Earthworms and arsenic

Arsenic poisoning in groundwater is a global calamity and millions of people are affected arsenic related diseases due to drinking groundwater contaminated with very high concentration of arsenic. Arsenic concentration > 10 ppb is recorded in groundwaters of theBengalBasinand nearly 60% of the population consuming such groundwater is affected by arsenic related diseases. This is more severe inBangladesh. This problem is existing for the last few decades. Children below the age of 12 are seriously affected.  Arsenic content in groundwater varies from less than 1 ppb to 3200 ppb. From water this problem has entered the food chain due to the prevalence of tube well irrigation in a large part ofWest Bengal. Over 5,50,000 bore wells operate now constantly pumping groundwater to the rice fields and the farmers have a happy, contended life since they can grow rice round the year without knowing the dark clouds that are lingering above them and their children!!.

Groundwater being used for irrigation has arsenic concentration about 10 to 32 ppm and it has been established now that the rice roots concentration all the arsenic from the irrigated water. The concentration of arsenic in the roots from rice cultivated in Nadia and Murshidabad districts of West Bengal varies from 56 to 136 ppm and the concentration of arsenic in rice grain is also high, over 3 ppm.

Since this practice of irrigation with arsenic rich groundwater is in place over centuries in India and other countries, and due to the age old practice of ploughing the roots back into the soil is being practiced by the farmers, the soil, over the period of time is accumulating arsenic to levels beyond ones imagination. Some of it enters the shallow aquifers due to subsequent crop cultivation seasons. 

There is no easy solution to this problem since its implementation needs political will. There are hundreds of short term solutions being proposed by a variety of scientists spending millions of rupees,  both from national and international organizations.  But the problem is not showing any downgrade trend but entering the subsequent generation of the population.

Several scientific papers appeared over the last two decades on the behaviour of earthworms in soils contaminated and uncontaminated with arsenic. Perhaps these papers may provide some clues to contain this problem. The solution to arsenic problem could be  earthworms! Earthworms are known to inhabit arsenic rich metalliferous soils and accumulate arsenic in their body and develop resistance to arsenic toxicity. The amount accumulated depends on the soil properties such as soil pH, organic content, microbial content etc. All of know that earthworms are the principal organisms responsible for mixing soil constituents. Countries where traditional agricultural practices are in place, farmers depend on earthworms to “plough” the soil before seeds are sowed. These animals help in soil fertility by removing partially decomposed litter (especially the stems and roots that were left after the harvest) from the soil surface, ingesting it and transporting it to the root zones. Since the earthworms are prey to many birds and animals, the toxic substances ingested by them from the soil is transferred to higher trophic levels. Arsenic concentration in soils is controlled by phosphorus, iron and organic carbon and redox state of the soils. Inorganic arsenic in soils are converted to organo-arsenic compounds by soil mirco-organisms. The toxic state of arsenic follows the order from most toxic to less toxic:

        arsenic gas> inorganic arsenic (III)> organic arsenic (III)> inorganic arsenic (V)> As (element).

 Earthworms are known to inhabit arsenic-rich metalliferous soils  and tend to accumulate arsenic in their bodies and are immune to arsenic toxicity. Chemoreceptor and sensory tubercles in the earthworms makes them very sensitive to chemical environment thus avoiding toxic environment.  But this sensitivity is selective! Experiments have shown that they are sensitive to sodium arsenate only when the concentration is > than 5000 ppm!! Although all species accumulate arsenic in the body, a few species like Eisenia fetida have a very high bio concentration factor ( ~ 10 – 18). This bio concentration factor is independent of the concentration of arsenic in the soil. It is not clear what control this factor. Again these features are earthworm species specific! The entire investigation suggests that, over a long period of time, earthworms are able to sequester arsenic in the tissues in less toxic form

 Again it all depends on the species. For example Lumbricus rubellus  can adopt both under toxic and non toxic conditions. Those adopted to non-toxic conditions can not survive in soils that have high arsenic concentration. Laboratory experimental results reported in the literature show that the above species adopted to toxic conditions are able to sustain arsenic concentration of 2000 ppm in soils and the tissues were able to accumulate 230 ppm of arsenic while the same species adopted to non-toxic conditions dies when exposed to such toxic conditions. Some earthworms of the same species develop yellow pigmentation when their tissues get enriched with arsenic. The above reported results on earthworms indicate that there are several species that are able to sustain high arsenic levels in the soils and some species are able to methylate arsenic in an organic form and pass the toxicity to higher trophic levels.

 Since earthworms are friends of the farmers, arsenic contaminated soils like those inWest Bengalcan find a solution to the perennial problem of arsenic contamination in irrigated soils.