Mekelle is one of the Ethiopian cities suitable for urban and peri-urban agriculture for cultivating leafy vegetables using wastewater. The consumption of unprocessed and processed leafy vegetables is also very high in the city. Wastewater samples collected from four experimental sites (ESs) in Hadnet district of the city had higher concentrations in 4 (i.e., Cd, Cr, Cu, and Mn) of the 10 heavy metals tested than the permissible limit established by pertinent standards. Spring water samples collected from another site called Kallamino, designated as a comparison site, also had higher concentrations in 4 (i.e., Al, As, Cd, and Cu) of the 10 heavy metals tested. However, the leafy vegetables grown in the least contaminated ES had higher concentrations in 6-7 of the 10 heavy metals tested. The lettuce and spinach samples had elevated concentrations of As and Al, respectively. The wastewater used to irrigate vegetable farms in Hadnet district is not safe enough. More importantly, the soils of the farms might have accumulated far more heavy metals. The cultivated lettuce, spinach, and cauliflower are highly contaminated. Thus, the use of wastewater for irrigating urban and peri-urban farms needs to be regulated.

  • Tests of wastewaters used in urban agriculture for heavy metal contents.

  • Tests of three leafy vegetables irrigated with wastewater for heavy metal contents.

  • Contamination of the three leafy vegetables with high level of 6 to 7 heavy metals tested.

Al

Aluminum

Alt.

Altitude

As

Arsenic

Cd

Cadmium

Cr

Chromium

CS

Control site

Cu

Copper

E

East

ES

Experimental site

FAO

Food and Agriculture Organization

Fe

Iron

g

Gram

H2O2

Hydrogen peroxide

Hg

Mercury

HNO3

Nitric acid

Lat.

Latitude

Long.

Longitude

masl

Meters above sea level

mg/L

Milligram per liter

Min

Minutes

mL

Milliliter

Mn

Manganese

N

North

ND

Not detected

NIST

National Institute of Standards and Technology

Pb

Lead

SD

Standard deviation

SPSS

Statistical Package for Social Science

Zn

Zinc

Heavy metals often enter into the environment due to anthropogenic activities. The toxicity, persistence, non-biodegradability, and bioaccumulation of the metals are regarded as the most severe environmental problems and human health risks (Souri et al. 2019). Toxic heavy metals enter the human body through the food chain and cause serious health problems. Human exposure to toxic heavy metals causes several health complications that can lead to death such as increased blood acidity, retardation of development, increased incidence of various cancers, interfering with the absorption of essential elements, and kidney damage.

Domestic and industrial wastewater is a critical medium of contaminant for the environment, which increases heavy metals and is disposed of into the environment (Souri et al. 2018, 2019). Wastewaters are discharged from wastewater treatment plants, industries, mining sites, power stations, and many other sites and facilities (Perrodin et al. 2013). Environmental contamination with heavy metals corresponds with the intensification of urbanization, agricultural activities, and industrialization that have the potential to accumulate heavy metals and contaminate the environment (Souri et al. 2018; Nkosi & Msimango 2022). Leafy and non-leafy vegetables irrigated with wastewater often accumulate heavy metals on their surfaces and tissues (Perrodin et al. 2013; Manzoor et al. 2018; Bahiru & Teju 2019; Rahman & Islam 2019; Gebeyehu & Bayissa 2020; Nkosi & Msimango 2022). The consumption of such produces contaminated with elevated levels of heavy metals has become a great concern for human health (Gebeyehu & Bayissa 2020).

Urban and peri-urban agriculture is being promoted as an important sector for livelihood and economic activities in many parts of the world including Ethiopia. Mekelle city, Tigrai, Ethiopia − endowed with many rivers and streams − is one of the fast-growing cities suitable for urban and peri-urban agriculture. In fact, urban and peri-urban agriculture for growing leafy and non-leafy vegetables is quite common in the city. Moreover, the consumption of unprocessed and processed leafy vegetables is very high. However, the quality of the produces has been of great concern. Some studies have reported that vegetables grown using the waters of some of the rivers and streams of the city have an excess accumulation of heavy metals (e.g., Gebeyehu & Bayissa 2020). Therefore, this study aimed to investigate the quality of water from unexplored rivers and streams used for irrigating leafy vegetables and the safety of three leafy vegetables grown there in terms of heavy metal accumulation. The findings of the study will help consumers to make good choices and take proper precautions when consuming leafy vegetables as well as relevant public entities to implement proper regulatory measures on urban and peri-urban agriculture.

