Salinity Distribution of Groundwater in Trincomalee Coastal Region

Abstract

Groundwater is the prime source for people living in dry zone of Sri Lanka, especially duringdry season for their drinking, domestic and agricultural activities. Groundwater has been exploited from earliest times in Sri Lanka mainly for domestic use and where readily available for irrigation as well. Almost 80% of the rural population in Sri Lanka relies on groundwater for their domestic needs today, because of its excellent natural quality and sustained availability throughout the year (Panabokke, 2007). Groundwater is frequently contaminated due to anthropogenic activities. However, some natural phenomena too are responsible for making groundwater unsuitable for drinking. Seawater intrusion is one such prominent process of groundwater contamination.

This study focuses on seawater intrusion of groundwater regime along a 22 km long coastal region of Trincomalee, fromKinniya to Nilaweli by evaluating the salinity distribution of groundwater. Groundwater plays a significant role as the major source of freshwater for Trincomalee district during the dry season. The main aquifer type in Trincomalee coastal region is called ‘shallow aquifer in coastal sand’. This aquifer is well marked in Nilaweliarea (Water Resources Board 1999-2001). People consuming water from the wells near the sea are highly affected by the salt (saline) water and undesirable properties of the water. Seawater intrusion has not only been affecting the groundwater, but also has changed the soil condition making it unsuitable for agricultural uses. Therefore, most of the farming lands are converted into fallow lands. After the tsunami in 2004 seawater intrusion has exacerbated more than the past. Therefore, seawater intrusion becomes a serious issue and has to be controlled for the future demands both water and land resources. Seawater mainly consists of cations like sodium, magnesium, calcium and major anion known chloride, which induce salinity and high el ectrical conductivity.Therefore, measuring the conductivity of groundwater may help identify any saltwater intrusion.

Methodology

Initially, existing data including aquifer types, geology and topography were studied with the help of previous researches and monitoring carried out in the area. Wells were selected along a profile perpendicular to the coastal region until the electric conductivity (EC) value became very low close to the drinking water standards. Water samples were collected in plastic bottles and capped tightly for subsequent laboratory analysis. Finally, 32 shallow well samples were selected for chemical analysis from the area of 22 km coastal belt from Kinniya to Nilaweli. Well locations were identified using GPS. On the spot measurement for EC of the water samples was carried out with Orion 3 Star EC meter. Water samples were analysed for Cl by using Silver Nitrate Titration method. Na was determined by Varian SpectrAA AAS facility available at the Uva Wellassa University. Spatial distribution of the chemical and physical parameters in the groundwater was plotted and interpolated using Inverse Distance Weighted method aided by ArcGIS 9.3 software to visualize the data.