A key function of terrestrial ecosystems is maintaining water quality by attenuating pollutants introduced from local land use and regional atmospheric deposition. This ecosystem service is critical in arid and semi-arid regions where urban runoff is managed to recharge groundwater for municipal supply. However, little is known about how solute sourcing, transport and biogeochemical cycling interact to control the runoff chemistry of urbanized catchments. Our study addresses the question: how does urban ecosystem function, with respect to solute sourcing and retention, vary across an urban land use gradient? We hypothesize that highly urbanized catchments will exhibit seasonal decreases in runoff solute concentrations, whereas catchments with low impervious cover will slowly source solutes as seasonal wetting increases. Using automatic water samplers we collected storm runoff during 2007 and 2008 from five distinct urban land use catchments within the Tucson basin: low, medium and high density residential, mixed land use and commercial. Samples were analyzed for anions, cations, nutrients, dissolved organic carbon (DOC) and metals. We compared reactive solute concentrations to a biologically inert solute (chloride, Cl-) and stable isotope values (δD and δ18O) of rainfall and runoff to distinguish physical and biogeochemical processes controlling the solute chemistry of runoff.
Strong seasonal Cl- flushing was observed at the commercial site contrasting with the low density site’s weaker flushing response. Increasing Cl- concentrations and δD and δ18O enrichment with respect to rainfall at the commercial site suggest evaporation during runoff, potentially concentrating solutes up to 25%. In contrast, δD and δ18O enrichment with respect to rainfall and a simultaneous decrease in Cl- at the low density site suggest watershed solute retention despite runoff evaporation. We observed nitrate production from the low density site, nitrate retention at the commercial site and seasonal DOC decreases across all catchments. The least and most urbanized catchments (low density and commercial) had the highest concentrations of DOC, copper and mercury, suggesting that metal mobilization was facilitated by DOC. Our data show that urbanized catchments exhibit large differences in their ecosystem function and subsequent solute sourcing or retention. Therefore, ecosystem services of urban catchments can not be evaluated based on land use alone, hydrologic flow path and catchment characteristics must be incorporated into our conceptual understanding of urban biogeochemistry.