The dynamics of transportation, accumulation, diminishing and distribution of ⁹⁵Zr in a simulated aquatic ecosystem was studied by isotope-tracer technique, and the fitting equation was established by application of a closed five-compartment model. The results showed that when ⁹⁵Zr was introduced into aquatic system, it was transported and transformed via depositing, complex with other ions, adsorption and absorption by aquatic living organisms, resulted in redistribution and accumulation in part of the organisms. After introduction, the specific activity of ⁹⁵Zr in the water decreased sharply to a certain value in a short time and then decreased slowly. The sediment accumulated a large amount of ⁹⁵Zr by ion exchange. The water hyacinth (Eichhornia crassipes) could also adsorb a large amount of ⁹⁵Zr in a short period of time. Snail (Bellamya purificata) and fish (Carassius Auratus) had a poor capacity of adsorbing ⁹⁵Zr. The amount of ⁹⁵Zr in the snail flesh was greater than that in the shell, and the distribution of ⁹⁵Zr in the fish was found mainly in the viscera. The amount of ⁹⁵Zr in individual compartment of the system was affected with time.
Three glass tanks with dimensions of 60cm 40cm 50cm were constructed. 8kg of paddy soil on powdery loam was filled into each pool. The tanks were flooded with 60l water. 10 ml of ⁹⁵ZrF4 with a specific activity of 4.32×10⁵Bq/ml was introduced into the water after one week. The water was stirred gently to get a homogeneous distribution of radionuclide. The radionuclide specific activity in water was 72.00Bq/ml. Fifteen water hyacinth (Eichhorma corssipes (Mart) Solms), and 30 snails (Bellamya purificata) and 15 fish（Carassius anratus）were put into each tank. Water was added with intervals of 3 days in order to maintain a constant height of water.
The samples were collected at time interval of 1, 2, 4, 6, 9, 15, 22, 29 and 34 days. Four 5ml aliquots of water were random collected from each pool, and disposable plastic cups were filled with them (20ml) for activity measurements. A fish was taken from each tank. The fish was divided into fin, viscera, liver, gill, skin, flesh, bone, head and roe, and each part was weighed. They were then cut into smaller pieces. Twenty gram samples from each part were put into the disposable plastic cups for measurements.
In the meantime, two sediment columns were collected from each tank using a sediment sampler. Each sediment column was sectioned into two equal parts, and each part was smashed and mixed thoroughly. Afterwards 20g of sediment samples were put into plastic cups for measurements. Every sample had 3 replications.
The ⁹⁵Zr emits β and γ particles when it decays. Those were measured with a multi-channel γ spectrometer (model BH 1224, Beijing Nuclear Instrumentation Factory). The counting error was controlled to be lower than 5%. The counts was calibrated with counting efficiency, diminish time, disintegration and other factors.
The results showed that the specific activity of ^95Zr in water decreased rapidly with time due to precipitation, adsorption to sediment and uptake by water calabashes, and snails and fishes. Most of ⁹⁵Zr in sediment was found concentrated in the surface layer. It was indicated that ⁹⁵Zr in water could not readily move downwards with percolating water before remained in surface sediment. The specific activity of ⁹⁵Zr in different parts of organisms in orders of root＞leaf in water hyacinth and flesh＞shell in the snails. The organ uptaking and adsorbing ⁹⁵Zr was mainly of intestines and stomach (viscera). ⁹⁵Zr absorbed by gill and fin was though a direct contact with water. The specific activity of ⁹⁵Zr in flesh, bone, liver and eggs was relatively lower, which were only a little higher than the background level. It was indicated that ⁹⁵Zr remained in intestines, stomach, gill and fin could not readily transport to inner organ such as flesh, bone, liver and eggs. The specific activity of ⁹⁵Zr in different parts of fish was arranged in order of viscera＞gill＞fin＞skin, fish scale＞bone, head, eggs＞flesh.
A closed five-compartment system model was applied to imitate the experimental data. For dynamics of specific activity in whole water, sediment, water hyacinth, snails, and fishes, it could be described with the following exponential regression equations respectively, the specific activity of water C₁=654.3447e^-2.2147t+0.3285e^-006690t+0.1119e-^0.0129t+0.1099e^-0.1848t+0.0006e^-0.3378t, and sediment C₂=85.7971e-^2.2147t+1.6024e^-006690t+91.8122e^-0.0129t+2.9428e^-0.1848t+0.3845e^-0.3378t, and water hyacinth C₃=(118995.7705e^-2.2147t+119516.3543e^-0.6690t+682.5725e^-0.0129t+656.1561e^-0.1848t+5.3418e^-0.3378t)/m₃(t), and snail C₄=(10198.6263e^-2.2147t+304.2514e^-0.6690t+679.3532e^-0.0e129t+155.6352e^-0.1848t+9201.0370e^-0.3378t)/m₄(t), and fishC₅=(213794.5841e^-2.2147t+4717.3065e^-0.6690t+6210.0565e^-0.0129t+212256.7591e^-0.1848t+23.4206e^-0.3378t)/m₅(t) were gained. The ANOVA showed that each regression equation can described the dynamics of accumulation and diminishment of ⁹⁵Zr in the aquatic ecosystem preferably.