Description of the study area

The study was conducted in two districts of Mekelle city called Hadnet and Kallamino. Mekelle, the capital city of Tigrai, Ethiopia, is located about 200 km southeast of the historic city Aksum (alt.: 1,900 to 2,200 masl; lat.: 13° 30′ 0″ N; long.: 39° 28′ 11″ E). The climate of the city is semi-arid, with a mean annual temperature of 16–20 °C and an annual rainfall of 600 mm (Gebreyohannes 2019). Hadnet and Kallamino watersheds are about 6 km apart, with a plateau separating them.

Design of the study

The study was conducted in five farmlands. The first four farmlands are located in Hadnet district (experimental site (ES)), while the fifth one was located in Kallamino (comparison site (CS)). The farmlands in the ES are irrigated with wastewater, while that in the CS is irrigated with spring water. The farmlands in the ES are located around or at Adi Haqi (ES1), Enda Mikael (ES2), Tsa'eda Agam (ES3), and Fitsum Birhan Eye Hospital (ES4). Thus, the study compared the concentrations of 10 heavy metals in the (a) wastewater and spring water samples and (b) leafy vegetable samples collected from the least polluted ES and the CS.

Sample collection

Five samples of wastewater were collected from each of the four farms of the ES. Likewise, 10 samples of spring water were collected from the farm in the CS. Also, samples of mature cauliflower (Brassica oleracea var. botrytis) (10), lettuce (Lactuca sativa) (10), and spinach (Spinacea oleracea) (10) were collected from ES1 and the CS. While the wastewater samples were collected in clean polythene bottles, the vegetable samples were collected in separate sterile plastic bags. The vegetable and wastewater samples were taken to the geology lab in the College of Natural and Computational Sciences, Mekelle University for analyses. The vegetable samples were thoroughly washed with sterile distilled water, air-dried for 1 h, and put in an electric oven at 70 °C for 24 h. Dried samples were ground with a mortar and pestle into a fine powder, sieved, and kept in polythene bags for further analyses.

Analyses of heavy metals in vegetable samples

Vegetable samples such as cultivated lettuce, spinach, and cauliflower were prepared for extraction and were digested using the wet digestion method of protocol developed by scholars (Verlicchi et al. 2012; Perrodin et al. 2013; Aydin et al. 2019). A 1.25 g digested vegetable sample was taken in a glass beaker and dried at 103 °C for 24 h. About 25 mL of HNO3 and three boiling chips were put into a destruction tube and heated at 100 °C for 1 h. The sample was subsequently heated at 125, 150, 175, and 200 °C for 15 min at each temperature. The solution was concentrated and cooled to about 5 mL. One milliliter of 30% H2O2 was added to the destruction tube and kept for 10 min. Then, 3 mL of 30% H2O2 and 25 mL of distilled water were added and boiled for 10 min to dissolve the sample. Finally, the sample was cooled and transferred to a 250 mL volumetric flask, settled for 15 h, and the absorbance of the clear supernatant was measured by atomic absorption spectroscopy. The precision and accuracy of the analyses were checked through repeated analysis against the NIST standard reference material 1570A.

Analyses of heavy metals in water samples

Each of the wastewater and spring water samples was subjected to the nitric acid digestion method (Verlicchi et al. 2012; Perrodin et al. 2013; Aydin et al. 2019). About 50 mL of the sample was transferred into a clean glass beaker, and then 10 mL of concentrated HNO3 was added. The beaker was placed on a hot plate at 80 °C and the solution evaporated until it reduced to 20 mL. Then, the content was cooled, and 5 mL of concentrated HNO3 was added and heated again on a hot plate. A small portion of concentrated HNO3 was cautiously added to the beaker until the solution appeared transparent. After that, distilled water was added to the beaker until the volume of the solution reached the 50 mL mark and was filtered through Whatman No. 42 filter paper. Finally, the concentrations of heavy metals were determined using a flame atomic absorption spectrophotometer (Varian FAAS-240). For quality control analyses, glassware was washed with 10% HNO3 and calibrated using standard solutions of each heavy metal. The precision and accuracy of analyses were checked through repeated analyses against the NIST standard reference material RM 1643E.

