Mapping Lead in Philadelphia organized free lead tests for a week. They tested over 350 samples, primarily from Philadelphia.

About 10% of the samples had > 1000 ppm lead (the point at which it becomes not so good for growing vegetables).

I color coded the markers based on lead levels and created this map:

View Philadelphia Lead Test Results in a larger map

It appears that Philadelphia has more lead than the suburbs.

Other than that it is hard to detect trends.

It might be that Center City has less lead, possibly as richer people, especially the type of people who would attend the Soil Kitchen event (aka Gardeners), are more likely to have bought garden soil that is low in lead. Rich people might also be more likely to have tested their house for lead and got rid of it.

By contrast garden soil that you buy has a lead level of around 0-40 ppm, some of which is from natural sources.

Urban Lead Poisoning

Urban Lead Poisoning has a lot of city and country maps of lead in the soil, including a map of lead "risk" (estimates not actual tests) for Philadelphia.