Arsenic risks increased by compost and fertiliser

Amending contaminated land with compost or phosphates could exacerbate the risks posed by arsenic contamination, according to a study of soil processes at a former industrial site at Merton Bank in St Helens, Merseyside.¹ With biodegradable wastes being increasingly diverted from landfill and used on brownfield sites, the findings have important implications for land remediation practices.

A team led by William Hartley from the School of Biological and Earth Sciences at Liverpool John Moores University challenged the assumption that arsenic has low soil mobility and bioavailability. It also considered the impact of modern land management practices on arsenic.

The team found the application of organic compost and phosphates to soils with a high arsenic content raised the element’s mobility, increasing risks to human health. Further investigation found that applying iron salts to the soil would reduce arsenic mobility, thus lowering the risks of exposure.

Merton Bank comprises 6.6 hectares of grassland covering a former industrial site. The site was previously the location of a chemical waste tip used by local glass and soda ash manufacturers. Both industries produced large quantities of arsenic-containing waste and arsenic trioxide was used as a clarifying agent in glass making.

An alkali works occupied part of the site as long ago as 1873, though it was gone by the turn of the century. The site remained derelict until the 1970s when it was remodelled. The alkali waste tips were levelled in 1978 and used to contour the site, which was then covered with 20 centimetres of acidic colliery spoil and a thin layer of soil. Fertilisers were applied to the soil in 1980, which was then seeded. Since then, the site has been managed as amenity grassland, providing a recreational green space for the local community.

Arsenic is carcinogenic to humans, with soil ingestion posing a major health risk. While concentrations of arsenic in uncontaminated soils are generally in the range of 2-53 parts per million, levels at Merton Bank have been recorded at up to 5,000ppm, far exceeding the 20 ppm soil guideline value for residential land use and the 500 ppm applicable to commercial and industrial land.

Despite the contamination, the site is not classed as contaminated land because arsenic mobility is low enough not to pose a health risk or a threat to groundwater. The researchers conclude that the arsenic has remained relatively stable in terms of environmental mobility, despite the previous remediation work falling below modern standards. They found that arsenic uptake by vegetation was low and the grass has reduced the risks of public exposure from windblown dust.

While arsenic mobility is low, it was found to be related to the level of dissolved organic matter and phosphate added through the application of compost and fertiliser. Conversely, arsenic immobilisation was correlated with the presence of iron oxides.

The researchers concluded that the risks to human health could be minimised through the avoidance of soil exposure and the application of compost or phosphates. They noted that such applications "would not appear to be sensible options for sustainable site management," while the addition of iron oxides would be beneficial. They added that natural processes were unlikely to resolve the contamination issues, but also would not cause any exacerbation of risk.

Arsenic has been a pertinent issue at other leisure amenity sites in the north-west of England. In 2007, Halton Borough Council determined part of a former municipal golf course in Widnes as contaminated land posing a public health risk. The site had also been used as a waste tip by the local chemical industry, particularly soda works. While remediation of part of the site was carried out in the 1970s, it proved inadequate and the golf course was closed in 2004 (ENDS Report 388, p 16 ).