Saturday, 9 August 2014

Arsenic levels in soil around cattle-dipping sites in the Vhembe District of Limpopo Province, South Africa.

Arsenic-based cattle-dips were used in South Africa from their first becoming available in1893 until their banning in 1983, and were widely used to control East Coast Fever, a Tick-born disease introduced with Cows imported from Kenya during the South African War of 1899-1902. Cattle-dipping as a control measure became compulsory under the terms of the Stock Disease Act of 1911, and was continued past the eradication of East Coast Fever in 1960, as a way of controlling Foot and Mouth Disease (it is still practiced, though arsenic-based dips are no longer used). Arsenic is a toxic metal which does not biodegrade, persists in the environment and accumulates readily in body tissues. It is known to be carcinogenic as well as causing arsenocosis, a specific disorder relating to ingestion of the metal which can cause a variety of skin problems, gangrene and cancer of the kidneys and bladder. 

The Vhembe District of Limpopo Province is home to the Vhavenda people, who traditionally practice a cattle-based economy. The Vhavenda people clashed several times with the independent Republic of South Africa prior to the South African War, and were subjugated by the British in 1905, their territory being formally integrated into the colony of South Africa in 1910. The area was declared an ‘independent homeland’ from 1979 till 1994, when it was reintegrated into modern South Africa. The Vhavenda people were on the whole hostile to the colonial administration interfering with traditional cattle-rearing practices, but adopted cattle dipping from 1915 onwards as its efficiency against East Coast Fever became apparent. The use of arsenic-based dips is believed to have been stopped in the area in about 1955. Dipping in the area was carried out under the supervision of the Department of Native Affairs, with technical support from the Division of Veterinary Services.

Studies from the 1980s onwards have indicated raised arsenic levels around former cattle dipping sites in the United States, Australia and New Zealand, and in 2007 Billy Moremedi and Jonathan Okonkwo of the Department of Environmental, Water and Earth Science at Tshwane University of Technology published paper in the Journal of Agriculture, Food and Environmental Sciences, which revealed that arsenic was present in the soil around a cattle dip at Ka-Xikundu Village near the Luvuvhu River in Vhembe District, at levels of around 1000 mg per kg of soil, against a background level of 0.15 mg per kg of soil in the area. South Africa does not currently have regulations on heavy metal levels in soils used for agriculture (though these are likely in the future), but 1000 mg per kg of soil is very high. China currently bans agriculture on soils with arsenic levels higher than 30 mg per kg of soil, and Australia and New Zealand require action if soil around former arsenic-dip sites contains more than 20 mg of arsenic per kg of soil and is higher than the background soil level for the area, or if it exceeds 70 mg per kg of soil, regardless of background soil levels.

In a paper published in the South African Journal of Science on 21 July 2014, Marubini Ramudzuli of the Department of Geography, Geoinformatics & Meteorology at the University of Pretoria, and André Horn of the Department of Geography at Unisa describe the results of a study which tested arsenic levels at 10 cattle-dipping sites across the Vhembe District (out of a total of 54 sites known to have been used). The soil was sampled at distances of 5 m, 20 m and 100 m from the dipping tanks; 5 m being within the splash zone of the tank, 20 m within the holding pen where cattle waited after being dipped, and 100 m acting as a control to establish arsenic levels in the local soils. All samples were taken from a depth of 300 mm.

Photo of a dip tank showing the sampling distances at points 1 (5 m), 2 (20 m) and 3 (100 m). Ramudzuli & Horn (2014).

The first site tested was at Tshivhulani in the central highlands of Vhembe, where the soils are predominantly deep red clays, and where cattle dipping began in the early 1920s, and is believed to have continued for about 32 years. This site produced an arsenic level of 30.18 mg per kg of soil at 5 m, 0.19 mg at 20 m and 0.1 mg at 100 m, making the concentration at 5 m more than 3000 times higher than the concentration at 100 m. The dip tank at Tshivhulani is only 100 m from the nearest water course, and was identified as being in need of repair by a government report in 1951.

The second site tested was at Khubvi, also in the central highlands, where the soils consist of highly weathered and compacted red clays. This tank is known to have been established in 1923. This site produced an arsenic level of 3.65 mg per kg of soil at 5 m, 3.69 mg per kg at 20 m and 3.60 at 100 m. The site of the Khubvi tank has been converted for use as a maize field, and it is possible that this has led to the movement of soil in the area, averaging out arsenic concentrations.

