Assessment of chromium contamination in the surface water and soil at the riparian of Abbay River caused by the nearby industries in Bahir Dar city, Ethiopia

Chromium is an element occurring naturally in the earth's crust in the form of compounds or as ions in water and is a common contaminant of surface water and groundwater (Bartlett & James 1988). Chromium is also released to the environment from anthropogenic sources and it is the major contributors of chromium contamination in the environment (ATSDR 2008). Chromium can enter the atmosphere as a result of fossil fuel burning, steel production, stainless steel welding, and Cr manufacturing, whereas, discharges into water and soil can result from industrial processes such as electroplating, tanning, dyeing, water treatment, or disposal of coal ash (USEPA 1995).

Chromium exists in the environment in two most stable oxidation states, Cr (III) and Cr (VI), which are likely to be interconvertible in natural waters and in soils (Schroeder & Lee 1975; Handerson 1994). However, these two forms of chromium are mainly different in their physicochemical and toxicological properties. Cr³⁺ is soluble in acidic solutions and it precipitates as the hydroxide in alkaline solutions (Rai et al. 1987). Chromium (III) is less toxic, but in higher concentrations it is likely to produce genotoxic DNA effects in the cell nucleus (ATSDR 2012). Cr (VI) mainly exists in water as chromic acid (H₂CrO₄), hydrogen chromate (HCrO₄⁻), chromate (CrO₄²⁻) and dichromate (Cr₂O₇²⁻) depending on the pH and total chromium concentration (Palmer & Puls 1994). The distribution of Cr (VI) species as the function of the pH, are H₂CrO₄ below a pH of about 1, CrO₄²⁻ above a pH of about 6, and HCrO₄⁻ at pHs between 1 and 6, when the concentrations of Cr (VI) are relatively low. The dichromate ion (Cr₂O₇²⁻) is a dimer of HClO₄⁻, which dominates when the concentration of Cr (VI) exceeds approximately 1 g/L. Cr(VI) is an extremely strong oxidant and highly soluble within a wide range of pH that makes it easily bioavailable (Kotas & Stasicka 2000). It is highly toxic and causes cancer, damages the kidney, liver and gastritis in humans (ATSDR 2008, 2012).

Leather and textile manufacturing processes consume large quantity of water and a variety of chemicals. These industries discharge a significant amount of wastewater which can severely damage the environment if not well treated. The large volume of wastewater generated contains different types of chemicals such as chromium (Cr) particularly used in tanning, printing and dyeing processes (Stein & Schwedt 1994).

In Bahir Dar City there are three industries established close to Abbay River, namely Bahir Dar Textile Share Company, Davimpex Enterprise Bahir Dar Tannery and Habesha Tannery PLc. The former industry has modern treatment plant consisting of primary, secondary and tertiary (electrochemical) treatment processes to remove chromium and other pollutants from the wastewater. While the later two industries have no well established functional treatment plants at the time of sampling and discharge their wastewater directly into Abbay River. The objectives of this study were to evaluate the concentration of total chromium in the surface water and soil at the riparian of Abbay River and describe its impact on the environment.

The study area is found in Bahir Dar, capital city of Amhara Regional State, located in North West of Ethiopia at a distance of 565 km from Addis Ababa. Its geographical location is at about 11°36′0″N latitude and 37°24′0″E longitude at an elevation of 1,800 m. It is a warm climate region with maximum rainfall during June to September. Bahir Dar is one of the fast growing cities in the country and becoming the source of pollution of Lake Tana and Abbay River. Although the city is not an industrial Zone, there are two tanneries and one textile industry planted close to the river. The study area covers approximately 8.5 km starting from Abbay bridge going down to Sevatamit.

Sampling site 1 (S1) is located near Abbay Bridge, in the upstream side of the three plants and supposed to be free from chromium contamination, because it does not receive industrial effluents and is taken as control. The second sampling Site (S2) was situated approximately 400 m from S1 and it is at the junction point of Bahir Dar Textile wastewater effluent to the river. Sampling site 3 (S3) is situated approximately 200 m after S2 and Site 4 (S4) is located 2 km down away from S3 near St. Abune Teklehaimanot Monastery. Sampling Site 5 (S5) is situated at the discharge point of Bahir Dar Tannery wastewater to the river, approximately 2 km from S4 and site 6 (S6) is situated approximately 200 m down S5. Sampling site 7 (S7) was located approximately 1 km down S6, at the junction point of Habesha Tannery wastewater at the river. Sampling Site 8 (S8) is located approximately 200 m from S7 and the last sampling site S9 is situated 3 km from S8, downstream of the three industrial plants near to Sebatamit village.

Water and soil samples were collected in the riparian of Abbay River in April, 2015, September, 2015 and January, 2016. Water samples were collected on the surface of the river approximately 3–5 cm depth near the margin of the river. Water samples were collected using HDP plastic bottles after soaking them in 10% nitric acid overnight followed by rinsing with distilled water. The samples were transported to Bahir Dar University Chemistry Laboratory, acidified with concentrated nitric acid to a pH < 2 and preserved in a refrigerator at 4°C.

