Solutions and reagents used include: distilled water, 37% hydrochloric acid (Sigma-Aldrich Chemicals, USA), 65% nitric acid (Riedel-De Haen, Germany).

Flame atomic absorption spectrophotometer (Model: GBC Avanta Ver. 2.02/AA6600), analytical weighing balance (Model: Mettler AE200), hot plate (Model: Corning PC-351), Soil sieve (Model: Endecotts Ltd - stainless steel with 2mm mesh and brass frame).

Study was conducted along the stretch of Epie Creek, from Igbogene to Okutukutu axis of Yenagoa City. Yenagoa metropolis with latitude 5˚23'N and 5˚30'N, and Longitude 6˚58'E and 7˚04'E is located in Bayelsa State, within the south-south region of the oil-rich Niger Delta zone of Nigeria.

Three replicate water samples were randomly collected from six locations along the Epie Creek. Surface waters were directly collected from the superficial layer into sampling containers. The sites were: up-Igbogene (control) (latitude 05.0386, longitude 06.4033); Igbogene (latitude 05.0255, longitude 06.3996); Akenfa (latitude 05.0019, longitude 06.3787); Agudama (latitude 04.9791, longitude 06.3675); Edepie (latitude 04.9616, longitude 06.3660); and Okutukutu axis (latitude 04.9543, longitude 06.3448). The sampling stations were selected based on their proximity to effluent discharge points in which waste dump sites were used as point sources of pollution along the river, while up-Igbogene axis was sampled as control. The grass species were found sprouting within the water environment but at close proximity to the river embankments. From each of the field locations, three replicate grass samples were collected.

Samples were collected in the dry season month of December 2017. The water samples were collected from the sampling stations in clean, sterilized 250 mL plastic bottles. They were then acidified with 5 mL concentrated nitric acid and transported to the laboratory for heavy metal analysis. In the field, the containers were severally rinsed with habitat water at each sampling point prior to collection.

Each sample was collected by submerging the receiving container into the river at about 100 mm to 300 mm below the surface with the open end aligned against the flow direction of water current. Similarly, the plant (Sacciolepisafricana) samples were collected from around point source waste dump sites. Afterwards, vegetation samples were placed in aerated polyethylene bags in order to retain their fresh condition while been transported to the laboratory.

All laboratory wares were washed with laboratory detergent, soaked in 10% hydrochloric acid solution overnight, thoroughly rinsed in distilled water, and then dried at 85^oC. Exactly 100 mL of unfiltered water was strained through whatmann filter paper, transferred into a 150 mL glass beaker, before adding 5 mL of concentrated nitric acid solution. Afterwards, the acidified water was evaporated to near-dryness on a hot plate. A further 5 mL of concentrated nitric acid solution was added before the sample mixture was continuously heated for 15 minutes. Sample was concentrated to about 5 mL before leaving it to cool at room temperature. Thereafter, the acid digest was filtered into 100 mL volumetric flask and the volume made up to mark with distilled water. The filtration step aids in the removal of silicate and other insoluble materials prior to the aspiration of acid extracts for Pb, Cr, and Fe analysis on the atomic absorption spectrophotometer (AAS). Instrument detection limits were 0.02 mg/L, 0.006 mg/L and 0.09 mg/L for Pb, Cr and Fe respectively. The solution was stored in 125 mL polypropylene bottle and aspirated through the nebulizer unit of the instrument. Concentrations of the respective metals were reported in mg/l units 2021.

The plant samples were isolated into their leaf, stem and root sections. All the plant parts were washed with distilled water to remove dirt, dust, and other contaminants. Furthermore, the vegetation samples were washed with distilled water and dried at room temperature (22°C to 25°C) before further subjecting them to oven-drying at 60˚C. The dried plant parts were crushed, powdered and homogenized. The powdered samples were stored in polyethylene sampling bags for further processing. Subsequently, a muffle furnace was used to dry-ash the plant samples. Five grams of each dry-ash sample was weighed into a conical flask. Each of the weighed samples was mixed with 10 mL of 1 N hydrochloric acid (HCl) and 10 mL of 1 N nitric acid (HNO₃). A mixture of the ash solution was predigested at about 50˚C on a hot plate. Heating continued until white fumes evolved, resulting in a clear brown-colored solution. The heat was further intensified to 120^oC for few minutes until sample was adequately concentrated. Afterwards, the cooled ash solution was filtered and made up to mark in 50 mL volumetric flask using distilled water. Filtrates were made through whatmann No. 1 filter paper and collected into clean and sterilized polypropylene vials. Subsequently, the filtered sample (ash) solutions were aspirated into the atomic absorption spectrophotometer (AAS) and the concentrations of test elements such as lead (Pb), iron (Fe), and chromium (Cr) were determined at wavelengths of 217.0, 248.3 and 357.90 nm respectively. Subsequently, concentrations were recorded in mg/kg units 22232425.

The concentrations of Pb, Cr and Fe in the different sample matrices were determined using FAAS (GBC Avanta PM, A6600, Australia). The operational condition of equipment is provided in Table 1 below.

In order to determine the association and variation between test metals in surface waters and vegetation of Epie Creek, descriptive statistical analysis was carried out using statistical package for social science (SPSS) version 20. Data was expressed as mean ± standard deviation. The range (minimum and maximum) of the values obtained across the sampling points was also presented. One way analysis of variance (ANOVA) was used to show significant variation at P = 0.05. Where significant variation occurred, Waller-Duncan statistics was used to compare mean values of each test parameter under investigation. Heavy metal distribution in water and vegetation was correlated using Spearman’s rho correlation matrix.

