In the location's words:

This insider tip for all Bonn residents is located south of Troisdorf. It takes around 20 minutes by car and 40 minutes by public transport to reach this recreational oasis. Unlike the other lakes in the area, there are no kiosks or waterskiing facilities here, but a place in the middle of nature where swimming is expressly permitted. The water quality here is regularly tested. There are hiking trails around the lake and the small inlets are perfect for a stroll with a thick tome.