The total hydrocarbon content removal in nutrient amendment and natural attenuation at day 56 showed percentage removal of 89 and 74, respectively. Total heterotrophic and total hydrocarbon utilizing bacterial count increased progressively in all the treatment options. The nutrient amendment showed greater removal of the hydrocarbon pollutants. The physicochemical analyses at day 56 were within the World Health Organization standard for drinking water.
Keywords: Aerobic biodegradation; Petroleum hydrocarbon; Groundwater; Nutrient amendment; Natural attenuation
Groundwater is an important source of water for agricultural and domestic use especially in developing countries like Nigeria, due to long retention time and natural filtration capacity of aquifers . It is less contaminated as compared to surface water . However petroleum hydrocarbons can be introduced into groundwater via oil spills, leaking or unplugged oil wells, the disposal ponds of waste petroleum products, abandoned oil refinery sites, pipe line ruptures, incomplete combustion of fossil fuels and accidental discharge during transport in tanks and ships failures .
In the present study, degradability of hydrocarbon, organisms involved and the physicochemical characteristics of the laboratory contaminated groundwater under natural attenuation and biostimulation conditions were determined.
In the course of the research it was also observed that the groundwater sample Treated (Nutrient amendment) showed a higher percentage removal of the total hydrocarbon content (89%) compared to un-Treated (natural attenuation) which showed 74% removal at the end of the fifty six days. The contributions of natural attenuation to the bioremediation of impacted media such as water have been reported at other times too .
Bioremediation can be effective only where environmental conditions permit microbial growth and activity. In some cases, the environment can be modified to support or accelerate microbial growth, for example, by fertilizer application . The benefit of nutrient amendment was also observed by Abu and Ogiji (1996), who noted that the response of the indigenous hydrocarbon degrading microorganisms to the bioremediation treatment was positive and differed according to the type and concentration of the nutrients added.
The increase in counts of the heterotrophic population is in agreement with results obtained by other researchers that hydrocarbon pollution does not enrich only hydrocarbon utilizers but also other populations that utilize breakdown products of hydrocarbons . The growth in the heterotrophic count and hydrocarbon utilizing bacteria were also observed to be more in nutrient amendment sample than untreated sample as the experiment progressed. Other researchers [8,4,5] have also demonstrated the use of inorganic nutrients in bioremediation of hydrocarbon impacted media with overall positive results.
Generally, it is believed that microbes preferably degrade or metabolize C8-C15 nalkanes followed by C16-C36 n- alkanes due to simplicity of these hydrocarbons . Most of the sharp peaks observed in the original sample were either reduced or not found in the Treated sample after fifty six days.
The observed pH values throughout the study are of special consideration since microbial populations are highly dependent on this parameter . The results are in agreement with observation made by other workers that a pH range of 6-8 provides better conditions for mineralization of hydrocarbons since most bacteria capable of metabolizing hydrocarbons develop best at pH conditions close to neutral .
The dissolved oxygen concentrations measured in the flasks in the course of the study were lower than the initial concentration of 8.0 mg/l. Oxygen is a microbial electron acceptor and a redox indicator. High oxygen (>2 mg/L) shows aerobic conditions and oxygen will be the preferred electron acceptor until depleted. Nutrient amendment enhanced oxygen uptake.
The reduction in conductivity in treated and un-treated sample from initial of 63 μs/cm to 43 μs/cm and 63 μs/cm to 26 μs/cm after fifty six days, respectively suggests that there was uptake and exchange of ions in the samples in course of the study confirming that biodegradation of hydrocarbon was achieved. The maximum permitted conductivity by Nigerian Standard for drinking water quality is 1000 μs/cm .
The reduction of these nutrients as the experimental period progressed suggests utilization by microorganisms.
These results clearly show that a large diverse aerobic bacterial population capable of utilizing organic carbon is present in groundwater.