Groundwater pollution: occurrence, sources, behavior, transport and fate of mercury, benzene and nitrate in soil–groundwater systems in the Eastern Niger Delta, Nigeria

Abstract

The Benin Formation aquifer in the eastern Niger Delta of Nigeria faces many threats from anthropogenic activities. These activities include indiscriminate sewage disposal, the usage of pit latrines, and several impacts of oil and gas activities capable of releasing various pollutants into the environment. The Benin Formation aquifer is the primary source of drinking water for most of the region. Hence, the release of pollutants into the aquifer may cause adverse health effects on the population. Despite the area's high level of anthropogenic activities, the sources and processes affecting various groundwater pollutants remain understudied. This study investigated, for the first time, the occurrence, sources, transport and fate of mercury, benzene and nitrate in the groundwater of the eastern Niger Delta, Nigeria. The investigation was conducted across five communities (i.e., Alesa, Ogale, Ebubu, Alode and Okochiri). Results may be transferable to other locations with similar incidences.

At a site impacted by crude oil refining in Okochiri, hydrocarbon contamination in the ground and drinking water was already known. However, the concentration, source, speciation and mobility of mercury (Hg), an oil refining byproduct, was unknown. To address this, groundwater at the impacted site was investigated. Hg levels in groundwater along the wastewater discharge outlet (WDO) of the refinery ranged from 0.2 and 6 µg/L. 63 % of the Hg occurred as Hg bound to particles larger than 0.45 µm (Hgpart). Operationally-defined Hg speciation shows that 33 % occurred as inorganic, reactive Hg2+, and 4 % as dissolved organic matter-bound Hg2+ despite the high DOC (up to 47 mg/L) and BTEX (up to 2,888 µg/L) levels, indicating that the DOC, which is mainly hydrocarbon, does not bind Hg. Overall, Hg levels in groundwater were elevated where oil and gas production wastewater was introduced into the aquifer.

The Hg concentration in sediments collected at the wastewater discharge point (WDP) was up to 529 µg/kg, and the carbon (C) content reached 40 %. The batch leaching experiments showed that up to 23.5 % of THg in the quartz-dominant sediment could be mobilized into groundwater under oxic conditions. In general, Hg retention in the sediment is controlled by the sediment’s natural organic matter (NOM).

The ground and drinking water in Alode, Ogale, and Okochiri, where several oil and gas production facilities are present, showed elevated BTEX levels, of up to 3,904 µg/L, 1,573 µg/L and 2,888 µg/L, respectively. Similarly, benzene levels were up to 3,500 µg/L, 1,300 µg/L and 2,700 µg/L in Alode, Ogale and Okochiri, respectively. Here, benzene was the dominant BTEX compound. DOC, which reflects the total hydrocarbon load in the groundwater, was up to 47 mg/L in Alode, 49 mg/L in Ogale and 33 mg/L in Okochiri. Benzene was detected in private supply wells (PSW) close to an underground petroleum pipeline passing through the three communities. Its concentration decreased with increasing distance from the pipeline, indicating probable leakage of petroleum from the pipeline into the aquifer.

The aquifer’s dissolved oxygen level (up to 7.5 (95 %) mg/L) and BTEX biodegradation capacity (up to 2.11 mg/L for DO) appear sufficient to support the BTEX degradation. However, the degradation may be weakened by the high groundwater temperature (32.5 °C). As a result, based on the calculated benzene (0.128 to 0.693) and BTEX (0.086 to 0.556) daily attenuation rates for this study, up to 66.5 years and 85 years would be required to attenuate benzene and BTEX. Thus, active remedial measures are required to reduce the time required for complete benzene attenuation in the affected groundwater.

Given the extensive animal husbandry, pit latrine usage, and indiscriminate municipal and domestic waste disposal practices, the groundwater and municipal sewage were investigated for nitrate (NO3−) in Alesa, Ogale, Ebubu, Alode and Okochiri. NO3− levels in the groundwater were highly variable: up to 142 mg/L in Alesa, 211 mg/L in Ogale, 120 mg/L in Ebubu, 55 mg/L in Alode and 10 mg/L in Okochiri. The major ion comparison showed that 71 %, 90 %, 87 %, and 92 % of the groundwater samples exceeded the reference site concentrations for Ca2+, Na+, K+, and Cl−, respectively, suggesting anthropogenic (sewage) influence. Similarly, the NO3−/Cl− ratio showed that 100 % (Alesa), 77 % (Ogale), and 84 % (Ebubu) of NO3− in the groundwater is derived from the ongoing anthropogenic activities (sewage). Furthermore, the isotopic evidence (15N-NO3− and 18O-NO3−) confirmed that the NO3− originates from sewage-derived NH4+ nitrification. Despite the high dissolved oxygen (DO) level, up to 8.9 mg/L, our results have shown evidence of denitrification in the area. This suggests that both biogeochemical processes occur simultaneously in the groundwater. However, the high DO and NO3− levels do not support a complete NO3− attenuation in the affected communities, hence the need for safe domestic and municipal sewage management practices. Consequently, there is a risk of NO3− export from shallow aquifers into the nearby surface waters in Okochiri and into the coastal ocean.

It can be concluded that the ongoing anthropogenic activities in the eastern Niger Delta have contributed a substantial amount of pollutants (i.e., mercury, benzene and nitrate) to the ground and drinking water at levels above the WHO guideline values. The (1) high concentration of the pollutants, (2) their continuous release into the aquifer, and (3) the impact of the high groundwater temperature on the prevailing redox conditions in the aquifer appear to weaken the natural attenuation of these pollutants from the aquifer. To protect the health of the growing population in the affected areas, measures to encourage immediate discontinuous use of the contaminated wells and alternative safe water sources (e.g., bottled, sachet, or a centralized water supply system) should be explored. Also, the local authorities need to implement measures to ensure a decline in the concentration of the investigated pollutants, including the remediation of the contaminated sediments and groundwater.