The salinity of the groundwater surrounding fish farms in the northern region has been on the rise as fish farmers have been using large quantities of salt in their ponds, a survey finds. Bangladesh Water Development Board (BWDB) surveyed the groundwater quality of around 40 monitoring wells in Puthia, Bagmara and Durgapur upazilas of Rajshahi; Singra and Naldanga upazilas in Natore; and Atrai upazila in Naogaon.
A monitoring well is used to measure or monitor the level, quality, quantity or movement of subsurface water. The survey found that the EC (electric conducivity: the quantity of dissolved salt in water measured by an electronic device) of the water in those wells ranged from 400 to 1,400. Anwar Zahid, director of groundwater hydrology of BWDB who led the survey, said, “We have found the EC ranging from 400 to 1400 in those wells. Water with EC up to 1,000 can be considered fresh water but the normal EC for groundwater should not exceed 200-300.”
The density of salinity in the monitoring wells, which are usually 80-100ft deep, located near fish farms was seen to be high, he said. The survey showed Rajshahi’s Durgapur upazila to have the highest level of salinity in its groundwater. Conducted in four wells in the upazila, the survey results showed the EC in those wells ranged between 800 and 1,400, meaning the amount of salt ranged between 520 and 911 mg per litre of water.
The second highest level of salinity, between 953 and 1,218 EC, was found in the well near Rajbari in the same district’s Puthia upazila. “We have spoken to the local farmers and came to know that they have been using considerable amounts of salt in their fish ponds for over 20 years to get rid of certain bacteria. Local fisheries officers also acknowledged this,” Anwar said. Md Ilias, a fish farmer of Durgapur upazila, said they use the salt to get rid of bacteria that cause skin diseases on the fish during winter.
“We dump around 50kgs of salt in our three-acre pond twice every winter.” In some areas, farmers have been doing the same for around 40 years, and the salt reaches the groundwater during its recharging process, according to the BWDB official. Groundwater recharge is a hydrologic process where water moves downward from surface water to groundwater.
On the other hand, the survey found that the salinity level in the monitoring wells of Bagmara upazila were not overwhelming as the number of ponds is less. Meanwhile, the salinity was also less in Naogaon Atrai upazila and Natore’s Singra upazila because fish farming had begun only recently in those places, Dr Anwar explained. In the wells in Atrai, the EC was between 347 and 410, while in Singra, they were from 400 to 600.
Abdul Wahed, deputy director of Rajshahi division’s fisheries department, said there is no doubt the farmers use salt in their ponds. “However, the amount isn’t so much that the groundwater would become saline.”
Anward Zahid said that while the salt used is still within a tolerable level, as fish farming is gaining popularity, the continued use of salt would become a matter of grave concern. “The water in our coastal zone have already turned saline due to sea water intrusion. Now, the groundwater in our northern region having increased levels of salinity has become a new cause for our concern.” Salinity, he said, is already a big worry for Bangladesh.
Saline water has intruded into at least 100 rivers and affected around 15 percent of the country’s total croplands in the south-western coastal districts due to climate change and human intervention mainly from neighbouring India. According to a Soil Salinity Map, by the Soil Resource and Development Institute, based on data from 2009, showed that in 1973 at least 0.83 million hectares of land was saline. That area has now expanded to 1.05 million hectares in 93 upazilas, which makes up more than half the coastal land mass in the Ganges floodplain. Meanwhile, a recent study by Khulna University showed that more than half of the arable lands in 21 coastal districts are suffering from salinity.