This research was carried out in Chunya District, situated within the Lupa Goldfield at coordinates 8˚33’23.69’’ S and 33˚26’1.88’’ E. Ranking as Tanzania’s second largest goldfield after the Lake Victoria Goldfield, the Lupa Goldfield spans an area exceeding 3,000 km². Historical data reveal that during the colonial era, the region yielded approximately 30 tonnes of gold and 8 tonnes of silver ( Mnali , 2001 ), with an additional 41.24 tonnes of gold extracted between 2015 and 2020 according to the Tanzania Mining Commission (TMC, 2021. The soils around the mining sites in the study area are mainly used for agricultural activities, particularly the cultivation of seasonal crops, including maize and common beans ( Mnali , 2001 ). Figure 1 shows the Chunya district and the five villages where this study was conducted.

The soil sampling procedure used a hybrid sampling approach combining cluster and simple random sampling techniques ( Pennock et al., 2007 ). Five villages Chokaa, Matundasi, Makongorosi, Itumbi, and Mbugani served as clusters, and in each village, a simple random sampling method was used to select two sites for this study. From each site, soil samples were collected at the tailing deposit (0 m), then at 50 m, 100 m, 150 m, and 200 m points along the transect line. At each point, three soil subsamples were collected using a soil auger at 0 - 30 cm, and these were used to constitute a single composite sample following the procedure described by Shapiro & Kranz (1992) . Additionally, one negative control sample was collected 6 km away from any mining activity to provide a baseline for comparison. GPS coordinates for all sampling points were recorded using a Garmin ETREX 22 device to support accurate spatial referencing and mapping ( Huising & Mesele, 2022 ). All samples were air-dried, homogenized, and sieved to remove debris and ensure uniform particle size before digestion as outlined in the procedure by Saha et al. (2017) .

The procedure described by Sharma et al. (2004) was adopted, where ten grams of air-dry soil were placed in a 50 mL beaker, then 25 mL of deionized water for the ratio (1:2.5) was added to the beaker and mixed well using a mechanical shaker for 30 minutes. After leaving the suspension to settle for 15 minutes, EC and pH were measured using an EC meter and a pH meter, respectively. Between each EC and pH reading, the electrode was rinsed with distilled water, and standardization was done after every 25 samples.

The acid digestion protocol was employed to determine the concentration of heavy metals ( Hseu , 2004 ). Ten grams of each sample were placed in a 250 ml conical flask, followed by the addition of 10 ml concentrated nitric acid (HNO₃).

The mixture was heated at 100˚C for 15 minutes to initiate organic matter breakdown, then cooled. Subsequently, 5 ml of concentrated hydrochloric acid (HCl) was added, and the solution was reheated for 30 minutes until brown fumes ceased, indicating completion of the initial digestion.

After cooling, 2 ml of distilled water and 3 ml of hydrogen peroxide (H₂O₂) were added to oxidize residual organic matter. If effervescence persisted, an additional 1 ml of H₂O₂ was added. The final digestion step involved adding 10 ml of concentrated HCl and heating until the volume was reduced to approximately 5 ml. The digested mixture was filtered, diluted to 100 ml with distilled water, and stored in acid-washed containers for analysis. Before the quantification of heavy metals in digested soil and tailings samples, the Atomic Absorption Spectrophotometer (AAS) was meticulously calibrated to ensure analytical accuracy and reproducibility. Calibration was conducted using certified standard solutions of the target elements—lead (Pb), mercury (Hg), cadmium (Cd), arsenic (As), and zinc (Zn)—prepared at predetermined concentrations. Mercury was quantified using Cold Vapour Atomic Absorption Spectrometry (CVAAS), while Graphite Furnace Atomic Absorption Spectrometry (GFAAS) was used for quantification of cadmium, zinc and lead, and Hydride generation AAS was used for quantification of arsenic.

Each standard solution was introduced into the respective AAS, and the corresponding absorbance values were recorded. The absorbance readings demonstrated a consistent and proportional increase with concentration, confirming the expected linearity of the calibration response.

