A colleague of mine - Martin Reiss - wrote his PhD on the fauna of springs in the German lower mountain region and also is an expert on aquatic cave fauna - both of these areas cover the groundwater macrozoobenthos.
You can find him here on Researchgate and I am sure he can help you with finding more literature on assessment systems.
Aside from that I cannot help you that much as I am working in surface water bodies. But I know that there is the EU groundwater directive and if it is anything like the water framework directive for surface water bodies it should lead you to methods and standards used for the ecological assessment of groundwater systems.