The harvesting and burning of turf and peat in Ireland was a major producer of toxic dioxins in centuries past, writes Dick Ahlstrom
Researchers have discovered a surprising source for dioxin pollution released long before the growth of the chlorine-based plastics industry. The burning of turf harvested near the coastline produced many kilograms of this persistent toxin every year.
Dr Andy Meharg and Dr Kenneth Killham of the School of Biological Sciences, University of Aberdeen were puzzled by the source of dioxins found in soils archived since the 19th century. This was long before the output of dioxins and furans - toxic and environmentally persistent chlorine-containing compounds - rose sharply after the 1950s. They decided to look at the single most important energy source in the parts of Scotland and Ireland where timber was scarce, turf. Their results are published in the journal, Nature.
Dioxins are contaminants produced in the manufacture of polychlorinated biphenyls and organochlorine pesticides. Municipal waste incinerators also produce dioxins, although these chemicals are formed when any chlorine containing substance is burned.
Turf, particularly when harvested near coastlines, is rich in salt and therefore chlorine, the authors point out. "We therefore tested the smoke and ash from burning peat for contamination with dioxins," they write.
Coastal peats were burned extensively over thousands of years across Britain and Ireland, they say. There were large populations of subsistence farmers making use of this fuel, with an estimated population in the highlands and islands of Scotland between 260,000 to 420,000 during the 18th and 19th centuries.
They used detailed anthropological reports for the Isle of Hirta, St Kilda, to examine how much dioxin could have been produced per household. In those years the typical Hirta household had four to five people and turf consumption stood at about 20 tonnes per household per year. They matched this population and consumption with dioxin outputs derived from laboratory tests. A kilo of peat ash contains about 61 billionths of a gram of dioxin and by their calculations the highlands could have produced about one kilo of dioxin per year.
"This is a remarkable figure for this remote region of Britain," they write. "Modern estimates of dioxin production by domestic coal combustion are 5.1 kilos per year for the entire United Kingdom, with municipal waste incinerators, the main dioxin source, producing 10.9 kilos per year."
In many coastal communities the peat ash was collected and added to arable soil to improve its fertility. "Hirta, which was evacuated in 1930, has high-chloride peat, and its soil has been deepened over the millennia by the addition of peat ash," the authors note.
They recovered peat ash dating back to between 1830 and 1850 and confirmed it contained dioxins that could only have come from peat burning. The ash had high levels of tetracholorodibenzofurans (TCDFs), accounting for 30 per cent of all dioxins and furans.
"Even with the superimposition of modern dioxin deposition and the differential loss of individual dioxins over time, the arable soil of Hirta still carries the signature of past peat ash with high levels of TCDF compared with fresh peat," the authors write.
"Today, 70 years after the island was evacuated, the dioxin signature accrued over centuries of peat burning is still evident, a reminder that modern humans were not the first to generate large quantities of dioxins."