Abstract
The existence and levels of certain potentially toxic elements (PTEs) in the
environment can be associated to anthropogenic activities and innate sources. The
accumulation of PTEs in water, soil, sediment, and vegetables can pose a severe
threat to the respective organisms and lead to human health risks upon exposure.
Therefore, there is a need to evaluate the transport, understand the fate in the
environment, and determine the exposure effects of these PTEs. This study aimed to
determine the transport and fate of chromium (Cr) and arsenic (As) in the Mokolo River
using the health risk assessment approach.
The study involved sampling water, soil, sediment, and vegetable samples (pumpkin,
chomolia, spinach, thepe, onion and mochina) from the Mokolo River and its vicinity.
The digestion and extraction of soil, sediment and vegetable samples were
successfully executed. The determination of total concentrations of As and Cr were
carried out by analyzing acidified water samples and digested samples, while
speciation analysis of Cr and As were conducted by analyzing all samples after
microwave assisted extraction of Cr and As species. Quantification of total
concentrations and speciation analysis were conducted using inductively coupled
plasma-mass spectrometry (ICP-MS) and high-performance liquid chromatography
hyphenated to inductively coupled plasma-mass spectrometry (HPLC-ICP-MS). After
determination of total concentrations and elemental species levels, the health risk
assessment comprising non-carcinogenic and carcinogenic effects was carried out for
both adults and children consuming the vegetables.
The total concentrations of Cr and As were observed between 0.220 to 4.31 μg/L and
0.168 to 0.612 μg/L, respectively. The observed total concentrations were beneath the
maximum permissible level (MPL) of 50 and 10 μg/L recommended by World Health
Organization (WHO), South African National Standards (SANS) and Department of
Water Affairs and Forestry (DWAF) for drinking water. In sediments, the total
concentration of Cr ranged from 6.63 to 491 μg/g. The concentrations suggested
possible contributors such as surface runoff and contamination by industrial activities.
During the high flow season (18 November 2020), the Cr concentrations in all sites
surpassed the recommended probable effect level (PEL) value of 90.0 μg/g, whereas
in the low flow season (26 August 2021), the levels of Cr in samples from all sites were
beneath the interim sediment quality guideline (ISQG) value of 37.3 μg/g. The
recommended ISQG value of 5.90 μg/g for As was not surpassed during the low flow
season. During high flow season, sites 1, 2, 3, and 9 surpassed 5.90 μg/g and
suggested moderate biological effects on aquatic species and the food chain. Species
affected via the food chain include larger fish, crustaceans, and birds. In the high flow
season, high concentrations were observed from site 8 for As(III) at 255 ng/g and
dimethylarsenic acid (DMA) at 958 ng/g, while at site 3, the recorded value for
monomethylarsonic acid (MMA) was 352 ng/g and for As(V) found to be 803 ng/g. The
presence of DMA and MMA was due to microbial activity resulting from sewerage
effluents and chemical reactions within sediments. Concentrations of Cr(VI) ranged
from 0.211 to 1.49 μg/g for both seasons. The presence was attributed to conversion
of Cr(III) to Cr(VI) via photo-oxidation and pollution from industrial activities.
In soil, the total concentrations of Cr and As ranged from 82.6 to 125 μg/g and 0.0453
to 32.6 μg/g, respectively. The observed concentrations could be linked to the use of
fertilizers and atmospheric deposits introduced through the burning of coal by power
stations in the vicinity. The soil quality guideline value of 64.0 μg/g for Cr was
surpassed and posed a great risk to the vegetables growing in the soil and, therefore,
affecting food security. The soil quality guideline value of 12 μg/g for As was surpassed
only in site 3 of farm 1 (F1S3), while the soil screening value of 48 μg/g was not
surpassed. In vegetables, the total Cr concentration ranged from 1.02 to 18.4 μg/g,
while As concentrations ranged from 0.0205 to 0.663 μg/g. The recommended MPL
for Cr, which is 0.5 μg/g, was exceeded in all vegetable samples and rendered the
vegetables unsafe for human consumption. The ability of a vegetable sample to
accumulate Cr and As was assessed by determining the bioaccumulation factor. It
was observed that spinach from site 1 of farm 1 (F1S1) and site 1 of farm 2 (F2S1)
were slight accumulators of Cr, while other vegetables did not show accumulation of
Cr. Vegetables such as spinach from site 2 of farm 2 (F2S2-S) and chomolia from site
1 in farm 2 (F2S1-C) were capable of accumulating As under natural environmental
conditions.
Non-carcinogenic health effects were evaluated using the total hazard quotient (THQ).
In the high flow season, consuming vegetables (all spinach, chomolia, pumpkin and
thepe) containing Cr and As would result in health risks in adults and children as the
THQ > 1. In children, serious health risks would occur as the THQ > 10 was observed
for Cr contaminated vegetables. In the low flow season, the THQ threshold of 1 was
exceeded mostly when consumed by children than adults. In children, the THQ was
above 1 for Cr in all vegetables, while for As, it was not exceeded for farm 1-site 1
spinach (F1S1-S), farm 2-site 1 spinach (F2S1-S), farm 2-site 2 chomolia (F2S2-C),
and farm 2- site 2 onion (F2S2-O). Overall, the non-carcinogenic risks were greater
when vegetables containing the same concentration are consumed by children than
adults. Similarly, the carcinogenic risk assessment was evaluated for children and
adults. For high flow season, the incremental lifetime cancer risk (ILCR) threshold of
1× 10-4 was exceeded for adults and children consuming Cr and As containing
vegetables. In the low flow season, the ILCR value was also exceeded for adults and
children except for adults consuming As contaminated vegetables such as farm 2-site
1 spinach (F2S1-S), farm 2-site 2 chomolia (F2S2-C), and farm 2-site 2 onion (F2S2-0)