Contamination of Soil and Groundwater with Toxic Heavy Metals near Lapite Dumpsite in Ibadan, Nigeria

Technical Summary: Contamination of Soil and Groundwater with Toxic Heavy Metals near Lapite Dumpsite in Ibadan, Nigeria

Problem Statement
Municipal solid waste (MSW) management in developing nations, particularly in Ibadan, Nigeria, presents significant environmental challenges due to uncontrolled disposal practices. The Lapite dumpsite, an open, unlined facility serving a rapidly growing urban population, lacks adequate containment measures. Consequently, there is a high risk of leachates containing toxic heavy metals percolating through the soil matrix into underlying aquifers and nearby residential well water. While previous studies have identified heavy metal contamination in such environments, there was a need to assess the specific impact of combustion processes and long-term accumulation on both soil profiles (at varying depths) and groundwater quality over a decade.

Methodology
The study employed a comparative analysis of soil and water samples collected from the Lapite dumpsite and a control site (a botanical garden with virgin land).

• Sampling Timeline: Data was collected in May 2010 and revisited in July 2020 to evaluate temporal changes in metal concentrations.
• Soil Sampling: Ten random locations at the dumpsite were sampled at three depths (0–15 cm, 15–30 cm, and 30–45 cm) using a soil auger. Control samples were taken from two locations at the botanical garden.
• Water Sampling: Well water was collected from eight households in proximity to the dumpsite.
• Analytical Procedures:
  • Physicochemical: Soil pH was measured electrometrically, and particle size distribution was determined via the Bouyoucos hydrometer method.
  • Digestion: Soil samples were digested using HNO3-HCl (1:3, v/v) following US EPA method 3050B. Water samples were digested with concentrated nitric acid.
  • Quantification: Concentrations of Chromium (Cr), Cadmium (Cd), Lead (Pb), Nickel (Ni), Zinc (Zn), Copper (Cu), and Manganese (Mn) were analyzed using a Shimadsu AA-6200 flame atomic absorption spectrophotometer.
  • Statistical Analysis: Descriptive statistics (mean ± SD) and Pearson's correlation coefficients were used to establish relationships between soil and water metal levels. The Geoaccumulation Index ($I_{geo}$) was calculated to quantify pollution levels.

Key Results

• Soil Characteristics: The soil at the Lapite dumpsite is predominantly sandy (73.2–87.2%), which reduces its capacity to retain dissolved contaminants compared to clay-rich soils. The soil pH ranged from 6.0 to 6.5, indicating acidic conditions similar to the control site.
• Heavy Metal Concentrations in Soil (2020):
  • The study found elevated levels of heavy metals in the dumpsite soil compared to the control site. The concentration trend was Mn > Zn > Cu > Pb > Cr > Ni > Cd.
  • Specific mean concentrations for 2020 included: Mn (1580 ± 21 µg/g), Zn (1520 ± 10 µg/g), Cu (710 ± 12 µg/g), Pb (120 ± 5 µg/g), Cr (30.1 ± 4.3 µg/g), Ni (28.4 ± 3.6 µg/g), and Cd (2.14 ± 1.1 µg/g).
  • Temporal Trends: Over the ten-year period, metal levels increased by varying degrees: Cr (19.4%), Cd (50.7%), Pb (7.5%), Ni (18.8%), Zn (1.33%), and Mn (3.0%).
  • Depth Distribution: Generally, metal concentrations were highest in the topsoil (0–15 cm) and showed complex variations with depth, though some metals (Cr, Pb, Ni) showed higher concentrations at 15–30 cm in 2010, potentially due to biological mixing (caterpillar activity) or leaching patterns.
• Groundwater Contamination:
  • Well water samples near the dumpsite contained elevated concentrations of Cr, Pb, Ni, Zn, Cu, and Mn, exceeding World Health Organization (WHO) guidelines.
  • Cadmium was the only metal below the detection limit (0.001 µg/mL) in the water samples.
  • A significant positive correlation was observed between heavy metal levels in the soil and the corresponding levels in the well water for most metals, except for Lead (Pb).
• Pollution Indexing: The Geoaccumulation Index ($I_{geo}$) indicated that the soil was "highly contaminated" with Zinc and Copper, "moderately contaminated" with Cadmium and Lead, and "uncontaminated to moderately contaminated" with Manganese.

Significance and Claims
The paper asserts that the Lapite dumpsite is a confirmed source of heavy metal pollution affecting both the local soil profile and the groundwater utilized by nearby residents. The study highlights that the uncontrolled combustion of waste and the lack of containment at the dumpsite have led to the leaching of toxic metals into aquifers.

The authors conclude that the continued use of the dumpsite without proper management poses deleterious effects on human health and the environment. Specifically, the contamination of groundwater renders it unhygienic for drinking purposes. The research underscores the urgent need for surveillance around dumpsites in developing countries to prevent the transfer of heavy metals from soil to water and subsequently to the human system through ingestion. The findings serve as a critical baseline for understanding the long-term environmental degradation caused by unmanaged solid waste in Ibadan.