A project I am reviewing is proposing a detention basin with additional storage in large pipes underground. The detention basin discharges to storm sewer. What are some of the issues that would need to be addressed regarding how the waterproof liner for the detention basin is constructed? There appears to be two manholes for access and an inlet structure within the basin. How does additional groundwater around the manholes (if construction does not address the watertightness on the surface around the manholes) or at the construction limits of the basin?

underground storage using 36" storm pipe. it is purely storm water but eventually discharges into a combined sewer system a couple miles down stream (contamination from surcharge would not be an issue). in addition to underground pipe storage, a basin was proposed on top of it with a landfill type waterproof liner. there is a catch basin and two access manholes within the limits of the proposed liner. i guess my question is what kind of problems arise if the liner is not properly constructed to be watertight around the edges and open structures. if additional water seeps in due to poor construction, that additional water through the ground would not be so much an issue or would it? as the basin is deeper than 1.5' we require waterproof liners, but i would like to question that requirement may be overkill as the risk for contamination is low. the risk of debris getting underground is low (it is in an urban residential area).