Super typhoon induced high silica export from Arakawa River, Japan

Area of Science:

• Environmental Chemistry
• Riverine Geochemistry
• Water Quality Monitoring

Background:

• Sewage treatment plants (STP) are recognized as significant contributors to dissolved silicate (DSi) in riverine systems.
• Understanding silica dynamics, including both dissolved (DSi) and particulate (PSi) forms, is crucial for aquatic ecosystem health.
• The impact of extreme weather events, such as typhoons, on nutrient loading in rivers requires detailed investigation.

Purpose of the Study:

• To quantify dissolved silicate (DSi) and particulate silica (PSi) concentrations and loading in the Arakawa River.
• To identify the primary sources of DSi, particularly the role of sewage treatment plant (STP) effluents.
• To assess the impact of extreme flooding events, exemplified by super Typhoon Hagibis, on silica dynamics in the river.

Main Methods:

• Continuous monitoring of DSi and PSi concentrations in the Arakawa River and STP effluents over a one-year period (October 2018 - October 2019).
• Inclusion of data from a major flooding event following super Typhoon Hagibis.
• Calculation of silica loading rates under ordinary and flooding conditions.

Main Results:

• STP effluents were identified as the dominant source of DSi during periods of ordinary water levels.
• During flooding, DSi concentrations decreased by approximately 25%, while PSi concentrations increased over sixteenfold compared to normal conditions.
• Silica loading during flooding events, driven by both elevated DSi and PSi, significantly exceeded the calculated annual loading under normal conditions.

Conclusions:

• Riverine silica loading is highly influenced by hydrological events, with flooding dramatically altering the DSi/PSi ratio and total load.
• STPs play a key role in DSi input during non-flood periods, but extreme events introduce substantial particulate silica.
• Effective management strategies for riverine silica require consideration of both chronic inputs from STPs and episodic loading during major flood events.