Dispersal Mechanisms of Trace Metal Elements in the Environment: The Case of Mineral Wastes Stored in Tshamilemba District of the City of Lubumbashi, DR Congo

Abstract

Since 2001, the Tshamilemba quarter, located in the City of Lubumbashi (DRC), has been home to copper- and cobalt-producing plants, which generate great amounts of mineral waste, the storage of which has resulted in environmental pollution. Previous studies conducted in the Tshamilemba district have identified the weathering process of stored mineral wastes as the main source of trace metal elements (TMEs) involved in the contamination of soil and well water, and have highlighted the population exposure to cobalt. This study strives to identify or establish the dispersal mechanisms of pollutants in the environment that contaminate soil, surface water and edible plants. This study measured major physicochemical parameters, determined TME concentrations in samples (soil, water and edible plants) and established, based on data from soil sample analysis mathematically processed using Matlab 7.1 software, the spatial distributions of TMEs, in both the upper and deep soil (20 cm). The soil sample analysis revealed an average pH of 7.69 and a value of 9.1 for the near-white crusts collected at some spots. In the soil, TMEs were present in upper layers (Co, Cu, Zn and Fe) and the deep layers (Co, Cu, Pb, Zn and Fe) at phytotoxic concentrations. TMEs were observed in water samples at concentrations (Cu, Co, Mn, Zn and Pb) surpassing the quality standards for drinking water. This also applies to edible plant samples of Saccharum officinarum (Co, Cd, Ni, Mg and Pb) and Musa acuminate (Cd, Co, Pb, Zn, Mn and Ni). TMEs disperse in the environment as airborne particles from aerial erosion and as dissolved species in run-off water, mixed with acidic, metal-rich waters spreading from the weathering of stored mineral waste. TMEs contaminate the surrounding soil near to the surface water and build up in edible plants. Therefore, fear among the population about the environment pollution in Tshamilemba is well justified. Understanding the dispersal mechanisms of TMEs is of paramount importance to better control and to contain mineral pollution and design strategies for minimizing the effects on human health.

1. Introduction

Mining is one of the industrial activities that leads to the most serious environmental issues, experienced in many regions of the world [1,2,3,4,5,6,7,8,9]. This is exemplified by the improper management of mineral waste and wastewaters generated due to mining activities over many years in the DRC. This practice has resulted in the pollution of soil and water resources in the former Katanga region, including in the Tshamilemba district which is studied in this paper [4,10,11,12,13,14]. Indeed, the environmental footprint of mining activities conducted in the Katanga region is such that TMEs have contaminated arable soils with a serious threat to wildlife and human health [4,15,16]. This situation has mainly resulted from the fact that the vast majority of mineral processing and hydrometallurgical plants in the Katanga region have not always had an appropriate management system for mineral waste. Furthermore, at most hydrometallurgical plants, liquid effluents rich in metals were frequently discharged into the environment after extraction of useful metals contained in ores [4,12,17,18]. Consequently, solid waste generated by mining operations and mineral waste discarded by hydrometallurgical processes were often stored without any environmental protection measures [13,19,20]. As for liquid effluents and mine waters, they were mostly discharged into rivers without prior treatment to remove toxic pollutants that might be present [13,15,18,21,22,23]. These waste management practices in the mining industry have resulted in the contamination of surface and ground water and neighboring soil [24].The above reveals that mining activities remain the major source of serious environmental issues in the Katanga region [13,15,18]. Indeed, in areas where mineral ores are extracted in view of producing valuable metals, mining operations are always accompanied with various environmental impacts that endanger wildlife and human health [13,25].

In the city of Lubumbashi, for instance, since the resumption of industrial mining activities in 2001 [23,24], the population living in the Tshamilemba district has been complaining about the persistent deterioration of the environment. Indeed, in this district, the appearance of near-white crusts on the walls of houses and in some spots of the soil surface has been observed. Furthermore, the population has complained about the change in air quality, resulting in them experiencing nostril irritation during breathing. This is in addition to the poor quality of the water [17] abstracted from wells, and the harm can be seen on edible plants grown in private gardens. Previous studies on the pollution that threatens the environment of the Tshamilemba district have identified sulfur-rich acid mine drainage, from improperly stored mineral waste, as the main source of TMEs involved in the contamination of soil and well water [17,25]. Other studies, essentially relying on urine analysis, highlighted the exposure of the population to cobalt in the Tshamilemba district [12]. A study conducted by other researchers [23] attempted to reprocess mineral waste in view of recovering copper and cobalt as a strategy for minimizing the mineral processing environmental footprint. Findings from previous studies revealed the risks of the food chain impacted by toxic metals [17] as a serious threat to wildlife and human health [16,25]. However, no study has aimed at identifying the routes or establishing the mechanisms through which pollutants liberated by stored mineral waste spread in soil and near to surface waters.

This study was especially prompted by fear among the population [25] in terms of their health, following the presence of near-white crusts on the soil and walls of houses in the Tshamilemba district after the rainy season, and nostril irritation when breathing air during the dry season [25]. This study strives to ascertain whether fears among the population [25] about environmental pollution are justified. Consequently, an emphasis was placed on the search for TMEs in samples collected in the Tshamilemba district and on the identification of the mechanisms through which pollutants liberated by mineral wastes improperly stored for years disperse in the environment and endanger wildlife and human health.

