Assessment of sub-surface hydrocarbon contamination resulting from fuel storage depots, Shimanzi area Mombasa Kenya

Abstract

The objective of this investigation was to establish whether there are hydrocarbon contaminants present in soils and groundwater within the depots by quantitative analysis, determine the extent of contamination, and determine what measures are required to control or minimise hydrocarbon pollution of the soils I groundwater at the depots and surrounding areas. A total of 86 soil vapour points were surveyed and Volatile Organic Compounds (VOC) measurements taken using a Photo Ionization Detector (PID) at the depots, with 52 points done at Shell, 23 at Caltex and 21 at Gapco. The results of the soil vapour survey indicate the highest reading to be 80ppm, 147.3ppm and1849.1ppm at Gapco, Shell and Caltex Depots respectively. The results of soil samples submitted to the laboratory indicate values ranging from 5.9mg/kg to 918mg/kg of TPH at Shell Depot, 1500mg/kg to 26,000mg/kg of TPH at the Caltex Depot and 52mg/kg to 250mg/kg of TPH at Gapco Depots. The results of water samples collected from the monitoring wells existing at the Shell revealed profound variation in the condition of contamination of grmmdwater within the same aquifer. Two monitoring wells had free phase hydrocarbon product present while the remaining two did not reveal any presence of contamination. The results of the groundwater laboratory analysis exhibited non-detectable concentrations of BTEX in all the monitoring wells and TPH concentrations were recorded confirming the contamination by heavy fuels as depicted in the soil results. At Caltex depot all the monitoring wells had free phase hydrocarbon product, a strong smell of hydrocarbon, and recorded very high TPH concentrations of 6~~6.00mg/kg, 810,000mg/kg and 790,000mg/kg. Monitoring wells with free phase hydrocarbon present had high . . values of 6,600mg/kg and 810,000 mg/kg respectively. The least impacted site recorded values of <50mg/kg and- some. traces of BTEX to the tune 0.30mg/kg indicating some levels of contamination. Based on the soil vapour survey results, soil analytical results and water analytical results within the depots; it is concluded that actual contamination exists within the depots.