Spatial and temporal assessment of groundwater quality indicators and hydrogeological characterization of a karstic aquifer in Western Turkey

A case study is presented that characterizes the hydrogeology of a karstic aquifer system nearby the city of Izmir, Turkey. The main objective of this study is to provide an assessment on the spatial distribution of specific groundwater quality parameters, and to gain insight into their temporal evolution. The study area is the Nif Mountain karstic aquifer system located to the southeast of the city of Izmir that is an important recharge source for the densely populated surrounding area. The 1000 km study area is within the boundaries of the third largest city, and in the vicinity of one of the most industrialized areas of Turkey. Therefore, it is under significant environmental stresses due to residential, industrial and agricultural activities. Field work was conducted as a series of field excursions to study the hydrogeology, and to mark sampling points for consecutive groundwater sampling. A total of 59 sampling points were selected constituting of 25 wells and 34 springs. Sampling was performed in April and September 2006, during the wet and dry season of the year to assess possible temporal changes in water quality and isotopic composition. Samples were analyzed for several groundwater quality parameters including major ions, arsenic, boron, heavy metals and isotopic composition. In addition to geochemical characterization, discharge rates of some springs were measured. It is found that the hydrogeological structure is fairly complex with springs having a wide range of discharge rates. High-discharge springs originate from allochthonous limestone units and surface outcrops of conglomerate and sandstone units. On the other hand low-discharge springs are formed at the contacts of claystone and limestone units, and at the contact zones of allochthonous limestone and flysch units. High-discharge springs typically have rates higher than 200 L/s in the winter months, with a maximum of about 1000 L/s. However, a 65% decrease was observed for the summer months. Based on stable isotope analysis data, an oxygen18deuterium relationship is obtained that lies somewhere between the Mediterranean meteoric and mean global lines. Tritium analyses confirm that low-discharge springs originating from contact zones have longer circulation times compared to high-discharge karstic springs. Isotopic composition of groundwater does not change throughout the year, except for tritium. Furthermore, spatial assessment of geochemical data revealed a correlation of sampling elevation with nitrate, chloride and electrical conductivity (EC). This result implies that groundwater quality significantly deteriorates as water moves from the mountain to the plains. The same “elevation effect” was also observed for the CaCO3 concentrations, therefore indicating continuous rock dissolution as groundwater flows through the system. Heavy metal, arsenic and boron concentration are generally below drinking water quality standards with a few exceptions occurring in residential and industrial areas located at the foothills of the mountain. Comparative evaluation of winter and summer quality data shows an overall increase in electrical conductivity values. Nitrate contamination was more sporadic and concentration changes in time were less predictable. Although lower contaminant concentrations due to dilution effects were expected for the winter months, the internal hydrodynamics and the ongoing dissolution processes in the karstic aquifer system are suspected to seclude any temporal trends. It is concluded that the Nif Mountain overall has a significant potential to provide high quality water, although the available groundwater quantity is expected to drop significantly for the summer months, due to decrease in discharge and also because of deterioration of groundwater quality. From a water quality point of view, direct consequences of anthropogenic activities were observed, which requires close monitoring of Nif Mountain’s pristine water resources. The study revealed that less groundwater recharge in the dry period of the year does not always translate to higher concentrations for all groundwater quality parameters. It was also evident from the study results that water circulation times, lithology, quality and extent of recharge also play an important role on the alteration of groundwater quality.