Answer to Question #1852 Submitted to "Ask the Experts"

Q

Phosphogypsum was used for a roadbed in every street within a subdivision in 1987; the roads have not been improved since that time because of the content of the radiation. The highway department refuses to tear up the existing roads but will only apply blacktop on top of the original road surface because of some previous lawsuit regarding phosphogypsum prior to when the subdivision was developed. This subdivision is in a city southeast of Houston, Texas. Due to tropical storm "Allison" last year the subdivision was in standing water of nearly three to four feet for more than two weeks before subsiding. What health threats can the phosphogypsum pose and what effects can this have on the water system? The water utility is privately owned and not a part of the city's water system. A black/orange sludge appears nearly every few days in sinks and toilets.

A

It is not possible to give a specific or even a very satisfactory general answer to this question. Potential exposures would depend strongly upon local conditions such as the amount of gypsum involved, the concentration and mobility of the radionuclides, details of the local hydrogeology, sources of drinking water, and many other factors. However the following material is found in Dr. Brian Birky's answer to "Ask the Expert" Question #629. "Phosphogypsum (PG) was recently addressed with an international perspective in the journal Phosphorus & Potassium (Karakunen and Vermeulen 2000). The authors believed it necessary to reassess the radiological hazards of phosphate products and PG following the issue of new safety standards against dangers from ionizing radiation (96/29/Eurotom). The activity concentrations in many PG stacks exceed the limits and cannot be used as raw material in the construction industry, but the radioactivity levels in PG produced from igneous phosphate rocks is below the new limit. Uranium has normally been enriched by seawater in all marine-deposited phosphates, which results in higher activity concentrations than igneous deposits. Radium-226 activity concentrations in PG stacks produced by the wet phosphoric acid process fall within the range of 10 to 1,300 Bq/kg with daughter nuclides also in that range. The radon emanation factor (fraction of radon that escapes) is typically about 0.2. In 1992, the EPA ruled that PG intended for most applications, including agricultural and construction use, must have a certified average ²²⁶Ra concentration of no greater than 370 Bq/kg. Consequently, most of the PG produced in the United States is prohibited from use and is stockpiled in stacks. In theory, the ²²⁶Ra concentration of 370 Bq/kg yields an effective dose of about 1 mSv/y through the significant pathways. This effective dose limit is estimated using an index based on not only the ²²⁶Ra activity concentration, but also those of ²³²Th and ⁴⁰K as well. Typical PG from many parts of the world will exceed this index, as do several building materials of nonphosphatic origin. In Florida alone, more than 900 million tons of PG are stacked in more than 25 stacks. Thirty million new tons of PG are produced each year. Most of that PG is restricted by the EPA for any application. Many products could be made from phosphogypsum that is under the EPA radioactivity limit or if the basis for that limit is reviewed and revised. Some examples are soil amendments (for the sulfur), vitrification for glass and ceramic products (roofing tiles, etc.), roadbeds, landfill cover, and oyster cultch. Roadbed PG can be used as a binder for base course mixtures. PG mixtures are easier to work with than clay mixtures, and the operation cost, including equipment time, for PG roads is lower than that of clay roads. Rain during construction does not cause excessive delays because the compacted mixture does not absorb water to any great extent. Shrinkage cracks, frequently occurring in clay roads, are greatly reduced. The stability of compacted PG mixtures is superior to that of clay mixtures. In studies of experimental roads it was concluded that gamma levels from the roadbeds do not yield doses that approach the limit of 1 mSv for a member of the public. There are no significant effects of radon levels from the roads. Furthermore, investigation of groundwater and soil has shown no significant increase in ²²⁶Ra levels due to the presence of the roadbeds. Reference Karkunen, J.; Vermeulen, S. Natural radioactivity of phosphates and phosphogypsum. September/October 2000. Phosphorus & Potassium. Fertilizer International No. 378. pp 75-81." Further information: Florida Institute of Phosphate Research 1855 West Main Street Bartow, Florida 33830 863-534-7160 Fax:863-534-7165 Thomas F. Gesell, PhD