Thallium can be found in the water and soil. Higher levels come from the use of petroleum based fuels and products. Was the urine test that you performed a pre and post provocation test?
I would recommend that you filter your water with a whole house system. It might be a good idea to get a water quality report for your city. Use only organic produce as this reduces the exposure to chemical based pesticides, herbicides, etc.
A very good cost whole house water filter can be found at http://www.chemicalfreewater.net