Data analyses

Acquired data were analyzed using inferential statistical methods with SPSS Version 23.0 software. Mean (±SD) values of heavy metal concentrations of the samples of wastewater, spring water, and leafy vegetables were computed using the analysis of variance and were compared against the values of the permissible limits established by the Food and Agriculture Organization (FAO 2004).

Concentrations of heavy metals in the waste and spring waters

Among the four sites of Hadnet district, copper (Cu) was recorded at the highest concentration above the permissible limit of the FAO (2004) at ES2 (1.1140 ± 0.0635 mg/L) followed by ES3 (1.0860 ± 0.0841 mg/L), ES4 (0.9420 ± 0.0390 mg/L), and ES1 (0.6120 ± 0.0249 mg/L) (Table 1). According to the FAO standards, the concentrations of Hg, Pb, Zn, Mn, Fe, and Al in the wastewater were within the safe limit. However, the concentrations of Cu (0.5460 ± 0.0241 − 0.7149 ± 0.0058 mg/L), Cr (0.6120 ± 0.0249 − 1.1140 ± 0.0635 mg/L), Cd (0.0620 ± 0.0045 − 0.1182 ± 0.0176 mg/L), and Mn (0.1246 ± 0.0112 − 0.1602 ± 0.0035 mg/L) were found to be above the permissible limit. On the other hand, the spring water of the CS had Al (0.1330 ± 0.0116 mg/L), As (0.0350 ± 0.0053 mg/L), Cr (0.2617 ± 0.0186 mg/L), and Cu (0.3752 ± 0.0191 mg/L) above the safe limit.

Table 1

Concentrations of heavy metals in the waste and spring water samples

Heavy metalsConcentration (mg/L) (mean ± SD)
FAO
ES1 (n = 5)ES2 (n = 5)ES3 (n = 5)ES4 (n = 5)CS (n = 10)
Al 0.0013 ± 0.0005 0.0011 ± 0.0002 0.0017 ± 0.0008 0.0010 ± 0.0001 0.1330 ± 0.0116 0.008 
As 0.0019 ± 0.0002 0.0018 ± 0.0002 0.0022 ± 0.0012 0.0039 ± 0.0002 0.0350 ± 0.0053 0.004 
Cd 0.0800 ± 0.0128 0.1182 ± 0.0176 0.0924 ± 0.0056 0.0620 ± 0.0045 0.2617 ± 0.0186 0.010 
Cr 0.5460 ± 0.0478 0.7194 ± 0.0058 0.6300 ± 0.0316 0.5460 ± 0.0241 0.0250 ± 0.0053 0.050 
Cu 0.6120 ± 0.0249 1.1140 ± 0.0635 1.0860 ± 0.0841 0.9420 ± 0.0390 0.3752 ± 0.0191 0.050 
Fe 0.5520 ±0.0541 0.6040 ± 0.0336 0.7760 ± 0.0385 0.6240 ± 0.0208 1.5120 ± 0.0148 260.000 
Hg 0.0027 ± 0.0005 0.0044 ± 0.0006 0.0009 ± 0.0001 0.0020 ± 0.0001 0.0001 ± 0.0003 0.006 
Mn 0.1310 ± 0.0288 0.1246 ± 0.0112 0.1602 ± 0.0035 0.1456 ± 0.0112 0.0128 ± 0.0013 0.100 
Pb 0.0012 ± 0.0004 0.0021 ± 0.0002 0.0011 ± 0.0002 0.0021 ± 0.0002 0.0259 ± 0.0336 0.100 
Zn 0.6560 ± 0.0513 0.6800 ± 0.0381 0.5760 ± 0.0643 0.7360 ± 0.0297 0.0534 ± 0.0059 2.000 
Heavy metalsConcentration (mg/L) (mean ± SD)
FAO
ES1 (n = 5)ES2 (n = 5)ES3 (n = 5)ES4 (n = 5)CS (n = 10)
Al 0.0013 ± 0.0005 0.0011 ± 0.0002 0.0017 ± 0.0008 0.0010 ± 0.0001 0.1330 ± 0.0116 0.008 
As 0.0019 ± 0.0002 0.0018 ± 0.0002 0.0022 ± 0.0012 0.0039 ± 0.0002 0.0350 ± 0.0053 0.004 
Cd 0.0800 ± 0.0128 0.1182 ± 0.0176 0.0924 ± 0.0056 0.0620 ± 0.0045 0.2617 ± 0.0186 0.010 
Cr 0.5460 ± 0.0478 0.7194 ± 0.0058 0.6300 ± 0.0316 0.5460 ± 0.0241 0.0250 ± 0.0053 0.050 
Cu 0.6120 ± 0.0249 1.1140 ± 0.0635 1.0860 ± 0.0841 0.9420 ± 0.0390 0.3752 ± 0.0191 0.050 
Fe 0.5520 ±0.0541 0.6040 ± 0.0336 0.7760 ± 0.0385 0.6240 ± 0.0208 1.5120 ± 0.0148 260.000 
Hg 0.0027 ± 0.0005 0.0044 ± 0.0006 0.0009 ± 0.0001 0.0020 ± 0.0001 0.0001 ± 0.0003 0.006 
Mn 0.1310 ± 0.0288 0.1246 ± 0.0112 0.1602 ± 0.0035 0.1456 ± 0.0112 0.0128 ± 0.0013 0.100 
Pb 0.0012 ± 0.0004 0.0021 ± 0.0002 0.0011 ± 0.0002 0.0021 ± 0.0002 0.0259 ± 0.0336 0.100 
Zn 0.6560 ± 0.0513 0.6800 ± 0.0381 0.5760 ± 0.0643 0.7360 ± 0.0297 0.0534 ± 0.0059 2.000 

