I’m not sure where you’re located, but I would compare a month without any rainfall versus this high one you got. Is there a correlation (e.g. more rain = larger sewer bill)? With several months of bills along with historical rainfall data you should be able to reach a conclusion.
If this Park is a master meter only setup then the water usage should be close to the flow meter usage sent from the city. If the sewer flow reading is tons higher than the water usage then you have an infiltration problem and you need to find out if it’s old clay pipes (as per above) and / or if the tenants are contributing to the problem by tying their gutters into the sewer system.
Have a look at state regulations too - I don’t think you can pass along infiltration charges to the tenants and you have to use that as part of the business case when deciding to replace the old pipes. That cost should be reflected in your offer for the Park since it will have a horrible increase on your expense ratio.
And finally, I would look to see if you can hook up a submetered sewer flow meter to see how flow coincides with rainfall in a given month (assuming the bill provided does not detail that already).