The dumping of waste on soils has been found to increase their phthalate esters and metals concentrations 12345678. Soil contaminants may be divided into two groups: organic contaminants, which contain carbon, and inorganic contaminants, which do not contain carbon 9. The organic contaminants of greatest concern are industrial in origin, and include agricultural pesticides and non-pesticide compounds like phthalate esters; whereas most important inorganic contaminants are metals, originating from industrial processes 2810111213.

The disposal of waste poses major environmental and public health problems in cities across the world 5141516. This has become a source of concern for rural and urban planners 1718192021. With increasing populations, urbanization and spatial growth in South Africa, most open dumps are now close to residential areas. The Muledane open dump in Thohoyandou is now in the heart of Muledane housing estate. Its location presents serious threats to, amongst other things, the quality of ground and surface water via contaminant leaching and runoffs 45121422. It has been documented that organisms inhabiting contaminated soils take up pollutants, which can bioaccumulate in the complex food chain 9222324. Despite this, open dumps remain the preferred management method for the disposal of municipal solid waste in many developing countries 1920. Lack of space for dumping solid waste has become a problem for many urban managers, who are concerned about the increasing costs of waste disposal and the possible hazards to water supply and air quality 1025.

The dialkyl or alkyl aryl esters of 1, 2-benzenedicarboxylic acid – commonly called phthalate esters – have a myriad of commercial uses, and are considered ubiquitous environmental contaminants. Globally, over 8 billion tonnes of phthalate esters are used each year primarily as additives to poly (vinyl) chloride (PVC) plastics, industrial solvents and components of many consumer products 26. Phthalates have been implicated as possible carcinogenic and teratogenic agents for humans 27282930. However, it is the possible action of phthalates as endocrine disrupters in humans that has caused the most serious concern 27313233. The toxic effects to humans and wildlife of metals such as cadmium and lead are well known. Even in small amounts humans do not require this category of metals 34. Similarly, elevated levels of essential metals like zinc, manganese and iron have been found to be toxic 1035. Calcium, on the other hand, enhances the toxicity and bioavailability of metals like cadmium and zinc 3637.

The objective of this study was to evaluate the levels of some phthalate esters (dimethyl phthalate, (DMP), diethyl phthalate, (DEP), dibutyl phthalate (DBP), diethyl hexyl phthalate (DEHP)) and metals (lead, cadmium, manganese, zinc, iron, calcium) in the Muledane open dump soils in relation to control soil samples. This is expected to provide baseline data that will assist the municipal and provincial authorities in adopting and enforcing a sustainable waste disposal and management system especially in South Africa's emerging urban settlements.

The mean soil pH values are 6.55 ± 0.44 and 6.10 ± 0.72 (Table 1) for the open dump and control samples respectively. These values fall within the range of 6.00–7.00 expected for most tropical soils 1138. The open dump soil's mean pH value of 6.55 ± 0.44 is however lower than the values 7.40 – 10.4 previously reported for some Lagos dumpsites 17. Soil pH is a very important factor in controlling the mobility and availability of metals. High pH reduces the plant availability of most metals 39. With a t_cal value of 1.18 compared to the t_tab value of 2.31, the soil pH values are non-significantly different at 95% confidence level (Table 1).

The mean percentage moisture content shown in Table 1 reveals values ranging from 9.29 ± 1.51 to 10.89 ± 2.83. These relatively low values are expected of tropical soil sampled before the onset of heavy tropical rains 11. However, the open dump and control soil samples exhibit non-significant differences at 95% confidence level, an indication of some degree of similarity in soil texture.

The phthalate esters were eluted from the gas chromatographic column in the order: DMP, DEP, DBP, and DEHP. The following retention times (min) were obtained: DMP, 2.25 ± 002; DEP, 3.03 ± 0.03; DBP, 5.73 ± 0.03; DEHP, 0.98 ± 0.05. The method detection limit (MDL) and instrument detection limit (IDL) are as follows (ng/kg): DMP, 40.17 and 20.46; DEP, 48.89 and 20.46; DBP, 87.52 and 3.80; DEHP, 14.61 and 1.09 respectively for MDL and IDL. The percentage recoveries from spiking experiments are: DMP, 89.08 ± 0.51; DEP, 89.95 ± 0.34; DBP, 88.72 ± 0.55; DEHP, 89.04 ± 0.48. The unrestricted dumping of domestic and medical waste materials made of plastic is the likely source of phthalate esters in the soil samples 5820.

Table 2 provides a list of phthalate esters separated and detected in the soil samples as well as the phthalate pollution index (PPI). Similarly, the t-test, statistical analyses of phthalate esters concentration in the open dump and control soil samples are shown in Table 3. The DMP, DEP, DBP and DEHP levels (mg/kg) in the open dump samples are 0.31 ± 0.12, 0.21 ± 0.05, 0.30 ± 0.07 and 0.031 ± 0.01, respectively. However, the control soil samples have values (mg/kg) of 0.20 ± 0.12, 0.18 ± 0.02, 0.23 ± 0.05 and 0.02 ± 0.01 respectively for DMP, DEP, DBP and DEHP.

