Our surveyor Frank Iebba said the soil is fine, just sand and gravel (*). They actually had trouble with the perc test pit collapsing. Newton nominally requires us to build an elaborate drainage system of plastic pipes routed from all the gutter downspouts to a large concrete drywell in the backyard. Based on the soil, and a basement that's been dry since 1926, we really don't need to do this; perhaps we can work out a suitable compromise.
(*) the landscaper who put in the front retaining wall a couple years ago hit sand several feet down, and immediately halted digging; he thought it might be the warning sand that the gas company sometimes backfilled gas lines with. "No," I said, "around here we call that topsoil."