Detail Abstract

Analysis of Dissolved Oxygen during Overturn period of Selective Withdrawal Operation in a Water Supply Reservoir

Abstract

Dissolved Oxygen (DO) and Manganese (Mn) have a significant role in indicating the quality of drinking water. In general, low DO causes a reduction of particle Mn to dissolved Mn and high DO helps in the oxidation of dissolved Mn. Mn concentration can cause aesthetic problem (0.1 mg/L), issues for piped water supply system (0.2 mg/L) and toxicity (over 0.4 mg/L). Understanding the DO and Mn cycle is important to help the management of a water supply reservoir because of the effect of DO level in the oxidation and reduction process of Mn.</br>   This study aims to examine the water quality condition of Ogouchi Reservoir in Tokyo, Japan which underwent a Selective Withdrawal (SW) operation from 1992 to 2001.  Initial investigation shows that the reservoir exhibited similar thermal structures annually but displayed variation in DO and Mn. Annual DO increase and Mn upwelling at the bottom during winter overturn (Dec-Mar) occurred during 1992-1996 and 2000-2001. But, from 1997 to 1999, the continuous low concentration of DO at the bottom resulted in an increase in Mn. Therefore, the next part of the study focused on winter overturn to examine DO changes in order to understand the process of Mn.</br>   Thermal gradient (TG), the difference in surface and bottom temperature over depth, and Schmidt Stability Index (SSI), the resistance of water column to mixing are used to explain DO changes. The relationship of the winter average of DO with TG and SSI shows an R² of 0.76 and 0.71, respectively. These results indicate that TG and SSI can partially explain DO behavior but they appear to be insufficient to determine the timing of the increase. The TG and SSI of the years with DO increases drop to 0.0035 ⁰C/m and 30 J/m², respectively. These values are the possible threshold value causing the change in DO. However, the TG and SSI of 1999 also dropped below the thresholds but DO remained low. Upon examining the week in DO change happened, it was found that the accumulated wind power during the time when DO remained low (37 to 77 W/m²) is lower than during that when DO increased (92-198 W/m²). Based on this, wind can be the driving factor of DO change. Therefore, if TG is at most 0.0035 ⁰C/m and SSI decreases to 30 J/m², DO increase can happen given the presence of approximately 90 W/m² accumulated wind power during that time.