The water hyacinth (Eichhornia crassipes (Mart.) Solms.) plants in lakes and reservoirs have gained considerable attention in tropical and sub-tropical parts of the world due to its rapid growth. The amount of nutrients released from the dead plant materials is of particular interest. Thus, decomposition of water hyacinth plant parts under aerobic conditions was studied in the laboratory. Roots, petioles, and leaves of water hyacinth were enclosed separately in one litre polypropylene bottles which contained 500 ml of lake water. To study the influence of bacteria on the decomposition, antibiotics were added to half of the bottles. We observed that decomposition of leaves and petioles without antibiotics were relatively rapid through day 61, with almost 92.7 and 97.3% of the dry mass removed, respectively. Weight loss due to bacterial activities during 94 days decomposition was 22.6, 3.9, and 30.5% from leaf, petiole, and root litter. Decomposition of litter in lake water indicated that after 94 days 0.6, 0, and 0.6 g m )2 of leaf, petiole, and root N was dissolved in leachate, while 23.1, 14.4, and 6.0 g m )2 of leaf, petiole, and root N was either volatilized or remained as particulate organic N. Moreover, 0.2, 0, and 0.1 g m )2 of leaf, petiole, and root P remained dissolved in the leachate, while 3.1, 3.4, and 1.1 g m )2 of leaf, petiole, and root P was either precipitated or remained as particulate organic P. The carbon dynamics during the decomposition indicated that 7.4, 28.8, and 3.7 g m )2 of leaf, petiole, and root C remained dissolved in the leachate after 94 days while 228.0, 197.6, and 107.4 g m )2 of leaf, petiole, and root C was either diffused or remained as particulate organic C. These findings are useful for quantifying the nutrient cycles of very shallow lakes with water hyacinth under aerobic water environment. Further examination of the fate of the plant litter as it moves down in deep anaerobic water environment, is necessary to understand the leaching process better.

The problems in sustainable groundwater management are of major and vital importance in the areas that comprise shallowly weathered and rarely fractured rocks with thin soil mantle, as recharge and yield are relatively low in these areas due to low porosity and permeability. In order to find out better ways and means to evaluate, develop and manage groundwater resources in these regions, an attempt Was made to understand the groundwater system of Vavuniya region aquifer through numerical modelling and to examine the behaviour of the aquifer under various operating conditions. The results reveal that the groundwater usage has already reached its optimum level in this region and immediate action is required not only to control further expansion of groundwater exploitation but also to regulate groundwater withdrawal especially in low rainfall years. Further, the influences of the river and subsurface dams in groundwater system were examined and the results show that the non-perennial river has less influences and the subsurface dams certainly have an impact on groundwater system but this has to be studied further in detail, in order to minimize the negative impact and utilize the merits.

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