The effect of an artificial destratification system on the water quality of Chichester Reservoir - the role of organic carbon

Abstract

Decreased dissolved oxygen, increased soluble iron and manganese concentrations are common effects of hypolimnetic anoxia in stratified reservoirs. Artificial destratification systems are applied to break stratification and create consistent temperature and dissolved oxygen levels throughout the reservoir. However, destratifiers can also cause unwanted changes in physical, chemical and biological processes that occur in the water column, which could deteriorate the water quality. Organic carbon, a major energy source for biological decomposition reactions, plays a vital role for reservoir water quality. However, uncertainty exists about how organic carbon degradation processes interact with the operation of destratifiers. Understanding the role of organic carbon during the operation of destratifier is important to manage and optimise the performance of destratifier. In this thesis, water quality data from a one-year monitoring program in Chichester dam is analysed. This is done by analysing: 1) water quality data trends and; 2) how the operation of destratifier affects water quality parameters at different sampling location under different scenarios. A fieldwork investigation was then applied to determine organic carbon content, reactivity and degradation processes in the water column. The results show that: 1) compared to the middle area which is not affected by the destratifier, the destratifier at the near dam wall area can effectively: a.) break stratification and achieve a well-mixed water column which; b.) reduces temperature and dissolved oxygen level differences between surface and bottom, and increases dissolved oxygen levels near the bottom; 2) two fluorescence dissolved organic matter components were found in the water column: processed and unprocessed humic/fulvic substances. These include a mixture of autochthonous and allochthonous humic/fulvic substances. Ratios of processed to unprocessed humic/fulvic substances range from 0.59 to 0.62. 5-day biological oxygen demand measurements show low organic matter decomposition in the water column. It is concluded that poor water quality at the near dam wall area is associated with: 1) deteriorated water being brought to the near dam wall area from sediments; 2) resuspension of iron, manganese from sediment due to the operation of destratifier in the near dam wall area. Soluble iron, manganese and ammonium in the water column originate from the anaerobic decomposition in sediments.