CHALLENGES IN ESTIMATING PUBLIC RADIATION DOSE RESULTING FROM LAND APPLICATION OF WATERS OF ELEVATED NATURAL RADIOACTIVITY P. LU * , R. AKBER ** , A. BOLLHÖFER *** * EWL Sciences, Darwin, Australia ** Safe Radiation Pty Ltd, Calamvale, Australia *** Supervising Scientist Division, Department of the Environment, Darwin, Australia Abstract Ranger Uranium Mine is located in the northern wet/dry tropical region of Australia. The mine implements a comprehensive water management programme and land application of water from its retention pond 2 (RP2) is a part of that programme. The land application areas are located in common woodlands within the mine lease boundary — the combined area is about 336 ha. RP2 contains runoff from the waste rock and low grade ore stock piles and the water is therefore elevated in natural radionuclide activity concentration. When the water is applied to the land, the radionuclides are adsorbed within a few centimetres of the surface. A comprehensive project is underway to estimate the radiation dose likely to be received by the critical group during occupancy of these areas after rehabilitation. The distribution of radionuclides and the traditional use of the land are such that direct application of commonly used parameters in dose estimates through pathway analysis may not be adequate. This paper describes the site, identifies the special aspects of the distribution of radioactivity on the ground and their influence on radiation dose estimation through the different exposure pathways. 1. INTRODUCTION Ranger Uranium Mine is located in the Alligator Rivers uranium province in the northern tropical region of Australia. Mining at Ranger commenced in 1980 and is still continuing. The main uranium deposits at Ranger are hosted in altered schists and silicified carbonates. Acid leaching is used to extract uranium. The total lease area of Ranger Uranium Mine is 7908 ha. It is located near the township of Jabiru. The lease area is surrounded by Kakadu National Park which is World Heritage listed both for its natural value and for its cultural significance due to ancient Aboriginal habitation of the area. The climate in the Alligator Rivers Region is monsoon-like with distinct wet and dry seasons. Almost all of the rain falls during November to March and the dry season lasts from about May to September. The mean average rainfall for the past 32 years at Jabiru Airport was 1579 mm and the mean annual evaporation (2628 mm) exceeds the rainfall [1]. The region is subject to severe weather events such as cyclones and floods. Seasonality adds to distinctive features in the physiography of the land.The seasonal and often unpredictable rainfall patterns pose water management challenges for Ranger Uranium Mine. Runoff water from areas of above background natural radioactivity is collected in retention ponds. Retention Pond 2 (RP2, Fig. 1) catches water mainly from the waste rock and low grade ore stock pile areas at Ranger and, consequently, it exhibits elevated activity concentrations of naturally occurring radionuclides. The pond is about 12 m deep and 24.86 ha in surface area. The water volume retained at any time in the pond varies — from nearly empty to its full capacity of 1 100 000 m 3 . Some water from the pond is used to sustain mining and milling operations, but since 1985, large volumes have been disposed of through a technique of land application to common woodland within the mine lease.