Urban stormwater quality II. Comparision of three New Zealand catchments

The variability in water quality of urban runoff was assessed by comparing the concentration distributions and variations of various parameters over storm events in three catchments. The parameters chosen assessed nutrient, total oxygen demand, sediment, and toxic metal levels. Two catchments were located in Wairau Valley, Takapuna, Auckland. The smaller (1.49 km²), with residential/commercial landuse, formed part of the larger (11 km²) with predominantly residential/industrial/developing land. The third was a 1.14 km² residential catchment in Hillcrest, Hamilton. The greatest dissimilarity occurred between the Hillcrest and the two Auckland catchments. Concentrations of suspended solids, total phosphorus, dissolved reactive phosphorus, Cr, Ni, and Zn were significantly higher in the Auckland catchments, whereas chemical oxygen demand, organic nitrogen, Pb, and volatile suspended solids were not significantly different from Hillcrest. NH₄ ⁺ levels were higher in Hillcrest. Storm flows diluted NO₃ ^‐ concentrations in Hillcrest but increased NO₃ ^‐ concentrations in the Auckland catchments, resulting in comparable levels in all three catchments. It is proposed that most of the differences are attributable to higher specific flows and subsoil erosion in the Auckland catchments coupled with dissimilarities in soil concentrations and characteristics. The differences in the variation of NO₃ ^‐ concentrations were probably because of septic tank seepage in the Hillcrest catchment. The largest catchment differed from the two smaller catchments in having higher Cu and slightly lower TN and NO₃ ^‐ levels. All three catchments showed some similarities: Pb, Zn, and probably Cu and Cd were clearly enriched in the suspended sediment over background soil levels, and P, organic matter (COD, ON, VSS), Cr, and Ni were not enriched. It was concluded that geographical differences were more important in determining stormwater quality than urban infrastructure.