Arsenic in groundwater – arsenic in rice?
22 August 2007
It is estimated that more than 1000 tonnes of arsenic per year enters Bangladesh’s fields via irrigation water. In two new ES&T publications, researchers from Eawag, the ETH Zurich and the Bangladesh University of Engineering and Technology report on the fate of this toxic element in water and in agricultural soils.
Each year more than
1000 tonnes of arsenic enters the rice paddies of Bangladesh. |
In Bangladesh, high concentrations of naturally occurring arsenic in groundwater have for many years seriously compromised the safety of drinking water supplies. As groundwater is increasingly also been used to irrigate agricultural land, arsenic could also be ingested in food. It has been estimated that 1360 tonnes of arsenic per year enters the country’s rice paddies through irrigation water. As part of a Swiss National Science Foundation project, researchers from Eawag and the ETH Zurich, together with colleagues from the Bangladesh University of Engineering and Technology, are currently studying chemical processes in water and the variability of arsenic concentrations in soils. They have found that arsenic concentrations in irrigation water remain constant along the channels between the groundwater well and the field inlet, independent of channel length. |
In the fields, however, where the water flows
more slowly or comes to rest, arsenic concentrations decrease with increasing
distance from the inlet. At the point where the water enters the field, they
are up to three times higher than at more distant points. The same pattern is
also found in soils, with concentrations decreasing in a fan-like manner away
from the inlet. In each case, the highest arsenic concentrations were measured after
irrigation in the top few centimetres of the soil.
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Although a large proportion of the arsenic is
remobilized by monsoon flooding over a period of months, the project shows that
arsenic builds up over the years. Here, the type of irrigation plays an
important role, as well as the levels of arsenic in pumped groundwater. In principle,
the groundwater could first be channelled into a fallow field so as to lower
the arsenic content on the subsequent cultivated fields. Fortunately, arsenic transferred
from the soil into rice plants has so far mainly been deposited in the |
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roots and to a much lesser extent in grains; thus, according to the current state of knowledge, arsenic concentrations measured in rice grains do not represent a significant toxicological problem, compared with drinking water. Nonetheless, the scientists from the three participating institutions conclude that, in monitoring programmes, attention must be paid to the distribution of arsenic in soils and to seasonal variations in concentrations.
- Environmental Science & Technology Science News article: Rice paddies map arsenic problem
- Environmental Science & Technology article: Spatial Distribution and
Temporal Variability of Arsenic in Irrigated Rice Fields in Bangladesh:
Part 1. Irrigation Water [pdf, 481 KB]
Part 2. Paddy Soil [pdf, 506 KB]