Bench Mark Groundwater Study in Weli Oya Diversion Project Area Sri Lanka

Groundwater is an interactive water sources for agriculture, industry and the human. Effective management of groundwater is essential to the national economy and society as well. Better understanding of the interaction between surface water and groundwater as well as aquifer behaviours facilitate water resources management and it is important for the determination of migration pathways for contaminants to the aquifer. Since the base line data available in the past has been limited groundwater monitoring in Weli Oya diversion project area has commence to achieve its objectives such as; understanding the interaction between surface water and groundwater by monitoring groundwater level and the temporal variation of the groundwater quality within the study area. The study area belongs to the Upper part of the Walawa Basin lies in dry zone of Sri Lanka which is the low rainfall region (Mean annual rainfall is in the range of 1000 mm 150 mm) of the country in comparison to the other part of the country. Before commence the study all the existing hydrogeological, hydrological, meteorological geological, structural and socio economic data related to the study were collected and analyzed. Based on these data seven, thirteen and six number of operating agro wells, dug wells and tube wells were selected from the study area for intensive monitoring. Water level of each selected wells were monitored every fortnight while hydrochemical analysis were carried out in monthly basis. Further, water quality of 52 number of tube wells, where water flowing through the fracture zones, operating in the related GND, was being monitored for dry and wet periods during the study. It was found that the Weli Oya diversion benefits not only for the irrigation but it also to replenish the regolith aquifer system of the area. Research revealed the surface water diversion contributed to raise the groundwater quality of the shallow aquifer system which is used specially for the human consumption. Moreover the monitoring data depicts the degree of groundwater surface water interaction depend on the factors such as topography, underline geology, subsurface hydraulic properties temporal and spatial variation of precipitation and local groundwater flow pattern.