Long-term Predictions of Phosphorus Dynamics in Kranji Reservoir

Transcription

1 Long-term Predictions of Phosphorus Dynamics in Kranji Reservoir Introduction Kranji reservoir, which is located in the northeast of Singapore, has been subjected to different trophic states since it was built. The reservoir was rapidly undergoing eutrophication during the 1960's and 1970's due to large volumes of water pollutants emanating from predominantly farming, industrial, urban and semi-urban areas. Since 1976, industrial and domestic discharges have been controlled by legislation, a vast network of sewers and wastewater treatment. The water quality was substantially improved. However, the results are not as expected. The eutrophication is still sustained and trophic status in Kranji reservoir is between eutrophic and mesotrophic. The reason is that the phosphorus, which is stored in sediments during the period of heavy pollution, is continuously released to support the eutrophication process in the reservoir. It is not clear how long the eutrophication will sustain and how much phosphorus is released from the sediment to overlying water during that period. Optimal control and management of eutrophication in the reservoir require more comprehensive studies from an integrated point of view to provide symbolic logic which is capable of expressing the functions and relationships of phosphorus dynamics associated with environmental processes and predicting the release flux. The utilisation of mathematical models is an attempt to realise these purposes as they can quantitative describe the sediment-water interaction system and predict the impacts of elemental changes on water ecosystems. Mathematical models of sediment phosphorus and long-term predictions A set of mathematical models has been developed to simulate the phosphorus dynamics in aquatic sediments. The models, consisting of six interacting differential equations, have the following general matrix form: t [ i ] y = [ y ' ( t, y )] i where i = 1 to 6 corresponding to dynamic variables of dissolved phosphorus (DP), exchangeable particulate phosphorus (EP) and organic phosphorus (OP) respectively in the aerobic and anaerobic layers. The sediment phosphorus models were applied to the Kranji reservoir to understand the dynamic characteristics of phosphorus in aquatic sediments and to predict internal loadings of phosphorus over a long-term period up to 30 years. The results will be greatly helpful for the management of reservoir eutrophication over a long-term period. Determinations of model parameters The parameters to be used in the proposed sediment phosphorus models include dynamic variables and state constants. The former are the external variables including the organic loading and environmental factors and the latter characterise the properties of sediments and the physical, chemical and biological mechanisms regulating phosphorus dynamics. The dynamic variables in Kranji reservoir were obtained by field surveys. The state parameters with respect to the characteristics of sediments, sorption and biological decomposition were determined by laboratory 1

2 work. Because no data were available to estimate the transport parameters of diffusion, mixing and burial processes, they were adopted from the literature. The obtained average parameters and values of external variables are listed in Tables 1 and 2. Table 1. State parameters used for the long-term prediction Parameters Aerobic layer Anaerobic layer Sorption: Kinetic rate constant 2.90 (day -1 ) 2.63 (day -1 ) Langmuir isotherm constant 5.97 (l/mg) 4.61 (l/mg) Sorption minimum 6.65 (mg/g) 3.72 (mg/g) Decomposition: Rate constant of labile fraction (day -1 ) (day -1 ) Labile fraction 1.91 (%) 1.59 (%) Rate constant of refractory fraction (day -1 ) (day -1 ) Refractory fraction 7.16 (%) 7.50 (%) Table 2. Annually averaged values of external variables External variables Organic matter Dissolved oxygen Dissolved phosphorus Temperature Calibrated depositional flux Values (mg/g) 2.7 (mg/l) 3.7 (µg/l) ( C) 0.51 (g/m 2.day) Because information on the recovery processes of Kranji reservoir is not available, four hypothetical scenarios are used for predictions: organic loading remaining at the present level, being decreased by 5%, 10%, 25% and 50% annually. Similarly, during the simulation period, the dissolved oxygen, phosphorus and temperature are kept constant whereas the organic matter remains constant in the first five years and then annually decreases by 5%, 10%, 25% and 50% respectively. Prediction results and discussion The predicted results of phosphorus internal loadings for the long period of 30 years are shown in Figure1. If current conditions of organic loading remain unchanged, the resulting internal loading of phosphorus released to the overlying water will remain almost without variation and be in a steady state level. However, if organic loadings reduce yearly, the internal loadings will decline gradually at a very slow rate. The quickest response is observed when the reductions of organic loading are larger than 25%. The periods to reach 50% decrease of internal loading are approximately 25, 15, 8 and 5 years when organic loadings are reduced by 5%, 10%, 25% and 50% respectively. The response speeds of predicted internal loading declinations are dependent on the reduction rates of organic loading. The larger the reductions of organic loadings, the higher are the declinations of the phosphorus 2

