Siiri Suursoo will defend her PhD in environmental technology "Natural Radioactivity in Cambrian-Vendian groundwater - Analytical Procedures, Monitoring of Drinking Water Treatment Technology, and Associated Changes in Groundwater Quality" on 2 October 2019.
Dr. Madis Kiisk (Institute of Physics, University of Tartu)
Prof. Franz Josef Maringer (University of Natural Resources and Life Scineces, Austria)
One of the most important public water supplies in North-Estonia, the Cambrian-Vendian (Cm-V) aquifer system, lays on the uranium and thorium rich crystalline basement rock and is known to be contaminated with naturally occurring radionuclides. It has been estimated that ca. 18 % of Estonian inhabitants (230 000 peoole) consume drinking water where radiological parameters exceed parametric values given by regulations. The capacity to conduct radiological analysis of water in Estonia was very limited when the requirements for radiological parameters in drinking water where adopted into local legislation. To fill the gap, analytical procedures for radium measurements were developed and validated. As a part of this thesis, procedures based on simultaneous determination of Ra-226 and Ra-228 by gamma spectrometry were accredited. This now enable to offer a reliable measurement service. In February 2012, the first Estonian water treatment plant specially designed for radium removal started operation in Viimsi. The facility was monitored for three years. The thesis demonstrated clearly that NORM (naturally occurring radioactive material) generation is difficult to avoid when Cm-V groundwater is used for drinking water production. In addition to Ra isotopes, which are the only radionuclides of concern in groundwater, radon-222) and thorium-228) become of interest in the treatment process because they are generated by the decay of Ra-226 and Ra-228 in the filter material. As for now, the treatment facility has been able to provide water compliant with the drinking water requirements. Yet the intensive use of groundwater brings about changes in its quality. Cl and Ra are critical parameters to monitor for sustainable management of the Cm-V groundwater. Possible explanations for the increase of Cl are intrusion of sea water into the aquifer or inflow of highly saline water from the crystalline basement accompanied by inflow of meteoric water. Ra concentration may rise either because of migration of Ra into the system or by changes in geochemical conditions that may lead to desorption of Ra from sediment surface. The present study proved that the monitoring frequency set for radiological parameters in local regulation is not sufficient for ensuring stable drinking water quality in case of aquifer systems as sensitive as Cm-V.