To further validate the calibration procedure, a reagent blank (negative control digest) was incorporated to account for potential background interference. Additionally, verification standards were employed throughout the analytical run to monitor instrument stability and performance.

Subsequently, calibration curves for each metal were constructed by plotting absorbance against concentration. These curves served as the analytical benchmark, enabling the accurate determination of metal concentrations in unknown samples based on their absorbance values.

Using R software, one-way ANOVA was used to analyze the data and Tukey’s Test was used to separate the means.

For Itumbi village site 1, the results indicate that the concentrations of cadmium (0.11 - 2.77 mg∙kg⁻¹) and mercury (1.25 - 0.43 mg∙kg⁻¹) in the soil were above the allowable limit (0.8 mg∙kg⁻¹ for Cd and 0.5 mg∙kg⁻¹ for Pb), while the remaining metals were below the allowable limit. The results also indicate a significant difference in Cd, Pb, and Zn concentrations between the 0 - 200 m distance, where a non-significant difference in concentration from 0 - 200 m distance was recorded for As (p ≤ 0.05). The results also show a decrease in soil pH and EC with distance from 0 to 200 m ( Table 1 ).

For the Itumbi site 2, a similar result was recorded, where the concentration of cadmium (0.21 - 3.78 mg∙kg⁻¹) and mercury (1.01 - 2.87 mg∙kg⁻¹) in the soils was above the allowable limit, while the remaining metals were below or within the allowable limit. A significant difference in heavy metal concentration from 0 to 200 m distance from the tailing deposit was recorded for Cd, Pb, and As, whereas a non-significant difference in concentration from 0 - 200 m was recorded for As (p ≤ 0.05) ( Table 2 ).

For site 1, the results indicate that the concentrations of Cadmium (4.0 - 1.22 mg∙kg^-1) and Mercury (2.66 - 1.29 mg∙kg⁻¹) in the soil were above the allowable

limit for Cd and Hg, respectively. This was contrary to other metals whose concentrations were within the allowable limit. The results also indicate a significant difference in Cd, Pb, Zn, and Hg concentrations between the 0 – 200 m distance, where a non-significant difference in concentration from 0 – 200 m distance was recorded for As (p ≤ 0.05). The results also show a decrease in soil pH and EC with distance from 0 to 200m ( Table 3 ).

For site 2, a similar result was recorded where the concentration of Cadmium (1.33 - 3.76 mg∙kg⁻¹) and Mercury (1.26 - 2.66 mg∙kg⁻¹) in the soils was above the allowable limit, while the concentration of other metals was below or within the allowable limit. A significant decrease in heavy metal concentration from 0 to 200 m distance from the tailing deposit was recorded for Cd, Pb, and Hg, whereas a non-significant decrease in concentration from 0 - 200 m was recorded for As at p ≤ 0.05. There was also a decrease in the average pH and EC with an increase in distance from 0 - 200 m ( Table 4 ).

The results for site 1 indicate that all tested heavy metals recorded higher concentrations between 0 - 200 m compared to the negative control point. The results also indicate that Cadmium (Cd) was above the allowable limit (4.3 - 1.43 mg/kg), while for Hg the distance from 0 - 150 m recorded concentrations above the allowable limit (2.62 - 0.88 mg/kg) except for the 200 m distance from the tailing deposit, whose concentration was slightly below the allowable limit of 0.43 mg/kg. This was contrary to other metals whose concentrations for distances between 0 - 200 m were within the allowable limit. The results also indicate a significant difference in Cd, Pb, Zn, and Hg concentrations between the 0 - 200 m distance, whereas a non-significant difference in concentration from 0 - 200 m distance was recorded for As. The results also show a decrease in soil pH and EC with distance from 0 to 200 m ( Table 5 ).