To achieve this aim, samples of soil, water from wells and edible plants were collected in the Tshamilemba district and subjected to analyses in view of determining the most relevant physicochemical parameters and their TME concentrations. The sample analysis data were mathematically processed using Matlab software 7.1 (The MathWorks, Inc., Natick, MA, USA) to establish the spatial distributions of toxic metals. The obtained results were compared to quality standards established by international organizations such as the World Health Organization (WHO) and the US Environmental Protection Agency (EPA) [3,26,27]. In parallel, spatial distributions of metals in the soil were established. The above will enable ascertaining if fear among the population [25] about pollution is justified and identifying the mechanisms or pathways through which TMEs liberated by stored mineral wastes disperse in the environment. Understanding the dispersal mechanisms of metals is expected to enable designing strategies [28,29,30,31] to better control and contain mineral pollution in the Tshamilemba district to minimize harmful effects on wildlife and the population health.

2. Materials and Methods

2.1. Location and Description of the Study Area

The Tshamilemba district is located near the center of the city of Lubumbashi, in the municipality of Kampemba. Tshamilemba is surrounded to the west by Ndjanja School, to the north by the mining company CHEMAF SARL and to the east by the Kabetsha district [12]. The north boundary of the study area (see the perimeter ABCD in Figure 1) is located close to an area used for the storage of mineral waste. These consist of solid residues from the hydrometallurgical processing of ores (3.2% Cu and 1.5% Co). According to [23], mineral waste contain on average 2.01% Cu and 0.32% Co present as chalcopyrite (CuFeS₂), chalcocite (Cu₂S), carollite (CuCo₂S₄), chrysocolla (CuSiO₃.2H₂O) and malachite ([CuCO₃.Cu(OH)₂]), accompanied by other minerals: pyrite (FeS₂), chlorites ([Mg₆Si₄O₁₀(OH)₁₀]), hematite (Fe₂O₃), limonite ([FeO(OH).xH₂O]), quartz(SiO₂) and clay minerals.

2.2. Description of Samples and Sampling Procedures

To achieve the research aims, we collected 24 surface soil samples, 24 deep soil samples, 4 samples of the near-white substances observed on the soil, 17 water samples, 5 samples of Saccharum officinarum (banana) and 5 samples of Musa acuminate (sugar cane).

2.2.1. Sampling of Soil, Plants and Well Water

Samples of well water and edible plants were taken around every point chosen for soil collection (Figure 2), as described below:

• Concerning the sampling of water, a weighed container was attached to a rope and placed in the well to collect water. Each water sample was kept in a clean polyethylene (500 mL) bottle;
• The soil was sampled at the surface and at a depth of approximately 20 cm. A 30 cm long steel pipe, with the inner diameter of 4 cm, was driven into the soil using a hammer until the desired depth was reached. Afterwards, it was removed to collect both the surface soil and the deep soil. Every time, two samples were prepared for the analysis and kept in small, well-coded plastics bags;
• To sample the Saccharum officinarum, sugar canes were collected and cut into small pieces. They were first washed with distilled water before being placed into clean plastics bags. As for Musa acuminata sampling, whole bananas were collected, peeled and cut into small slices. Afterwards, they were washed with distilled water and kept in plastics bags.

Figure 2. Study area with sampling points highlighted: (a) Soil samples. (b) Near-white crusts samples. (c) Soil samples for pH measurement. (d) Saccharum officinarum samples. (e) Musa acuminata samples.

Figure 2. Study area with sampling points highlighted: (a) Soil samples. (b) Near-white crusts samples. (c) Soil samples for pH measurement. (d) Saccharum officinarum samples. (e) Musa acuminata samples.

2.2.2. Sampling of Near-White Substances Observed on the Soil

Near-white crusts were collected using a spatula around the soil sampling points and the samples were kept in clean plastic bags. The whitish crusts are composed of saline matter that covers at some spots the soil of the Tshamilemba district. They are most noticeable when the soil becomes dry and most people believe they are made up of acid matters or something that may endanger human health. Once the soil is moistened, these crusts dissolve and disappear.

2.3. Samples Analysis Procedures Description

2.3.1. Soil and Water Sample pH Determination

To determine the soil samples’ pH, aliquots (30 g) were weighed using a Mettler Toledo mark analytical balance and placed in clean 500 mL polyethylene flasks. Subsequently, 300 mL of distilled water was added to each flask and the obtained mixture was magnetically stirred (500 rpm) using a Fisher Scientific apparatus to prepare a soil–water suspension. The stirring lasted an hour and the solid matter was allowed to settle to obtain clear supernatant water used for the soil pH measurement using a HANNA HI 2211 apparatus. The apparatus calibration was achieved using buffer solutions of different pH (4, 7 and 10) supplied by HACH Company, Germany. To analyze well water, the samples were poured into a 100 mL flask for direct pH measurement (without prior treatment) using the same apparatus as for the soil pH analysis.

