Stable isotope patterns reveal widespread rainy-period-biased recharge in phreatic aquifers across Greece

Abstract

The stable isotopic composition of water (δ18O, δ2H) is used to trace different components of the hydrological cycle, particularly interactions between precipitation, surface, and ground water. In Greece, insufficient spatial coverage for isotopes in precipitation hinders investigations of relationship between ground water and precipitation, information that is required to help quantify aquifer replenishment rates. We used precipitation, geospatial, ground water isotope data and General Linear Models (GLM) to predict the spatial distribution of isotopes in phreatic ground water in Greece. Prediction covariates of elevation, latitude, distance to nearest coastline, drainage basin (western vs. eastern Greece), and amount of precipitation predicted 62% of the isotopic variance in ground water. The GLM model yielded predictions of the isotopic composition of shallow aquifers, which allowed us to construct maps for use in hydrological and other forensic applications in Greece. A comparison of the stable isotope values predicted by our GLM to those of existing precipitation models revealed that phreatic aquifers in Greece are mainly recharged during the annual wet period, between November and March, as documented by the high overlap coefficients of 0.77 and 0.82, respectively. Rainy-period-biased aquifer recharge has implications for water quality and management issues, for example, nitrate pollution may be enhanced in the non-growing rainy period when crop uptake is the lowest, or for water resource management if climatic changes alter these temporal rainfall patterns.