Comments on “Constant‐concentration boundary condition: Lessons from the HYDROCOIN variable‐density groundwater benchmark problem” by L. F. Konikow, W. E. Sanford, and P. J. Campbell

Abstract

As one of the members of the pilot group that defined the salt dome case tackled in the paper I want to add some comments and respond to critical remarks on the HYDROCOIN test case definition. The case definition was set up with reference to a subsurface formation located above a salt dome in Lower Saxony in Germany. In the salt dome near Gorleben the German high level radioactive waste repository is projected. Safety assessment requires studies on the migration of radionuclides in the aquifers above the salt formation. For that purpose the flow pattern above the salt needs to be known [Biitow et al., 1985]. The concerned HYDROCOIN test case is a highly simplified representation of an aquifer above a salt dome cap. In the HYDROCOIN project [Organisation for Economic Co-operation Development (OECD), 1988] the definition of the saltdome problem has been worked out by Biitow [1984] assisted by L. Heredia at Technical University Berlin. In HYDROCOIN the salt dome problem was treated as a test case in level 1 (as case 5) and in level 3 (as case 4). In the level 3 task, mainly a sensitivity analysis for the former problem was required [Biitow and Holzbecher, 1986]. The reference case in level 3 is identical with the level 1 case except that diffusivity has a nonzero value: D = 1 x 10-8 m 2 s-. This change was made as a reaction to bad experience with zero diffusivity in the level 1 test case. Herbert et al. [1988] provide a good discussion about the difficulties. Most modelers who treated the problem after HYDROCOIN used their own values for diffu