see the following sites

http://www.epa.gov/ncea/iris/subst/0275.htm

http://www.ineris.fr/substances/fr/substance/1887

Yesterday, I set out to answer your original question about how to calculate the [radio]activity [concentration] of uranium in solution from its [mass] concentration.  I soon realized that you probably don't want the detailed answer I was thinking about posting.  Here's a simple answer: Multiply the specific activity of each pure isotope of uranium by its isotopic abundance as a mass fraction and add the results to get the specific activity of the element.  Multiply the specific activity of the element by its mass concentration to get the activity concentration.  

For example, if the uranium in solution has the isotopic composition of ordinary terrestrial uranium, the mass fractions of U-234, U-235, and U-238 in the uranium are 0.000053, 0.00711, and .0.99283, respectively.  The specific activities of the pure isotopes are 2.31 x 108 Bq/g, 8.00 x 104 Bq/g, and 1.2445 x 104 Bq/g, respectively ("Uranium Radiation Properties," http://www.wise-uranium.org/rup.html).  From these values, you can calculate the specific activity of ordinary uranium as 2.52 x 104 Bq/g.  Multiply that by the uranium concentration, with appropriate conversion factors, to obtain the activity concentration of uranium in solution (in whatever unit you want) from the mass concentration in ug/L.

In ordinary terrestrial uranium, U-234 and U-238 are responsible in equal amounts for most of the pure-uranium activity.  Uranium in natural waters often has significantly more U-234 than does ordinary terrestrial uranium, so to calculate the activity of uranium in such water with good accuracy you must know the U-234/U-238 activity ratio in the water.  If you know that ratio, replace the contribution of U-234 to the specific activity of ordinary uranium (2.31 x 108 Bq/g x 0.000053) with the product of the U-234/U-238 activity ratio and the contribution of U-238 to the specific activity of ordinary uranium (1.2445 x 104 Bq/g x 0.99284).  This calculation isn't exact, but it works because relatively large changes in amount of U-234 have no appreciable effect on the mass fraction of U-238.   

Uranium-238, a radioactive metal, is present in rocks, soil, and throughout the environment. Uranium-238 decays to form radium-226, which has a half-life of 1,600 years. Radium-226 then decays to form radon-222 gas, which has a half-life of 3.8 days1. Exposure to uranium can occur through the air, with an average daily intake estimated to be 0.0007 to 0.007 picocuries per day (pCi/d.). Higher levels of exposure generally occur through food consumption, with average levels of 0.6 to 1.0 pCi/d, or through the drinking water, with average levels of 0.6 to 1.0 pCi/d3. Radium is found in soil, water, plants, and food at low concentrations. The greatest potential for human exposure to radium is through drinking water, where levels are usually less than 1 picocurie per liter (pCi/L) but higher levels (>5 pCi/L) have been detected3. The major source of radon exposure is through inhalation, with background levels in ambient air of approximately 0.1 to 0.4 pCi/L. Higher levels of radon are frequently present in indoor locations, such as homes, schools, or office buildings. Indoor radon levels measured in one study showed a mean level of 1.6 pCi/L. Studies have shown that 1-3% of single-family homes may exceed 8 pCi/L4. People who work at factories that process uranium, work with phosphate fertilizers, or live near uranium mines have a greater chance of being exposed to higher levels of uranium, radium, and radon than the general population2.

1.     Agency for Toxic Substances and Disease Registry (ATSDR). Case Studies in Environmental Medicine. Radon Toxicity. Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA. 1992.

2.      Agency for Toxic Substances and Disease Registry (ATSDR). Toxicological Profile for Uranium (Update). Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA. 1999.

3.      Agency for Toxic Substances and Disease Registry (ATSDR). Toxicological Profile for Radium.  Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA. 1990.

4.      Agency for Toxic Substances and Disease Registry (ATSDR). Toxicological Profile for Radon. Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA. 1990.

I recommend to study UNSCEAR Report 2000. You can download it easily from UNSCEAR home page.

Dear, you can also check the UNSCEAR  Report 2000

Intake of uranium from air, food and water is given with references by world Health organization.

Intake of uranium from air, food and water is given with references by world Health organization.

The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) released the new report on 8 February 2017, which is freely downloadable from

http://www.unscear.org/docs/publications/2016/UNSCEAR_2016_Report.pdf

This "UNSCEAR 2016" report has four annexes. Of these, Annex D is dedicated to uranium.

look here: http://digisrv-1.biblio.etc.tu-bs.de:8080/GGTSPU-555dc3ff26f53e90-3737-2442289-aHwiki2xWwMozWiA-LOD/docportal/servlets/MCRFileNodeServlet/DocPortal_derivate_00021802/Diss__Hassoun.pdf%3Bjsessionid=CD808D41B9C3830A20FD7438059C0F37