RESUMEN
This study investigates the toxicological effects of heavy metals on lithofacies of the subsurface in a drilled hydrocarbon well as well as, to the drilling crew and people in an environment. The pollution levels of selected heavy metals were considered alongside their ecological effects during dry and wet seasons. The health hazard potential of human exposures to the metals, were estimated in terms of intensity and time using the USEPA recommended model. The heavy metal concentration for each layer decreased across the lithofacies as follows; Layer 5> Layer 4> Layer 3> Layer 2> Layer 1. The average concentrations of the heavy metals present in the samples obtained from the formation zone, varied significantly and decreased in the order of Al> Zn> Ni> Pb> Cr> Cu> Cd> As> Hg. The highest concentration of Al, Cu, and Zn in this present study were within the maximum allowable limits whereas, those of As, Cd, Hg and Ni were all above their maximum allowable limits. Among the transition metals analysed, the maximum mean daily dose of Pb (9.18 × 10-6 mg/kg/d) and Cr (1.42 × 10-6 mg/kg/d) were confirmed susceptible to human carcinogens and environmental toxins. The estimated hazard quotient shows that the dermal pathway is the most likely route via which the drilling crew and people in the environment can get contaminated. The cancer risk values for the Pb (7.72 × 10-4), Cd (1.35 × 10-1), Ni (9.97 × 10-3), As (1.50 × 10-1) and Cr (3.16 × 10-3) are all above the acceptable values. The cancer risk contribution for each metal was in the order of As> Cd> Ni> Cr> Pb. Layer 5 had the maximum Geo-accumulation index for the heavy metals considered. This higher Geo-accumulation index noted at the depth in Layer 5 may be attributed to the effect of water basin with turbidity currents, deltas, and shallow marine sediment deposits with storm impacted conditions. Also, the pollution from lead (Pb) in the dry season was maximum with an Igeo value> 5 for all the lithofacies considered because of the low background concentration of the metal. During the wet season, the heavy metal pollution rate was moderate for Zn whereas, it was extremely polluted with respect to Pb. The ecological risk potential of Pb shows that the associated ecological risks range from 536 - 664 in the wet season (i.e. extremely strong) and 2810 - 3480 in dry season (extremely strong). The high level of Pb pollution found in the area at such shallow depth may be due to the sedimentary folds possibly caused by the full spectrum of metamorphic rocks and primary flow structures at shallow depths. This was used to identify the environmental sensitivities of the heavy metals during the dry and wet seasons.
RESUMEN
The accumulation and increase in radionuclide activities of NORMs beyond permissible levels, will lead to health hazards and environmental damages if proper measures are not taken to control their occurrence as well as protect the lives of drillers and the environment. Therefore, evaluations and risk assessments of subsurface lithofacies is inevitable in order to protect people and the environment. Lack of existing Federal environmental regulations to address the presence of NORMs in oil and gas exploration activities in Nigeria, gives credence to this study. However, before these regulations can be developed, adequate research knowledge is needed to better understand the occurrence and distribution of Norms in subsurface lithofacies, as well as quantify the hazards posed by these NORMs to the people in the environment. This study then investigates the occurrence of natural radiation in lithofacies of an oil field region in Niger-Delta area using Hyper Germanium (HPGe) detector. Six (6) samples of different subsurface layers of lithofacies were collected during drilling, and analyzed. The results showed that the measured activity concentration of 238U decreased as the depth increased; the activity concentration of 232Th ranged between 11.8 ± 9.29 Bq/kg and 23.1 ± 8.43 Bq/kg, while the activity concentration of 4 K ranged from 161.8 Bq/kg to 245.4 Bq/kg. The estimated radiological risks such as absorbed dose rates, annual effective dose rates, radium equivalent index, external hazard index and internal hazard index were determined. The mean values for the estimated radiological parameters were 12.32 nGyh-1, 15.1049 Svy-1, 44.7720 Bqkg-1, 0.1209 and 0.1318 respectively. The gamma index estimated for the samples used were within the standard values recommended by Unscear, 2000. Significantly, this study reveals a distinctive decrease in 232Th activity with depth within the area under consideration. Based on the compared results, the measured radioactive concentrations and estimated radiological risks were below international reference values.
