Physicochemical Characterization of Leachate and Groundwater at the Tam David-West Road Dumpsite, Port Harcourt, Nigeria: Nature, Magnitude and Spatial Distribution of Contaminant Loading

materials were analysed alongside every batch of environmental samples to check for contamination, instrument drift and analytical accuracy. Analytical precision was accepted where the relative standard deviation between duplicates was below 10%. 2.5 Data Analysis Laboratory results were organised in Microsoft Excel 2021 and summarised using descriptive statistics (mean, minimum and maximum) for each parameter, by season and by sampling location, to characterise the magnitude and spatial distribution of contaminant loading. Mean concentrations were compared against Federal Environmental Protection Agency (FEPA, 1991) effluent limits, for leachate, and World Health Organization (WHO, 2022) drinking- water guideline values, for groundwater, to evaluate the environmental significance of the observed concentrations. 3. Results 3.1 Physicochemical Characteristics of Leachate The physicochemical characteristics of leachate sampled across the five leachate points (LS1- LS5) in the dry and rainy seasons are summarised in Table 1. Leachate pH ranged from acidic in the dry season (mean ~6.1) to near-neutral in the rainy season (mean ~6.6), indicating a system transitioning from the acidogenic towards the methanogenic phase of waste stabilisation. Organic pollution indicators were markedly elevated in both seasons: mean BOD was 760 mg/L and 590 mg/L , while mean COD was 785.4 mg/L and 606.6 mg/L , giving a BOD/COD ratio of approximately 0.97 in both seasons, indicative of an unusually fresh and highly biodegradable waste mass. Ammonia averaged 82.0 mg/L and 68.4 mg/L , while chloride, nitrate, sulphate and phosphate were all higher in the dry season than in the rainy season, consistent with reduced dilution during periods of low rainfall. Figure 1 illustrates the seasonal contrast in the principal organic and nutrient parameters. Table 1: Mean physicochemical characteristics of leachate, Tam David-West Road dumpsite (n = 5 points per season) Parameter Unit Dry season Rainy season FEPA limit pH - 6.11 6.60 6-9 BOD mg/L 760.0 590.0 - COD mg/L 785.4 606.6 - Ammonia mg/L 82.0 68.4 - Chloride mg/L 59.3 20.9 600 Nitrate mg/L 27.2 14.3 20 Sulphate mg/L 47.1 16.8 - Phosphate mg/L 3.7 1.8 50 Figure 1: Seasonal variation in organic and nutrient loading of leachate 3.2 Heavy Metal Concentrations in Leachate Table 2 presents the heavy metal concentrations recorded in leachate. Iron averaged 0.824 mg/L and 0.786 mg/L , exceeding the FEPA effluent limit of 0.05 mg/L by more than fifteen-fold in both seasons. Cadmium was the most environmentally concerning metal detected, averaging 0.894 mg/L and rising to 1.440 mg/L , well above levels considered safe for aquatic and human exposure. Chromium (0.164-0.170 mg/L) approached or slightly exceeded typical regulatory guidance, while copper (1.762-1.806 mg/L) and zinc (1.710-1.858 mg/L) were also elevated, consistent with inputs from metal scraps, batteries and electrical waste within the mixed waste stream. Figure 2 compares these concentrations with applicable FEPA limits. Table 2: Mean heavy metal concentrations in leachate, Tam David-West Road dumpsite (n = 5 points per season) Metal Unit Dry season Rainy season FEPA limit Iron (Fe) mg/L 0.824 0.786 0.05 Copper (Cu) mg/L 1.806 1.762 2.0 Zinc (Zn) mg/L 1.710 1.858 1.53-1.66 Chromium (Cr) mg/L 0.164 0.170 0.20 Cadmium (Cd) mg/L 0.894 1.440 0.003-<1 Figure 2: Seasonal heavy metal concentrations in leachate compared with FEPA limits 3.3 Physicochemical Characteristics of Groundwater Table 3 summarises groundwater physicochemical characteristics from the five boreholes (BH1-BH5), arranged in order of increasing distance from the dumpsite. A clear spatial gradient was evident: BH1, the closest borehole, recorded the highest electrical conductivity (150 μS/cm dry; 12 μS/cm rainy), total dissolved solids (105 mg/L dry), total hardness, chloride (69.4 mg/L) and nitrate (34-35 mg/L), while values declined progressively towards BH4 and BH5, the most distant boreholes, which approached background conditions. Mean electrical conductivity across all boreholes was 90.6 μS/cm and 21.6 μS/cm ; mean total dissolved solids were 63.4 mg/L and 11.0 mg/L ; and mean nitrate was 27.4 mg/L and 6.9 mg/L . Groundwater pH was slightly acidic overall (mean 5.25 dry; 5.93 rainy), falling below the WHO/NSDWQ range of 6.5-8.5 in several boreholes, particularly during the dry season. As with the leachate, nearly every parameter was higher in the dry season than in the rainy season, reflecting reduced recharge-driven dilution. Figure 3 illustrates these seasonal contrasts. Table 3: Mean physicochemical characteristics of groundwater, boreholes within 1000 m of the dumpsite (n = 5 boreholes