Ice Core evidence for Cadmium pollution

It's a commonly known fact that ice cores, whether they be from Greenland or Antartica, are an important part of our understanding of past conditions. Until recently, our understanding of heavy metal pollution in the planet's history has been intermittent. In 1996 the first reliable time series of four heavy metal pollutants, Lead (Pb), Zinc (Zn), Cadmium (Cd) and Copper (Cu) was published after closely studying a GRIP ice core. Hong et al. (1996) found that concentrations of these metals had been highly variable and there was early large scale pollution before the industrial revolution in the Greek, Roman and medivial eras.

Having been the first time series published of heavy metal pollution from an ice core, it was found that the climatic variability of these metals was strongly anti-correlated with those of the oxygen isotope, O18. As seen in the figure below, in the first half of the GRIP core, 0-80 kyr B.P., the high frequency observed for Cu, Pb, Cd and Zn was in contrast to the low and steady concentrations of O18. In the subsequent years, 80-160 kyr B.P., the concentrations of Cd, CU, Pb and Zn were very low but concentrations of O18 were very high and more variable. More O18 in the ice is a common indicator of a colder climate, this would allow one to conclude that heavy metals are more persistent and variable in the atmosphere when the climate is warmer, such as in the first half of the record and recent history.

Hong et al. (1006) look closely at the climatic record portrayed in their GRIP ice core before turning to analyse the sources of heavy metal pollution in the atmosphere. It was found that large scale pollution could be attributed to wind-bourne soil and rock particles, sea salt spray, volcanoes and wild fires. Cu and Pb were likely to be the product of soil and rock dust during the study time period (0-160 kyr B.P.). Hong discussed that the large variations depicted in the time series in the most recent inter-glacial, the holocene, was most likely the product of dramatic changes in environmental conditions. The retreat of northern ice sheets, such as the Eurasian and American, meant opening up and exposure of large areas of fresh rock and land. This would account for the variation in Pb and Cu which originated from rock and soil particles. Sea ice also retreating leaving more exposed ocean and coastal areas, thus more sea spray.

As far as cadmium goes, the variation is mainly accountable because of more dust and soil particle sources being exposed, which is a significant source, but also large contributions from the growing continental biosphere. The progressive expansion of vegetation and northward advance with retreating ice sheets meant that the Cd ratios increase observed between 13,000 to 9300 years ago was primarily a result of increasing plant matter on the continent. From reading my previous post one can presume this is because more plants means a larger Cd potential reservoir on the continent. Cadmium levels increased until 9300 kmyr B.P. but then decreased despite further advance of vegetation, this has been attributed to changes in atmospheric circulastio influencing the movement of air-bourne cadmium.

The paper by Hong et al. (1996) was another good indicator review of heavy metal pollution during the last glaciation and during deglaciation, they offered explanations for the reasons behind the variablitily to pollution. Yet the paper lacked a comprehensive discussion of the differnt sources of heavy metals into the atmosphere or how changes in atmospheric transport could influence heavy metal content in the ice core.

These results show that cadmium has indeed been an integral part of our environment for thousands of years and has been collecting in soils and plants before the industrial revolution. Although crops have only recently become a health threat it is important to recognise natural sources and past aptterns of cadmium pollution to better understand future fluctuations and their role in our health.

Reference: Hong, S., J-P. Candelone, C. Turetta and C.f. Boutron (1996) 'Changes in natural lead, copper, zinc and cadmium concetrations in central Greenland ice from 8250 to 149,100 years ago: their association with climatic changes and resultant variations of dominant source contributions,' Earth and planetary science letters 143: 233-244.

Cadmium Toxicity

Cadmium is a non-essential element that is found in naturally occurring high abundance in fertilizers. Cadmium has been able to monopolize similar reservoirs as mercury by collecting in tropic levels. This bioaccumulation is due to wide use of cadmium rich fertilizers, sewage sludge and industrial uses of cadmium, as well as the plants possessing a naturally high tolerance for the substance, unlike mercury. Similar to arsenic, cadmium moves readily within the soil, resulting in wide distribution and easy absorption into plants. The high tolerance in plants means their cadmium levels are greater than those found in animals or humans and thus only a small amount of contaminated plant matter needs to be ingested for a toxic dose (Satarug et al., 2003).

