ᐅ Sealing a soakaway pit – what materials to use?

As shown in the picture, there is groundwater standing in my soakaway pit. The water comes from below and appears whenever the groundwater level rises. When the groundwater level drops again, the water in the pit also drains out.

I have a question for the experts.
The soakaway pit is made entirely of concrete, meaning it was formworked and poured with concrete. Unfortunately, at one described corner on the upper left, the work was not done carefully enough. I have tried several times to seal that corner, but I haven’t succeeded.
I used a rubber mortar (a two-component mortar for sealing all kinds of leaks) from the company MEM Bauchemie. I applied it at least three times, but it still does not seal properly. Would it maybe make sense to chisel out the corner a bit more and then fill it with something? But with what?

So my question to you is: how can I solve this problem?
I would be very grateful for any help or suggestions to fix this issue.
Here is a picture, if it works.

Kind regards,
Dieter