A Crown report says there could be a long-term build up of uranium in farmlands from using phosphate mined from the Chatham Rise.
But the company proposing the sea-bed mining, Chatham Rock Phosphate, says the uranium content is not a health hazard and is little different from that in Moroccan products already used here.
The Crown has outlined in its submission to the Environmental Protection Agency concerns it has over Chatham Rock Phosphate's (CRP) marine consent application to vacuum phosphate nodules from the sea floor. The nodules from the Chatham Rise would primarily be used as fertiliser on New Zealand farmland.
The Crown's submission said the concentration of uranium in the CRP product was about four times higher than that of the imported Moroccan product currently used.
"Uranium is a chemically toxic and weakly radioactive heavy metal . . . naturally found in all rocks and soils.
"Over time, uranium breaks down into 13 other radioactive elements (decay products) such as radium, polonium and lead . . . some of these decay products tend to be more radioactive than uranium itself."
The submission went on to say that there is general agreement that use of phosphate fertiliser with high levels of uranium would increase soil uranium levels over time.
"Depending on the total content of uranium and its decay products and the manufacturing process, uranium and its decay products in phosphate rock might present a food safety risk".
The submission cited an Institute of Environmental Science & Research (ESR) study which estimated that if "all of the CRP product was used on New Zealand farmland, agricultural soils would potentially exceed Canadian and United States standards/guidelines for uranium after a period of 40-50 years of continuous use."
For the Ministry for Primary Industries to assess whether the fertiliser met "fit for purpose" criteria, it required further information including "impacts on soil buildup of uranium and its decay products, and impacts on the food supply chain," the Crown application said.
CRP managing director Chris Castle agreed that all rock phosphate contained uranium.
"The uranium levels of our rock phosphate are not significantly different than those of imported rock phosphate."
Castle said "scientific reports note uranium from the Rise is not considered to be a health hazard".
"Historic Chatham Rise data (Kolodny & Kaplan) shows an average of 181 ppm (parts per million) - including an outlier of 450 - in the nodules. Thirteen recently acquired Chatham Rise samples (Dorado Discovery 2012) show an average of 156 ppm.
"Moroccan rock phosphate, the main source of rock used to make super phosphate, contains a range usually between 100 to 150 ppm but can be significantly higher depending on the source mine."
Castle said as far as he was aware the Crown had not held recent fertiliser tests to establish actual uranium levels.
The Crown was also seeking as a condition of the application, if it was approved, independent baseline testing of heavy metal levels present in Chatham Rise fish stocks.
"This would provide a base measure against which to assess changes in contaminant levels due to mining activity."
George Clement, chief executive of fisheries management organisation Deepwater Group, was "pleased to see EPA has now decided to seek independent expert advice on the potential impacts of the ‘radioactive elements, such as uranium, strontium and caesium' in the phosphate entering into our marine food chain."
The EPA wrote to CRP seeking further information which it wants by August 4.
The application would be heard by an EPA-appointed Decision-making Committee.