For site 2, a similar result was recorded where the concentration of Cadmium (1.63 - 4.42 mg∙kg⁻¹) and Mercury (1.17 - 3.72 mg∙kg⁻¹) in the soils was above the allowable limit, while the concentration of other metals was below or within the allowable limit. A significant decrease in heavy metal concentration from 0 to 200m distance from the tailing deposit was recorded for all tested heavy metals at (p ≤ 0.05) ( Table 6 ).

The results indicate that the concentration of Cd and Hg was above the allowable limit (1.07 - 4.24 mg/kg) and (1.17 - 3.39 mg/kg) for Cd and Hg, respectively. This was contrary to other metals whose concentrations for distances between 0 - 200 m were within the allowable limit. The results also indicate a significant difference at p ≤ 0.05 in Cd, Pb, Zn, and Hg concentrations between the 0 - 200 m distance. The results also show a decrease in soil pH and EC with distance from 0 to 200 m ( Table 7 ).

For site 2, similar to site 1, it was recorded that the concentration of Cadmium (1.61 - 4.24 mg/kg⁻¹) and Mercury (1 - 2.31 mg/kg⁻¹) in the soils was above the allowable limit, while the concentration of other metals was within the allowable limit. A significant decrease in heavy metal concentration from 0 to 200m distance from the tailing deposit was recorded for all tested heavy metals (p ≤ 0.05) ( Table 8 ).

The results indicate that the concentrations of Cd and Hg were above the allowable limits (2.85 - 0.89 mg∙kg⁻¹) and (1.98 - 1.53 mg∙kg⁻¹), respectively. There was a significant difference in all tested metal concentrations between the 0 – 200 m distance. The results also show a decrease in soil pH and EC with an increase in distance from 0 to 200m ( Table 9 ).

Similarly, site 2 recorded higher concentrations of Cd and Hg (3.65 - 0.98 mg/kg) and (2.43 - 1.60 mg/kg), respectively, which were above the allowable limit, while the concentration of other tested metals was within the allowable limit. A significant decrease in heavy metal concentration from 0 to 200 m distance from the tailing deposit was recorded for all tested heavy metals at (p ≤ 0.05). There was also a decrease in soil pH with an increase in distance from the tailing deposit ( Table 10 ).

Concentrations of Pb at the tailing heap (0 m and 200 m)

At the tailing deposit site (0 m), the concentration of Pb ranged from 24.66 mg/kg at Matundasi site 2 to 41.33 mg/kg at Itumbi village site 2. The results also show that for most sites, there was a small difference in the concentration of Pb ( Figure 2 ).

At 200 m ( Figure 3 ), the lowest Pb concentration was recorded in Mbugani village site 2 (15.87 mg/kg), while the highest concentration was recorded in Itumbi village site 2 (28.34 mg/kg) ( Figure 3 ). The results show a significant decrease in Pb concentration from 0m to 200m (p ≤ 0.05).

Concentrations of Cd at the tailing heap (0) and 200 m

At 0 m ( Figure 4 ), the concentration of Cd ranged from 2.7 to 4.42 mg/kg, where the lowest concentration was recorded in Itumbi village site 1, while the highest was recorded in Mbugani village site 2. However, a small difference was recorded (Graph 4) in the concentration of Cd for most of the studied sites.

At 200 m ( Figure 5 ), the lowest Cd concentration was recorded in Itumbi village site 1 (0.11 mg/kg), while the highest concentration was recorded in Mbugani village site 2 (1.63 mg/kg). The results show a significant decrease in Pb concentration from 0 m to 200m (p ≤ 0.05).

Concentration of Hg at the tailing heap (0 m) and 200 m

The results indicate that the lowest concentration of Hg at 0 m ( Figure 6 ) was recorded at the Makongorosi site 2 (1.61 mg/kg), while the highest concentration was recorded at the Mbugani village site 2 (3.72 mg/kg) ( Figure 6 ), the concentration of which was above the allowable limit. The results indicate a small range of Hg concentrations among the studied sites, with the lowest value recorded at Mbugani Village Site 1 (0.88 mg/kg) and the highest at Chokaa Village Site 2 (1 mg/kg) ( Figure 7 ). The results show a significant decrease in Hg concentration from 0 m to 200 m for all sites.