2.3.2. Spectrophotometric Measurement of Chemical Species Concentrations in Water

Each water sample was poured into a 10 or 25 mL test tube. A colorimetric reagent capsule (composed of a complexing agent) was added to water to analyze a specific chemical species. To analyze sulfates, for instance, the colorimetric reagent utilized was sulfaver (a trade name), phosver for phosphates, nitraver for nitrates, etc. These reagents were supplied by HACH Company, Germany. Once the complexing agent was added to water, the test tube was vigorously agitated for 1 min for sulfate analysis, 2 min for phosphate analysis, 3 min for nitrate analysis, etc. Subsequently, the mixture (sample and reagent) was introduced into the bowl of the HACH spectrophotometer. Once the apparatus was switched on, the analysis automatically started and the spectrophotometric measurement result of the concentration of each chemical species was displayed on the screen in mg/L.

2.3.3. Analysis of Suspended Solid Matters in Water Samples

To determine the suspended solid matters content, 100 mL of the water sample was subjected to vacuum filtration. A Büchner flask provided with funnel containing a Whatman (589/2) filter paper was utilized to filter each sample using a Buchi vacuum pump V-300. Subsequently, the filter paper loaded with the suspended solid matter was dried inside a HERAEUS UT12 oven for 2 h, with the temperature kept at 105 °C [27]. The filter paper weight difference (in milligrams) before and after before filtration is equal to the suspended solid matters content corresponding to the analyzed water volume.

2.3.4. Electrical Conductivity Analysis of Water Samples

The electric conductivity measurements were performed using a SK10B general purpose conductivity + ATC electrode connected to a CONSORT C933 multi parameter analyzer. The sample (100 mL) was poured into a 250 mL beaker and the measurement electrode was thoroughly rinsed with distilled water and calibrated with a 3M KCl standard solution [27] supplied by CONSORT Company, Belgium. Once the electrode was introduced into the sample, the measured electric conductivity was displayed on the screen.

2.3.5. Water Samples Turbidity and Color Measuring

The turbidity measurements were carried out using a DR/890 colorimeter. Water samples, each of 10 mL, were poured into test tubes to be inserted into the bowl of the colorimeter. Once the lid was closed and the colorimeter was switched on, the water turbidity measurement was displayed on the screen in Nephelometric Turbidity Units (NTU) [26,27]. Prior to the turbidity measurements, the colorimeter was calibrated using double-distilled water and a formazin standard solution (4000 NTU, 100 mL) supplied by HACH company, Germany.

As for the color measurement of water samples, the same DR/890 colorimeter was utilized as per the same principle as the turbidity measurements. The measurement was value expressed in True Color Units (TCU) or Hazen [26,27].

2.4. Trace Metal Elements Analysis in Samples

2.4.1. Soil Samples Preparation and Mineralization

The measurement of metal concentrations (Cd, Co, Cu, Pb, Zn, Fe, Mn and Ni) in the soil samples was carried out using the ICP-MS technique based on atomic emission spectrophotometry. An atomic emission Perkin Elmer 500 spectrophotometer was utilized and the standard solutions were supplied by Perkin Elmer (Germany). Soil samples to be analyzed were first pretreated through drying and grinding prior to mineralization. The solid samples’ pretreatment consisted of first drying them in air, or in an oven at a temperature not exceeding 40 °C [30]. These dried samples were subjected to grinding and passed over a 2 mm sieve [30]. Oversize particles (>2 mm) were again subjected to grinding. Subsequently, the obtained matter was converted into powder so the particle size was less than 250 μm. As for the sample mineralization [32], this was carried out using 0.25 g aliquots of the already prepared soil powder. The powders were poured into 50 mL Pyrex beakers prior to be attacked with aqua regia. This is an acid mixture compound of 6 mL of concentrated hydrochloric acid (36%) and 2 mL of concentrated nitric acid (65%). The acid attack of the samples was carried out at 95 °C for 75 min. Each beaker was placed on a lab-sand heating block until smoke was released. The acid attack of the soil samples was repeated twice dry and once without not dry. Solutions originating from the sample acid dissolution were put aside to cool. Afterwards, each cooled solution was adjusted to 50 mL using distilled water before being diluted to measure the concentration of each metal using the ICP-MS technique based on atomic emission spectrophotometry.

2.4.2. Water Sample Preparation and Trace Metal Element Analysis

The sample preparation consisted of their filtration to avoid obstructing the spectrophotometer’s capillary during the aspiration phase to feed the atomization chamber. The metal analysis (Cd, Co, Cu, Pb, Zn, Fe, Mn, Ni, Cr, Ca, Mg, Na, K, As and Se) in samples was carried out using the ICP-MS technique based on atomic emission spectrophotometry. The filtered water samples were poured into 100 mL volumetric flasks. Once a sample entered the spectrophotometer’s capillary, the nature and concentration (in mg/L) of each analyzed metal was displayed on screen.