Soil samples were taken about 5 cm depth of the surface close to water sampling sites and sealed in clean polyethylene bags. These samples were transported to the laboratory and dried in an open air for 2 days and placed in an oven at 105°C overnight. The dried samples were crushed using mortar and pestle and sieved with 250 μm stainless steel mesh. The finely ground samples were stored in polyethylene bags and preserved in a refrigerator at 4°C to the time of digestion. All the materials used in sampling and preparation were cleaned by socking them in 10% nitric acid for 24 hours.

The pH of water samples were analysed using portable Hanna pH meter (HI8424) after calibrating it using standard buffers 4, 7 and 10. To determine total chromium in water samples, first the samples were digested using concentrated nitric acid following standard procedures for the examination of water and wastewater (APHA 1998). Finally the samples were filtered through Whatman filter paper grade 42, England and the chromium concentration was determined using Ultima 2 Inductively Coupled plasma Optical Emission Spectrometry (ICP-OES), Horiba Scientific.

The soil pH was determined using Hanna pH meter from a suspension produced by mixing 10 g of soil sample with 25 mL of 1 M KCl solution in the ratio of 1:2.5 after automatic string for 30 minutes.

To determine total chromium, 1 g of dry soil was mixed with 10 mL of 1:1HNO₃ in 200 mL flask. The flask was covered with a watch glass during digestion. The samples were heated to 95°C ± 5°C and refluxed for 15 minutes without boiling. After the sample was cooled, 5 mL of concentrated HNO₃ was added; the cover was replaced, and refluxed for 30 minutes. This process continued until no brown fumes were given off by the sample. The solution was allowed to evaporate to approximately 5 mL without boiling in the above mentioned temperature ranges. Then the sample was cooled and 2 mL of water and 3 mL of 30% H₂O₂ was added. The cover was replaced and warmed to start the peroxide reaction. Addition of 1 mL H₂O₂ continued until effervescence is minimal or until the general sample appearance is unchanged. Heating of the acid-peroxide digestate continued until the volume has been reduced to approximately 5 mL. After cooling, it was diluted to 25 mL with distilled water. Particulates in the digestate were removed by filtration, using whatman filter paper grade 42 (USEPA 1996). The soil extracts were analysed for chromium total using ICP-OES. Concentrations in the soils were reported on dry-weight basis.

Statistical analysis was done using IBM SPSS software version 21. To test the significant difference between means, one way analysis of variance (ANOVA) was used to compare the level of contamination of chromium at different sampling stations both in the soil and water body.

The quality of data was controlled through use of standard operating procedures, calibration with standards, analysis of reagent blanks, recovery of known additions and analysis of replicates. All analyses were carried out in triplicate, and the results were expressed as the mean ± SD.

Four standard solutions were prepared with a concentration of 0.1, 0.5, 2 and 5 mg/L from Cr (NO₃)₃.9H₂O following the same procedure as the sample and analysed using ICP-OES. The calibration curve, intensity Vs concentration was drawn with linear fit. The correlation coefficient (R) value obtained was with a minimum 0.9907 and maximum 0.9998 during measurement.

It was done on one sampling site (S2) for both water and soil recovery tests. 1 mg/L standard chromium salt solution was spiked to water sample. Analysis was made before and after spiking using the mean of triplicates, the recovery was 95.8%. Analogically the recovery of soil sample was determined by adding 1 mg/kg of standard Chromium salt solution. The recovery was 89.7%.

The limit of detection (LOD) is three times the standard deviation of the blank (LOD = 3 × Sb). The average SD of eight replicate blanks was 1.315 ng/mL and LOD of total chromium was 3.95 ng/mL.

There was a significant variation in the level of total chromium in the water samples taken at different sampling sites at the periphery in the course of Abbay River. Sampling site S1 showed the lowest mean concentration of total chromium in three different periods of sampling, i.e. 0.007 ± 0.002 mg/L, because this site is located above the industries and it was taken as control. The concentration of chromium increased at S2 (0.052± 0.025) as compared to S1, this might be due to discharge of dyes which contain chromium from Bahir Dar Textile Share Company. At sampling site S5, the total chromium concentration increased to a mean of 0.207 ± 0.268 mg/L, due to discharge of chrome containing tannery effluent from Davimpex Enterprise Bahir Dar Tannery. Sampling site S7 showed the highest concentration of chromium (8.420 ± 6.639 mg/L) as compared to all sampling sites, this was due to direct discharge of tannery effluent from Habesha Tannery Plc. The chromium concentration in the downstream of industries showed a gradual decrease in the mean total concentration of chromium as shown in Table 1 below. The last sampling site (S9) showed nearly comparable value as the control, this was due to dilution of industrial wastewater with the river water.

