Here is a source detailing the threats of storing nuclear waste on the surface level:

https://www.bund.net/themen/atomkraft/atommuell/zwischenlager/

Google translate: https://www-bund-net.translate.goog/themen/atomkraft/atommuell/zwischenlager/?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en-US&_x_tr_pto=wapp