Off site stormwater handling

Can we obtain this credit for a site that already has natural drainage systems surrounding it, i.e. outside the site boundary? The water that falls on the site is taken care of in a natural way. There are currently ditches around the site that lead runoff water to nearby ponds. The stormwater runs down to the pond and is infiltrated there. The site area is de-watered via ditches and culverts to a stream as well as a lake. Rainwater that falls on e.g. car parks and sources of pollution (in this case diesel aggregates and tanks) will be 'cleaned' on site, i.e. only cleaned water will run off to the ponds, streams etc. It is then let out into the stormwater system. The stormwater flow will be delayed to ensure that the current rate will not increase in a 10 year storm event The offsite pond is dimensioned for a 10 year storm. The site area is included [in the area that the system can take care of] and the amount of runoff will be accounted for, by including detention depositories in the design. Will this be acceptable for obtaining this credit?