per season) Parameter Unit Dry season Rainy season WHO limit EC μS/cm 90.6 21.6 382-512 TDS mg/L 63.4 11.0 1000 pH - 5.25 5.93 6.5-8.5 Turbidity FAU 1.6 2.4 <5 Total Hardness mg/L CaCO3 40.8 10.4 <200 Chloride mg/L 52.9 11.4 250 Nitrate mg/L 27.4 6.9 50 Sulphate mg/L 2.1 0.2 250 Phosphate mg/L 6.7 1.4 0.1 Figure 3: Seasonal variation in borehole (groundwater) physicochemical parameters 3.4 Heavy Metal Concentrations in Groundwater Table 4 presents groundwater heavy metal concentrations. Iron was the only metal to exceed its guideline value at every borehole, averaging 4.55 mg/L in the dry season and 1.25 mg/L in the rainy season, more than fifteen times the WHO limit of 0.3 mg/L even during the wetter, more dilute season, and reaching a maximum of 6.10 mg/L at BH2 in the dry season (Figure 4). Copper, lead, manganese and zinc were present at low concentrations, generally below or only marginally above guideline values, while aluminium was not detected in any sample. Lead exceeded the WHO limit of 0.01 mg/L only in isolated dry-season samples (maximum 0.02 mg/L at BH4), suggesting early-stage, spatially limited impact rather than widespread contamination. Table 4: Mean heavy metal concentrations in groundwater, boreholes within 1000 m of the dumpsite (n = 5 boreholes per season) Metal Unit Dry season Rainy season WHO limit Iron (Fe) mg/L 4.550 1.252 0.3 Aluminium (Al) mg/L 0.000 0.000 0.05-0.2 Copper (Cu) mg/L 0.066 0.009 2 Lead (Pb) mg/L 0.010 0.003 0.01 Manganese (Mn) mg/L 0.004 0.001 0.4 Zinc (Zn) mg/L 0.018 0.003 5 Figure 4: Mean borehole iron concentration versus WHO guideline value 4. Discussion 4.1 Nature and Magnitude of Contaminant Loading in Leachate The physicochemical characteristics of the leachate confirm that the Tam David-West Road dumpsite continues to function as an active source of contaminant generation. The slightly acidic to near-neutral pH range observed indicates a system in transition from the acidogenic towards the methanogenic phase of landfill stabilisation, during which organic acids produced by decomposition progressively give way to more neutral conditions as methanogenic microorganisms become dominant (Christensen et al., 2001). Similar acidic leachate conditions have been reported in other active African dumpsites undergoing anaerobic decomposition (Nyirenda & Mwansa, 2022). This acidity is environmentally significant because lower pH enhances the solubility and mobility of heavy metals, thereby increasing the potential for their transfer into groundwater. The high BOD and COD values, and the resulting BOD/COD ratio of approximately 0.97, indicate that the leachate is unusually fresh and highly biodegradable, more characteristic of a young, actively tipped waste mass than of a site that has progressed substantially towards stabilisation. This finding is consistent with Reddy et al. (2026), who reported comparable biodegradability characteristics in actively decomposing tropical landfill systems, and reflects the continuous, unmanaged addition of fresh municipal waste onto older material at the dumpsite. Elevated ammonia concentrations further confirm extensive degradation of nitrogen-containing organic matter; ammonia is widely recognised as one of the dominant nitrogen species in landfill leachate arising from anaerobic decomposition of proteins and amino acids (Kjeldsen et al., 2002). Chloride, nitrate, sulphate and phosphate concentrations were all higher in the dry season than in the rainy season, a pattern attributable to reduced dilution when infiltration and recharge are lower. Chloride is widely regarded as a conservative tracer of leachate contamination because it is highly soluble and undergoes limited chemical transformation in the subsurface (Freeze & Cherry, 1979); its elevated concentration in the leachate therefore signals substantial potential for downstream groundwater impact. Elevated nitrate is consistent with nitrification of ammonium and decomposition of organic nitrogen (Ezeikpe, 2024), and elevated phosphate with the decomposition of food waste and domestic refuse components (Mor et al., 2006); by contrast, the comparatively low sulphate concentrations recorded may reflect sulphate reduction under the anaerobic conditions typical of an actively decomposing waste mass (Christensen et al., 2001). 