Groundwater Quality Assessment Using GIS Based on some Pollution Indicators over the Past 10 Years (2005–2015): a Case Study from Semi-Arid Harran Plain, Turkey

Harran Plain is the largest plain within the area administered under the Southeastern Anatolia Project (known as the GAP project in Turkey), covering an area of 1500 km 2 . This article presents results of groundwater quality assessment based on some pollution indicators such as electrical conductivity (EC) and nitrate (NO 3 − ) values in Harran Plain between 2005 and 2015. In order to show the change of EC and NO 3 − contamination in 20 observation wells located on the plain, seasonal average concentration distribution maps based on 2005 and 2015 were created. The results were then evaluated based on national and international standards and classifications. As a result; EC and NO 3 − values decreased throughout the plain from 2005 (max 8235 μS/cm, 720 mg/L) to 2015 (max 2510 μS/cm, 327 μS/cm). The observation wells where EC and NO 3 − values registered as high in 2005 were W8-Kisas, W11-Ugurlu, W14-Kizildoruc, W15-Yardımli, W16-Ozlu, and W20-Ugrakli, while in 2015, it was W11-Ugurlu and W14-Kizildoruc. The positive effects of the measures taken and improvements in the plain such as irrigation systems, correct fertilizer applications, closed drainage systems, and product pattern change have realized positive results with the reduced NO 3 − and EC levels. However, a different situation was observed for NO 3 − at the points where nitrate was low in 2005, with a comparatively slight increase recorded in 2015. The reason for this minimal increase was considered to be continued fertilizer application, applied fertilization method, and increased urbanization in addition to uncontrolled and over-irrigation.