In 2003, Soisungwan Satarug and associates published a detailed paper discussing the effects of cadmium pollution and toxicity in the non-occupationally exposed population. They reviewed several studies and trials that documented the effects of cadmium toxicity on the body, which was then compared with their own study on liver and kidney samples that had been exposed through major food groups. Tabacco, willow and the sunflower plant have all been found to have high tolerance levels against cadmium. This means their bio-reservoirs are larger and thus allow cadmium to be readily passed up the food chain. In Australia, a study in the early 90s found that sheep grazing on fertilized pasture had high levels of cadmium in their kidneys compared to those grazing on unfertilized pasture. Correspondingly, Taiwanese and Japanese rice grown on contaminated soils was documented as having 0.21-2.16 and 1.67-5.38 mg/kg of cadmium, respectively, compared with rice grown in Queensland which only had 0.05mg/kg of cadmium.

Satarug et al. (2003) moves on from a review of studies to look at WHO/FAO cadmium health regulations. They establish that any crop grown on coils that are either contaminated or naturally rich in cadmium are the main source of toxic exposure in the general population. In Australia, the recommended daily allowances of cadmium are 0.1 ug/kg body weight per day. The WHO and FOA recommend no more than 70 ug per day and considering that most indiciduals ingest and average of 30 ug per day (based on estimates typical food products and diets) this fall within safe limits. However, in some areas and products where food items were reported to contain higher levels, ingestion was estimated to be as high as 90 ug per day, way above the safe levels. These food products were identified as primarily vegetables and cereals.

A detailed list of problems that were caused as a result of cadmium exposure were included in the paper, these incorporated diabetic renal complications, hypertension, osteoporosis, leukemia and cancer in several other organs; lung, kidney, breast, bladder, pancreas to name a few. They conclude that cadmium has a profound effect on the burden of health and the connections have now been well established and documented. In Japan, cadmium exposure in contaminated areas was found to increase mortality by 40-80%. A follow up study, 15 years later, showed researchers that the higher standardized mortality rate for cardiovascular disease was higher in contaminated areas of the Kakehashi river basin in a sample size of 2840 residents, the large sample of residents in this area meant minimizing the risk of anomalies and give a clear picture of cadmium pollution in the basin.

The paper contains a plethora of example studies to reinforce the impact of cadmium on the human body at high levels. Many of the example studies are reinforced by multiple studies and have large sample sizes meaning a higher quality of results. It is comprehensive in its review of the risk cadmium poses to health burden and the consequences of cadmium exposure. Regions of the world including, Australasia, have been identified as having high levels of cadmium. On the other hand, despite the comprehensive overview and arguments for the significant role of cadmium in the non-occupationally exposed population, there is no mention of how these risks can be monitored, controlled or even prevented. The journal article is a good foundation for future developments in both research and policy.

Summary and Introduction to Cadmium

One can deduce so far that heavy metals are commonly found in our environment yet are found in concentrations to small to cause adverse effects to public health. That is until commercialisation of resources of the local environment takes place. In the case of Mercury, mercury poisoning was a little occurring problem until fish become exposed to the toxic substance through industrial waste. The USA has large problems with mercury causing neurological damage and infertility, it was only decades after the Minimata disaster that many authorities woke to the grave realisation that mercury had dire health, and ultimately economic, consequences.

On a similar scale to Minimata, Bangladesh, predominantly the Ganges delta, has suffered mass arsenic poisoning with many people becoming critically ill and unable to support families. Both arsenic and mercury are responsible for mass poisoning from natural sources. Arsenic was the result of years of deposition in the Ganges aquifer during the last glaciation and was not correctly checked before being cleared for domestic water use. Mercury, on the other hand, was responsible for an early mass extinction of early species during the evolution of our current atmosphere causing the evolution of toxic metal defences in the biology of modern day animals.