ES, experimental site; CS, comparison site; SD, standard deviation; FAO, Food and Agriculture Organization.

Concentrations of heavy metals in vegetables irrigated with wastewater

Wastewater acquired from ES1 was relatively less polluted as compared to those acquired from the other three ESs. Thus, the investigation into the concentration of heavy metals in the three leafy vegetables was carried out by taking samples from this least polluted site. The three leafy vegetables grown in ES1 were found to have accumulated Al (0.0100 ± 0.0000 mg/L), As (0.0100 ± 0.0000 − 0.0300 ± 0.0000 mg/L), Cd (0.1500 ± 0.0300 − 0.5800 ± 0.0400 mg/L), Cr (1.8500 ± 0.2900 − 3.4900 ± 0.3800), and Fe (434.0000 ± 6.6700 − 503.4000 ± 12.3600) above the safe limit established by the FAO. Moreover, whereas Pb was found above the safe limit in lettuce and spinach, Zn was above the safe limit in lettuce and cauliflower (Table 2). The concentrations of some of the heavy metals are very high (e.g., As, Cd, Cr, and Pb). These imply that the heavy metal accumulation in such leafy vegetables will be much higher in the more contaminated sites of ES2, ES3, and ES4.

Table 2

Concentrations of heavy metals in leafy vegetables collected from ES1

Heavy metalsConcentration (mean ± SD) (mg/L)
FAOLevels of Contamination
Lettuce (n = 10)Spinach (n = 10)Cauliflower (n = 10)
Al 0.0100 ± 0.0000 0.0100 ± 0.0000 0.0100 ± 0.0000 0.006 1.67 times 
As 0.0300 ± 0.0000 0. 0100 ± 0.0000 0.0100 ± 0.0000 0.001 10.0 − 30.0 times 
Cd 0.5800 ± 0.0400 0.1500 ± 0.0300 0.3400 ± 0.0300 0.100 1.50 − 5.80 times 
Cr 3.4900 ± 0.3800 2.3100 ± 0.3300 1.8500 ± 0.2900 1.000 1.85 − 3.49 times 
Cu 0.8000 ± 0.0800 0.9200 ± 0.2200 1.3200 ± 0.2800 73.000 − 
Fe 450.0000 ± 7.9100 503.4000 ± 12.3600 434.0000 ± 6.6700 425.00 1.02 − 1.18 times 
Hg ND ND ND 0.010 − 
Mn 18.8000 ± 4.9700 17.2000 ± 1.9200 8.0000 ± 2.2400 500.00 − 
Pb 0.6600 ± 0.1300 0.6100 ± 0.1100 ND 0.300 2.03 − 2.20 times 
Zn 119.0000 ± 3.8100 ND 114.0000 ± 76.2800 100.00 1.14 − 1.19 times 
Heavy metalsConcentration (mean ± SD) (mg/L)
FAOLevels of Contamination
Lettuce (n = 10)Spinach (n = 10)Cauliflower (n = 10)
Al 0.0100 ± 0.0000 0.0100 ± 0.0000 0.0100 ± 0.0000 0.006 1.67 times 
As 0.0300 ± 0.0000 0. 0100 ± 0.0000 0.0100 ± 0.0000 0.001 10.0 − 30.0 times 
Cd 0.5800 ± 0.0400 0.1500 ± 0.0300 0.3400 ± 0.0300 0.100 1.50 − 5.80 times 
