Enriched dissolved organic carbon export from a residential stormwater pond

In urban watersheds, stormwater retention ponds are intermediate junctions that capture, store, and discharge stormwater, and provide an organic-rich environment that transforms and retains nutrients and other constituents. This study investigated the concentrations and loads of dissolved and particulate organic carbon (DOC and POC) in discharges from a stormwater retention pond that receives runoff from a residential catchment. We installed an autosampler, a flowmeter, and a rain gauge at the outlet (weir) of the stormwater retention pond and collected samples from 13 storm events during the 2016 wet season (May-September). Results showed the dominance of DOC (11.2 mg L^-1) over POC (0.6 mg L^-1) in the pond discharges. The elevated DOC levels in the pond were close to eutrophic lakes and ponds (~10.3 mg L^-1), but not statistically different from urban runoff at a nearby site. High-frequency monitoring of pond discharge waters showed that DOC concentrations peaked at the beginning of storm events due to initial surface runoff following a rainstorm (first-flush effect). Rainfall samples analysis suggested that precipitation accounted for a small fraction of DOC pool, but carbon to nitrogen (C/N) ratios supported that in situ aquatic sources could dominate DOC inputs in some storms. Relative to DOC, the first-flush effect was even more apparent for POC, and POC inputs from in situ aquatic sources were more common based on C/N ratios. The calculated export of total organic C (TOC = DOC + POC) was 22.5 kg ha^-1 over the observed events, and the estimated export was 33.8 kg ha^-1 over the 2016 wet season. Our data suggest that reducing high DOC export from residential stormwater ponds warrant controls on both inputs from the watershed and in situ aquatic sources.