Sizing mitigation wetlands in agricultural watersheds

Area of Science:

• Environmental Science
• Ecology
• Water Quality Management

Background:

• Agricultural watersheds face significant nitrate pollution challenges.
• Wetlands are crucial for mitigating nitrogen pollution but their optimal size is debated.
• Field studies for determining wetland size often yield high variance in rate constants.

Purpose of the Study:

• To assess the utility of microcosms in supplementing field studies for wetland size determination.
• To establish reliable parameters for calculating the necessary wetland area for nitrate mitigation.
• To compare nitrate removal kinetics between field and microcosm settings.

Main Methods:

• Field studies in San Joaquin Valley wetlands to measure nitrate removal efficiencies and rates.
• Flow-through microcosm experiments using sediments and plants from a field site.
• Kinetic analysis of nitrate removal, including determination of half-saturation constant (K(m)) and maximum removal rate (J(max)).
• Comparison of first-order rate constants derived from field and microcosm data.

Main Results:

• Field studies showed mean nitrate-nitrogen mass removal efficiencies of 10-34% and areal rates (J) of 142-380 mg-N m(-2) d(-1).
• Microcosm experiments yielded a first-order rate constant (10.4 cm d(-1)) in close agreement with the field value (11.9 cm d(-1)) but with significantly lower variance (<16%).
• The microcosm provided kinetic parameters (K(m) = 43.8 mg/L, J(max) = 4.11 g m(-2) d(-1)) for nitrate-nitrogen.

Conclusions:

• Microcosms provide a more precise method for determining nitrate removal kinetics compared to field studies alone.
• A reliable first-order rate constant derived from microcosms suggests 1.3-3.6% of watershed land should be managed as mitigation wetland.
• The required wetland area is dependent on desired nitrate reduction levels and wetland design efficiency (e.g., plug-flow approximation).