Net changes in nutrient concentrations below a point source input in two streams draining catchments with contrasting...

Area of Science:

• Environmental Chemistry
• Stream Ecology
• Water Quality Assessment

Background:

• Wastewater treatment plants (WWTPs) are significant point sources of nutrients in aquatic ecosystems.
• Understanding nutrient processing in streams receiving WWTP effluent is crucial for water quality management.
• Agricultural runoff represents a major non-point source of nutrient pollution.

Purpose of the Study:

• To investigate the impact of WWTP effluent on nutrient dynamics (ammonium-Nitrogen, nitrate-Nitrogen, DIN, SRP, DOC) in agricultural and non-agricultural streams.
• To estimate nutrient processing rates (Snet, Vf) below a WWTP outfall.
• To compare the relative influence of WWTP inputs versus diffuse agricultural inputs on stream water chemistry.

Main Methods:

• Field sampling of water chemistry at multiple dates along stream reaches below a WWTP.
• Measurement of chloride-corrected ambient concentrations of NH(4)(+)-N, NO(3)(-)-N, DIN, SRP, and DOC.
• Estimation of nutrient processing length (Snet) and mass transfer coefficient (Vf) based on net changes.

Main Results:

• In agricultural streams, diffuse nutrient inputs from fields overshadowed the WWTP's impact, exceeding the stream's processing capacity.
• Non-agricultural streams showed clear longitudinal trends for NH(4)(+)-N and NO(3)(-)-N below the WWTP.
• Observed NH(4)(+)-N decrease coupled with NO(3)(-)-N increase in non-agricultural streams indicates nitrification of WWTP-derived ammonium.

Conclusions:

• WWTP effluent significantly influences nutrient cycling in non-agricultural streams, primarily through ammonium nitrification.
• Agricultural streams are more susceptible to diffuse pollution, which can overwhelm the system's capacity to process nutrients from point sources.
• WWTP inputs can create localized 'hot spots' conducive to chemoautotrophic activity in receiving streams.