2.4.3. Plant Sample Preparation and Trace Metal Element Analysis

Sugar canes were used to prepare the samples of Saccharum officinarum. The bark was removed and the remaining matter was cut into small pieces and crushed using an agate laboratory pestle and mortar. The resulting matter was manually pressed to collect the sweet juice, which was filtered and diluted through addition of distilled water to prepare a solution for the analysis of metals using the ICP-MS technique based on atomic emission spectrophotometry. As for Musa acuminata, the bananas were peeled and cut into small pieces which were dried in an oven at 40 °C and crushed with an agate laboratory pestle and mortar. The obtained powder was attacked with aqua regia; that is, 0.25 g aliquots were placed into 50 mL Pyrex beakers to which was added an acid mixture compound of 6 mL of concentrated hydrochloric acid (36%) and 2 mL of concentrated nitric acid (65%). The acid attack of the aforementioned powder was also conducted through direct wet digestion [33] through heating at 95 °C on a lab-sand heating block. The solution from the acid attack of the Musa acuminata sample was diluted prior to spectrophotometric analysis of metals (Cd, Co, Cu, Pb, Zn, Mn and Ni) as was the case for the Saccharum officinarum sample.

3. Results

3.1. Physicochemical Characteristics of the Near-white Crusts Covering the Soil

The results from the physicochemical analyses of the near-white crusts observed on the soil in the Tshamilemba district are given in

Table 1. Physicochemical characteristics of the near-white crusts covering the soil.

Sample Number	Analyzed Parameter
	pH	Sulfates (mg/kg)	Phosphates (mg/kg)	Nitrates (mg/kg)	Cu (mg/kg)	Zn (mg/kg)	Fe (mg/kg)	Cr (mg/kg)
1	9.7	80	30.0	35.0	2.74	1.16	3.30	0.53
9	8.6	80	30.0	35.1	2.60	1.15	3.25	0.55
17	8.7	77	29.6	34.5	2.80	0.99	2.25	0.50
23	9.4	80	29.8	33.5	2.73	1.11	3.29	0.40
WHO STD	-	250	50	50	0.5	5	0.3–3	0.1

below.

The results of the near-white crust samples pH measurement gave values between 8.5 and 9.8. Sulfates, phosphates and nitrates were observed in the near-white crusts at concentrations lower than quality standards. The observed concentration ranged from 77 to 80 mg/kg for sulfates, around 30 mg/kg for phosphates and was equal to 35 mg/kg for nitrates. In contrast, metals such as zinc, copper, chromium and iron were found at concentrations far beyond the limits set by quality standards.

3.2. Physicochemical Characteristics of Water Samples

Table 2. Physicochemical parameters of water samples from the Tshamilemba district.

Sample Number	Analyzed Parameter
	pH	Temperature (°C)	Electric Conductivity (µS/cm)	Turbidity (NTU)	Colour (Hazen)	Matters in Suspension (mg/L)	Sulfates (mg/L)	Phosphates (mg/L)	Nitrates (mg/L)
1	7.01	24.5	1034	0	0	0	0	0.33	0.2
7	7.25	24.5	1110	19	111	19	2	1.81	3.4
8	7.04	24.0	1003	10	35	10	1	0.55	2.0
10	7.16	23.6	950.1	22	50	7	2	0.61	2.8
11	7.24	24.3	1050	8	94	18	0	0.35	3.2
12	7.25	22.2	1020	15	53	16	0	0.64	1.2
13	7.19	23.0	500	13	20	19	2	0.57	2.7
14	7.03	24.3	273.2	6	44	6	1	0.58	1.9
15	7.11	24.4	830.0	11	59	11	1	1.06	3.5
16	7.29	24.5	1111	25	168	20	3	1.85	3.3
17	7.17	24.0	650	9	40	20	3	0.42	3.4
18	7.03	23.7	1000	5	12	15	3	0.90	0.1
19	7.13	23.3	660.2	8	101	12	1	0.57	0.05
20	7.29	24.5	663.2	3	18	6	0	0.48	2.7
22	7.07	22.8	280.9	3	20	18	0	0.71	3.3
23	7.05	24.4	450.5	14	41	13	1	1.04	2.5
24	7.28	22.5	725	9	30	7	3	0.56	3.0
WHO STD [24]	6.5–8.5	≤35	≤2000	≤5	≤20	≤20	≤250	≤50	≤50

below provides the physicochemical characteristics of water sampled in Tshamilemba district.

The measurements of the pH, electrical conductivity, temperature, suspended solids and the nitrates, phosphates and sulfates contents gave values in accordance with the WHO’s quality standards for drinking water [26]. However, 76% of the water samples presented color and turbidity greater than the quality standard [26].

3.3. Soil Samples pH Measurements

Soil sample pH measurements led to the results given in

Table 3. Measured values of the pH in soil samples.

	Soil 1	Soil 7	Soil 11	Soil 14	Soil 15	Soil 19	Soil 20	Soil 22	Soil 23	WHO STD
pH	7.6	8.0	7.88	8.05	7.75	7.92	7.61	7.45	7.23	6 < pH < 7

below.

The measurement of pH of the soil samples collected in Tshamilemba district gave values between 7 and 8.1, with an average value of 7.69. Inside the study area, the soil showed alkaline or slightly alkaline character, depending on the sampling point location.

