Delineating groundwater-surface water interaction in a lake watershed (Sapanca Lake, NW Turkey)

Characterization of groundwater-surface water interaction in the Sapanca Lake basin (eastern Marmara region of Turkey) is critical for sustainable management of freshwater resources in the region. In this study, the groundwater-surface water interaction in the Sapanca Lake basin has been evaluated using multi-method approaches including hydrogeological, hydrogeochemical and environmental isotope analyses, seepage meter measurements and lake water budget analysis. Alluvial and karst aquifer systems represent the most productive aquifer systems in the Sapanca lake basin. Groundwater flow in the karst aquifer is relatively fast and primarily governed by NE and NW trending fractures developed by active tectonism in the region. The alluvial aquifer system at the south of the lake basin acts as a transition zone between the lake and the karst aquifer. It is recharged not only by infiltration but also periodically from streams and significantly from mountain-front recharge mechanisms through the karst aquifer. Although the existence of direct discharges into lake from the marbles of karst aquifer were not noted in field studies, both lake water budget calculations and seepage measurements on the lake bed revealed that groundwater contribution in lake water budget was about 34% of total inflow. Seepage measurements showed that the contribution of the alluvial aquifer to the recharge of the lake remained negligible. This study demonstrates the existence of karstic discharges into the lake occurring through deep fracture-controlled flow paths developed in karst aquifer, and their importance in the groundwater recharge mechanism of Sapanca Lake.