Couple of things here:
If you say this park was "farmland up until 1970", then this park is too new for silver. I doubt the piles or scraped surface is worth hunting, unless you're into clad.
Unless you had the date wrong and meant 1870? if so diggummup is right: You want to hunt the scraped areas, not the piles of turf. Afterall, the top 5 or 6" is the newer stuff (in undisturbed park turf), and it's the deeper stratas you want to hunt anyhow.
Next problem though: you're saying the digging is for sewers/water. Therefore this isn't really a "scrape" (as some of us initially interpretted the word "scrape" to mean). Rather, this is trenching. So really, that type of digging isn't nearly as good for park turf hunting, as scraping would be. Because think about it: in trenching, they might go 5 or 6 ft. deep, yet only a foot or two across. So you can see that because everything below the top 1 ft. (Assuming an old park that is. And only a few inches for a 1970 park) is going to be sterile, as it's below the human influence/sinkage level. This means that with trenching spoils, the vast majority of the soil is sterile.
The word "scrape" on the other hand, is normally used for the type of tractor work you might expect in cases of scraping 6" off an entire turfed park, to prepare for artificial turf, or to prepare to put in an ice-skating rink, or tennis courts, or parking lot, etc.... Those are the perfect scrapes if done in 100+ yr. old parks
?? I get all excited just thinking about it. but I have a day job during daylight hours