The next metal that will be discussed is Cadmium, a naturally occurring heavy metal that is commonly found. Levels of dangerous cadmium have started to appear in the environment, like mercury, since industrialisation and its main sources are sewage, fertilisers and other agricultural products. Similar to mercury, cadmium's most effective poisoning method is via the food chain, this is because plants generally have a much higher cadmium tolerance level than animals and as such create cadmium reserves. The next post will discuss the work by Satarug et al. (2002) where cadmium was studied in the non-occupationally exposed population.

Arsenic around the world

World population is reaching record highs, 6 billion and counting, and the most sought after resource of the 21st century will not be oil but clean water. In 1990 the WHO revised its estimates of access to safe drinking water, they concluded that 43% of the world's population is lacking adequate sanitation and 22% do not have access to clean drinking water. Heavy reliance is beginning to come from underground aquifers as surface water begins to become scarce.

Nordstrom (1998) discussed the risks associated with groundwater and heavy metal pollution. His review in the policy forum outlined that groundwater utilization comes with high risks, such as the 36 million people in the Bengal Delta now suffering from arsenic. Arsenic is not found in high abundance in the Earth's crust and is strongly associated with pyrite, a prolific mineral. Areas that have high levels of arsenic are Kamchatka, New Zealand, Japan, Alaska, California and Wyoming, areas where there are young basalt rocks. Environments such as closed basins in arid and semi arid climates and areas that have aquifers with strong reduction properties are all prone to arsenic collections. Nordstrom does, however, highlight that although these environments are condusive to arsenic it doesn’t help us to predict a high or low concentration, primarily due to the heterogeneous spread in the aquifers. Arsenic is highly soluble and as such its concentrations and distributions in aquifers can change within a few years. This results in constant monitoring to pre-empt arsenic entry into ground water enterprises in high risk areas.

Review of Arsenic

So far, I have discussed the role of Mercury and its affects on our health, I now wish to look briefly at Arsenic. Arsenic is classed as a metalloid which means it shares both metal and non-metal qualities and is ranked the 20th most common element in the Earth's crust.

Arsenic and humans have had a fairly dark relationship for centuries. It was immortalised by Plato when he described the use of Hemlock (a natural resource of arsenic) in the death of Socrates. Due to its lack of odour or smell, and small qunatities being fatal, it is a highly effective killer.

The most infamous aresenic poisoning that occured on a large scale, is the case of Bangladesh. Shallow aquifers, less than 300m deep, supply more than 90% of the drinking water to Bangladesh and West Bengal. It has been known for years that up to one million wells using these water reserves contain dangerous levels of arsenic. In many of these wells the arsenic content reaches 1000 mg l-1, 9950 mg l-1 above the bangladeshi limit and the WHO limit (Nickson et al., 1998).

Arsenic occurs in the Bangladesh and West Bengal aquifers due to reductive disolution of arsenic rich iron oxyhydroxides, which originate from the reduction of sulphate based metals after weathering. Though it is important to note that the arsenic rich wells are primarily isolated to the ganges delta aquifers. This suggests that weathering upstream of Bangladesh transported the arsenic-rich oxyhydroxides in sediments downstream to the delta in the late pliestocene, an epoch covering the last glaciation (Gibbard and van Kolfschoten, 2004). The sediments displaced by glaciation and de-glaciation have accumulated and been chemically reduced for centuries in the genges delta to produce arsenic rich water reserves.

Nickson et al. (1998) discuss how understanding the past geological structure of arsenic depisits and transport allow a better understanding of arsenic distribution in Bangladesh. They propose a predictive model to help influyence and advise future water developments on the aquifer to minimise the risk of future poisoning.

References:

Nickson, R., J. McArthur, W. Burgess, K.M. Ahmed, P. Ravenscroft and M. Rahman (1998) 'Arsenic Poisoning of Bangladesh Grounwater,' Nature 395: 338.

Gibbard, P. and van Kolfschoten, T. (2004) 'The Pleistocene and Holocene Epochs' in Gradstein, Ogg, Smoth and Gilbert (eds.) A geological time scale, Cambridge Press, Cambridge.