Composition of dissolved organic matter in urban runoff depends on source and landscape position

Background/Question/Methods: The sources and transport of organic matter (OM) through watersheds are significantly altered by urbanization. Dissolved OM (DOM) has multiple influences on the ecology of freshwater ecosystems and the suitability of water for human uses. Therefore, the dynamics of DOM in waters draining from urban landscapes is of significant interest. We investigated two questions in this study: 1) How variable is the composition of the DOM pool at different positions within the urban stream continuum (i.e. roof gutters, catch basins, stormwater pipes, headwater channels, larger channels)? And 2) to what degree is the DOM pool of an urban stream a conservative mixture of upstream sources (vs. novel materials derived from in stream processing)? In order to characterize the composition of DOM pools, samples were taken of the runoff from house gutters (collected by residents of Durham, NC), the runoff from washing roads and stormwater pipes, and standing water from catch basins and stream baseflow. We used the EEM-PARAFAC approach to characterize the composition of the colored DOM (CDOM) pool and quantify variability among samples representing different CDOM sources from different positions on the landscape.

Results/Conclusions: Grass clippings and leaves contained similar amounts of leachable DOC (18.94±6.03 mg C/g and 14.56±5.70 mg C/g, respectively; p-value=0.36), but grass clippings had significantly higher leachable TDN as compared to leaves (1.41±0.46 mg N/g and 0.16±0.07 mg N/g, respectively, p-value=0.04). Urban soil contained significantly less leachable DOC and TDN than leaves or grass clippings (0.0676±0.0002 mg C/g, 0.0253±0.0013 mg N/g). We found little variability in CDOM properties within source type (i.e. leaf leachate CDOM properties from different locations were relatively similar, but were distinct from other sources such as road wash runoff, soil leachate, etc.). These results indicate that position within the urban stream continuum determines the relative importance of differences in the source vs. in-system processing for determining the composition of DOM in urban waters.  In these highly engineered urban flowpaths, position within the urban stream continuum may be related to residence time, which in turn may partially determine the degree of processing urban DOM undergoes before reaching downstream water bodies including rivers and reservoirs. The amount and type of DOM present can impact the ecosystem services performed by these water bodies, such as drinking water provisioning, nutrient removal, and habitat for aquatic life.