Abstract:
Auto-garages are seen currently as potential sources of heavy metal contamination to
urban soil. This study aimed to investigate the contamination of soils in garage and
auto-mechanical workshops by selected heavy metals and to evaluate associated
human and ecological risks in Ambo town, Ethiopia. Risk assessment models were
applied to measure the pollution levels of heavy metals and estimate of their health
risk of human exposure. To achieve objectives, 24 soil samples (0–15 cm soil depth)
were collected using soil auger from randomly selected seven (7) auto-garages and
one references site in Ambo town, Ethiopia employing judgemental/purposive
sampling technique. The reference soil samples were collected from uncontaminated
or less disturbed sites. Wet digestion technique was employed to prepare the soil
samples for the quantification of heavy metal levels using flame atomic absorption
spectrometric (FAAS) technique. The optimum acid volume ratio was 2:2:1 for
H2O2:HClO4 and aqua-regia, respectively with the digestion time of 3:30hrs and
digestion temperature of 220 ˚C.The LOD and LOQ values lied in the range of 0.01–
0.1 mg.kg−1
and 0.02 – 0.34 mg.kg−1
, respectively. The observed relative standard
deviation (%) and recovery (%) values for the samples ranged from 0.0002 to 1.89
and 90.55-99.98%, respectively. The average concentration of heavy metals found in
the auto-garage soil samples were ranged as. 24.3-315.9,179.9-1438.8, 76.3-176.9,
485.3-1849.9, 25985.8-68064.6, 2342.9-6098.64, 70.79-1095.85, 187.7-1615.83,and
1275.59-41768 in mg/kg for Cd,Co,Cr,Cu,Fe,Mn,Ni,Pb,and Zn, respectively.These
values exceeded the soil quality guidelines established by FAO/WHO and EU, except
Cr,Fe and Ni at ABR.G,ABU.G and ISH.G site which is below the permissible limit.
The results of pollution indexes such as contamination factor (CF), degree of
contamination (CD), modified degree of contamination (mCd), enrichment factor
(EF) and geo-accumulation (Igeo) indicated that the the studied auto-garage soils
contamination status lied between moderate to severe contamination by the studied
heavy metals. The results of the evaluated ecological risk indexes (ecological risk
factor (ErF = 4.78 to 2181) and potential ecological risk index (PERI = 694 to
6392)) showed that there are slight risk to extremely strong risk due to individual
metal elements and their mixture, respectively. The human health risk assessment
results showed that lead (Pb; HQoral >1) and mixture of the studied metals (THI > 1)
can pose non-carcinogenic health risk (HQoral >1) . Similarly, the carcinogenic risk
assessment result indicated that Cd,Cr,Ni and Pb metals can pose slight to high
cancer risk to adult population (total cancer risk (TCR) range from 1.42x10-5
to
3.3x10-3
). The study highlights that open auto-garages in Ambo town are greatly
contaminating their surrounding soils by the heavy metals and threating health of
humans and the environment which necessitating immediate intervention to avert the
problem.
