Chennai: Researchers from Anna University and Central Ground Water Board have found that the groundwater in Cauvery delta region is facing a major threat due to the contamination and a possible intrusion of seawater.
Based on numerical modelling, they also estimated that Cauvery river has to flow for a period of 90 days to improve the groundwater quality in the region. In a research paper published in International Journal of Environmental Science and Technology recently, they observed that the chloride and nitrate concentrations and the presence of heavy metals in the groundwater. Heavy metals such as silicon, lithium, lead, manganese, silver, nickel, aluminium, iron, cadmium, zinc, chromium were found in the study area, which is 260 square kilometres in the delta region.
The number of days of river flow will have an impact on the groundwater head and as well as the decrease in the concentration of the solutes. The chloride and nitrate transport simulation indicate that the high concentration of ions in the coastal area was due to insufficient river flow.
“Recharge plays a major role in maintaining the quality of groundwater in delta region for which we need a minimum flow of water in the river. The study identified that Cauvery river needs 90 days of flow to improve the already deteriorated groundwater quality in the region,” said Professor L. Elango, head, Department of Geology, Anna University and one of the authors of the paper.
Simulation of solute transport until the year 2020 indicates that the dilution of groundwater will take place only if the river flows at least for 90 days in a year. “If the flow is less than 90 days, the farmers in the river basin need to use the groundwater wisely by going for modern methods such as System of Rice Intensification (SRI),” he said.
If the accumulation of the ions in this region continues, it can force the farmers to change the cropping pattern to salt-tolerant crops. “There is no seawater intrusion in Cauvery delta region. But, if the present situation continues, the seawater intrusion will happen as farmers are using groundwater for irrigation due to the lack of water flow in the river,” said M. Senthilkumar, one of the authors of the paper who has developed the numerical modelling for the study.
“We need at least 30 days flow in the river to refill the groundwater and reduce the salinity. If the river flows for 90 days continuously, the delta region will preserve its groundwater and the presence of the major ions in the ground and water also will be considerably reduced,” he observed.
Owing to the modern day issues of irregular patterns of rainfall due to climate change and increased demand of groundwater for various purposes, the proper management of this critical aquifer system is very important for sustainable agricultural production.
“This can be achieved by maintaining river flow at least for 90 days in a year, regulation of fertiliser use and creating awareness of sustainable use of groundwater in this area,” the researchers suggested in their paper.
The farmers have said the non-perennial river has not flowed continuously for more than two weeks in the last six years. “There was no continuous water flow in the Cauvery river for last six years. The last time when the river flowed continuously for more than a month was in 2011-12,” said P.R. Pandian, president of the Coordination Committee of All Tamil Nadu Farmers Associations.
The delta region has only ponds and puddles. “The sand in the river bed only preserves the groundwater. So, only if the water flows in the river, the groundwater could be recharged or saved. The river needs to flow at least one month,” he added.
“The river needs 2.5 tmcft water a day to flow. If it has to flow for at least 90 days we need 225 tmcft water,” he added. It may be noted that the Supreme Court while reducing the allocation of water to Tamil Nadu has advised the state to tap ground water.
TNAU suggests alternate crops and methods for Cauvery delta
Due to the lesser flow in the Cauvery river, the Tamil Nadu Agricultural University has suggested alternate crops and methods which will reduce the use of water in the Cauvery delta region.
“We have suggested 62 alternate crops which are location specific to replace the rice cultivation during the shortage of water. Of 25 lakh acres, around 8 lakh acres the farmers are using the System of Rice Intensification to bring down the water usage. Through this method, we can save 25% to 40% water,” K.Ramasamy, Vice-Chancellor, Tamil Nadu Agricultural University, Coimbatore.
The University also suggested alternate irrigation methods such as drip water irrigation, spray water irrigation and artificial rain guns during the period of water shortage.
The researchers warned that if the groundwater is not judiciously used in the region then the seawater intrusion will render is unusable.
The groundwater modelling
The model was used to predict the changes in the solute concentration of both chloride and nitrate when the river flows for 30, 60 and 90 days a year. It is observed that the dilution takes place with the increase in river flow days.
Similar to nitrate, the chloride concentration also decreases with increase in the number of days of river flows. The solute transport modelling indicates that this aquifer system is under threat primarily due to lack of sufficient duration of flow in the river.