Elevated mercury (Hg) in groundwater caused by oil and gas production

Between 1965 and 2017, the Port Harcourt Refining Company (PHRC) refined crude oil in the eastern Niger Delta and groundwater hydrocarbon contamination in the area is known. However, nothing is known about the concentration, source, speciation and mobility of mercury (Hg), a potential byproduct of oil refining. To address this, groundwater samples were collected along the wastewater discharge outlet (WDO) and around the PHRC. The Hg concentrations in groundwater near the WDO varied between 0.2 and 6 μg/L, compared to less than 0.01 μg/L at distances greater than 300 m from the WDO and reference sites away from the refinery. Up to 63 % of Hg were present as Hg bound to particles larger than 0.45 μm (Hg_part), suggesting the prevalence of Hg transport in the suspended colloidal phase in the aquifer. Operational-defined Hg speciation shows that 33 % of the total Hg (THg) occurred as inorganic, reactive Hg²⁺. In comparison, only 4 % occurred as dissolved organic matter-bound Hg²⁺ despite high DOC and BTEX concentrations of up to 47 mg/L and 2888 μg/L, respectively. Notably, the DOC is predominantly petroleum hydrocarbon, generated from the ongoing oil and gas activities at the site. This suggests that hydrocarbon-based-DOC does not bind Hg.

Sediment samples collected from the wastewater discharge point (WDP) contained Hg concentrations of up to 529 μg/kg, and the carbon (C) content reached 40 %. Sediment batch leaching experiments showed that up to 23.5 % of the Hg in the quartz-dominant sediment can be mobilized into groundwater under oxic conditions. Despite the presence of petroleum hydrocarbon, Hg retention was significantly controlled by the sediment's natural organic matter (NOM). Hence, the discharged oil and gas production wastewater due to crude oil refining released Hg into the aquifer, where NOM ultimately controls fate and transport.