Assessing the role of cation exchange in controlling groundwater chemistry during fluid mixing in fractured granite at Aspo, Sweden

Technical Report · Sat Jun 01 04:00:00 EDT 1996

DOI:https://doi.org/10.2172/270703· OSTI ID:270703

Viani, B E; Bruton, C J

Geochemical modeling was used to simulate the mixing of dilute shallow groundwater with deeper more saline groundwater in the fractured granite of the Redox Zone at the Aespoe underground Hard Rock Laboratory (HRL). Fluid mixing simulations were designed to assess the role that cation exchange plays in controlling the composition of fluids entering the HRL via fracture flow. Mixing simulations included provision for the effects of mineral precipitation and cation exchange on fluid composition. Because the predominant clay mineral observed in fractures in the Redox Zone has been identified as illite or mixed layer illite smectite, an exchanger with the properties of illite was used to simulate cation exchange. Cation exchange on illite was modeled using three exchange sites, a planar or basal plane site with properties similar to smectite, and two edge sites that have very high affinities for K, Rb, and Cs. Each site was assumed to obey an ideal Vanselow exchange model, and exchange energies for each site were taken from the literature. The predicted behaviors of Na, Ca, and Mg during mixing were similar to those reported in a previous study in which smectite was used as the model for the exchanger. The trace elements Cs and Rb were predicted to be strongly associated with the illite exchanger, and the predicted concentrations of Cs in fracture fill were in reasonable agreement with reported chemical analyses of exchangeable Cs in fracture fill. The results of the geochemical modeling suggest that Na, Ca, and Sr concentrations in the fluid phase may be controlled by cation exchange reactions that occur during mixing, but that Mg appears to behave conservatively. There is currently not enough data to make conclusions regarding the behavior of Cs and Rb.

• Research Organization: Lawrence Livermore National Lab., CA (United States)
• Sponsoring Organization: USDOE, Washington, DC (United States)
• DOE Contract Number: W-7405-ENG-48
• OSTI ID: 270703
• Report Number(s): UCRL-JC--121527; CONF-9506374--1; ON: DE96013186
• Country of Publication: United States
• Language: English

Related Subjects

05 NUCLEAR FUELS
CALCIUM
CESIUM
DEPTH
GEOLOGIC FRACTURES
GEOLOGIC MODELS
GROUND WATER
HYDROLOGY
ION EXCHANGE
MAGNESIUM
MIXING
NUMERICAL DATA
POTASSIUM
RADIOACTIVE WASTE DISPOSAL
RADIOACTIVE WASTE FACILITIES
ROCK-FLUID INTERACTIONS
RUBIDIUM
SALINITY
SITE CHARACTERIZATION
SODIUM
STRONTIUM
SWEDEN
UNDERGROUND DISPOSAL
WATER CHEMISTRY