Concentrations of As at the tailing heap (0 m) and 200 m

The concentration of As at the tailing heap ( Figure 8 ) ranged from 0.78 mg/kg to 1.83 mg/kg, where all concentrations were within the allowable limit. The highest concentration was recorded in Chokaa village, while the lowest concentration was recorded in Itumbi village.

At 200 m, the lowest As concentration was recorded in Matundasi village site 1 (0.15 mg/kg) while the highest concentration was recorded in Itumbi village site 2 (0.45 mg/kg) ( Figure 9 ). The results show a significant decrease in Pb

concentration from 0 m to 200 m.

Concentrations of Zn at the tailing heap (0 m) and 200 m

At 0 m, both site 1 from Mbugani village and site 2 from Itumbi village recorded the lowest concentration of Zn (14.51 mg/kg), while the highest concentration of Zn was recorded in Matundasi village site 2 (27.74 mg/kg) ( Figure 10 ). The results show a wide range of differences in the Zn concentration at the tailing deposit site for the studied sites.

At 200 m, the lowest Zn concentration was recorded in Itumbi 1 village site (5.98 mg/kg), while the highest concentration was recorded in Mbugani village site 1 (13.99 mg/kg) ( Figure 11 ). The results show a significant decrease in Pb concentration from 0 m to 200 m for all sites.

The combined influence of soil pH and electrical conductivity (EC) governs how heavy metals move from the tailings toward surrounding soils. Close to the tailings (0 - 50 m), EC is very high (up to about 2800 µS cm⁻¹) and pH is neutral to slightly alkaline (≈7 - 8) (refer to Table 1 and Table 2 ). According to Nordstrom et al. (2015) , tailing heaps often have high pH (alkaline) and high Electrical Conductivity (EC) levels due to the specific chemical reagents used during mineral processing and the natural mineral composition of the ore. Under these conditions, the abundance of dissolved salts promotes ion exchange and desorption, allowing lead (Pb), cadmium (Cd), zinc (Zn), and mercury (Hg) to enter the soil water and migrate laterally. However, as water flows and percolates through the soil profile, soluble salts are leached out and EC rapidly decreases to below 40 µS cm⁻¹ at 200 m. At the same time, pH declines to weakly acidic values of about 5.5 - 6.5. Acidification favours the adsorption of metal cations onto clays and iron or manganese oxides or their precipitation as stable minerals, which sharply lowers their dissolved concentrations.

This two-stage process—initial mobilization followed by immobilization—explains the statistically significant drop in heavy-metal concentrations with distance observed at all sites. The mechanism is consistent with laboratory and field findings reported by Violante et al. (2010) , who demonstrated that decreasing pH promotes adsorption of Pb, Zn, and Cd on soil minerals and thereby reduces their mobility; by Grobelak and Kowalska (2020) , who showed that acidification and loss of soluble salts limit heavy-metal movement and bioavailability; and by Kumkrong et al. (2022) , who found that high electrical conductivity in gold and silver tailings accelerates the early leaching of Cd and Zn, while the sharp decline in EC downstream limits further migration. Similar conclusions were drawn by Gitari et al. (2018) , who observed that lower pH and lower ionic strength downstream of abandoned copper-mine tailings led to natural attenuation of toxic metals, and by Jiang et al. (2021) , who confirmed that acidic conditions and reduced EC enhance the fixation of heavy metals and reduce their leaching potential.

Collectively, these studies support the present findings that neutral to alkaline, saline tailings facilitate the release of heavy metals near the source. As soils become less saline and slightly acidic with distance, natural immobilization processes significantly restrict the migration of heavy metals.

