Geosigma (Sweden)

Earth and Space Science Open Archive posterOpen AccessYou are viewing the latest version by default [v1]SKB Task Force GWFTS: Task 9 - Increasing the realism of solute transport modelling in fractured mediaAuthorsBjörnGyllingiDPaoloTrincheroiDJosepSoleriDJamesCrawfordKerstiNilssoniDBillLanyonJan-OlofSelroosiDAnttiPoteriSee all authors Björn GyllingiDCorresponding Author• Submitting AuthorGylling GeoSolutionsiDhttps://orcid.org/0000-0002-2464-6725view email addressThe email was not providedcopy email addressPaolo TrincheroiDAmphos21iDhttps://orcid.org/0000-0003-1351-2788view email addressThe email was not providedcopy email addressJosep SoleriDIDAEA-CSICiDhttps://orcid.org/0000-0003-0741-249Xview email addressThe email was not providedcopy email addressJames CrawfordKemakta Konsult ABview email addressThe email was not providedcopy email addressKersti NilssoniDGeosigma ABiDhttps://orcid.org/0000-0001-7140-5863view email addressThe email was not providedcopy email addressBill LanyonFracture Systems Ltd.view email addressThe email was not providedcopy email addressJan-Olof SelroosiDSKB (Svensk Kärnbränslehantering AB)iDhttps://orcid.org/0000-0001-9251-5367view email addressThe email was not providedcopy email addressAntti PoteriPosiva Oyview email addressThe email was not providedcopy email address

<p>Radionuclides usually migrate slower than the flowing water due to sorption and matrix diffusion. The performance assessment assumes that retention takes place mostly in the vicinity of the deposition holes. REPRO (<em>REtention Properties of ROck matrix</em>) experiments analyzed the matrix retention properties of the rock matrix under realistic conditions deep in the bedrock in ONKALO underground characterization facility at Olkiluoto, Finland. The objective was to investigate tracer transport in the rock matrix, which was representative to the near-field of the final disposal repository of the spent nuclear fuel, and to demonstrate that the assumptions made in the safety case of the deep geological spent fuel repository were in line with site evidence.</p><p>REPRO is composed of several supporting laboratory and <em>in-situ</em> experiments which investigate the retention properties under different experimental configurations. The first <em>in-situ</em> experiments were water phase diffusion experiments performed 2012-2013. Through Diffusion Experiment (TDE) studies diffusion and porosity properties of rock matrix in stress field of repository level and sorption properties of nuclides in intact rock circumstances.</p><p>The TDE experiment has been performed in three parallel drillholes drilled near to each other. Breakthrough of the radioactive tracer is monitored with on-line measurements and samplings along and perpendicular to the foliation. The non-sorbing radioactive isotope traces of HTO and <sup>36</sup>Cl, as well as slightly sorbing <sup>22</sup>Na and strongly sorbing <sup>133</sup>Ba and <sup>134</sup>Cs were used. TDE was designed to control advective flow, as it had caused problems in previous <em>in-situ</em> tests.</p><p>Supporting laboratory studies were performed for drillcore samples sampled from the experimental drillholes. In these laboratory experiments, i.e. porosity, permeability and diffusion coefficients of the drillcores were determined using different methods.</p><p>The TDE experiment was carried out from 2016 to 2019. A breakthrough was seen in the timeframe predicted by scoping calculations carried out. REPRO has produced data and knowledge to the safety case and the performance assessment. According to the preliminary results, values measured in the laboratory are applicable also in larger scale and <em>in-situ</em> conditions.</p>

Chlorophyll-a (chl-a) concentrations are often used as a proxy for water quality problems as well as phytoplankton blooms. Available chl-a models range from simple phosphorus loading models to complex regression and dynamic models. A comparison of multiple regression models was made with genetic programming (GP) techniques to predict chl-a concentrations over a large range of 104 Swedish lakes. Independent variables used were lake area, mean depth, iron, latitude, ammonium, nitrogen + nitrate, pH, phosphate, secchi depth, silicon, temperature, total phosphorus, total nitrogen and total organic carbon. GP is a method based on the Darwinian evolution theory. This implies that a program will be able to test different mathematical equations, iterating and improving each equation using fundamental ideas from evolution theory to increase the predictive power. A good correspondence was found between the multiple regression and the GP modelling approach. No significant improvement of the predictive power was found using GP, and it is therefore recommended that multiple regression methods should be preferred when predicting chl-a concentrations as these models tend to be less complex and the modelling approach is easier to use. Results from GP were in some cases more accurate compared to multiple regressions; however, the best model was created by multiple regressions which used concentrations of total phosphorus, total nitrogen and latitude as independent variables. These findings will be an important note for limnologists and modelling managers when developing future models of chl-a concentrations in lakes.

Three radially converging in situ migration experiments over a distance of 5 m were performed in a single fracture at a depth of 400 m in the Aspo Hard Rock Laboratory. Injection and breakthrough curves were obtained for uranine,H3HO (HTO), 22Na+, 42K+, 47Ca2+, 58Co(II), 82Br−, 85Sr2+, 86Rb+, 99mTc (no breakthrough), 131I−, 131,133Ba2+ and 134,137Cs+. The in situ experiments lasted for nearly 1.5 years, with single experimental times up to 10,000 hours. The tracer concentrations span over seven orders of magnitude between injection and sampling under practically undisturbed chemical conditions. Dynamic ranges in the breakthrough curves of up to four orders of magnitude were obtained. Thus, the use of radioactive (especially γ-emitting) tracers showed to be a most useful tool for in situ tracer experiments. The relative retardation sequence obtained in the field experiment was Na 90%, were obtained for uranine, HTO, Br, I, Na and Sr and lower recoveries for Ba, Rb, Cs and Co. However, there were indications that there would have been higher recoveries of these elements if it had been possible to continue monitoring over longer experimental times. The low recoveries of Cs and Co indicate either slowly reversible or non-reversible sorption behavior. The laboratory diffusion experiments showed lower diffusivities and porosities and somewhat lower sorptivity of all studied tracers in the site-specific rock samples, dominated by mylonite, than in the diorite host rock. Matrix diffusion and associated sorption within the rock matrix is indicated in the in situ experiments, although this can not be verified without modeling that involves such processes.