Contaminated Wells
Many village wells in Asia are contaminated by pesticides and nitrates
Pesticides Used in Rice Production
The cultivation of high-yielding rice varieties is associated with the heavy use of pesticides. Pesticides are lost through both runoff and percolation, causing the pollution of water resources. It has been known for some time that pesticides are contaminating lakes and rivers, and affecting aquatic life. More recently, we have begun to realize that they are also polluting the drinking water of many farm families in Asia.
Shallow Aquifers
Those at highest risk are rice farmers who live near their paddy fields, and who take their drinking and household water from wells fed by relatively shallow aquifers. Studies in Malaysia and the Philippines have shown similar patterns of pesticide pollution of wells. Endosulfan and monocrotophos are the main culprits. Even more dangerous are the traces of organochlorine insecticides such as dieldrin which are found in some wells. Although these are mostly now banned, some have a long half-life and persist for many years.
In view of the high risk to human health, there is an urgent need to prevent and correct pesticide pollution. It would be very helpful to improve the efficiency of pesticide applications. At present, only 15% of applied pesticides hit the target.
When pesticides are recommended for use by farmers, we need to consider more than the effectiveness and the cost. We should also consider the solubility, mobility and degradation of the different pesticides available on the market. It is these which determine how much pollution a pesticide is likely to cause.
Nitrates
Fertilizers are a common cause of contamination of groundwater. A quarter of drinking wells in Malaysia contain higher than acceptable levels of nitrates, while 9% contain high levels of phosphorus.
Groundwater pollution with nitrates is a concern all over the region, especially in rural areas producing horticultural crops. These generally receive high levels of fertilizer, much of which is wasted. One detailed study in Japan has compared the application rates of nitrogen with the uptake by various crops. Results showed that excess applications of nitrogen fertilizer were very common in vegetable production. Celery was an outstanding example. Of the nitrogen fertilizer applied to the crop, 732 kg/ha was not being absorbed by the plants.
The mean amount of non-absorbed N was considered the N fertilization load. There was a clear relationship between the N load and the percentage of wells with a high level of nitrate in their water.
Index of Images
Figure 1 Thatched Hut of Villager
