An assessment of heavy metal exposure risk associated with consumption of cabbage and carrot grown in a tropical Savannah region

Figures & data

Figure 1. Location and layout of sample sites in Santa, North West region, Cameroon

Table 1. Study sites with their coordinates (UTM) and descriptions

Areas	Vegetables	Latitude	Longitude	Elevation (m asl)	Site description
Akum	Carrot	5°51ʹ07.1”	10°09ʹ55.4”	1884	Both farms located on a slope of 0–5% with a stream below the farms. 3 km from the main road.
	Cabbage	5°51ʹ01.0”	10°09ʹ52.4”	1883
Mbei	Carrot	5°49ʹ14.5”	10°09ʹ05.7”	1853	Farms located on a slope of 0–2%, surrounded by a range. 1 m from the road. Having a nearby river.
	Cabbage	5°48ʹ57.5”	10°09ʹ13.2”	1814
Matazem	Carrot	5°46ʹ28.2”	10°08ʹ34.8”	1711	Farms surrounded by celeries and potatoes farms, with a dam as a source of irrigation. 5 km from the main road.
	Cabbage	5°46ʹ22.6”	10°09ʹ12.2”	1641
Moforbe	Carrot	5°46ʹ49.0”	10°09ʹ26.9”	1644	Farms surrounded by cabbage and lettuce farms. 2 km from the main road with a nearby river.
	Cabbage	5°46ʹ50.3”	10°09ʹ22.7”	1642

Note: m asl—metres above sea level

Table 2. Reference dose values for heavy metals consumption

Heavy metal	As	Cd	Co	Cr	Cu	Fe	Mn	Ni	Pb	Zn
Reference Dose (RfD)	0.0004	0.001	0.0003	1.5	0.04	0.7	0.014	0.02	0.004	0.3
Source	United States Environmental Protection Agency and Integrated Risk Information System (2006)	Codex Alimentarius commission (2013)	United States Environmental Protection Agency and Integrated Risk Information System (2006)	Codex Alimentarius commission (2013)				United States Environmental Protection Agency and Integrated Risk Information System (2006)	Codex Alimentarius commission (2013)

Table 3. Soil physicochemical properties of the study areas in Santa, Cameroon

Properties	Mean values	Sources
Soil texture	40% loamy: 10% clay loam: 10% sandy clay	Okha (2016)
Total N	2.43%	This work
Soil pH	5.6	Okha (2016)
C/N ratio	13.96	This work
Organic matter	4.68%	Okha (2016)
Ca	1.66 Cmol/kg	Okha (2016)
Mg	1.31 Cmol/kg	Okha (2016)
K	0.67 Cmol/kg	Okha (2016)
Na	0.02 Cmol/kg	Okha (2016)
CEC Cmol/kg	11.37 Cmol/kg	Okha (2016)

Table 4. Critical values of nutrient and soil properties

Properties	Critical values
	Very low	Low		Medium	High	Very high
OM	<1	1–2		2–4.2	4.2–6	>6
Total N	<0.5	0.5–1.25		1.25–2.25	2.25–3.0	>3.0
C/N	<0.5 = good, 10–14 = medium and >14 = poor
Ca Cmol/kg	<2	2–5		5–10	10–20	>20
Mg Cmol/kg	<0.5	0.5–1.5		1.5–3	3–8	>8
K Cmol/kg	<0.1	0.1–0.3		0.3–0.6	0.6–1.2	>1.2
Na Cmol/kg	<0.1	0.1–0.3		0.3–0.7	0.7–2.0	>2
pH	2.3–6.0 = moderately acid; 6.0–7.0 = slightly acidic; 7.0–8.5 = moderately alkaline
CEC Cmol/kg	0–20	21–40	41–60		61–80		81–100

Source: Beernaert and Bitondo (1992)

Table 5. Fertility index and rating of soils

Rating	Fertility Index (FI)
Very low	0–50
Low	50–75
Medium	75–100
Very high	150–300

Source: Beernaert and Bitondo (1992)

Table 6. Different fertilizers used in the cultivation of cabbage and carrot in Santa, Cameroon

Properties	Types of fertilizers	Percentage (%)
Kind of fertilizer	Only Inorganic	2
	Only Organic	0
	Both Organic and inorganic	98
Type of inorganic fertilizer	NPK	48
	Urea	18
	NPK + Urea	28
	Phosphate fertilizer	6

Table 7. Chemicals mostly used by farmers for the cultivation of cabbage and carrot in Santa, NWR, Cameroon, their heavy metal content and their level of toxicity

