During operation and maintenance of Pressurised Heavy Water Reactors (PHWRs) at Kakrapar Atomic Power Station (KAPS), low level radioactive liquid waste is generated and released to the aquatic ecosystem (Moticher lake). The silt and aquatic weed (Hydrilla verticillata) samples collected from different locations in Moticher lake were analysed for ¹³⁷Cs, ¹³⁴Cs, ⁶⁵Zn, ⁶⁰Co, ⁵⁴Mn and ⁴⁰K during 2007-2008. A wide variation in activity levels of ¹³⁷Cs, ¹³⁴Cs, ⁶⁵Zn, ⁶⁰Co, ⁵⁴Mn and ⁴⁰K in silt and weed samples were observed in aquatic system of KAPS. The activity buildup in the silt is confined to a small area in the Moticher lake. The activity levels were found to be insignificant at 1 km away from discharge point (upstream and downstream). An attempt was made to evaluate the radiological dose to aquatic weed (Hydrilla verticillata), which was found to be well within the dose limit prescribed by US DOE. The total radiological dose due to the naturally occurring radionuclide (⁴⁰K) is comparatively higher than that of other reactor released gamma emitting radionuclides.