By Hannah Corsini, Freelance Writer

Published October 12th, 2022

Environmental researchers in southwestern Spain have highlighted an alarming persistence of mercury and lead in bird populations located near mining areas. Given the diverse breadth of flora and fauna to which the Almadén and Sierra Madrona towns are home, this presents vast and worrying implications for the future generations of wildlife looking to inhabit this area.

Mercury contamination in Almadén

The mines of Almadénㅡa UNESCO World Heritage siteㅡhave a 500-year history plagued by exploitation and environmental and social harm; in the 16th century, for example, the labour of African slaves and prison convicts was used to extract mercury from the mine.

The mercury was then used by Spanish colonists to extract silver and gold from ores in Mexico and Bolivia, releasing highly toxic pollutants in the process.

Mercury is a highly potent neurotoxin, known to cause issues in the neurological development of the children of pregnant women exposed to it. Although mining in Almadén ceased in 2003 due to the prohibition of mercury mining in Europe, it is still considered to be one of the most mercury-contaminated areas in the world.

In a new study published in September 2022, researchers assessed the mercury and lead exposure in fish and birds whose habitat surrounds the abandoned mining area of Almadén.

‘Mercury is a highly potent neurotoxin, known to cause issues in the neurological development of the children of pregnant women exposed to it.’

Mercury has previously been found to adversely affect the health of fish and birds, and the researchers demonstrated that Almadén is not exempt from the effects of aquatic and terrestrial pollution. The study sampled birds of the Guadalmez river, 13 kilometres downstream from the Almadén mines.

Although mercury contamination in Almadén birds was lower than other researchers have previously found in other highly-polluted areas such as the Guangxi province of China; an abandoned cupric-pyrite mine in southern Portugal and gold-mining sites in northwestern Colombia, there was an undeniably alarming presence of mercury contamination exceeding that of previously surveyed sites of pollution and nearly eight-fold higher than that of their control site.

At this site, they surveyed 15 species, 69 birds in total and found that 28% of birds had a feather mercury level higher than 1.43 μg g−1, this amount has previously been found to reduce migration success and survival in birds such as the blackpoll warbler.

Further, they found that 13% of the birds (9 out of 69) had a feather mercury level higher than 3 μg g−1, which corresponded with reduced nest success, and 9% had a feather mercury level higher than 5 μg g−1, which has been associated with reproductive disorders.

Their findings also indicated that mercury was least likely to accumulate in granivorous birdsㅡbirds which feed on grainㅡand most likely to accumulate in piscivorous and insectivorous birds, having ‘found the highest feather mercury levels in piscivorous birds.’

The species most vulnerable to mercury contamination was the Cetti’s warbler (Cettia cetti), followed by the kingfisher (Alcedo atthis).

‘[Their findings] showed the highest feather mercury levels in piscivorous birds.’

Lead contamination in Sierra Madrona

The province of Ciudad Real was historically also home to several lead mines, including those of Sierra Madrona, another site surveyed by the researchers.

Lead is capable of accumulating in keratin structures in the feathers of birds, and often reflects dietary exposure of birds to the element. In birds, lead produces haematological, neurological, immunological, reproductive and behavioural disorders. Using the same method, the researchers analysed lead and other trace metal levels in the feathers of birds at a dam about 1.5 kilometres east of a Sierra Madrona mine.

Approximately, 14% (6 out of 43) of the birds sampled at the Sierra Madrona site had a lead concentration higher than 4 μg g−1, a threshold known to be associated with various health effects such as drooping of wings, loss of appetite, lethargy, weakness, emaciation and impaired locomotion, balance and depth perception.

The average lead levels were once again lower than those found in the polluted Guangxi region of China, but still concerningly high: 1.596 μg g−1 compared to the average level in the control site of 0.340 μg g−1.

‘Lead is capable of accumulating in keratin structures in the feathers of birds.’

In contrast to the findings at the Almadén site, lead was most likely to accumulate in granivorous and omnivorous birds, at more than twice the average level found in insectivorous and piscivorous birds.

Previous researchers have hypothesised about the circumstances in which lead is ingested by birds. They believe that granivorous and omnivorous birds ingest dust and soil along with their food and that this often confers lead contamination, which would explain the increased incidence of lead accumulation observed at the Sierra Madrona site.

Contamination from other metals

Birds from Sierra Madrona had higher feather concentrations of magnesium and manganese than those captured at the control site, whilst birds from both sites had more than twice the level of nickel and thallium than those of the control site.

Excessive amounts of manganese have been shown to produce immune system defects in birds, and the toxicity of thallium is well-documented, with adverse effects on organs such as the liver, brain and kidneys, which can be potentially fatal in humans as well as birds. Nickel exposure in birds has been demonstrated to reduce growth rates, and at high concentrations causes significant mortality.

When accounting for birds’ dietary groups, most trace metal contamination results showed similar disparities to those of lead. For example, the highest feather aluminium levels were found in granivorous birds, followed in order by omnivorous, insectivorous and piscivorous birds. The same pattern was found in cobalt, nickel and iron levels. This could indicate that these birds are particularly vulnerable to metal accumulation.

‘Granivorous and omnivorous birds ingest dust and soil along with their food and this often confers lead contamination.’

Birds are generally considered to be good bioindicators because they occupy higher levels of the food chain, feeding on lower trophic levels which may be susceptible to contamination. As such, they can accumulate high levels of contaminants and their sensitivity makes them excellent reflectors of habitat contamination.

This means that the results obtained by researchers at the Almadén and Sierra Madrona mining districts are particularly significantㅡthey demonstrate metal pollution, which is likely to have adverse effects on a vast range of wildlife in these areas.

In 2014, a calf reared for beef in Sierra Madrona died of lead poisoning, and other cows were assessed to be at risk. The results of this study indicate that contamination is still present in the areas surrounding the Sierra Madrona and Almadén mines.

This has worrying implications for the citizens of Ciudad Realㅡboth human and non-humanㅡand clearly, future work needs to be done to remediate the effects of past ore exploitation on the environment in order to ensure the health and survival of future generations of flora and fauna within the province.

Featured Image: Sparty Lea / Agustín Povedano | Flickr & Javier Martin | Wikimedia Commons

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