Cr 3.4900 ± 0.3800 2.3100 ± 0.3300 1.8500 ± 0.2900 1.000 1.85 − 3.49 times 
Cu 0.8000 ± 0.0800 0.9200 ± 0.2200 1.3200 ± 0.2800 73.000 − 
Fe 450.0000 ± 7.9100 503.4000 ± 12.3600 434.0000 ± 6.6700 425.00 1.02 − 1.18 times 
Hg ND ND ND 0.010 − 
Mn 18.8000 ± 4.9700 17.2000 ± 1.9200 8.0000 ± 2.2400 500.00 − 
Pb 0.6600 ± 0.1300 0.6100 ± 0.1100 ND 0.300 2.03 − 2.20 times 
Zn 119.0000 ± 3.8100 ND 114.0000 ± 76.2800 100.00 1.14 − 1.19 times 

SD, standard deviation; FAO, Food and Agriculture Organization; ND, not detected.

Concentrations of heavy metals in vegetables grown in spring water

With the exception of Al in spinach and As in lettuce, the concentrations of the heavy metals in the vegetables irrigated with spring water at the CS are below the permissible limits (Table 3). Even the concentrations of Al in spinach (0.0080 ± 0.0008 mg/L) and As in lettuce (0.0020 ± 0.0006 mg/L) are only slightly above the safe limits. The concentrations of Al, As, Cd, and Cu were above the permissible limits in the spring water of this site (Table 1).

Table 3

Concentrations of heavy metals in the vegetables collected from the CS

Heavy metalsConcentration (mean ± SD) (mg/L)
FAO
Lettuce (n = 10)Spinach (n = 10)Cauliflower (n = 10)
Al 0.0020 ± 0.0004 0.0080 ± 0.0008 0.0020 ± 0.0004 0.006 
As 0.0020 ± 0.0006 0.0010 ± 0.0004 ND 0.001 
Cd 0.0790 ± 0.0134 0.0580 ± 0.0148 0.0862 ± 0.0062 0.100 
Cr 0.9000 ± 0.0381 0.8500 ± 0.0762 0.8400 ± 0.0822 1.000 
Cu ND ND ND 73.000 
Fe 251.0000 ± 9. 6177 179.8000 ± 10.0349 194.4000 ± 8.8487 425.000 
Hg ND ND ND 0.010 
Mn 0.9020 ± 0.2056 0.7700 ± 0.1723 0.8780 ± 0.0626 500.000 
Pb 0.2670 ± 0.0120 0.0432 ± 0.0160 0.0720 ± 0.0192 0.300 
Zn 83.6000 ± 5.5946 ND ND 100.00 
Heavy metalsConcentration (mean ± SD) (mg/L)
FAO
Lettuce (n = 10)Spinach (n = 10)Cauliflower (n = 10)
Al 0.0020 ± 0.0004 0.0080 ± 0.0008 0.0020 ± 0.0004 0.006 
As 0.0020 ± 0.0006 0.0010 ± 0.0004 ND 0.001 
Cd 0.0790 ± 0.0134 0.0580 ± 0.0148 0.0862 ± 0.0062 0.100 
Cr 0.9000 ± 0.0381 0.8500 ± 0.0762 0.8400 ± 0.0822 1.000 
Cu ND ND ND 73.000 
Fe 251.0000 ± 9. 6177 179.8000 ± 10.0349 194.4000 ± 8.8487 425.000 
Hg ND ND ND 0.010 
Mn 0.9020 ± 0.2056 0.7700 ± 0.1723 0.8780 ± 0.0626 500.000 
Pb 0.2670 ± 0.0120 0.0432 ± 0.0160 0.0720 ± 0.0192 0.300 
Zn 83.6000 ± 5.5946 ND ND 100.00 

SD, standard deviation; FAO, Food and Agriculture Organization; ND, not detected.