The concentrations of chromium (Cr), lead (Pb) and iron (Fe) in the water and plant (cupscale grass) samples from Epie creek (up-Igbogene (control)) and the other five sample locations (Igbogene, Akenfa, Agudama, Edepie and Okutukutu) are presented in Table 2 and Table 3.

The result shows a wide range of concentrations of the selected heavy metals. The concentrations of iron (Fe) in mg/L are: 4.05 ± 0.06, 5.06 ± 0.04, 3.42 ± 0.06, 4.05 ± 0.04, 12.06 ± 0.06 and 5.44 ± 0.05 respectively for up-Igbogene (control), Igbogene, Agudama, Akenfa, Edepie and Okutukutu sampling sites. Iron was the most significant mineral concentration across all sample locations while lead and chromium were below the instrument detection limits of 0.02 and 0.006 mg/L respectively. Iron levels exceeded the stipulated WHO permissible limit of 1.0 mg/L. The result obtained from this study showed an increasing level of iron in the surface water environment as compared to the concentration range of 0.32 to 2.52 mg/L that was previously reported 26. The spike in iron may be from the turbid appearance of water samples emanating from soil runoffs or leachates from increasing anthropic occurrences of waste dumpsites. A greater mobility factor of 0.84% for iron in dumpsite environments as compared to 0.71% in uncontaminated control habitats has been reported in a previous study 27.

Results obtained for the metals (Cr, Fe and Pb) in water samples were compared with WHO maximum permissible limits. The permissible limits of Cr, Fe and Pb for water according to WHO are 0.1 mg/L, 1.0 mg/L and 0.05 mg/L respectively 28. Consequently, the concentrations of Cr and Pb were recorded within WHO permissible limits while Fe values exceeded stipulated regulatory comparison (Table 4). The results of Cr and Pb obtained in this study partially corroborated 20 where Cr recorded mean values of 0.27 ± 0.05 mg/L and 0.23 ± 0.01 mg/L in the Middleton creek, Bayelsa state, during the dry and wet seasons respectively. In addition they reported Pb amounts below measurable detection limit of the instrument (0.02 mg/L). On the other hand, 21 had reported Pb values ranging between <0.001 and 0.007 mg/L in the surface waters of Kolo creek. Contrastingly, the concentrations of Pb and Cr significantly exceeded WHO limits in oil contaminated surface waters of Azuabie creek 29. The trace or non-detectable levels of Pb and Cr along Epie creek is an indication that activities releasing these micropollutants into the environment are rare.

The concentrations of Cr, Pb and Fe in the various plant sections; roots, stem and leaves, obtained from all the sample and control locations are presented in Table 3a.

Results indicate that the concentration of iron in vegetation sample was in the order; root > leaf > stem. The study further shows that the concentrations (mg/kg) of lead (Pb) in most of the sample locations and up-Igbogene (control) were below measurable detection limit of the instrument for various parts of the plant (Table 3).

Results show that the highest concentrations of iron (Fe) and chromium (Cr) were found in the plant roots, followed by the leaves, and lastly the stem across all sampling sites. There was significant difference in the concentration of iron and chromium in the roots, leaves and stem for all sample sites. The concentration of metals in the plant tissues revealed the trend; roots > leaves > stem. This may have resulted from the embedding of roots in soil and direct absorption of minerals from bottom soil which ultimately leads to elevated concentrations. The absorbed minerals are transported to the leaves where they are used for photosynthesis whilst being accumulated in leaves. The stem only allows transfer of minerals from the roots to leaves, leading to decreasing element concentrations in the stem.

The FAO/WHO maximum permissible limit for Fe, Cr and Pb in leafy vegetable is 425.0 mg/kg, 0.10 mg/kg and 0.3 mg/kg respectively 30. For iron (Fe) concentrations, values ranged between 365.53 mg/kg and 1,012.85 mg/kg (leaves), 196.63 mg/kg and 492.57 mg/kg (stem) and 2,399.30 mg/kg and 9,423.17 mg/kg (roots). The most significant iron concentrations were recorded at the root section of the grass species. Apart from stem sample collected at Agudama sample station, all other stem samples were found at concentrations within the FAO/WHO regulatory limit of 425.0 mg/kg for iron (Fe) (Table 3b). Trends across both sampling locations were observed to be similar for Fe mineral. Apart from the samples of Edepie stem, Igbogene stem and root, and Okutukutu stem which showed Cr values below the instrument detection limit of 0.006 mg/l, chromium (Cr) levels ranged from 0.25 to 3.19 mg/kg (leaves), <0.006 to 0.95 mg/kg (stem) and <0.006 to 8.83 mg/kg (roots) across all sampling locations, with the root section of Sacciolepisafricana plant collected from Edepie axis reflecting the most significant concentration of 8.83 mg/kg. Most of the chromium concentrations obtained in this study exceeded FAO/WHO recommendation of 0.10 mg/kg. Apart from the observed presence of lead (Pb) in Akenfa leaf (0.21 mg/kg) and Akenfa stem (0.34 mg/kg), all other sample locations recorded lead (Pb) values below the instrument detection limit of 0.30 mg/kg. The levels of iron (Fe) and chromium (Cr) in common edible vegetable leaves and stems sold in Yenagoa metropolis has been assessed previously 22. Iron (Fe) values reported in both studies are similar, with reported concentrations ranging between 307.6 mg/kg and 1,051.31 mg/kg. On the other hand, chromium (Cr) level in this study was lower when compared to the range of 8.12 mg/kg to 31.72 mg/kg reported for the commonly consumed vegetables. The dissimilarities in chromium (Cr) composition of plant tissues from the two different studies may have resulted from the variations in the trace/heavy metal bioaccumulation potential of the diverging plant species as well as the difference in geology of field area.