Biosolids from five treatment plants in greater Vancouver, British Columbia, reveal DMP, DEP, DBP and DEHP values (mg/kg) of 0.13 ± 0.34, 0.15 ± 0.14, 0.15 ± 0.15 and 2.70 ± 2.70 respectively 3. These values – except for DEHP – are found to be lower than the levels reported for the Muledane open dump soil samples (Table 2). Nevertheless, contaminated soils in Ontario, Canada, as reported 9 have DMP, DEP and DBP levels of < 15 mg/kg, and DEHP burden of < 250 mg/kg. A statistical comparison of DMP, DEP, DBP and DEHP concentrations in the respective soil samples reveal non-significant differences at p < 0.05. A similar trend has been observed before, and may be taken as an indication of low levels of organic contaminant pollution in the open dump 38. Some degree of plastic recycling activities in the vicinity of the open dump may be responsible for this. Of concern, however, is the detection of DEHP in the soil samples, ranging from 0.02 to 0.05 mg/kg in the open dump soils, and 0.02 to 0.03 mg/kg in the control soil samples. DEHP is regarded as having the most deleterious endocrine disrupting capability, resulting in the European Union's (EU) having restricted its use 40.

The maximum admissible concentration (MAC) for the other low molecular weight phthalates, such as DMP, DEP and DBP, has, however, been established by most countries 3032. The phthalate pollution index (PPI) for the soil samples is relatively low at 0.12 – 0.20 (open dump) and 0.09 – 0.14 (control). It is instructive to note, however, that the PPI is higher for the open dump soil samples. Although non-significant levels are recorded between open dump and control samples, regular monitoring is imperative, due to the expected increase in urbanization and industrialization of Thohoyandou in the years ahead.

The metal recovery studies to assure quality gave the following the percentage recoveries: Pb, 90.21 ± 0.004; Cd, 95.40 ± 0.005; Mn, 89.94 ± 0.003, Zn, 93.14 ± 0.002, Fe, 91.64 ± 0.004; Ca, 94.71 ± 0.003. The burden of metals (Table 4) in the soil samples collected from the open dump is generally higher than those of samples collected from the control site. This trend has been observed before in open dump soils 158.

The mean lead, cadmium, manganese, zinc, iron and calcium levels for the control site are (in mg/kg): 0.04 ± 0.01, 0.004 ± 0.001, 3.94 ± 3.33, 0.11 ± 0.05, 11.69 ± 6.19 and 0.04 ± 0.03 respectively. Whereas the values for the open dump samples are (mg/kg): 0.07 ± 0.04, 0.003 ± 0.001, 5.02 ± 1.92, 0.31 ± 0.12, 11.62 ± 9.48 and 0.21 ± 0.13 for lead, cadmium, manganese, zinc, iron and calcium respectively. There exist statistically significant differences (95% confidence level) in the concentrations of zinc in the soil samples (Table 5). This may be taken as an indication of zinc pollution in the open dump. Waste materials in the open dump containing metals are likely sources of the additional burden when compared to the control levels 8112228. The metal concentration levels obtained here were generally lower than the average world values reported earlier 181423. This may not be unconnected with the fact that the Thulamela municipality is still evolving economically.

The concerns regarding zinc are its toxicity to aquatic species, its ability to bioaccumulate, and the interrelationship of zinc with cadmium and lead 414243. Similarly, calcium, iron and zinc play major roles in interfering with cadmium adsorption in the human intestine 44. Soil losses of soluble calcium salts gradually lead to decalcification of the soil and thus to lower pH. This process may be accelerated under wet conditions 3945. Nevertheless, the levels of lead, cadmium, manganese and iron are non-significantly different at 95% confidence level. It is noteworthy that the burden of lead and cadmium in the open dump is relatively high enough to raise public health concerns. These metals are not required even in small amounts by living organisms 3346. The metal pollution index (MPI) shown in Table 4 reveals values ranging from 0.13–0.45 for open dump soil samples, and 0.08–21 for the control samples. These values are relatively high and could be a cause of future concern when related to pollution indexes reported earlier in literature 4748.

The paucity of data on the levels of organic and inorganic pollutants and limit values in the country necessitates reference to values in other countries and organizations. Consequently, results from this study may constitute baseline data to fill this gap.

The study showed that the dumping of wastes in the open dump has the potential to pollute the environment, especially via dispersal. This is revealed by the statistically non-significant differences in the levels of phthalate esters and metals (except for zinc), in the open dump and control soil samples.

We can conclude, however, that the phthalate esters and metals may not presently constitute a serious pollution problem. Nevertheless, the presence of the phthalates and metals as contaminants at these relatively low levels is of concern from the point of view of bioaccumulation. The increased levels of metals and phthalate esters in the soil pose risks to public health, plants and animals, with serious implications for the development of housing, industrial estates and agriculture.

There is need for the Thulamela municipal government to evolve a sustained pollutants monitoring program. Similarly, the open dump should be upgraded to a landfill to cope with the economic growth in the locality.

AA: design, coordination, sampling, bench work, statistical analyses and drafting of manuscript. MD: design, coordination, sampling, statistical analyses and drafting of manuscript. PS: sampling, bench work, statistical analyses and drafting of manuscript. OO: design, drawing of figures, statistical analyses and drafting of manuscript.