Impact of fly ash deposition on plants diversity, metal content, and associated risk on grazing ruminants in the vicinity of Morupule power station, Botswana

Figures & data

Figure 1. Sampling map of soils and plants within the vicinity of the Morupule fly ash dumpsite.

Figure 2. Prevailing wind direction in Mahalapye from 1960 to 2016, representing Palapye area (department of meteorology, Botswana).

Table 1. Estimates of parameters used in the calculation of daily dietary intake of grazing ruminants from plants in the vicinity of Morupule power station.

Parameters	Cattle	Goat	Sheep
Diet Composition
Grass	75%	20%	74%
Woody Species	23%	78%	21%
Herbs	2%	2%	5%
Average daily dry matter consumption (kg day^-1)	16.9	0.90	2.50
Average daily soil intake (kg day^-1)	0.41	0.10	0.10

Adapted from Manyiwa et al. (2021).

Table 2. Physicochemical properties and metal concentration for soils found in the vicinity of morupule power station¹.

Distance (km)	Direction	pH	EC (µS/cm)		Particle size						Organic carbon (%)		CaCO3 (%)		Total N (%)
	NE to SW				Silt	Clay (%)		Sand	Texture
40		5.72e	22c		7.1	24.6		68.3	SCL		1.2e		0.71d		0.55a
20		5.24ef	6c		6.0	25.6		68.4	SCL		1.65bc		0.85d		0.48b
10		6.38d	33c		14.1	18.2		67.7	SCL		1.51d		0.86d		0.39c
5		5.26ed	8c		11.2	22.5		66.3	SCL		1.09e		1.02c		0.41b
2		6.21d	24c		12.3	25.8		61.9	SCL		0.61f		1.68c		0.1d
1		7.76b	21c		18.4	16.5		65.1	SL		0.46f		2.22b		0.2d
0.5		8.39a	4255a		–	–		–	–		0.02g		10.01a		0d
0.5		7.7b	2468b		–	–		–	–		0.01g		10.02a		0d
1		7.75b	263c		9.2	16.8		74.0	SL		1.5d		3.85b		0.01d
2		7.83b	77c		12.5	21.8		65.7	SCL		1.59 cd		2.07b		0.38c
5		7.01c	385c		12.3	28.5		50.2	SCL		1.67bc		1.91b		0.45b
10		4.92ef	18c		8.4	25.1		66.5	SCL		1.9b		0.62d		0.42b
20		4.82f	9c		15.1	15.0		69.9	SCL		1.82b		0.55d		0.58a
40		5.19ef	30c		12.7	22.4		64.9	SCL		2.62a		0.52d		0.61a
p-value	SW to NE	0.001**	0.000***								0.000***		0.001*		0.000***
		Metal concentration (mg kg^-1)
Distance (km)	NE to SW	Cr		Mn			Cu			Zn		As		Pb
40		44.00bc		268.00d			23.63 cd			184.59a		3.73b		34.33a
20		37.00c		112.00de			24.33 cd			165.37a		3.69b		2.70d
10		29.00c		77.00e			24.09 cd			131.08ab		3.74b		4.30c
5		63.00b		350.00c			21.72d			129.18ab		2.54b		4.40c
2		27.00c		71.00e			20.33d			0.00e		3.00b		2.90d
1		32.00c		75.00e			16.50e			0.00e		2.63b		4.40c
0.5		129.00a		1033.00a			21.00d			25.20d		6.80a		22.00b
0.5		106.00a		897.00b			21.33d			25.90d		6.90a		29.47ab
1		39.00c		78.00e			28.00c			0.00e		2.68b		6.70c
2		25.00c		73.00e			21.01d			0.00e		2.45b		5.28c
5		32.00c		77.00e			54.42b			21.91d		3.59b		4.90c
10		38.00c		219.00d			85.61a			21.91d		3.70b		3.83 cd
20		35.00c		71.00e			33.57c			24.41d		3.88b		3.57 cd
40		72.00b		493.00c			32.66c			48.34c		3.65b		4.27c
p-value	SW to NE	0.001**		0.000***			0.001**			0.001**		0.005*		0.003*

¹ Means within the same column followed by the same letter(s) are not significantly different from each other based on Tukey’s test at 5% level of significance. * = significant, ** = highly significant, *** = very highly significant, SCL: sandy clay loam; SL: sandy clay, at 0.5 km distance the area is covered with pure fly ash.

Table 3. Palatable plant species from the vicinity of the morupule power station.

Species name	Life form
Panicum kalaharense	Grass
Vachellia nilotica	Shrub
Ximenia americana	Tree
Eragrostis pallens	Grass
Diospyros lycioides	Shrub
Senegalia. nigrescens	Tree
Grewia flavescens	Shrub
Terminalia sericea	Tree
Vachellia tortilis	Shrub
Asparagus nelsii	Tree
Eragrostis rigidior	Grass
Bauhinia petersiana	Shrub
Dichrostachys cinerea	Tree
Cenchrus ciliaris	Grass
Eragrostis rigidior	Grass
Urochloa trichopus	Grass
Aristida stipoides	Grass
Grewia bicolor	Shrub
Chloris virgata	Grass
Asparagus africanus	Shrub
Digitaria sanguinalis	Grass
Colophospermum mopane	Tree

Table 4. Diversity index, species richness, and species evenness along the vicinity of morupule power station.

