Irrigation with reclaimed water in an urban watershed and nutrient levels in adjacent streams

Reclaimed water, used commonly for lawn irrigation, can be a complicating factor for watershed managers addressing sources of nonpoint source pollution. This research examined concentrations of total nitrogen (TN) and total phosphorus (TP) in the reclaimed water effluent from the City of St. Petersburg wastewater treatment plant (WWTP) (secondary treatment), the Pinellas County WWTP (tertiary treatment), and four tributary sites in the highly urbanized Joe's Creek watershed in Pinellas County, Florida. Two of the four tributary sites are control sites (35-10 and 35-11). The Bonn Creek tributary is within the tertiary treatment service area, and Miles Creek tributary is within the secondary treatment service area. For the 6-year study period, results show that Bonn Creek's mean TN concentration of 0.92 mg/L was lower than Miles Creek's (1.077 mg/L), and both control sites were the lowest (0.66 mg/L and 0.71 mg/L, respectively). Similar results were found for TP concentrations. Bonn Creek's TP concentration was 0.084 mg/L slightly lower than Miles Creek's (0.096 mg/L), and both control sites were the lowest (0.064 mg/L and 0.07 mg/L, respectively). Potential TN and TP loadings from each WWTP to their service areas were calculated, as well as potential TN loadings to each landscape. PRACTITIONER POINTS: We investigated TN and TP concentrations from secondary and tertiary WWTPs and streams that receive runoff from two neighborhoods using reclaimed water from these plants for landscape irrigation. As expected, reclaimed water from the secondary WWTP had higher TN and TP concentrations. The surface water concentrations of TN and TP in the adjacent streams within the reclaimed service areas were higher than the concentrations at the control sites outside of the reclaimed service areas. The nitrogen applied to landscapes in the secondary WWTP reclaimed service area was ten times higher than the nitrogen applied to landscapes in the tertiary WWTP reclaimed service area.