Saturday, 15 July 2006

Assessment of the Origin of Surface Salinity Problems in the Surroundings of Evaporation Ponds in a Semi-Arid Environment.

Thomas Baumgartl, Kate Secombe, Mansour Edraki, and David Mulligan. Centre for Mined Land Rehabilitation, Earth Sciences, The Univ of Queensland, Brisbane, 4072, Australia

Disposal of produced formation waters in the oil industry in evaporation ponds often results in environmental impacts in the surroundings of the ponds caused by the saline nature of these waters, if the hydraulic containment in these ponds is not guaranteed. This study on salinity problems in a semi-arid environment in the centre of Australia is assessing the origin of surface salinity in the vicinity of evaporation ponds. The objective is to eventually develop rehabilitation strategies to hand back environmentally impacted land, which is commonly used by the stakeholder as grazing land. The primary objective of the assessment is to identify the nature and cause of the high surface salt enrichments in a soil environment with naturally high concentrations of the main cations. The thorough understanding of the processes leading to salt accumulation is a main prerequisite for the development of successful management procedures for rehabilitation. The investigations are based on the understanding of the soil chemical condition and the pond water-ground water characteristics. Soils have been characterised by the determination of the spatial distribution (depth within a profile and different distances from the pond) of concentrations of the main cations. The focus of the geochemical characterisation of the waters is the determination of the isotopic signatures of main salts and oxygen as well as the determination of the geochemical properties by comparing pond water and groundwater samples in the surroundings of the evaporation ponds. Results show higher signatures of oxygen isotopes at certain locations close to the evaporation ponds and indicate seepage from the ponds, which is affecting the groundwater. The results of the soils investigation show that the salt concentration in the soil is naturally very high with high amounts of Na in deeper soil depths. In the vicinity of the evaporation ponds mainly Na is accumulated at the surface causing problems to the establishment of vegetation. The totals of the ion concentrations within a profile show that within a distance of 250 m no statistically secured difference can be found. The calculation of the salt balance shows, that despite a high input of salts by the produced formation waters into the evaporation ponds, it seems that seepage from the ponds into the surrounding soils did not increase the salt load substantially. The combination of the water and soil study lead to the hypothesis, that the main cause of the elevated concentrations of Na at the surface are a result of the altered hydraulic regime in the vicinity of the ponds creating capillary contact of the subsurface water to the ground surface rather then an input of salts into natural soils as a result of seepage. The proof of this hypothesis will have a substantial implication on the strategy for rehabilitation.

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