	Chemical	Active ingredient	Percentages (%) of respondents using these chemicals	Heavy metals	Toxic class
Pesticides	Action (H)	Diuron	30.1	-	III
	Round up (H)	Glyphosate	12.3	As (Lajmanovich et al., 2019)	III
	Mancozan super (F)	Metalaxyl	5.5	-	III
	Ridomil Plus (F)	Mefenoxam	32.9	-	III
	Gramaxone (P)	Paraquat	16.4	-	II
Inorganic fertilizers	Urea fertilizer	Urea	46	Ni, Pb, Cd (Benson et al. 2014)
	NPK (20:10:10) fertiliser	N, P and K	76	Pb, Cd, Cu and Mn (Milinovic, et al. 2008)
	Phosphate fertilizer	Phosphate	6	Cu and Zn (Benson et al. 2014)

Class II = Toxic, Class III = Harmful

Table 8. Physicochemical properties of soils from the different study sites in Santa, NWR, Cameroon

Parameter	Mbei	Moforbe	Akum	Matazem
Soil pH	4.66	4.20	6.10	5.95
Organic Carbon %	4.50	4.20	6.40	7.50
Organic Matter%	8.10	8.12	7.29	9.10
N %	0.70	0.58	0.52	0.58
C/N	13.48	11.41	12.41	11.65
Ca (Cmol/kg)	2.0	1.5	1.5	2
Mg (Cmol/kg)	1.20	2	0.90	2.60
K (Cmol/kg)	0.60	0.40	0.70	0.80
Na (Cmol/kg)	0.02	0.01	0.01	0.02
CEC	9.5	9.2	12.4	16
Available P(mg/kg)	45.5	46.2	38	31.1

Table 9. Soil texture across the different study sites in Santa, NWR, Cameroon

Soil texture Sites	Coarse sand	Fine sand	Silt	Clay
Akum	23.12	39.99	27.89	9.00
Mbei	18.02	49.57	16.00	14.00
Moforbe	11.02	51.08	16.00	14.00
Matazem	12.00	58.67	23.00	11.8

Table 10. Mean concentration of soil heavy metals where cabbage and carrot were cultivated in Santa, NWR, Cameroon

Heavy metals	Cabbage soil (mg/kg)	Carrot soil (mg/kg)	Food and Agricultural Organization/World Health Organization (2007) (mg/kg)
As	1.16	1.55	0.5
Cd	0.87	0.91	0.07
Co	2.04	2.19	10
Cu	4.61	4.65	6
Cr	9.70	10.07	65
Fe	841.40	843.93	150
Mn	27.10	45.68	437
Ni	2.29	2.27	75
Pb	3.03	3.08	10
Zn	11.47	9.16	50

Figure 2. Correlogram of heavy metals concentration in carrot and soil physicochemical parameters

Figure 3. The principal component analysis of heavy metal load and soil physicochemical parameters in carrot

Figure 4. Correlogram of heavy metals concentration in cabbage and soil physicochemical parameters in cabbage

Figure 5. A Biplot for the principal component analysis of heavy metal load and soil physicochemical parameters in cabbage

Table 11. Mean transfer factor of the various heavy metals in cabbage and carrot across the different sample sites in Santa, NWR, Cameroon

Heavy metal/sites	Soil	Akum	Mbei	Matazem	Moferbe	Mean
As	Carrot	0.29	0.19	0.00	0.42	0.22
	Cabbage	0.19	0.83	0.47	0.34	0.46
Cd	Carrot	0.16	0.15	0.19	0.19	0.17
	Cabbage	0.15	0.20	0.15	0.18	0.17
Co	Carrot	0.64	0.56	0.67	0.67	0.64
	Cabbage	0.83	0.75	0.66	0.57	0.70
Cu	Carrot	0.53	0.57	0.55	0.51	0.54
	Cabbage	0.56	0.56	0.60	0.53	0.56
Cr	Carrot	0.13	0.12	0.12	0.12	0.12
	Cabbage	0.56	0.13	0.15	0.13	0.24
Fe	Carrot	0.01	0.03	0.02	0.01	0.02
	Cabbage	0.12	0.01	0.22	0.05	0.10
Mn	Carrot	0.07	0.11	0.13	0.22	0.13
	Cabbage	0.03	0.50	0.37	0.28	0.29
Ni	Carrot	0.57	0.49	0.48	0.50	0.51
	Cabbage	0.16	0.52	0.66	0.45	0.45
Pb	Carrot	0.40	0.51	0.40	0.40	0.43
	Cabbage	0.52	0.43	0.48	0.44	0.47
Zn	Carrot	0.63	0.29	0.44	0.35	0.43
	Cabbage	0.44	0.33	0.17	0.40	0.34
Mean		0.35	0.36	0.35	0.33