For instance, at the Makongorosi site 1, Pb concentrations decrease from approximately 36 mg∙kg⁻¹ at pH 8 and EC 2800 µS cm⁻¹ at the tailings point to 24 mg∙kg⁻¹ at 200 m, where pH drops to 6 and EC to 2 µS cm⁻¹. Chokaa exhibits a similar trend, with Cd declining from 4 mg∙kg⁻¹ at pH 8.3 and EC 1056 µS cm⁻¹ near the source to 1 mg∙kg⁻¹ at 200 m, where pH and EC are 6.5 and 28 µS cm⁻¹, respectively.

The results indicate that the Pb concentration between 0 - 200 m for all sampling points was within the permissible limits set by environmental guidelines, such as the WHO/FAO standard of 100 mg/kg for agricultural soils ( WHO (2008) ). This result is consistent with the study by Kaaya et al. (2025) , who reported low concentrations of Pb in Geita mining sites in Tanzania, which could cause negligible health risks. This lead concentration found in the study area is lower than the elevated concentrations reported in both South Africa and Ghana (80 - 510 mg/kg).

Pb is known to have low mobility in soil; hence, elevated concentrations are expected at point sources of contamination, which are tailing deposits ( Shu et al., 2014 ; Martín et al., 2014 ; Rodríguez et al., 2009 ). The results show a significant decrease in Pb concentration with the decrease in soil pH and electrical conductivity; for instance, the concentration of Pb at the tailing heap (0 m) was 41.33 ± 0.54 mg/kg while the soil pH and electrical conductivity were 6.13 and 950 µS cm⁻¹, respectively. The concentration decreased to 28.34 ± 0.55 mg/kg at 200 m when soil pH and electrical conductivity reduced to 5.7 and 25 µS cm⁻¹ in Itumbi village site 2 ( Table 2 ).

Potentially, Pb poses a serious health risk to both adults and children when it is at high concentrations; it is associated with impaired fertility and complicated pregnancy, cardiovascular diseases, impaired kidney function, high blood pressure, and neurodevelopmental effects ( WHO, 2011 ). Also, Gimmler et al. (2002) and Abdulkareem et al. (2015) revealed that exposure to high Pb concentrations is linked to numerous health and environmental issues, such as renal, neurological, and developmental effects (Haque et al., 2021), soil pollution, and phytotoxicity.

The study recorded higher concentrations of Cd than the permissible limit (0.8 mg∙kg⁻¹) for most of the studied sites, whereby the highest value was 4.42 mg∙kg⁻¹, while the lowest concentration was 0.11 mg∙kg⁻¹. This concentration is, however, slightly below the Tanzania general reported increasing concentration of Cd (6.4 - 11 mg/kg) in mining areas ( Fikri et al., 2023 ).

However, the result is consistent with area-specific studies such as Kaaya et al. (2025) , who reported a higher concentration of Cd in Ifugandi (13.67 mg∙kg⁻¹) and Bululu (20.13 mg∙kg⁻¹) Gold mining sites in Tanzania. Another study by Kaggwa et al. (2024) reported potentially higher concentrations of Cd in tailing heaps (e.g., a mean of 11.83 ± 3.01 mg/kg at Nyarugusu-Geita, Tanzania) and the concentration reduced to 0.553 - 0.770 mg/L in mining drainages. Similarly, the lateral mobility of Cd was found to be governed by soil pH and electrical conductivity; for instance, in Matundasi village site 1, the concentration at the tailing heap (0 m) was 2.85 mg∙kg⁻¹ while soil pH and electrical conductivity were 8.2 and 77 µS cm⁻¹ respectively, whereas at 200 m, the concentration reduced to 0.98 mg∙kg^-1 while soil pH and electrical conductivity also reduced to 6.0 and 25 µS cm⁻¹ respectively ( Table 9 ).

When compared to other heavy metals such as Pb and Cu, Cd exhibits high relative mobility, often found in acid-extractable fractions in the soil, increasing the risk of contamination. This has been confirmed by this study, where higher concentrations of Cd above the permissible limit (0.8 mg/kg) were even found at the furthest distance (200 m) from the tailing heap. A study by Merrington and Alloway (1994) indicated that more than 4.2 kg Cd/a was being transferred from the tailing heaps via the streams in the form of dissolved load.

