Göksu Delta is an important wetland area in Turkey and has been declared a first-degree site and has been included in the Ramsar list. Agricultural activities continue for 12 months of the year due to the favorable climate in the delta and these activities are the most important income source for the people living in the region. Investigations to date have identified the presence of nitrogen pollutants in delta groundwater. However, studies have yet to be conducted on the source of nitrogen contaminants. This study has identified source(s) of nitrogen through the combined use of isotope methods and groundwater modeling.¹⁵N and ¹⁸O stable isotope analysis were done for identifying the source(s) of nitrogen pollution. We used a three-dimensional Visual MODFLOW model to simulate the alluvial aquifer around the delta. After establishing the groundwater fate and transport model (MT3DMS), we investigated the source of nitrate pollution by particle tracking analyses (MODPATH). The results of δ¹⁵N showed that nitrogen in delta groundwater was greatly affected by agricultural activities, but was mainly sourced from domestic wastewater. Similar to isotopic results with the particle tracking model, 5 years ago it was determined that nitrate ions came from domestic wastewater of Silifke city and surrounding settlements that lack wastewater treatment facilities.