"Accidental" urban wetlands are environments formed not through deliberate restoration or management activity, but as a result of land use and water infrastructure decisions by municipalities. These wetlands are often formed in abandoned industrial, residential, or commercial areas that are low-lying in the landscape, where overland flows from storms and municipal water use can accumulate and support wetland soils and plant communities. Their positioning within the urban matrix means that accidental wetlands have the potential to both receive and modify materials from surrounding urban areas. Abandoned industrial sites have been estimated to occupy 20,200 km2 in U.S. cities, but the function and capacity of pollutant removal in unrestored urban wetlands has been little studied. Little is known about how management and design intention or interventions influence most wetland functions, as compared to no interventions or design. Ecosystems such as created wetlands, which are deliberately constructed to support specific biogeochemical processes, typically include structural characteristics that mimic those of natural systems, where contaminant processing is high. Design and management of vegetation, soils, and hydrology is meant to facilitate particular wetland functions (e.g., nutrient removal). I argue, however, that accidental wetlands may in some cases provide a greater variety of functions than designed urban environments, because the latter can be over-designed for a limited set of specific functions.
Results/Conclusions
Research I have conducted in the southwestern U.S. (Salt River in Phoenix, Arizona) demonstrates that designed and accidental wetlands tend to receive high volumes of nutrient-enriched municipal, storm, and rainwater, and demonstrate the capacity for high levels of nutrient removal. Although their soils and hydrology differ greatly from native and constructed wetland systems in the same region, accidental urban wetlands studied in Arizona appear to support redox conditions that facilitate high rates of microbial denitrification within a few decades following formation. Percent removal of NO3- by accidental wetlands was generally high, and matched or exceeded removal in native or constructed wetlands in the same region. The wetlands were often a net source of dissolved PO43-, but on average removed 21–28% of P entering the wetlands. These high levels of function suggest that structural elements (soil structure, hydroperiod) mediating beneficial microbial processes in urban wetlands may in some cases develop relatively quickly in the absence of design and management.