4.2 Heavy Metal Composition and Environmental Significance The heavy metal composition of the leachate confirms that the dumpsite receives mixed waste inputs, consistent with common sources such as batteries, electronic waste, paints and metal scraps (Alloway, 2013). Iron concentrations, although environmentally significant, may partly reflect natural weathering of lateritic soils in addition to corrosion of metallic waste, a pattern also reported in other tropical lateritic landfill settings (Odukoya & Abimbola, 2010). Copper and zinc were relatively elevated in both leachate and, to a much lesser extent, groundwater, and are commonly associated with galvanised materials, batteries and electrical components; while both are essential micronutrients at trace concentrations, excessive accumulation can become toxic to aquatic organisms and to humans (Alloway, 2013). Cadmium was the most concerning metal identified, exceeding permissible environmental limits in the leachate in both seasons and increasing further in the rainy season. Cadmium is highly toxic even at low concentrations and is recognised for its persistence and bioaccumulative behaviour; prolonged exposure has been associated with kidney dysfunction, skeletal disorders and carcinogenic effects (WHO, 2022). Similarly elevated cadmium levels have been reported at other unmanaged dumpsites in developing countries where waste segregation is poorly practised (Ezeikpe, 2024). Chromium concentrations approached or slightly exceeded recommended limits, plausibly reflecting inputs from paints, dyes and leather-related waste; because chromium mobility is strongly influenced by pH, the slightly acidic conditions recorded in the leachate may enhance its solubility and transport (Alloway, 2013). The coexistence of elevated organic matter and heavy metals is itself significant, since dissolved organic matter can enhance heavy metal mobility through complexation, increasing the persistence and migration potential of metals within groundwater systems (Duan et al., 2024). 4.3 Spatial and Seasonal Distribution of Groundwater Contamination The clear decline in electrical conductivity, total dissolved solids, chloride, nitrate and iron with increasing distance from the dumpsite provides direct evidence that the observed groundwater quality deterioration is attributable to a locally sourced, waste-derived contaminant loading rather than a regional geochemical background. Boreholes located closer to the dumpsite consistently recorded the most degraded water quality, a spatial pattern that has also been documented at other Nigerian dumpsites (Akinbile & Yusoff, 2011; Odukoya, Arowolo & Bamgbose, 2019). The slightly acidic pH recorded in both leachate and nearby groundwater further supports the interpretation that contaminant migration is actively influencing groundwater chemistry, since acidic conditions increase the dissolution potential of heavy metals and enhance their persistence within the aquifer. The consistently higher concentrations recorded in the dry season across almost every parameter, in both leachate and groundwater, demonstrate the dominant control that rainfall- driven dilution exerts on measured contaminant concentrations at this site: reduced recharge during the dry season concentrates dissolved constituents, while increased infiltration during the rainy season dilutes them even as it may simultaneously increase the total mass of contaminant flux into the subsurface. This has practical implications for any future monitoring programme, which should sample across both seasons to avoid misrepresenting either the peak contaminant concentrations reached in the dry season or the greater hydraulic loading that occurs during the rains. The detection of iron at concentrations exceeding the WHO guideline value at every borehole, together with more localised and lower-magnitude impacts from lead, copper, manganese and zinc, indicates a groundwater system that is already measurably impacted by the dumpsite, with the potential for further deterioration should waste disposal continue without engineered containment. Because groundwater remains the principal source of domestic water for many peri-urban communities in this part of Port Harcourt, these findings carry direct implications for public health, particularly given the tendency of iron and other metals to persist and, in the case of nitrate and heavy metals, to pose specific risks such as methemoglobinemia in infants and chronic accumulation-related disease (WHO, 2022). 5. Conclusion This study characterised the physicochemical properties of leachate and groundwater within and around the Tam David-West Road dumpsite, Port Harcourt, in order to establish the nature, magnitude and spatial distribution of contaminant loading associated with the site. The leachate was found to be chemically aggressive, near-neutral to slightly acidic, and heavily loaded with biodegradable organic matter, ammonia and heavy metals, particularly iron and cadmium, at concentrations that greatly exceeded applicable FEPA effluent limits in both the dry and rainy seasons. Groundwater quality declined systematically with proximity to the dumpsite, with the closest boreholes recording markedly elevated conductivity, dissolved solids, nitrate and iron relative to more distant locations, and iron exceeded the WHO guideline value at every borehole sampled. 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