Ability of restored wetlands to reduce nitrogen and phosphorus concentrations in agricultural drainage water

ABSTRACT:

Runoff from artificially drained agricultural lands is a common source of excessive nitrogen and phosphorus to downstream waters. Restored wetlands receiving crop field drainage water are shown to lower concentrations of both nitrogen and phosphorus. Case studies in Maryland, Illinois, and Iowa indicate that wetlands can remove up to 68% of nitrate-nitrogen and 43% of phosphorus from drainage water, although performance varies considerably. Performance comparison across sites indicates that large wetlands relative to the contributing drainage area most effectively improve water quality. Time series data representing periods of both relatively high and low inflow indicate that performance is highly sensitive to retention time, with greatest nutrient removal during flow conditions that facilitate retention times of at least one to two weeks. Where wetlands are incorporated into forested riparian buffer strips, additional water quality benefits are shown.