3.4. Concentrations of Trace Metal Elements in Samples

3.4.1. Trace Metal Elements in Water Samples

The results from the chemical analysis of water samples collected in Tshamilemba district are given in

Table 4. Trace metal element concentrations in water samples.

SPL	Chemical Element Concentration (mg/L)
	Cd	Co	Cu	Pb	Zn	Fe	Mn	Ni	Cr	Ca	Mg	Na	K	As	Se
1	0.000	0.000	0.03	0.000	0.000	0.12	0.205	0.001	0.38	37.14	15.72	8.195	1.037	0.026	0.003
7	0.039	0.030	0.06	0.003	0.000	0.35	0.898	0.004	0.07	28.84	16.71	82.38	2.477	0.027	0.047
8	0.000	0.014	0.08	0.001	0.000	0.04	1.582	0.017	0.04	49.67	28.90	47.81	4.259	0.027	0.003
10	0.000	0.899	0.000	0.000	0.000	0.000	1.294	0.011	0.001	27.20	58.88	29.87	0.768	0.048	0.017
11	0.000	0.002	0.000	0.000	0.000	0.000	1.789	0.045	0.002	22.65	32.74	8.479	4.742	0.047	0.000
12	0.000	0.008	0.000	0.000	0.000	0.191	0.485	0.018	0.000	19.82	35.89	30.49	1.962	0.035	0.000
13	0.000	2.004	0.000	0.000	0.160	0.037	0.289	0.069	0.000	15.50	16.21	60.87	3.958	0.036	0.000
14	0.000	3.244	0.08	0.012	0.163	0.17	3.197	0.107	0.04	27.22	39.61	7.965	1.215	0.041	0.000
15	0.000	0.085	0.05	0.000	0.000	0.21	0.263	0.008	0.04	14.36	8.147	18.44	1.033	0.000	0.017
16	0.000	0.958	0.08	0.000	0.000	0.16	1.302	0.010	0.05	25.68	55.72	29.00	0.696	0.021	0.000
17	0.000	1.258	0.000	0.000	0.000	0.348	1.367	0.073	0.003	17.84	54.56	61.25	1.025	0.028	0.000
18	0.000	0.708	0.000	0.000	0.000	0.062	2.849	0.021	0.000	31.80	43.02	40.61	2.081	0.000	0.036
19	0.000	0.024	0.018	0.000	0.000	0.000	0.143	0.004	0.002	5.520	7.607	13.77	1.201	0.020	0.000
20	0.002	0.015	0.09	0.000	0.000	0.43	0.559	0.011	0.06	26.61	15.37	51.50	2.641	0.035	0.013
22	0.000	0.006	0.000	0.001	0.000	0.062	1.289	0.012	0.001	27.20	50.23	70.84	3.582	0.021	0.018
23	0.000	0.051	0.000	0.000	0.000	0.173	3.021	0.009	0.000	33.12	39.58	45.12	2.456	0.020	0.041
24	0.000	0.269	0.000	0.000	0.000	0.062	2.968	0.178	0.000	45.63	19.78	25.62	2.784	0.001	0.035
QS	(3–5)10−3	0.1	0.5	0.01	5	5	0.5	0.2	0.1	50	0.5	-	-	0.1	0.02

SPL: Sample; QS: Quality Standards from [3,26,27].

below.

The mean concentrations of cobalt (0.563 mg/L), magnesium (31.68 mg/L) and manganese (1.382 mg/L) were above the standards [26,27] set for drinking water. Concentrations did not exceed quality standards in the case of calcium (29.73 mg/L), copper (0.028 mg/L), iron (0.036 mg/L), nickel (0.041 mg/L) and selenium (0.01 mg/L). A single water sample (number 18) presented a cadmium concentration 10 times greater than the lowest limit of quality standards used.

3.4.2. Trace Metal Elements in Soil Samples

Table 5. Trace metal element concentrations in the Tshamilemba district surface soil samples.

Sample	Cd (mg/kg)	Co (mg/kg)	Cu (mg/kg)	Pb (mg/kg)	Zn (mg/kg)	Fe (mg/kg)	Mn (mg/kg)	Ni (mg/kg)
SS1	0.000	30	562	0	0	12,333	0	0
SS2	0.000	140	580	170	1610	13,850	320	20
SS3	0.000	65	308	80	745	10,781	259	17
SS4	0.000	50	279	30	360	11,710	200	10
SS5	0.000	73	254	45	508	12,352	214	11
SS6	0.000	47	396	100	420	10,891	300	15
SS7	0.000	86	475	60	1655	13,750	307	16
SS8	0.000	90	270	30	170	12,490	260	10
SS9	0.000	69	500	50	960	11,454	288	13
SS10	0.000	95	509	65	820	12,200	254	12
SS11	0.000	80	480	50	1200	12,490	298	14
SS12	0.000	55	431	75	564	13,100	310	16
SS13	0.000	98	366	48	450	12,356	311	10
SS14	0.000	77	421	150	708	11,999	245	19
SS15	0.000	63	290	65	1333	10,500	268	20
SS16	0.000	45	263	49	645	12,421	312	11
SS17	0.000	32	560	45	1589	13,803	282	19
SS18	0.000	120	245	32	194	10,510	265	20
SS19	0.000	34	453	30	196	11,746	301	18
SS20	0.000	76	378	33	260	12,748	274	13
SS21	0.000	51	296	30	185	13,800	304	17
SS22	0.000	100	515	90	1600	12,830	315	19
SS23	0.000	97	400	160	1001	13,801	290	20
SS24	0.0000	82	356	157	985	12,350	315	18
WHO QS	4–85	35	100	84–420	300–7500	20	-	50

QS: quality standards.

presents the results of the analysis of TMEs in the surface soil samples.

