Monitoring and modeling dissolved oxygen dynamics through continuous longitudinal sampling: a case study in Wen-Rui Tang River, Wenzhou, China

Synoptic water sampling at a fixed site monitoring station provides only limited snap-shots' of the complex water quality dynamics within a surface water system. However, water quality often changes rapidly in both spatial and temporal dimensions, especially in highly polluted urban rivers. In this study, we designed and applied a continuous longitudinal sampling technique to monitor the fine-scale spatial changes of water quality conditions, assess water pollutant sources, and determine the assimilative capacity for biochemical oxygen demand (BOD) in an urban segment of the hypoxic Wen-Rui Tang River in eastern China. The continuous longitudinal sampling was capable of collecting dissolved oxygen (DO) data every 5 s yielding a similar to 11 m sampling interval with a precision of +/- 0.1 mg L-1. The Streeter and Phelps BOD-DO model was used to calculate: (1) the oxygen consumption coefficient (K-1) required for calibration of water quality models, (2) BOD assimilative capacity, and (3) BOD source and load identification. In the 2014 m river segment sampled, the oxygen consumption coefficient (K-1) was 0.428 d(-1) (20 degrees C), the total BOD discharge was 916 kg d(-1), and the BOD assimilative capacity was 382 kg d(-1) when the minimum DO level was set to 2 mg L-1. In addition, the longitudinal analysis identified eight major drainage outlets (BOD point sources), which were verified by field observations. This new approach provides a simple, cost-effective method of evaluating BOD-DO dynamics over large spatial areas with rapidly changing water quality conditions, such as urban environments. It represents a major breakthrough in the development and application of water quality sampling techniques to obtain spatially distributed DO and BOD in real time. Copyright (c) 2012 John Wiley & Sons, Ltd.