Table 12. Geo-accumulation index, contamination factor and pollution load index for studied heavy metals in soils used for the cultivation of cabbage in Santa, NWR, Cameroon

Cabbage				Igeo
Sites\Heavy metals	Igeo-As	Igeo-Cd	Igeo-Co	Igeo-Cu	Igeo-Cr	Igeo-Fe	Igeo-Mn	Igeo-Ni	Igeo-Pb	Igeo-Zn
Akum	0.89	0.20	0.30	0.30	0.50	1.57	159.20	0.42	0.64	1.17
Mbei	0.77	0.18	0.34	0.28	0.44	1.27	34.97	0.39	0.58	1.06
Matazem	0.66	0.19	0.39	0.29	0.45	1.35	99.30	0.41	0.58	1.10
Moferbe	0.77	0.19	0.44	0.29	0.45	1.46	67.86	0.44	0.61	1.501
Contamination Factor
Sites/Heavy Metals	CF-As	CF-Cd	CF-Co	CF-Cu	CF-Cr	CF-Fe	CF-Mn	CF-Ni	CF-Pb	CF-Zn	PLI
Akum	4.43	1.02	1.52	1.48	2.50	7.86	795.83	2.10	3.21	5.83	7.08
Mbei	3.87	0.93	1.74	1.40	2.21	6.34	174.83	1.92	2.92	5.89	5.31
Matazem	3.28	0.94	1.96	1.43	2.26	6.74	496.50	2.06	2.91	5.51	6.20
Moferbe	3.87	0.99	2.19	1.45	2.27	7.32	339.33	2.22	3.06	7.50	6.57

Table 13. Geo-accumulation index, contamination factor and pollution load index for studied heavy metals in soils used for the cultivation of carrot in Santa, NWR, Cameroon

Carrot
Igeo
	Igeo As	Igeo Cd	Igeo Co	Igeo Cu	Igeo Cr	Igeo Fe	Igeo Mn	Igeo Ni	Igeo Pb	Igeo Zn
Akum	0.91	0.21	0.39	0.30	0.43	1.34	169.27	0.37	0.62	0.33
Mbei	1.09	0.22	0.46	0.29	0.49	1.34	149.87	0.42	0.60	1.19
Matazem	0.86	0.20	0.38	0.29	0.50	1.46	138.23	0.46	0.61	1.21
Moferbe	1.29	0.19	0.37	0.29	0.50	1.54	151.80	0.42	0.64	1.13
Contamination Factor
	CF-As	CF-Cd	CF-Co	CF-Cu	CF-Cr	CF-Fe	CF-Mn	CF-Ni	CF-Pb	CF-Zn	PLI
Akum	4.57	1.06	1.92	1.46	2.14	6.71	846.33	1.83	3.09	1.65	5.93
Mbei	5.43	1.08	2.31	1.44	2.43	6.70	749.33	2.1	2.98	5.96	7.38
Matazem	4.30	0.96	1.92	1.44	2.52	7.26	691.17	2.25	3.05	6.04	7.02
Moferbe	6.43	0.96	1.85	1.47	2.50	7.70	759.00	2.09	3.20	5.64	7.42

Table 14. Mean heavy metal concentrations in carrot and cabbage cultivated in Santa, NWR, Cameroon, compared with Food and Agricultural Organization/World Health Organization (2007) standards

Heavy metals	Cabbage (mg/kg)	Carrot (mg/kg)	Food and Agricultural Organization/World Health Organization (2007) (mg/kg)
As	0.51	0.38	0.20
Cd	0.15	0.16	0.30
Co	1.41	1.39	0.05
Cu	2.61	2.51	40
Cr	1.30	1.26	1.30
Fe	62.56	14.35	425
Mn	7.31	5.97	500
Ni	1.23	1.16	1.50
Pb	1.36	1.31	0.30
Zn	3.69	3.51	60

Table 15. Mean values for the daily intake of metals and the health risk index of carrot and cabbage cultivated in Santa, NWR, Cameroon

HRI
Heavy metals	Carrot	Cabbage
As	0.46	0.62
Cd	0.08	0.00
Co	2.26	2.31
Cr	0.00	0.00
Cu	0.02	0.02
Fe	0.04	0.04
Mn	0.21	0.26
Ni	0.03	0.03
Pb	0.16	0.17
Zn	0.01	0.01

Figure 6. The relationship between transfer factor and heavy metal concentration in the soil and in the vegetables (carrot and cabbage)

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