This study has led to three important observations. First, the vegetables grown in farms irrigated with wastewater have a higher accumulation of heavy metals than those grown in farms irrigated with spring water. This is quite natural and many studies have come up with similar observations. For example, higher concentrations of heavy metals were observed in radish, spinach, turnip, cauliflower, and carrots grown in farms irrigated with wastewater as compared to those irrigated with spring water (Arora et al. 2008; Ahmad et al. 2016).

The second important observation is that the leafy vegetables grown in ES1 have higher levels of accumulation in 6 or 7 of the 10 heavy metals than the wastewater used to irrigate it. This implies that the wastewater is neither the direct nor the only source of the heavy metals for the vegetables. The soil, as continuously exposed to contaminated wastewater, can accumulate heavy metals over the course of time much more than the flowing wastewater. The physical and chemical properties and compositions of the soils such as pH, organic matter content, cation exchange capacity, nutrient balance, and metal permissibility can also lead to the accumulation of heavy metals in soils (Qureshi et al. 2016; Yu et al. 2018; Weber et al. 2019; Yang et al. 2020). Moreover, despite the concentrations of many of the heavy metals in the wastewater being below the safe limit, what has reached the vegetables can accumulate in their tissues to far greater levels. The bioaccumulation of heavy metals in vegetables is affected by many factors such as climate, atmospheric depositions, heavy metal concentrations in soil, the nature of the soils of the vegetable farms, and the degree of maturity of the plants at the time of harvest (Bhargava et al. 2012; Ali et al. 2013). Therefore, enhanced levels of heavy metals can be accumulated in the leafy vegetables through the soil and by way of bioaccumulation. In line with this, the higher accumulation of the heavy metals despite their relatively lower concentration in the wastewater can also be due to their higher transfer factor. The heavy metals of Cd, Cr, Fe, Zn, and Pb, detected in the vegetables above the permissible level, have a high transfer factor from the soil and water to the vegetables (Luo et al. 2011; Verlicchi et al. 2012; Perrodin et al. 2013; Aydin et al. 2019).

The third important observation is the difference in the accumulation of heavy metals in different vegetable types. The bioaccumulation of heavy metals of lettuce is higher than spinach and cauliflower and the bioaccumulation of cauliflower is lower than both the lettuce and spinach. These variations may account for differences in morphologies and anatomies, absorption and assimilation capacities, and sensitivities to the heavy metals of the vegetables (Pandey et al. 2012; Xiong et al. 2014; Khan et al. 2015; Qureshi et al. 2016). The findings of the present study are similar to previous ones that demonstrated that leafy vegetables that grow quickly with high transpiration and absorption rates bio-accumulate higher concentrations of heavy metals quickly (Luo et al. 2012; Durowoju et al. 2016; El-nour 2020).

The three vegetables grown in the ESs in the Hadnet district of Mekelle city provided us with important indications that the soils of the farms are accumulating heavy metals. The observation that the fast-growing lettuce has higher bioaccumulation capacity implies that other fast-growing produce cultivated in farms irrigated with wastewater can have similar contamination levels. Thus, urban and peri-urban agriculture in Mekelle city for growing leafy vegetables has to be monitored not to use wastewater for irrigation. Likewise, the soils of the farms growing such vegetables need to be examined if they are suitable. Furthermore, consumers need to be cautious in their consumption of vegetables growing in urban and peri-urban farms.

The authors acknowledge the administrators and technicians of the geology laboratory of the College of Natural and Computational Sciences for allowing us to carry out the laboratory studies.

H.T.T., D.B.S., and G.G. were involved in conceiving, designing, and planning the study. H.T.T. carried out the collection and preparation of water and vegetable samples, the determination of heavy metal contents and data collection, the organization and analyses of the data, and the drafting of the manuscript. G.G. reviewed the draft manuscript and developed the final draft, and D.B.S. reviewed the manuscript for content, language, and style and readied it for publication.

This study was supported by Mekelle University, Mekelle, Ethiopia (G. No.: CRPO/MIT/005/08).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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