The mass balance and activity ratios of naturally occurring radium (Ra) isotopes (Ra-223, Ra-224, Ra-226, Ra-228) were investigated in Cockburn Sound (Western Australia) to further understand submarine groundwater discharge (SGD) into these coastal waters. Water samples from 11 marine stations and 20 groundwater sites (encompassing three aquifer layers) were analysed for Ra at four times, at the end of winter (September 2003), early summer (December 2003), late summer (March 2004) and mid-winter (July 2004). Variable isotopic signatures of groundwater suggested that vertical mixing may occur between different aquifer layers and two isotopically different water sources were identified in marine waters both inside and outside of Cockburn Sound. A mass balance of the long lived radium isotopes (228 Ra and 226 Ra) produced a range of discharge estimates from 0.8 x 10(7) L day(-1) in late summer to 2.7 x 10(7) L day(-1) at the end of winter. Signatures of radium isotopes in the coastal waters suggested that groundwater discharge was not confined to the shoreline and may have occurred from a number of aquifer sources at a temporally variable scale. (C) 2007 Elsevier B.V. All rights reserved.