NaC1 crystallization at the MgCl₂/NaC1 solution boundary–a possible natural barrier to the transport of radionuclides

Abstract

ABSTRACT. Concentration, conductivity, temperature, and flow logs from sixteen brine-filled shafts in northern Germany have shown that the brines in all former potash salt mines exhibit a very sharp stratification into lower Mg-rich brine, an upper layer of Na-rich brine, and groundwater at the top. Laboratory experiments have shown that, at the MgC12-brine/NaCl-brine boundary, both solutions become oversaturated with regard to NaC1, due to diffusion processes. NaC1 therefore crystallizes from the solutions and forms a salt plug in the boundary region, which considerably reduces further diffusion. It is concluded that the observed effects would also take place in shafts. The backfilling material helps to nucleate the halite crystals and provides a structure on which they might be supported. The results of these experiments show that the density boundaries in the brine bodies act as barriers against transport of matter while the formation of a halite plug growing independently at the MgClz/NaCl-brine interface acts as an additional barrier.

Water Management at Abandoned Flooded Underground Mines

“…(31) with ∆h head difference between the shafts, m h 1 height of water column in shaft 1, m ρ 1 density of mine water in shaft 1, kg m -³ ρ 2 density of mine water in shaft 2, kg m -³ Sander and Herbert (1985) conducted depth-dependent temperature and velocity measurements in several flooded German salt mines. Their investigations focused on the potential isolation of radioactive waste deposits from the anthroposphere.…”

“…Their investigations focused on the potential isolation of radioactive waste deposits from the anthroposphere. Sander and Herbert (1985) conclude: "from laboratory experiments and in situ tracer tests, it may be deduced that the transport of matter: (a) in static layers takes place by diffusion, i.e. It becomes clear that zones with nearly constant physicochemical parameters are dominated by turbulent flow and zones with gradients in the temperature or electrical conductivity are diffusion dominated (they call those layers 'static layers').…”

“…All the other examples given here are based on the investigations of Uerpmann (1980), Sander and Herbert (1985), GSF -Gesellschaft für Strahlen-und Umweltforschung (1986), Herbert (1989), Herbert and Sander (1989), and Czolbe et al (1992). From the examples in those publications, four salt mines, three in Lower Saxony (Hope and Ronnenberg near Hannover, Beienrode near Wolfsburg) and another one in Saxony-Anhalt (Peißen near Bernburg) are described.…”

“…His work is entitled "Hydrogeological Questions at Radioactive Waste Disposal" and is focused on the safe disposal of such waste. Furthermore, they observed convective flow in homogenous layers, and no flow between layers with density or temperature gradients (Herbert and Sander 1982;Sander and Herbert 1985). Herbert and Sander (1982), who were responsible for the Hope flooding experiment, had earlier established that the brines in flooded mines were commonly stratified into three layers: an upper layer with slightly saline or fresh water, a middle NaCl-saturated layer with a density of about 1.20 g cm -3 , and a lower MgCl 2 -rich layer with a density of 1.29-1.30 g cm -3 .…”

“…His work does not contain details about the single mines, therefore the missing data were added after personal communication with the author. In another paper (Sander and Herbert 1985), they concluded that the NaCl/MgCl 2 boundary can be successfully used as a barrier against transport as they noticed that crystalline halite (rock salt) was forming at the boundary. Between those three layers, they found intermediate layers.…”