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Coastal aquifers face greater saltwater intrusion due to sea level rise

Coastal regions are very fragile systems. They are continually exposed to various environmental factors that have a direct bearing on their functioning. Intrusion of sea water into coastal aquifers is one such aspect that needs to be addressed. A paper recently published by researchers from the Indian Institute of Science addresses the factors that aggravate the problem of saltwater intrusion, i.e. the movement of sea water into freshwater aquifers. The research was carried out to establish the extent of saltwater intrusion under different conditions.

Saltwater intrusion occurs naturally in most coastal aquifers. It is caused mainly due to the higher composition of minerals in saltwater than freshwater, making it denser, thereby exerting greater water pressure inland. This is, however, exaggerated by human activities such as groundwater withdrawal. Removing groundwater decreases the pressure underground in the aquifer, which can result in a higher degree of intrusion.

“I have considered parameters such as hydraulic conductivity, porosity, recharge rate and longitudinal dispersivity to simulated saltwater intrusion under natural conditions. Apart from these parameters freshwater draft is one of the major reasons for saltwater intrusion into coastal aquifer”, explains Priyanka, first author of the study and a research scholar from the Department of Civil Engineering, Indian Institute of Science.

Sea level rise was also considered as one of the parameters for the study. Global mean sea levels have been rising for the past few centuries and their rate has also increased. It has been predicted that the sea level would rise by almost a meter over the next century. This would adversely affect the coastal regions, as it will cause an increase in saltwater intrusion. “Climate change is one of the recent areas of interest among researchers; very few articles have been published to study the effect of sea level rise on coastal aquifers”, says Priyanka.

A hydrological model of an unconfined coastal aquifer was created. The aquifer was bounded by sea along the west, seasonal rivers along its northern and southern side and land on its eastern side. “In Southern India most of the rivers are seasonal. During non-monsoon period the salinity of these seasonal rivers increases due to low flow, which contaminates the adjacent aquifers. Aquifers adjacent to seasonal rivers are contaminated more than aquifers adjacent to perennial rivers”, justifies Priyanka when asked why only seasonal rivers were considered for the study. The amount of freshwater flow in rivers has a direct effect on the length of saltwater intrusion.

The analysis was done for a 100 year period. Simulations of groundwater flow and ‘mineral’ transport in the aquifer were created. The results demonstrated that saltwater intrusion was barely affected by natural surface water recharge. Hydraulic conductivity (the ease with which water is able to flow through the aquifer) however, had a profound effect on saltwater intrusion - a 100 percent increase in hydraulic conductivity caused a 14 percent spike in saltwater intrusion. Salinity of the aquifer changed with its porosity. A 15 to 20 percent increase in porosity brought its salinity down by 4 percent.

Another aspect of the study was to investigate the effect of groundwater pumping on saltwater intrusion. Wells were placed in the conceptual aquifer for this investigation. It was found that when they were located further inland, they had a lesser impact on saltwater intrusion.

Increased sea level rise over a 100 year period caused an upshot in saltwater intrusion by 25 percent than when at normal sea level. Combined effect of freshwater withdrawal (at a certain rate) and sea level rise could result in 57 percent increase in sea water intrusion over a period of a century.

This study helped in investigating general trends in groundwater dynamics. It will help provide a better picture in the effect of climate change and land use patterns on saltwater intrusion.

About the authors:

B.N. Priyanka is a research scholar at the Department of Civil Engineering, IISc.

A Mahesha is a Professor at the National Institute of Technology Karnataka, Surathkal, Mangalore.

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