Abstract
Prediction of the speciation of radionuclides in Boom Clay pore water and their retardation by interactions with Boom Clay require knowledge about the composition of Boom Clay pore water, the inventory of reactive solids and understanding of interactions between Boom Clay and pore water. Boom Clay material was obtained from undisturbed cores collected at the province of Zeeland and from drill cuttings retrieved from a drilling in the province of Limburg. Solid phase characterization included determination of carbon, nitrogen and sulphur contents, measurement of cation exchange capacity (CEC) and sequential extraction of iron phases. These analyses complement the geochemical characterization of Boom Clay samples from the Netherlands documented in the report OPERA-PU-5-2-1-TNO-1. Pore water was obtained by mechanical squeezing of Boom Clay material from Zeeland and has been analysed. Additionally, dilution experiments were performed in which the clay material was suspended with demineralized water or a 0.1 M NaHCO3 solution. In the latter experiments, the solution composition was monitored over a period of 30 days. The main focus of this study lies on comparing pore water and solid phase composition at the two locations in the Netherlands to those reported for Boom Clay in Belgium which has been extensively investigated and reported in various SCK-CEN reports.
In comparison with Boom Clay in Belgium and in Limburg, Boom Clay in Zeeland has generally lower contents in carbonates, pyrite, and organic carbon. The measured CEC values at both locations in the Netherlands vary between 7 and 35 meq/100g and are, for most samples, in the range reported for Boom Clay in Belgium (7-30 meq/100g). The CEC of Boom Clay in Zeeland tends to be higher than the CEC of samples from Limburg. Iron bound to silicates is the largest iron fraction in all samples and pyrite is generally the largest pool of reactive iron in samples from Limburg while, in Boom Clay from Zeeland, the amount of HCl extractable Fe(III) is often of comparable size as the amount of Fe in pyrite. Samples from Boom Clay in Zeeland have higher contents of iron
(hydr)oxides compared to samples from Limburg.
In contrast to the pore water in Belgian Boom Clay, which has a fresh water c haracteristic, pore water collected from the location in Zeeland has a strong seawater signature with chloride concentrations corresponding to 70-96 % of chloride concentrations in seawater. The pore waters have, in general, a deficit of marine cations (Na+, K+, Mg2+) while Ca2+ concentrations exceed those
of seawater. In some samples, sulphate is highly enriched, while sulphate is depleted in others. The pH is lower than in seawater and in some samples acidic pH values around 3.0 were measured. The time evolution of the solution compositions in the dilution experiments is often not exhibited by all experiments with the same material. The change in compositions does often not depend systematically on solid to solution ratio or whether bicarbonate or demineralized water was used.
When calcium carbonates are present in the solid, concentrations of cations, in particular of calcium, increase in experiments with demineralized water while calcium carbonate dissolution does not affect solution composition, in experiments with these samples, when bicarbonate solution is used. In most samples, sulphate is removed from solution during the experiment. One sample
from a core top, which has been obviously affected by drying and oxidation, showed very deviating behaviour during the dilution experiments.
Intrusion of seawater ions into Boom Clay at Zeeland is a dominant process controlling the major ion composition in the pore water. The cation assemblage is modified by exchange processes with adsorbed ions. Oxidation of pyrite and Fe(II) is another important process affecting pore water and solid phase composition. Indications are given that oxidation occurred in the pore water after sampling and in the cores during storage. However, it is also possible that oxidation is ongoing in situ or could have taken place in the past. It could be, that oxidation has occurred contemporaneously during partial erosion of Boom Clay in Zeeland before deposition of the Breda Formation. In particular, the relative low concentrations of pyrite, organic carbon and calcite in the Boom Clay in Zeeland can be an indicator for an oxidation event. Dissolution of pyrite can be coupled to the dissolution of calcium carbonates and lead to concomitant increase of dissolved sulphate and calcium, which, in turn, can eventually result in precipitation of gypsum. Potential for removal of dissolved sulphate upon interaction with Boom Clay has been detected, possibly involving microbial sulphate reduction. However, further research is required to identify the underlying process and to evaluate whether the process also occurs in-situ.
In comparison with Boom Clay in Belgium and in Limburg, Boom Clay in Zeeland has generally lower contents in carbonates, pyrite, and organic carbon. The measured CEC values at both locations in the Netherlands vary between 7 and 35 meq/100g and are, for most samples, in the range reported for Boom Clay in Belgium (7-30 meq/100g). The CEC of Boom Clay in Zeeland tends to be higher than the CEC of samples from Limburg. Iron bound to silicates is the largest iron fraction in all samples and pyrite is generally the largest pool of reactive iron in samples from Limburg while, in Boom Clay from Zeeland, the amount of HCl extractable Fe(III) is often of comparable size as the amount of Fe in pyrite. Samples from Boom Clay in Zeeland have higher contents of iron
(hydr)oxides compared to samples from Limburg.
In contrast to the pore water in Belgian Boom Clay, which has a fresh water c haracteristic, pore water collected from the location in Zeeland has a strong seawater signature with chloride concentrations corresponding to 70-96 % of chloride concentrations in seawater. The pore waters have, in general, a deficit of marine cations (Na+, K+, Mg2+) while Ca2+ concentrations exceed those
of seawater. In some samples, sulphate is highly enriched, while sulphate is depleted in others. The pH is lower than in seawater and in some samples acidic pH values around 3.0 were measured. The time evolution of the solution compositions in the dilution experiments is often not exhibited by all experiments with the same material. The change in compositions does often not depend systematically on solid to solution ratio or whether bicarbonate or demineralized water was used.
When calcium carbonates are present in the solid, concentrations of cations, in particular of calcium, increase in experiments with demineralized water while calcium carbonate dissolution does not affect solution composition, in experiments with these samples, when bicarbonate solution is used. In most samples, sulphate is removed from solution during the experiment. One sample
from a core top, which has been obviously affected by drying and oxidation, showed very deviating behaviour during the dilution experiments.
Intrusion of seawater ions into Boom Clay at Zeeland is a dominant process controlling the major ion composition in the pore water. The cation assemblage is modified by exchange processes with adsorbed ions. Oxidation of pyrite and Fe(II) is another important process affecting pore water and solid phase composition. Indications are given that oxidation occurred in the pore water after sampling and in the cores during storage. However, it is also possible that oxidation is ongoing in situ or could have taken place in the past. It could be, that oxidation has occurred contemporaneously during partial erosion of Boom Clay in Zeeland before deposition of the Breda Formation. In particular, the relative low concentrations of pyrite, organic carbon and calcite in the Boom Clay in Zeeland can be an indicator for an oxidation event. Dissolution of pyrite can be coupled to the dissolution of calcium carbonates and lead to concomitant increase of dissolved sulphate and calcium, which, in turn, can eventually result in precipitation of gypsum. Potential for removal of dissolved sulphate upon interaction with Boom Clay has been detected, possibly involving microbial sulphate reduction. However, further research is required to identify the underlying process and to evaluate whether the process also occurs in-situ.
Original language | English |
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Publisher | Covra N.V. |
Number of pages | 34 |
Publication status | Published - May 2015 |