3 internal loading. However, when the reductions of organic loading are larger than 25%, this function will be weakened. The calculated initial rates of yearly decreases of internal loadings for three cases are respectively 2.5%, 4.3%, 9.8% and 19.0%(Table 3). These rates decrease gradually in response to the time and tend to be the same. The internal loadings do not show a significant declination due to the fact that the stored sediment phosphorus will sustain the release process when external organic loads are reduced. P Internal loading (mg/m.day) Constant 10% reduction 25% reduction 50% reduction Figure 1. Long-term predictions of the internal loading of phosphorus for 30 years in Kranji reservoir Table 3. The indexes characterizing the recovery of internal loading Organic Annual decl. rate (%) Recovery Loading Initial Final (%) 5% reduction 2.5% 3.7% 63.4% 10% reduction 4.3% 4.4% 78.3% 25% reduction 9.8% 4.6% 85.5% 50% reduction 19.0% 4.5% 87.0% During the declination of internal loadings when the organic loading is reduced, the stored exchangeable particulate phosphorus is correspondingly consumed as shown in Figure 2. Unlike the decrease of internal loadings, the exchangeable particulate phosphorus decreases at a much slower rate to compensate for the reductions in the regeneration flux from the decomposition of organic phosphorus. This mechanism can be clearly seen from the simulated variations of adsorptiondesorption fluxes occurring in the anaerobic layer, as shown in Figure 3. The sediments act as the source when the organic loading remains at current conditions. However, when the organic loadings are reduced yearly, the adsorption fluxes of phosphorus in the sediments gradually reduce to zero and finally desorption takes place. The sediments become the source to sustain the phosphorus release. The reduction speeds are associated with the reduction rates of organic loadings. 3

4 Conclusion The results of long-term predictions of Phosphorus dynamics in Kranji reservoir indicate that internal loadings of phosphorus will remain approximately constant if the organic loading into sediments is kept unchanged. However, if this organic loading reduces yearly, the internal loadings of phosphorus will decline gradually at a very slow rate. The response speeds of this declination are dependent on reduction rates of organic loadings. The larger the reductions of organic loadings, the higher the declinations of phosphorus internal loadings EP (mg/g) Constant 10% reduction 25% reduction 50% reduction Figure 2. Simulated variations of exchangeable particulate phosphorus in sediments of Kranji reservoir for 30 years Adsorption/desorption flux (mg/m.day) Desorption zone Adsorption zone Constant 10% reduction 25% reduction 50% reduction Figure 3. Simulated adsorption-desorption fluxes in sediments of Kranji reservoir for 30 years The calculated initial rates of annual decreases of internal loadings are 2.5%, 4.3%, 9.8% and 19.0% respectively when organic loadings are reduced by 5%, 10%, 25 % and 50%. These rates decrease gradually in response to time and tend to be equal finally. The internal loadings predicted do not show a significant and immediate declination as the sediment phosphorus stored in sediments will sustain the release process when the external organic loading is reduced. 4

5 The exchangeable particulate phosphorus stored in sediments is predicted to decrease at a much slower rate. The sediments act as the source when the organic loading remains unchanged. However when organic loadings are reduced yearly, the adsorption fluxes of phosphorus in sediments gradually reduce and finally desorption takes place. The sediments in this case will become the source to sustain the phosphorus release. A Appan (cappan@ntu.edu.sg) Wang Hong 5