Resolving Spatial and Temporal Variability in Dissolved Organic Matter Characteristics Within Combined Agricultural and Waterways Conveyances

Dissolved organic matter (DOM) plays an important role in the aquatic environment and can have significant effects on aquatic organisms. Characterizing the composition of DOM within urban receiving waters and the contributions of DOM from urban stormwater runoff is important for understanding potential downstream water quality effects. We conducted this study to characterize the DOM in an urban water conveyance that receives stormwater inputs during runoff events. Baseflow samples were collected at upstream and downstream ends of a study reach, and stormflow samples were collected from outfalls discharging to the study reach. DOM was characterized by measuring dissolved organic carbon (DOC) concentration and excitation emission matrix spectroscopy (EEMS). In addition to the physical samples measured, in situ data was collected at the upstream and downstream monitoring locations with a suite of fluorescent DOM (fDOM) sensors and a mobile sensing platform was deployed during select times to capture a Lagrangian profile of water quality in the canal. During storm events, DOC concentrations were more than 3 times greater at the downstream site than those at the upstream site due to high contributions of DOC being discharged from outfalls. EEMS results and fluorescence indices indicated that DOM composition shifted during storm events from a more autochthonous, less degraded DOM in baseflow to more decomposed and terrestrially derived DOM in stormwater flows and that these changes were driven by outfall specific runoff contributions and DOM compositions. In situ data showed that fDOM is higher in baseflows during spring snowmelt and decreases throughout the irrigation season but experiences short and significant increases in fDOM during runoff events. Measurements of fDOM at the upstream monitoring location were lower than the downstream monitoring location during stormflows indicating that DOM from outfalls is elevating fDOM concentrations between the two sites. The mobile sensing platform allowed for the collection of high spatial resolution water quality data and revealed DOM contributions from unmonitored outfalls that would have otherwise gone unmeasured and also provides a visualization of the longitudinal mixing between canal and outfall contributions