In surface soil, the average concentrations of cobalt (727.3 mg/kg), copper (401.34 mg/kg), iron (12,344 mg/kg) and zinc (730.13 mg/kg) were greater than the quality standards [26]. The same applies to the concentration of lead, which is among the most toxic or dangerous chemical elements to human health. The spatial distributions of metals observed in the Tshamilemba surface soil samples are depicted in Figure 3. A careful analysis of these spatial distributions reveals the build-up of metals in the surface soil. They also enable a visualization of the points with high metal concentrations. These points can be viewed as sources from where metals have spread or accumulated in the top soil inside the study area. The above prompts looking for a link between the area where stored mineral wastes undergo weathering processes responsible for the spread of toxic metals into the environment and high metal concentrations observed at some spots inside the study area. This applies also to variations in the concentration of metals observed along a given direction inside the study area. Establishing the aforementioned link is a very useful start for seeking or suggesting the most plausible dispersal mechanisms of metals into the soil of the study area.

In deep soil (

Table 6. Trace metal elements in the Tshamilemba district deep soil samples.

Sample	Cd (mg/kg)	Co (mg/kg)	Cu (mg/kg)	Pb (mg/kg)	Zn (mg/kg)	Fe (mg/kg)	Mn (mg/kg)	Ni (mg/kg)
DS1	0.000	20	120	30	5540	23,830	130	10
DS2	0.000	97	174	55	765	1960	200	12
DS3	0.000	40	410	70	490	1432	190	10
DS4	0.000	61	308	43	2016	17,890	162	19
DS5	0.000	740	2120	60	290	3505	600	20
DS6	10.000	100	870	90	2530	22,740	300	10
DS7	0.000	610	1350	320	2150	35,170	350	10
DS8	0.000	45	902	67	800	6565	665	18
DS9	0.000	30	150	40	180	11,810	170	10
DS10	0.000	300	590	60	300	12,110	880	20
DS11	0.000	233	780	89	900	17,623	708	17
DS12	0.000	174	610	71	500	1501	541	15
DS13	0.000	121	510	55	699	1300	690	18
DS14	0.000	315	450	74	3508	32,874	610	16
DS15	0.000	28	569	62	2120	1660	765	13
DS16	0.000	86	800	54	1789	20,235	860	14
DS17	0.000	60	1302	92	2212	15,562	800	16
DS18	0.000	115	150	260	300	1854	855	20
DS19	0.000	700	405	88	2021	33,102	554	15
DS20	0.000	562	178	78	1456	29,468	402	13
DS21	0.000	74	2100	100	2003	21,401	350	19
DS22	0.000	602	1017	90	1500	4520	750	16
DS23	0.000	17	613	65	745	1698	576	16
DS24	0.000	19	511	57	787	1657	561	17
WHO QS	4	35	100	84	300	20	-	50

QS: Quality standards.

), the mean metal concentration was largely above the limits set by the standards [24] for cobalt (223.04 mg/kg), copper (717.73 mg/kg), iron (13,904.78 mg/kg), lead (85.34 mg/kg) and zinc (1513.65 mg/kg). Only sample 6 from deep soil presented a concentration of cadmium that was two times greater than the limit set by quality standards. As for manganese, its concentration in the deep soil was such that there is no set limit by the used quality standards.

A careful analysis of the spatial distributions of metals (Figure 4) reveals that, except for cadmium and manganese, toxic metals have also contaminated the deep soil. Consequently, higher concentrations of metals were observed at some points in the study area. The above makes it easy to better understand how metals behave in the soil compared to their source; that is, stored mineral wastes rich in sulfides undergoing weathering processes owing to an exposure to rainwater and air.

3.4.3. Trace Metal Elements in Plant Samples

The search for metals was also carried in two types of edible plant material.

Table 7. Concentrations of trace metal elements measured in Saccharum officinarum samples.

Sample Number	Cd (mg/kg)	Co (mg/kg)	Cu (mg/kg)	Pb (mg/kg)	Zn (mg/kg)	Mn (mg/kg)	Ni (mg/kg)
2	0.5	10.1	30.6	0.0	12.6	4.2	4.1
8	0.4	9.4	93.8	0.6	80.1	11.0	1.6
10	0.6	5.7	135.2	0.0	49.8	8.70	3.7
13	0.8	7.3	150.7	0.8	50.3	10.6	1.9
21	0.9	3.5	75.6	0.0	72.6	13.4	2.8
WHO SDT	0.05	0.05	100	0.1	100	0.20	0.20

below provides the results from the analysis of metals in the Saccharum officinarum samples.