Higher concentrations of Cd in the soil pose human health and environmental risks ( Ahmad et al., 2021 ; De Vries et al., 2007 ). Studies refer to Cd as an important environmental and industrial toxicant that has a very long half-life, ranging from 10 to 35 years. It is a multi-organ, multi-system poison that actively migrates through soil–plant systems ( Wang et al., 2021a ). Various studies have highlighted the human health risks of exposure to Cd, which include an increased risk of cancers, such as bladder, prostate, kidney, pancreatic, and breast cancers ( Qing et al., 2021 ; Tchounwou et al., 2012 ).

On the nervous system, Cd exposure is thought to contribute to the onset of central nervous system (CNS)-related disorders, such as Parkinson’s disease (PD), Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), Huntington’s disease (HD), and multiple sclerosis (MS).

The results indicate that the concentration of Hg at all sampling points was higher than the permissible limit (0.5 mg/kg). The highest value recorded was 3.72 mg/kg, while the lowest concentration was 0.88 mg/kg. The results indicate higher mobility of Hg from the tailing heap (0 m) to the soils, whereby higher concentrations above the allowable limits were even recorded at the furthest distance (200 m) from the heap. This result is consistent with various studies that have been conducted in Tanzania, which have reported concentration levels in soils and tailings ranging from 1.7 to 53.8 mg/kg ( Taylor et al., 2005 ; Herman & Kihampa, 2015 ; Sanga et al., 2023 ). The result is also in line with the findings by Gonçalves Jr. et al. (2017) and Fikri et al. (2023) , who justified that since mercury is commonly used in gold extraction, it is frequently found in elevated levels at ASGM sites in Tanzania, Ghana, and Ecuador.

Other metals such as nickel (Ni), cobalt (Co), manganese (Mn), and iron (Fe) have also been detected in high concentrations near mining areas across Africa, Asia, and Latin America ( Ogundele et al., 2021 ; Wiafe et al., 2022 ; Montalván-Olivares et al., 2021 ; Yabe et al., 2010 ).

Higher concentrations of Hg are reported because the metal is used mainly for the processing of primary gold quartz veins and supergene gold mineralizations. Gravimetric material flow analyses show that 70% - 80% of the Hg is lost to the atmosphere during processing, and 20% - 30% is lost to tailings, soils, stream sediments, and water. For instance, for every 1 g Au produced, 1.2 - 1.5 g Hg are lost to the environment ( Maglambayan et al., 2005 ).

Mercury mobility in soil is reported to vary greatly, from very low in soils with high organic matter and sulfur too high in sandy soils, and depends on its chemical form, soil properties like pH, organic matter, and sulfur content, and environmental factors such as redox potential ( Šípková et al., 2016 ; Boszke et al., 2008 ; Skyllberg, 2012 ).

Mercury tends to immobilize in the upper soil layers due to its affinity for soil minerals and organic matter, forming stable complexes that can reduce its potential to move to groundwater.

Higher concentrations of Hg in the environment above the permissible limit cause significant environmental and health risks ( Kim et al., 2016 ). For instance, in human health, Hg has become one of the most serious problems threatening human subsistence ( Clarkson, 2002 ; Tsuji et al., 2003 ). Mercury (Hg) is recognized as one of the most harmful metals with potential impacts on human health due to its volatility, its long life in the atmosphere, and its tendency to accumulate in living organisms ( Yu et al., 2016 ). Human poisoning can result from both short-term and prolonged exposure to mercury ( Carocci et al., 2013 ).

The results indicate that the concentration levels of As at all sampling points were within the permissible limit of 20 mg/kg. The highest concentration was 1.83 mg/kg ( Table 8 ), while the lowest concentration was 0.15 mg/kg ( Table 10 ). This result is contrary to the study by Kaaya et al. (2025) , who reported higher concentrations of As, i.e., 6.17 ± 4.41 mg/kg in Nyarugusu and 63.8 ± 53.2 mg/kg in Lwamgasa, both mining sites from Geita District in Tanzania. This result also indicates that As levels are extremely far below the reported elevated concentrations of As in Ghana (8305 mg/kg) ( Gonçalves Jr. et al., 2017 ).