The level of average total chromium at sampling sites, S5 and S7 were higher as compared to the maximum permitted values set for drinking water quality by Ethiopian Ministry of Water Resources (EMWR) and other different organizations as shown in the Table 2 below.

The increased average concentration of chromium in the river water at the junction point would be due to discharge of chromium containing effluents from industries beyond the standard limits set by WHO (1998) which is 1 mg/L.

According to single factor ANOVA, the average concentration of chromium in the boundary of the river shows significant variations, P < 0.001 at the sampling sites. It couldn't be denied the impact of Wastewater discharged from tanneries in the pollution of Abbay River by chromium salts. Different studies reported high contamination of water bodies with chromium salts. In Rio de Janeiro (Brazil), a maximum of 80 mg/L of chromium was reported in the river estuary waters of Iraja that was discharged from a chromium electroplating industry (Pfeiffer et al. 1982) and in Ganga river at Kampur (north of India), the water was contaminated with a concentration of total chromium over 150 mg/L (Khwaja et al. 2001).

The average pH of water samples taken were in the range of 7.56 to 9.12 as shown in Table 1 above. Lower pH was analysed at site S2, this would be due to the influence of textile wastewater discharge to the river and maximum pH value was observed at sampling site S7, near the discharge point of Habesha tannery to the river. The alkaline nature of the wastewater might be use of alkaline salts in the processing of leather during the time of sampling. Average water pH values of all samples but S7 were within the range of WHO guideline values of drinking water quality, i.e. 6.5–8.5 (WHO 2011).

The average total chromium concentration in the soil at sampling site, S7 was maximum (232.465 mg/kg) as compared to the control group(S1) as shown in Table 3 above. This might be due to discharge of chromium containing tannery effluent from Habesha Tannery beyond the permissible limit of industrial effluent to surface water. Sampling site S5 also revealed high concentration of Cr (31.82 mg/kg) next to S7. This was due to discharge of effluent from Bahir Dar Tannery. Soil samples taken close to industries showed high contamination of Cr in the soil. The results were compared with other similar studies. In Hyderabad (India), much higher concentrations of chromium were reported in soil samples collected at Balanagar industrial area in the range of 82.2 to 2,264 mg/kg (Machender et al. 2011). In the Białka River (Poland), sites located in the downstream of tanneries, the concentration of Chromium in the sediment found in the range of 700–1,600 mg/kg (Pawlikowski et al. 2006). In Brazil, it has been observed that the metallic chromium contamination in fluvial sediments in the downstream of tanneries ranged to a maximum 2,878.0 ± 78.0 mg/kg in the state of Minas Gerais (Jordao et al. 1997). In Algeria, the conc. of chromium in the sediment downstream of Jijel Tannery samples taken at Mouttas River reached to a maximum of 4,261.00 mg/kg (Leghouchi et al. 2009).

Soil average pH at sampling sites showed a minimum value 4.7 ± 0.529 near textile Share Company and maximum value 7.55 ± 0.105 near the discharge point of Habesha Tannery. According to USDA (1993) Natural Resources Conservation Service, soil pH ranges roughly from acidic (pH < 3.5) to very strongly alkaline (pH > 9.0). In this study, average pH values were in the range of USDA standard. Soil pH is a main characteristic in soil chemical properties because it governs many chemical processes. The fate of chromium ions in soil is dependent upon the redox potential and the pH of the soil. Chromium occurs in the soil predominantly as Cr (III) near neutral pH values with low solubility and reactivity resulting in low mobility in the environment and low toxicity in living organisms (Barnhart 1997). A lower pH of the soil favours reduction of Cr (VI) to chromium (III) (Raymond et al. 2015).

The study find out that water and soil samples taken in three periods of sampling at the periphery of upper head of Abbay River showed higher concentration of total chromium at the wastewater discharge points from industries compared to the control site. Higher concentration of total chromium at the discharge point of tanneries to the river indicated that tanneries are discharging higher concentrations of chromium containing wastewater beyond the permissible limit set by Ethiopian Ministry of Water Resources (EMWR) and WHO. A Sample taken near the wastewater discharge point of Habesha Tannery(S7) at Abbay River showed a maximum total chromium concentration of an average 8.420 ± 5.409 mg/L and 232.465 ±56.219 mg/kg in water and soil samples, respectively.

The fate of chromium ions in the environment is mainly dependent on pH. The pH of water samples were in the range of 7.45 ± 0.128 to 9.12 ± 1.545 during sampling periods, which shows slight variation set by WHO drinking water quality guideline (6.5–8.5). The pH of soil samples were in the range of 4.7 ± 0.529 to 7.55 ± 0.105, according to the USDA Natural Resources Conservation Service, Most measured values were in the optimum pH range with low solubility & mobility of chromium in the environment. But this may not be consistent due to variable nature of the wastewater depending on the unit process.

The authors are grateful to Bahir Dar University, Research and community Service, Biotechnology Research Institute (BRI) for the financial support.