interaction between the size of the contributing drainage basin, the depth of the central depression, and the rates of precipitation and evaporation.

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