Just out of curiosity, do you have some reason to believe that your soil is contaminated with lead? Chemical dump sites can have all kinds of bad stuff in them. Not just lead. Roadside beds, especially with alkaline soil, are almost always contaminated with lead, because of the lead that used to be added to gasoline, though not usually at levels that are considered hazardous. You might get in touch with your local city resource recovery office. They might do the test for free. Around here, they do (for both lead and arsenic and, while they're at it, major plant nutrients). Whyever should they do this for free? I suspect that they're looking for seriously polluted sites, and lots of federal money is available for remediation if they find any. Offering tests for plant nutrients gives them hundred of samples from interested gardeners, who are also interested in possible contaminants.

I was in your shoes last year so I can only imagine what you're feeling now :). My whole backyard is full of rock hard Californian clay soil (also with lots of rocks I have to pick out, and concrete chunks, and roof tiles, you name it!). During spring and summer it's like one giant brick, it's impossible to dig and you can see huge deep cracks everywhere. So as someone who knows what you have to deal with, my recommendation is lots and lots of organic matter! As much as you can possibly get your hands on! And like others have mentioned never leave the soil bare, any mulch you have on top will help retain moisture and slowly improve the structure of your soil. Slowly but surely you'll build a population of earthworms which will help you out (I also had 0 last year). You'll have to keep adding stuff every year too. I thought I had added enough organic matter to certain locations in my backyard only to find it had disappeared this year! (The soil was still clay, but it was much easier to dig since I'd already done it last spring). I would suggest making paths in your garden so you don't step all over the soil you worked so hard to improve and compact it, as clay so easily does (the oxygen you introduced while digging is important for root development). I also recommend not working the soil when it's too wet (and sticks together) and if it's too dry letting the water soak in will make it much much much easier to dig. The upside is, clay usually has lots of nutrients and once structure is improved will be wonderful stuff! Raised beds weren't an option for me (I live at home and my mom didn't want any) but I've only heard wonderful things about them from other people with clay soils so you might want to go that route (a lot less/no digging for you). You won't be stepping all over the soil compacting it either. And the last thing that comes to my mind is try seeing if you have a local community compost place. I had NO idea I could pick up FREE compost at my local recycling center (I can get up to 1 truckload a week!) until this year. Sometimes it has little bits of plastic in it, or maybe a golfball or some other random object but it's helped my backyard out immensely now that I haul in as much compost as I can carry per week free :) Our community compost gets tested too, so I'm not worried about diseases or anything. Whew, that was a long post! I hope that helps! Happy gardening! :)

Sorry to hear of this problem, and you are asking the right questions. kimmsr is right... and "ingestion" also includes passive ingestion of soil from hands or dust that gets into your mouth from the air. Children have a much higher passive ingestion rate, hence lower safe values for soil where children are present. If you are doing phytoremediation with sunflowers or anything else, the plants should be landfilled, not composted or reused in any way. That's if you actually plant something in the high lead soil that you know will take up lead. A lot of this stuff got where it is prior to regulations preventing it, so unless it was illegal burning and disposal post-1980 or so (RCRA), and if the seller didn't know about it, it is most likely not actionable by env. agencies. YMMV in your state. So, you gave us the lead numbers, and those are marginal for residential - my state uses 260, EPA uses 400, and you say you have "400 to 600 in the good area". What about the average across the yard, how many samples did you take, were they from 0-1 ft. or from 1 ft. discretely, and how high is it in the 'bad' areas? And what about the arsenic levels? Did you have that tested?

This year I successfully remediated 2 cubic meters of aminopyralid-contaminated 9 year old manure that was delivered spring of 2011. Aminopyralid and chlopyralid is similar to picloram. In the fall of 2011, I spread the manure over the 15' x 20' area of lawn/quack grass. Spring 2012, I pulled the quack grass which was easy as roots had entered the loose manure. This exposed the underlying soil and incorporated some of it with the manure. I build a sheet mulch bed around the perimeter. Next I drenched the contaminated soil with active aerated compost tea made with high nutrient compost and well spiked with activated effective microorganisms (EM - brand used ProBio Balance Plus, manufactured by SCD). I did this twice, deeply raking the surface to oxygenate the soil micro-organisms without destroying them. Then I planted the manure area with corn and the sheet mulch area with various squashes, both of which produce much biomass and are the least susceptible to pyralids. I also did a weed survey, finding less than 10 misshapen and malformed weeds of three species along the margins. Three weeks after planting, I did another soil drench with AACT and EM. I also ensured adequate watering (mostly rainfall with a bit of irrigation By mid July the corn was waist high and the squash were rampant enough that drivers heading up the street would brake suddenly for a better look. I harvested 40 ears of mid-quality corn and more than 120 pounds of high-quality squashes. The best was the weed survey completed at the end of August. Fourteen different species in robust health. The biomass will stand over winter, collecting snow and moisture, then in the spring, I'll chop it and add it back to the soil and do another AACT and EM soil drench. I'm wondering if perhaps AACT and EM along with biomass accumulators and ensuring soil moisture might help remediate your contaminated soil?