Predicting Nutrient Distribution Patterns in Some Coastal Lagoons in Ghana Using Mike 3 Model

Abstract

The study assesses the levels of phosphate, nitrate and ammonia in water and sediments from the Mukwe, Sakumono II, Gao and Laloi lagoons situated within the central coast of Ghana alongside other physicochemical parameters (pH, Dissolved Oxygen, Total Dissolved Solids and Temperature). Data collected over a six-month period was used in calibrating and predicting nutrient concentrations in the Sakumono II and Gao lagoons using the MIKE 3 model. High pH measurements beyond USEPA and Ghana’s EPA permissible limits of 6.5 to 8.5 and 6.0 to 9.0, respectively, were recorded in the Sakumono II and Mukwe lagoons whiles Gao and Laloi recorded pH values within these permissible limits. With the exception of the Laloi lagoon which had an average dissolved oxygen value of 5.1mg/l, dissolved oxygen concentration in all the other lagoons were lower than the EPA and WHO permissible limits of 5.0mg/l to 6.5mg/l and 5.0mg/l to 6.0mg/l, respectively. Total Dissolved Solid measurements in all the lagoons were above the EPA permissible limit of 500mg/l. Phosphate, nitrate and ammonia concentrations in all the four lagoons were generally above Ghana’s EPA and USEPA permissible limits. Discharge rates measured were highest for the Laloi lagoon and lowest for the Gao lagoon, which may be linked to the width and depth of the lagoons. Sediment nutrient concentrations were used together with the instantaneous sediment discharge formula to calculate the rate of discharge of sediment nutrient for the four lagoons. The results indicated that sediment nutrient concentration has a positive correlation with sediment discharge rate. Apart from few discrepancies in the results which was caused by poor rainfall data and non-point discharges, the correlation analysis indicated that MIKE 3 model was able to generally predict nutrient concentrations and show the distribution patterns in the Sakumono II and Gao lagoons. Average sediment nutrient concentrations recorded for phosphate during the dry and rainy season for Sakumono II, Mukwe, Gao and Laloi lagoons were (0.121 and 0.128mg/kg), (0.146 and 0.113mg/kg), (1.42 and 1.22mg/kg) and (0.112 and 0.103mg/kg) respectively. The average sediment nutrient concentrations recorded for ammonia during the dry and rainy season for Sakumono II, Mukwe, Gao and Laloi lagoons were (37.18 and 25.85 mg/kg), (41.49 and 37.46mg/kg), (4.58 and 4.31mg/kg) and (41.74 and 40.93mg/kg) respectively. The average sediment nutrient concentrations recorded for nitrate during the dry and rainy season for Sakumono II, Mukwe, Gao and Laloi lagoons were (14.54 and 11.55mg/kg), (15.15 and 15.41mg/kg), (3.83 and 3.46mg/kg) and (13.22 and 11.77mg/kg) respectively. Average sediment discharge rates recorded in the Sakumono II, Mukwe, Gao and Laloi lagoons for ammonia, nitrate and phosphate were (0.00000399 m3/s, 0.00000133m3/s and 0.0000000143m3/s), (0.0000377m3/s, 0.0000126m3/s and 0.00000131m3/s), (0.000021m3/s, 0.0000073m3/s and 0.0000000746m3/s) and (0.00001624m3/s, 0.0000537m3/s and 0.0000000465m3/s) respectively.