Distance (km)	Direction	S	N	Richness	Diversity index	Evenness
40	NE to SW	15	183	1.11b	1.86a	0.69d
20		13	305	0.74f	1.93a	0.75c
10		8	190	0.58g	1.27c	0.61d
5		6	49	0.86e	1.35c	0.75c
2		7	78	0.80e	1.66b	0.85b
1		7	78	0.79f	1.86a	0.95a
0.5		7	105	0.68g	1.48c	0.76c
0.5		Fly ash dumpsite
1		6	99	0.60g	1.75b	0.68d
2		10	89	0.95d	1.76b	0.80b
5		8	57	1.06c	1.78b	0.86b
10		9	60	1.16b	1.68b	0.76c
20		10	44	1.51a	1.89a	0.82b
40	SW to NE	10	81	1.11b	1.94a	0.84b
P-value				0.000***	0.003*	0.001**

S: total number of species in a community; N: total number individual plant in a community.

Figure 3. Heavy metal concentration found in dominant herb species growing within 40 km distance of the fly ash dumpsite. (n = 11; mean ± standard deviation). Critical concentration are Cr =75-100^a, Mn = 400^a, Cu = 100^b, Zn = 200b, as = 10-20^a, Pb = 20^b mg kg^-1; ^aHajar, Sulaiman, and Sakinah (2014) ^b Manyiwa et al. (2021).

Figure 4. Heavy metal concentration found in dominant shrub species growing within 40 km distance of the fly ash dumpsite. (n = 11; mean ± standard deviation). Critical concentration are Cr =75-100^a, Mn = 400^a, Cu = 100^b, Zn = 200^b, as = 10-20^a, Pb = 20b mg kg^-1; ^aHajar et al. (2014) ^b Manyiwa et al. (2021).

Figure 5. Heavy metal concentration found in dominant tree species growing within 40 km distance of the fly ash dumpsite. (n = 11; mean ± standard deviation). Critical concentration are Cr =75-100^a, Mn = 400^a, Cu = 100^b, Zn = 200^b, as = 10-20^a, Pb = 20^b mg kg^-1; ^aHajar et al. (2014) ^b Manyiwa et al. (2021).

Figure 6. Bioaccumulation factor of heavy metals in the dominating grass species growing within 40 km distance of the fly ash dumpsite (n = 11; mean ± standard deviation).

Figure 7. Bioaccumulation factor of heavy metals in the dominating herb species growing within 40 km distance of the fly ash dumpsite (n = 11; mean ± standard deviation).

Figure 8. Bioaccumulation factor of heavy metals in the dominating shrub species growing within 40 km distance of the fly ash dumpsite (n = 11; mean ± standard deviation).

Figure 9. Bioaccumulation factor of heavy metals in the dominating tree species growing within 40 km distance of the fly ash dumpsite (n = 11; mean ± standard deviation).

Table 5. Estimated daily intake of Cu of cattle, goat, and sheep grazing in the vicinity of Morupule power station.

Distance	40km	20km	10km	5km	2km	1km	0.5km	0.5km	1km	2km	5km	10km	20km	40km
Direction	Southwest													Northeast
Daily intake of Cu (mg day^-1)
Ruminants
Cattle	2881.67	1711.39	617.38	400.79	3400.18	2505.15	2993.16	Fly ash dumpsite	2123.06	2122.81	2095.79	900.15	2188.79	2159.81
Goat	90.63	112.63	107.77	73.03	169.50	138.01	141.81		118.62	100.80	103.32	146.78	124.28	129.44
Sheep	418.92	265.59	92.78	55.24	495.12	359.41	430.89		310.04	316.54	317.53	140.24	328.35	323.18
Daily intake of Cr (mg day^-1)
Ruminants
Cattle	8057.60	10425.44	2226.32	2029.01	11037.73	9108.42	7288.63	fly ash dumpsite	10038.26	9507.60	9041.44	8420.28	9048.91	10260.42
Goat	497.37	508.18	380.17	366.44	514.55	478.13	401.02		500.92	469.69	504.62	502.27	510.40	490.25
Sheep	1486.76	1542.25	341.09	274.05	1622.05	1352.53	1081.78		1486.92	1408.65	1330.19	1241.45	1354.71	1520.64

The maximum allowable daily intake of Cu for cattle = 1000-2000 mg day^-1, for goat = 150-180 mg day^-1, and for sheep = 230-250 mg day^-1 (Manyiwa et al. 2021); while for Cr the maximum allowable daily intake for cattle = 67.6 mg day^-1, goat = 10.0 mg day^-1, and sheep = 3.6 mg day^-1 (Popovic et al. 2010).

Figure 10. Heavy metal concentration found in dominant grass species growing within 40 km distance of the fly ash dumpsite. (n = 11; mean ± standard deviation). Critical concentration are Cr =75-100^a, Mn = 400^a, Cu = 100b, Zn = 200^b, as = 10-20^a, Pb = 20^b mg kg^-1; ^aHajar et al. (2014) ^b Manyiwa et al. (2021).

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