It is obvious that the mean concentration of metals in Saccharum officinarum samples exceeds the standards for cadmium (0.64 mg/kg), cobalt (7.2 mg/kg), magnesium (9.621 mg/kg) and nickel (2.82 mg/kg). Only zinc and lead were observed at concentrations lower than the quality standards. The concentration of lead was greater than the standards in samples 13 and 10. However, the consumption of Saccharum officinarum grown in the Tshamilemba district could expose humans to toxic metals capable of endangering their health [25]. The average concentration for copper (74 mg/kg) and zinc (53.05 mg/kg) did not exceed the limits set by quality standards.

The results from the search for toxic metals in the Musa acuminata samples are recorded in

Table 8. Concentrations of trace metal elements measured in Musa acuminata samples.

Sample	Cd (mg/kg)	Co (mg/kg)	Cu (mg/kg)	Pb (mg/kg)	Zn (mg/kg)	Mn (mg/kg)	Ni (mg/kg)
4	0.2	2.3	95.1	0.8	20.8	7.0	5.6
10	0.8	6.2	86.2	0.7	12.5	7.0	3.4
12	0.9	5.8	73.4	5.7	2.8	0.5	19.9
16	0.5	3.4	71.0	0.3	22.2	6.1	1.3
22	0.6	4.4	45.7	0.4	25.6	6.9	1.1
WHO SDT	0.05	0.05	100	0.1	100	0.20	0.20

.

The obtained results showed that the mean concentration of metals was higher than quality standards for cadmium (0.6 mg/kg), cobalt (4.42 mg/kg), manganese (6.54 mg/kg), nickel (2.84 mg/kg) and lead (0.54 mg/kg). Only in the case of copper and zinc was the average concentration below the limits set by the quality standards. The presence of metals in analyzed samples and, in the majority of the cases, at concentrations greater than the quality standards renders the bananas grown in Tshamilemba district unfit for consumption. Their consumption bears great health risks given that consumers are likely in the long term to be exposed to diseases such as osteoporosis and cretinism owing to the ingestion of cadmium and lead. Indeed, these metals are reputed to be especially harmful to the health of children and pregnant women [12].

4. Discussion

4.1. Chemical Composition and Plausible Origin of Near-white Crusts Collected on the Soil

Chemical analyses revealed that near-white crusts covering the soil in the Tshamilemba district consist of alkaline matter (Table 1). Near-white crusts could be reaction products or secondary minerals [34] resulting from the neutralization of metal-rich, acidic waters released by the weathering reactions of sulfides contained in stored mineral wastes. The above occurs due to the mineral wastes being exposure to air and rainwater resulting in acid mine drainage formation and spread [17]. The above findings demonstrate the population’s fear [25] for their health regarding the environment pollution in Tshamilemba district is justified.

4.2. Well Water Sample Quality and Implications on Consumer Health

The physicochemical characteristics of well water sampled in Tshamilemba district (Table 2) indicate, given their excess color and high electrical conductivity, the presence of impurities. Therefore, the water needs to be disinfected before drinking [35,36]. These impurities could consist of colloidal compounds and dissolved matter such as derivatives of organic acids (humate-fulvate compounds), reputed to be responsible for natural water’s color [35,36]. The excess color of the water samples can lead to the soiling of white clothes during washing or rinsing. As for the high levels of solid matter in suspension, they predispose well water to become an environment more conducive to the development of pathogen microorganisms capable of endangering human health [35,36]. Besides, electrical conductivities measured in some samples indicate that the water may contain dissolved organic matter (metal ion complexes) [37], which could in the long-term expose consumers to serious health issues such as kidney complications.

4.3. Soil pH and Its Role in the Near-White Crust Formation and Mobility or Spread of Metals

The alkaline range in which the pH measured in the soil falls (Table 3) allows us to determine how the near-white crusts observed on the soil in Tshamilemba district are formed and the extent of metal ion mobility in soil water. Indeed, this pH range satisfactorily supports the assumption that the near-white crusts result from the neutralization process [34] of acidic water liberated due to the weathering reactions of stored mineral waste. Indeed, the Tshamilemba district is made of alkaline soils [34]. The above presupposes the reduced mobility of dissolved chemical species (ions) or lowered bioavailability of toxics metals in the soil [37]. Consequently, metals could be present in the soil as hydroxides and complexes; that is, they may be contaminating the soil and building-up in edible plants that might be grown there. Indeed, most metals retained in the soil can undergo dissolution inside the plants’ rhizosphere, wherein the pH is ordinarily acidic even if their closer surrounding area is alkaline [37]. As for the alkaline soil pH observed in Tshamilemba, this is due to the fact that this district is found in the municipality of Kampemba; that is, an area wherein one finds dolomite-made bedrock basement [38,39] including the Kakontwe dolomite, dolomitic shale and limestone. This means that the measured pH will not be dictated by the soil acid pollution induced by the weathering of stored mineral wastes but will be strongly influenced by the bedrock pH [40]. However, it is of paramount importance that a comparative study is conducted to ascertain whether the neutralization potential of the soil bedrock found in Tshamilemba district is high enough to counterbalance and even decreasing the acidogenic capacity of sulfides contained in mineral wastes from the hydrometallurgical extraction of copper and cobalt [41,42].

