Natural background levels and threshold values of monitored parameters in the Cambrian-Vendian groundwater body,Estonia

The Cambrian-Vendian groundwater body plays an important role in drinking water supply of capital Tallinn and other settlements in northern Estonia. Water quality in this deep-seated confined groundwater body is influenced by the intrusion of present-day seawater and pumping-induced upward migration of deeper saline groundwater. The aim of this study is to evaluate the threshold values applying the methodology worked out within the EU 6th FP project “Background criteria for the identification of groundwater thresholds (BRIDGE)” and to compare the results with the values proposed by local authorities. A database containing 250 monitoring points was completed and used for the calculation of natural background levels (as the 90 and 97.7 percentile) in groundwater. The calculation of threshold values is based on natural background levels and reference values. In case of Cambrian-Vendian groundwater body the receptor is drinking water, thus limit values set by Estonian drinking water standard were used as reference values. The threshold values proposed by local authorities are more or less the same as calculated by BRIDGE methodology. Exceptionally different values are derived in case of chloride and ammonium.