Efficacy of Natural Wetlands to Retain Nutrient, Sediment and Microbial Pollutants
Journal of Environmental Quality , 37 , 2008

Knox, A.K., Dahlgren, R.A., Tate, K. W., Atwill, E. R.

Wetlands can improve water quality through natural processes including sedimentation, nutrient transformations, and microbial and plant uptake. Tailwater from irrigated pastures may contribute to nonpoint source water pollution in the form of sediments, nutrients, and pathogens that degrade downstream water quality. We examined benefi ts to water quality provided by a natural, fl ow-through wetland and a degraded, channelized wetland situated within the fl ood-irrigation agricultural landscape of the Sierra Nevada foothills of Northern California. Th e non-degraded, reference wetland signifi cantly improved water quality by reducing loads of total suspended sediments, nitrate, and Escherichia coli on average by 77, 60, and 68%, respectively. Retention of total N, total P, and soluble reactive P (SRP) was between 35 and 42% of loads entering the reference wetland. Retention of pollutant loads by the channelized wetland was signifi cantly lower than by the reference wetland for all pollutants except SRP. A net export of sediment and nitrate was observed from the channelized wetland. Decreased irrigation infl ow rates signifi cantly improved retention effi ciencies for nitrate, E. coli, and sediments in the reference wetland. We suggest that maintenance of these natural wetlands and regulation of infl ow rates can be important aspects of a best management plan to improve water quality as water runs off of irrigated pastures.