Hydrologic and chemical connectivity in urban accidental wetlands: implications for nitrate removal

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Accidental wetlands have emerged in the urban Salt River channel of Phoenix, Arizona. Stormwater pipes collect urban runoff that drains directly into the dry river channel, providing a new perennial water source that sustains wetlands. These systems were not planned nor are they actively managed by any group, hence the ‘accidental’ designation. Water delivered by storm drains is enriched in nitrogen, particularly nitrate (NO₃^–), a common surface-water pollutant. We investigated the chemical and hydrological connectivity between surface water and subsurface porewater to evaluate potential for nitrogen attenuation by microbial NO₃^– transformation. Microbially mediated NO₃^– reduction requires low oxygen and high organic carbon availability, conditions common in wetland soils. For microbial NO₃^– reduction to occur in the Salt River wetlands, the NO₃^–-enriched surface water must be connected to the subsurface soils and sediments. Surface–subsurface connectivity also may be expected to vary with surface vegetation cover and type. To evaluate hydrochemical connectivity, we measured surface water and subsurface porewater electrical conductivity and chloride (Cl^–) concentration as inert tracers. We measured dissolved oxygen and NO₃^– concentrations across three wetland vegetation cover types to test for spatially explicit differences in potential for NO₃^– reduction between the surface and subsurface zones. Results suggest that wetland surface water and subsurface porewater exchange dissolved constituents. Electrical conductivity and Cl^– concentrations were indistinguishable both vertically across surface and subsurface water samples as well as horizontally across vegetation cover types, indicating high chemical connectivity between zones. Dissolved oxygen concentration was significantly lower in the subsurface at 1.7 ± 0.69 mg L^-1 (mean ± SD) compared to the surface water at 4.4 ± 2.6 mg L^-1, indicating high potential for NO₃^– reduction under subsurface conditions. Nitrate concentration was an order of magnitude higher in the surface water than in the subsurface porewater, but there were no concentration differences between the wetland vegetation patch types. Given the chemical connectivity indicated by electrical conductivity and Cl^– concentration, the difference in NO₃^– between the surface and subsurface hint that the wetland soils act as a sink for NO₃^– delivered from stormwater drains. Thus these unplanned, unmanaged urban wetland systems may have high capacity to attenuate nitrogen delivered from the urban landscape.