Effect of oil exploration and production on the salinity of a marginally permeable aquifer system in the Thar Jath-, Mala- and Unity Oilfields, Southern Sudan

Abstract

The study area, located in the Subtropics west of the East African Rift, is characterized - on the basis of its hydrology - by an alluvial aquifer system that is made up of predominantly fi ne-grained early Tertiary and Quaternary sediments. These sediments were deposited by the River Nile in large lakes. Nowadays, there is little natural new water recharge from seeping precipitation, due to marginal permeability and the formation of extensive wetlands. In order to search for possible links between an increasingly high salinity in drinking water and the exploration and production of oil in Unity State, South Sudan, a total of 90 water samples from 76 different sampling sites were take n, 74 have been analysed in detail. The samples were taken from the surface water of swamps, from boreholes in various settlements in the area, from produced water storage basins, from production facilities of the oil industry and from mud pits at abandoned as well as from still active exploration drilling sites. Results of these analyses facilitated the identifi cation of six water types that were hydro- chemically distinguishable, and that were characterized by signifi cantly variable ionic relationships and salt contents. Waters unaffected by human activity are characterized by prograde quality development, controlled by water-rock reactions, and they range from a hydrogen carbonate type to a sulphate type. The total dissolved content increases only gradually with the geogenic, prograde evolution of groundwater close to the surface, and ranges in terms of electrical conductivity from 6,000 μS/cm) with simultaneous dominance of chloride and sulphate contents. This anion dominance occurs also partially in produced water storage basins of oil production facilities, especially in mud pits of exploration operations. Due to the direction of groundwater fl ow, the spatial distribution of salt contamination and the hydro-geological boundary conditions found in low water permeable alluvial deposits, the cause of high conductivities in drinking water wells can only be attributed to selective seepage of salt-containing water from the basins and mud pits of those three oilfi elds operating in the study area. Other sources of salt contamination, e.g. deepwater rises or groundwater affected by evaporation, were not identifi ed. In order to guarantee the supply of drinking water for the population in the region, initially two deep wells were sunk (down to 280 m and 300 m depth respectively), and drinking water was pumped from the deepest aquifer, which had not yet been explored and developed. The latter has sulphate-dominated water of low to medium conductivity, which circulates in thin lenses of sand as well as in predominantly silty, fi ne sand layers. A clay layer, approximately 50 m thick, separates the fi rst partly salinized aquifer from the underlying newly explored groundwater horizon. This horizon is supplied only by very slow processes of new accumulation (re-charge) processes, extending over decades. With Open Access Article