Geological assessment of heavy metals' soil contamination by drilling waste discharge, Palogue Fields, Melut Basin, South Sudan

Abstract

Oil and gas operations produce wastes that are potentially toxic to the environment. The waste deposited into reserve pits can contaminate soil, groundwater, and surface water with heavy metals and other toxic substances if improperly managed. This study was carried out to determine the potential contamination following their disposal. Samples of produced water and drilling waste disposed of in a reserve pit were collected at prescribed depths. Samples of soil around area of the reserve pit were collected at distance of 15m, 25m, 50m from the reserve pit. A reference soil sample also taken. All were analyzed for heavy metals using inductively coupled plasma mass spectrometry (ICP-MS). Water samples were also analyzed for physico chemical parameters (EC, pH, TDS and TSS). Mean pH values were found to range between 9.45 and 9.85, categorizing the drill waste as basic while electrical conductivity values range from 4.27 to 4.63mmhos cm-1 . Physico-chemical parameters were generally higher than limits given by regulatory authorities. Mean TDS and TSS, values were exceedingly very high (5313 mg/l and 847.5mg/l.) compared to the established limit. The trend of all the parameters showed that the values for produced water increased with depth. The mean concentration ranges for Cr, As, Cd, Zn, Ba and Pb in the reserve pit in mg/l were (2.18 – 3.73), (0.08 – 0.77), (1.04 – 1.28), (13.35 – 27.09), (0.82 – 2.32), respectively. In the reference soil, the range was (1.53-.4.97), (0.02-0.69), (0.99-1.74), (15.95-37.75), (85.58-376), (0.76-2.85) respectively. In the soil, the concentrations decreased with distance from the reserve pit. The pollution load indexes (PLI) for most of the metals were very high which imply that there is high pollution potential around the study area. These high levels of some of the physical-chemical parameters such EC, pH TDS, TSS and heavy metals in the drilling waste as seen in this study may be the source of environmental pollution which possibly originated from produced water and drilling muds. This high TDS, TSS and heavy metals will cause environmental pollution and health problems. For example, Pb can cause health problems to target organs like bones, brain, blood, kidneys and reproductive organs if its concentration exceeds 1.0 ppm. This therefore, underscores the need for due diligence in managing drilling waste discharges from the ongoing exploratory drilling activities in Palogue field, South Sudan.