Spatial and Statistical Analysis of Environmental Stable Isotopes in Precipitates and Groundwaters in Atalanti Coastal Watershed (Central Greece)

Abstract

Article history: Received 08 August 2019 Accepted 23 August 2019 Available online 23 September 2019 The results of a groundwater-meteoric isotopic research programme held during the period 2014-2017 in the area of Atalanti coastal area, Central Greece are presented in this essay. Thirty-six groundwater samples and twenty-seven rainwater samples from improvised rain collectors were collected during the period 10/2014-04/2015 and 10/2016-05/2017 respectively, analyzed in the laboratory of Isotope Hydrology of the University of Darmstadt in Germany for oxygen isotopes δ18O and Deuterium (δ2H or D). Oxygen and hydrogen stable isotopes were mainly collected in different hydrological conditions (peak precipitation amount in April, drought in October) at increasing distance from the source and at different altitude, from the upper part of the catchment to the lowest part nearby the coastline. These stable isotopes were employed as natural tracers to determine and identify the main recharge areas of the Atalanti drainage basin. Isotopic values (δ18Ο and D) were then defined in order to identify the groundwater flow path and groundwater residence time inside various geological-lithological formations. From the δ18O spatial distribution, the alluvial aquifer seems to be recharging from higher altitudes, through direct infiltration of modern precipitation and possibly laterally from the adjacent geological formations. These potential recharge areas were delineated by combining the isotopic and geological properties of the entire area with the contribution of GIS techniques. The precipitation’s isotopic characteristics indicated that wet seasons were relatively depleted compared to dry seasons due to the rainfall amount showing also a significant shift of both the slope and the d-excess (intercept) in the meteoric local water line (LMWL) compared with others such as the world meteoric, Hellenic and Eastern water lines as a result of the enrichment of heavier isotopes probably attributed to evaporation both during the rainfall and surface runoff before infiltration. Elevated d-excess values were observed, probably due to water vapour originating from the Aegean or the mid-eastern Mediterranean. The distance of the station from the shoreline and the altitude were the main factors imprinted in the isotopic signature of precipitation. Inferred groundwater recharge elevations, estimated on the basis of the local vertical isotopic gradient of δ18O ranged between 150 and 750 m a.s.l. Statistics including both linear and multivariate regression analysis was applied to the isotopic data in precipitation and groundwater samples to determine the local meteoric and groundwater water line. The measured stable isotope ratios range from -9.70‰ to -5.67‰ and -52.78‰ to -38.44‰ for δ18O and δD, respectively. Isotopic maps and cross-sections were drawn to visualize spatial gradients which reflected the evolution of the water composition at progressive distance from the source. Finally, the secondary processes seemed to be a major factor affecting the initial isotopic signature since they were related to the specific geological and hydrogeological setting including the evaporation of soil and sub-soil water as well as the groundwater recharge is by direct rainfall infiltration with primary evaporation before reaching the groundwater table.