Lead Concentrations and Risk Exposure Assessment in Surface Soils at Residential Lands Previously Used for Auto-Mechanic and Auto-Welding Activities in Port

: This study investigated lead concentrations in < 250 µm and < 75 µm of deposited dust and< 2000 µm, < 250 µm, and < 75 µm of surface soils at undeveloped residential lands leased to auto-mechanic artisans for a minimum of ten years and estimated exposure risk for children that will reside on the polluted lands after the lease periods. Soil-Pb levels ranges obtained were between 40.0 - 411 mg/kg, 62.0 – 435 mg/kg , and 61.0 – 491 mg/kg for < 2000 µm, < 250 µm and < 75 µm fractions, respectively. Dust-Pb levels ranges were between 138 – 1819 mg/kg, and 128 – 1584 mg/kg for < 250 µm and < 75 µm fractions, respectively. The estimated daily intake were in the range of 0.17 – 1.22 µg Pb Kg -1 body weight [BW]d -1 and 0.39 – 5.11 µg Pb Kg -1 BWd -1 for soil and deposited dust, respectively. This study has highlighted possible risks to occupants of lands previously used for auto-mechanic and auto-welding activities

Lead is included in the formulation of several motor oils. Leaded gasoline has been phased out, but is still used in parts of Africa (Tuakuila et al., 2010). Automechanic yards may still be point sources of lead pollution because unacceptable lead concentrations in the range of 1500-9300 mg/kg have been reported for gear and motor oils by Clausen and Rastogi, (1977). The legacy of the auto-mechanic activities is usually not investigated and remediated; consequently people that develop their residential homes at sites previously used for auto-mechanic services may be exposed to surface soil-lead through the inhalation of re-suspended dust and ingestion of garden food (e.g. Boisa et al., 2013;Laidlaw et al 2014;Mackay et al 2013;Zahran et al., 2013;Boisa et al., 2014). Exposure could also occur via borehole ground water, since Government treated pipe borne water is hardly available in cities of most developing countries including Nigeria.
Human exposure to motor garage chemicals has been suggested to induce genotoxic effects (Oktem et al., 2004;Shastri and Plant, 2011;;Eum et al 2013;Pottier et al., 2013;Weuve et al., 2013;Hong et al., 2015). Results of previous epidemiological studies conducted on adolescent workers indicated elevated blood lead levels for auto-mechanics and garage owners (Enander et al., 2004;Oktem et al., 2004;Tuakuila et al., 2010;Adela et al., 2012). Previous published studies (e.g. Adelekan and Abegunde, 2011) at mechanic garages in Nigeria investigated contaminant levels without considering exposure risk to the artisans on site or risk to population that may reside permanently at the emerging Brownfield areas. The aim of this study is to investigate lead concentrations in surface soils at residential lands leased to auto-mechanic and auto-welding artisans and estimate exposure risk to both artisans and people who will reside permanently on such lands.

MATERIAL AND METHODS
Study sites: Port Harcourt has large number of automechanic service stations that are situated at private lands and roadsides. The study was conducted at selected roadsides (RS) and residential yards (YD) used as auto-mobile mechanic service stations within Port Harcourt City, Nigeria. The criteria employed for the qualification of any of the sites was proximity to nursery and primary schools. Other factors considered were closeness to roadside commercial barbeque snacks points (e.g. for roasted plantain), children playgrounds and uncontaminated land (C). The zones of Port Harcourt NDOKIARI, BOISA; ERIC, JULIET OLOH captured in this study were Town (RS1), Borokiri (RS2, RS3 and YD1), Ikoku (YD2 and RS4), Rivers State University -RSU (RSC and YDC) and Oyibo (YD3, YD4, YD5, YD6 and YD7) (Figure 1). Details about the type of auto-mechanic services provided, public institutions and nature of surfaces at each of the study sites are listed in Table 1. The < 2000 µm fraction (51 g) of the different samples were quantitatively transferred into screw cap plastic bottles and organic matter was destroyed by adding hydrogen peroxide and sodium hexametaphosphate and the resulting soil analysed by hydrometer method (Rowell, 1994;Keller and Gee, 2006).
Organic matter: About 5 .00 g of the < 2000 µm fraction of soils were quantitatively transferred into previously weighed clean porcelain crucibles and heated in a muffle furnace at 400 o C for 3 hours. The resulting crucibles and their contents were allowed to cool in desiccators and weighed to determine the loss on ignition as a measure of the organic matter (Rowel, 1994;Schumacher, 2002).
Pseudo Total Analysis: Aqua regia digestion (HCl : HNO 3 in the ratio 3:1 v/v) were performed on 0.5 g subsamples and an aqua regia certified reference material (BCR 143R Sewage sludge amended soil), using hotplate for 2 hours. For each digestion reagent blanks were also prepared. The resulting filtrates were transferred into plastic sample bottles and subsequently analyzed for their Pb content by Flame (air-C 2 H 2 ) Atomic Absorption Spectrophotometer (AAS-Buck Scientific).