The third site tested was at Rambuda in northeastern Vhembe, and has a red, loamy soil with a high level of organic content. The Rambuda dip tank was established in 1940. This site producedarsenic levels of 3.53 per kg soil at 5 m, 3.63 at 20 m and 3.70 at 100 m, a small but steady increase moving away from the dipping site. The area around the Rambuda site is used for the manufacturing of mud bricks, making it highly likely that soils at the site have undergone considerable re-working since dipping was abandoned in the area. The former dip tank was not fenced off and was 30 m from the nearest home; it was also reported as being in need of urgent repair in the 1951 report. As well as being a site used for brick-making, the dip tank is within a field used for crop growing.

The fourth site tested was at Tshikuwi in the western area of Vhembe. It has a red, loamy soil with a high sand content, heavily weathered and compacted around the dipping site, which was established in 1940. This produced arsenic levels of 0.08 mg per kg soil at 5 m, 0.12 mg at 20 m and 0.02 mg at 100 m.

The fifth site tested was at Tshituni, also in the west of Vhembe, where the soil was gravely with some brown clay. This site was established in 1940. Arsenic levels at Tshituni were 0.02 mg per kg soil at 5 m, 0.06 mg at 20 m and 0.01 mg at 100 m.

The sixth site inspected was at Sambandou in the northwest of the Vhembe district, where the soil was a sandy loam with a very high organic content, and the tank was established in 1948. This produced an arsenic level of 46.76 mg per kg soil at 5 m, 6.88 mg per kg soil at 20 m and 1.09 mg per kg soil at 100 m. These were the highest levels found in the study, despite the dip having only been used for seven years, from 1948 to 1955.

The seventh site inspected was at Tshifudi in the central highlands of Vhembe, where the soil is a sandy loam with a high organic content, and where the dip tank was established in 1948. This produced an arsenic level of 3.85 mg per kg soil at 5 m, 0.23 mg at 20 m and 0.15 mg at 100 m. This tank was also identified as being in need of urgent repair in the 1951 government report.

The eighth site inspected was at Mukula in the central highlands of Vhembe, where the soil is a weathered and compacted red clay. This dipping site was established in 1948. This produced arsenic levels of 2.30 mg per kg of soil at 5 m, 1.2 mg at 20 m and 0.08 mg per kg at 100 m. The Mukula tank was also identified as being in need of urgent repair in 1951.

The ninth site inspected was at Thengwe in the northeast of Vhembe, where the soils are sandy with little organic matter. This tank was established in 1950. This site produced arsenic levels of 0.14 mg per kg at 5 m, 0.07 at 20 m and 0.09 at 100 m. 

The final site inspected was at Tshandama in the northeast of Vhembe, where the soil is also sandy with little organic matter and where the dipping site was also established in 1950. This site produced arsenic levels of 0.002 mg per kg of soil at 5 m, 0.003 mg at 20 m and 0.002 at 100 m, the lowest levels found in the study.

All of these test sites produced arsenic levels lower than that found by Moremedi and Okonkwo at Ka-Xikundu, with only two sites (Sambandou and Tshivhulani) producing arsenic levels in excess of statutory limits in China and Australia, and all other sites considerably below 10.0 mg per kg of soil, the average level in UK soils. However Ramudzuli and Horn caution that there is a distinct trend towards higher levels of arsenic concentration in clay and loam rich soils, where arsenic is known to move relatively little, and that the samples were taken at a depth of 300 mm, compared to the 100 mm depth used by Moremedi and Okonkwo, raising the possibility that arsenic levels might be higher at shallower depths at some of these sites. They also observe that the relationship between arsenic levels and risk is not completely straightforward; arsenic persists in higher levels in organic and clay rich soils, but is more bioavailable (i.e. more easily absorbed by organic life forms) from sandy soils.

Ramudzuli and Horn also observe that none of the sites used for historic or modern dipping in the Vhembe District is fenced off, and that children are often observed playing close to the dipping sites and even assisting with the dipping process. Moreover few dipping attendants in the district have received any formal training in handling dangerous chemicals until very recently, and protective clothing is seldom worn during the process.

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