4.4. Trace Metal Element Presence in Water Samples and Health Implications

Metals were observed in water (Table 4) at concentrations greater than the quality standards for drinking water [25,26]. These large concentrations could be due to the weathering of stored mineral wastes involved in the contamination of the near-to-surface water and the well water sampled in Tshamilemba district. The contamination of water by toxic metals in the Tshamilemba district has also been reported in previous research [17,25]. However, in contrast to water samples under consideration in this research, the pH measured in those studies varied from 4.9 to 6.7, revealing that samples consisted of acidic water abstracted from wells dug very near to the mineral waste storage site.

4.5. Trace Metal Element Presence in Surface Soil Samples and a Suggestion of Their Dispersion Model

As can be seen from results depicted in Table 5, cobalt, copper, iron, zinc and even lead polluted the surface soil of the Tshamilemba district. Cadmium was the only metal which was not observed in the surface soil at a concentration above the detection limit of the analytical apparatus used. This finding is in perfect agreement with its low presence in well water, where a measurable concentration was detected only in a single sample. Given its low concentration, cadmium seemed to be the least toxic metal observed in water and soil samples subjected to the mineral chemical analysis.

A careful analysis of Figure 3 reveals that metals are spread in certain manner within the surface soil of the Tshamilemba district; that is, depending on their types and their concentrations, they increase or decrease at specific points or along a given direction inside the study area. These phenomena can be explained by envisaging a dispersal mechanism based on the shift of acidic and metal-rich waters generated by the weathering of stored mineral wastes. The same mechanism includes the spread of metals involving run-off waters during their shift toward the study area. Firstly, it can be assumed that pollutants would have been driven as dissolved species until inside the study area, and secondly, they would have been precipitated in the surface soil due to a change in pH (alkaline soil) induced by the bedrock [37,38,39,40]. Indeed, according to [17], metals have been transported by runoff waters [25] when they shifted from the North and the East towards the study area (Figure 2). However, this dispersal mechanism cannot wholly explain the behavior or the variations in the concentrations of some metals which have arisen inside the study area. For instance, it is difficult to explain a mechanism in which lead builds up in the study area’s surface soil. In this case, other dispersal models should be considered such as aerial erosion of mineral waste stockpiles during periods of high winds. This would enable the assumption that the dispersion of metals also includes airborne particles that would have deposited on the surface soil inside and outside the study area. High metal concentrations which are observed in the Tshamilemba district could also result from a reduced mobility of their ionic species in the soil water given that the mean measured pH is alkaline [37,38,39].

4.6. Trace Metal Element Presence in Deep Soil Samples and a Suggestion of Their Dispersion Model

Excess concentrations of metals observed in samples from the deep soil (Table 6), as well as their behavior, can be explained using the same dispersion model as in the surface soil. However, the average metal concentration is below the limits set by the standards [24] only for cadmium and nickel. A carefully analysis of the spatial distributions of metals in deep soil (Figure 4), in cases where their concentrations exceed the limits of the quality standards, reveals that their spread took place in the study area (Figure 1 and Figure 2) along the North–South direction. This finding is supported by the one in [18], which suggested a link between the build-up of metals in the soil and water and the weathering process of stored mineral waste in Tshamilemba district brought about by their exposure to air and water, resulting in acid mining drainage. Indeed, the direction usually taken by the run-off waters in Tshamilemba district is such that they mix with percolates liberated by the weathering of stored mineral waste. Thus, the established spatial distributions (Figure 4) for studied metals enable comprehending how the storage of mineral wastes partakes in the formation of high toxic metal concentrations observed in soil and water samples under consideration in this study. The build-up of metals in the soil and water samples can be also explained by the topography of the terrain on which Tshamilemba district is built, which dictates the direction usually taken by run-off waters, i.e., from the mineral waste storage site towards dwellings [17].

5. Conclusions

This research aimed to better understand the dispersal mechanisms of metals liberated by stored mineral waste. The environmental pollution complained about by the population in Tshamilemba district is brought about by the presence of metals. Using the results from the sample analyses and metal spatial distributions established in this study area, the following conclusions can be drawn:

• The population fear about the environment pollution, of which harmful effects are visible, is well justified, since toxic metals were observed at high concentrations in the soil, surface water and grown edible plants.
• The spatial distribution of metals in the soil enables comprehending their dispersal mechanisms in the environment. The chemical species come from the run-off waters and the metal-rich, acidic waters liberated due to the weathering of stored mineral waste.
• Metals also spread in the environment as airborne particles due to the aerial erosion of mineral waste stockpiles during periods of high winds.
• A better understanding of the dispersal mechanisms of metals is a great step forward in the search for strategies to better control and to mitigate the mineral pollution experienced in Tshamilemba district so to minimize its harmful effects on wildlife and human health.