The low levels of arsenic in the study area can be attributed to the presence of little arsenic-rich ores, such as arsenopyrite ( Garelick et al., 2008 ; Majzlan et al., 2014 ). Additionally, limited oxidation and leaching activities of sulfide minerals can account for the low levels of As in the environment ( Lengke et al., 2009 ).

The lateral mobility of arsenic is variable and often relatively low, but it can increase significantly under certain conditions. For instance, in highly acidic and highly alkaline conditions, arsenic exhibits higher mobility, with greater movement observed in acidic environments, while in neutral pH conditions (pH ≈ 7), As is more stable and less mobile ( Campbell & Nordstrom, 2014 ; Patel et al., 2023 ). Water flow and contact time enhance leaching, while factors like organic matter and certain mineral compositions can either increase or decrease mobility ( Carrillo‐González et al., 2006 ).

According to Wang et al. (2021b) , arsenic was listed first among substances toxic to human health by the Agency for Toxic Substances and Disease Registry (ATSDR) from the Department of Health of the United States, and thus was recognized as an important carcinogen.

Higher concentrations of arsenic in the mining sites and soils are associated with serious environmental and health problems, including cancers, cardiovascular diseases, neurological damage, and skin lesions in humans ( Chen et al., 2009 ; Singh et al., 2007 ). It contaminates soil and water through waste products, potentially harming ecosystems and local communities. The effects depend on the arsenic species, dose, and duration of exposure ( Patel et al., 2023 ; Singh et al., 2007 ).

The results indicate that the concentration levels of Zn for all sampling points were within the permissible limit (200 mg/kg). The highest concentration of Zn was 27.74 mg/kg ( Table 10 ), while the lowest concentration was 5.98 mg/kg ( Table 2 ). This result is consistent with other studies conducted in other Tanzanian gold mining sites and tailing heaps, where the concentration is at higher levels (e.g., 101.9 ± 20.6 mg/kg) than surrounding soils (e.g., 68.67 mg/kg ( Machiwa , 2003 ). Another study by Pkemoi (2024) reported a Zn mean concentration of 54.17 - 118.07 mg/kg at the tailings. The concentrations of Zn in the study area are consistent with elevated concentrations reported in South Africa, showing (8.9 - 65.7 mg/kg), where levels for both areas fall within the permissible limits (below 200 mg∙kg⁻¹).

The low concentration of Zn in the study sites can be attributed to the nature of the gold ore composition, whereby if the ore is not rich in zinc, the tailings will naturally have low zinc content ( Rodríguez et al., 2009 ). However, leaching occurring under acidic conditions can lead to the dissolution of zinc from the ore, making it more mobile and causing it to be washed out of the mine and tailings ( Sethurajan et al., 2017 ; Shu et al., 2001 ). On the other hand, a lower concentration of Zn in the soils can be attributed to the soil pH, where the study sites are dominated by neutral to alkaline conditions, where Zn is least mobile.

According to Gangloff et al. (2006) , zinc mobility in soil is generally low, as it is tightly held by soil particles and organic matter. Zn movement is largely controlled by soil pH (least mobile in neutral to alkaline soils, while highly mobile in acidic conditions).

High levels of zinc in the environment have been associated with altering soil and aquatic microbial diversity, and can thus affect the bioavailability and absorption of other metals as well ( Hussain et al., 2022 ). In humans, exposure to excessive zinc levels leads to toxicity; this toxicity is, however, a treatable and non-life-threatening condition. Several symptoms cause distress to human activities and lifestyle, including fever, breathing difficulty, nausea, chest pain, and cough ( Hussain et al., 2022 ; Balch et al., 2011 ). In plant nutrition, excessive Zn levels are associated with decreased soil fertility and potential toxicity to plants ( Van and Nga, 2024 ).