Présentations Colloques

Session 8.04: Isotopic and residence time tracers
Raidla (orateur)
A δ+18O isoscape for the shallow groundwater in the Baltic Artesian Basin
When studying water reservoirs, it is critical to describe the stable isotopic composition of modern precipitation because it serves as an input signal to the isotopic composition of surface water bodies and aquifers. The network that monitors the isotopic composition of precipitation over space and time (GNIP) does not cover the Earth evenly. The Baltic Artesian Basin (BAB) can be cited as an example of a region, where historically only one GNIP station (Riga) provides data for over ca. 175000 kmC.**BAB is an important paleogroundwater reservoir but little has been known about the spatial variability of δ+D and δ+18O values in modern precipitation input across the region. To overcome this limitation, we hypothesized that the isotopic composition of shallow groundwater in the BAB could be used as a proxy for the weighted mean annual isotopic composition of local precipitation. However, our results reveal clear discrepancies between the isotopic composition of precipitation and shallow groundwater in the area. The isotopic composition of shallow groundwater is mostly biased towards isotopically depleted wintertime precipitation. We believe that the long daylight period in the BAB area during the summer growing season allows plants to transpire a substantial portion of the summer precipitation input, which causes the groundwater recharge to be strongly biased towards the spring snowmelt and autumn precipitation. The results suggest that when making palaeoclimatic interpretations in palaeoenvironmental research, we should not assume that δ+18O values of shallow groundwater are identical to those of precipitation.**The derived isoscape of the BAB shallow groundwater δ+18O values is characterized by a high spatial resolution and its estimated error is comparable to the analytical precision of the δ+18O measurements. Therefore, it can serve as a fairly accurate reference basis for further hydrogeological modelling and also for ecological, or even forensic applications. The variogram analysis revealed that the spatial dependence structure of shallow groundwater δ+18O values in the BAB area is characterized by a mixed geometric and zonal anisotropy.**