In general the level of lead should be less than 20ppm in soil. Although leaded gasoline was eliminated many years ago there were still high concentrations of lead in soils of homes close to highways and busy roads. Once in the soil it is very difficult to remove. If your 100pppm level is accurate I would expect it is coming from past contamination due to leaded gasoline emmissions.
If you by chance live in Frisco you may have contamination from the local lead smelter that has been in business for over 40 years. I worked for a solder company in Richardson (Greenville and Campbell area) that used lead and we had some emmissions in the first 10 years prior to installing filtration systems on all our furnaces. There were also a number of lead smelters in the Dallas area that not only emitted lead but the lead slag was used to fill in low spots in the roads before lead was determined to be so dangerous. The RSR plant in west Dallas had to pay to have inches of soil removed and replaced on surrounding homes and schools.
Another interesting possible contamination comes from old shooting ranges. Texas Instruments operated the Texins Rod and Gun Club in the 1960's-80's in Allen around the Bethany Park. I understand they sold it to the city of Allen and it cost T.I. more to remediate the soil than they made on the sale. I lived in Lucas during the remediation and witnessed many inches of soil removed. The trap and skeet range faced east so homeowners on Bethany Creek Blvd, Dearborn, Joshua, and Normandy may experience some higher lead levels in their yards. A lead pellet does not wash out of the soil.
The recommendations for remediation of your yard correlate with everything Howard preaches. Keep the pH higher than 6.5 with organic matter which will bind the lead. It is also recommended to add phosphorus as this also binds the lead. The University of Minnesota does not recommend using soil with lead above 100ppm for gardening but if you do, you would want to wash the crops extra well.http://www.extension.umn.edu/distributi ... G2543.html
I hope this helps! Other references:http://www.umass.edu/plsoils/soiltest/lead1.htmhttp://www.brown.edu/Research/EnvStudie ... /index.htmhttp://epa.gov/lead/pubs/leadinfo.htmhttp://www.uri.edu/ce/factsheets/sheets/lead.html