Experimental Quality Control:
The accuracy of the aqua regia digestion method was monitored using BCR 143R (aqua regia certified Sewage sludge amended soil). Excellent results were obtained for total Pb (171 ± 9 mg/kg) determined compared to certificate value (174 ± 5). All test samples were extracted in triplicate. For the determination of Pb, AAS was calibrated using at least 6 Pb standards (AccuTrace Reference Standard -AA29N-1) obtained from AccuStandard, Inc, USA. The instrument was re-calibrated after not more than 10 samples. Two quality control standards (high and low range) were analysed after no more than 10 unknown samples. All reported measurements are based on the average of three replicate analyses.  Table 2). The highest soil organic matter observed at the former domestic waste dumpsite is expected because of food and other human wastes that had interacted with the original surface soil. The lower soil organic matter observed for the other surface soil from mechanic workshops are expected since most waste generated are not organic and biodegradable in nature. Lead concentration in surface soils: Lead levels measured in the < 2000 µm fraction of the surface soil samples were found to range between 40 and 411 mg/kg, and the range was above the concentrations 24 and 29 mg/kg found in control samples (YDC and RSC, respectively) ( Table 3). The maximum concentration (411 mg/kg) indicated in this study is above that (298 mg/kg) reported by Ipeaiyeda et al., (2007) for a newer city, Iwo in Nigeria (  (Table 3). The concentrations observed for the re-suspendable size fraction were also higher than those for the < 2000 µm fraction. Excluding sample points YD2 and YD3 all other sample points indicated concentrations that were at least two times higher than those for the < 2000 µm. The finer fractions are usually expected to have higher loadings of contaminants, but results obtained in this study indicate some exceptions; samples obtained from RS2, YD2, YD3 and YD5 indicated higher Pb loading for the < 250 µm than the < 75 µm size fraction. The elevated lead concentrations observed for the more coarse size fraction samples (RS2, YD2 and YD3) may be due the fact that automobile body refinishing jobs were taking place alongside the car engine servicing. Since one of the activities at automobile body workshops involve scrapping of old automobile body surfaces the generated paint dust and metal fillings may have contributed the large lead loadings.

RESULTS AND DISCUSSION
Soil ingestion exposure assessment at automobile mechanic yards: In developed countries, currently human health risk assessment studies from soil are based on ingestible (< 250 µm) and fine particles of soil (< 100 µm) fractions (e.g. Boisa et al., 2013;Boisa et al., 2014). Using the < 250 µm size fraction Pb concentrations in surface soil and deposited dust, it was possible to estimate the potential the potential human exposure risk associated with the ingestion of any of the matrices. The method used here is based on calculating the Pb daily intake (Pb DI) from incidental ingestion.

*auto-mechanic waste dump sites
Pb DI (µg Pb Kg -1 body weight [BW]d -1 ) has been calculated for children (2-6 years) that may later occupy these emerging brownfields and pupils presently studying in schools sited within or around mechanic garages based on their pica habits as: Where; EC is aqua regia Pb concentration, µg/g; SIR is soil ingestion rate, mg/d (given as 100 mg/d for the age group of interest) (US EPA, 2011);ED is exposure duration, to be0.5 as exposure could be 12 hours per day, 365 days a year; BW is body weight, 17.8 kg (US EPA, 2011) for the age group of interest.
The automobile artisans workshops investigated in this study are adjourning either primary or nursery school. Some of the shops had additional businesses like food canteens and in few cases wives of artisans and their below school age children stay at the shops to assist. The soils and deposited dust sampled in this study are in contact with the population. However, because of the vulnerability of children this study sought to quantify the exposure to children in the age range of 2-6 years. Exposure estimate were NDOKIARI, BOISA; ERIC, JULIET OLOH calculated for topsoil and deposited dust samples using the concentration data (Table 3). The data for test samples (  , 1977). Our data highlights that a possibility of harm exist via exposure to the deposited dusts. Implications for occupiers of residence sited on Brownfields resulting from mechanic activities

*-JEFCA (2012)
Existing environmental risk assessment allows for the comparison of soil aqua regia extractable total of potentially harmful elements with environmental quality guidelines. This study has shown the possibility of link between Pb concentrations in surface soils and deposited dusts within automobile mechanic yards. Whilst direct soil ingested can be monitored in children, the deposition of suspended surface dust on children hands and subsequent ingestion might be difficult to monitor. All dust samples indicated higher Pb DI than the soil samples. Permanent residency at such land areas may expose inhabitants to unacceptable Pb intake. In order to minimize Pb intake risk management strategies such as soil encasement, re-vegetation and soil replacement will be required. Whilst attempts are made to have better understanding of sources of Pb at mechanic yards and effect contaminants management, it is essential that the danger of leasing landed properties for mechanic services be highlighted.
Conclusion: This study investigated the levels of Pb in surface soils and dusts obtained from eleven (11) land areas previously used for automobile mechanic and other associated services and two (2) control sites. Our data indicated elevated soil-Pb levels for the automobile service sites. Whilst published similar studies in developing countries were principally based only on the < 2000 µm particle size fraction of soils, this study in addition of the classical < 2000 µm size fraction of soil also considered two other size fractions (< 250 and < 75 µm) and dust matrix.
The higher Pb levels observed in the dusts samples highlights the relevance of this matrix. Risk assessment for accidental ingestion of soil and dust based on the < 250 µm fraction indicated more daily intake doses of Pb from dust than soil. Two sites;(1). a site used for diesel engine and battery services, and (2) a site used for car engine service and automobile body refinishing work indicated daily Pb intake doses that exceeded current tolerable daily intake value. The utilization of dust data for the estimation of children exposure doses may however be overprotective of children's health.
Most of the sites investigated were engaged in multiple automobile mechanic and welding services; therefore it was difficult to account for the specific activities responsible for high Pb loadings. There is therefore need to conduct a source apportionment studies of potentially harmful elements at automobile yards and sites. Data from such studies may assist in identifying services associated with higher lead loadings.