Enriched dissolved organic carbon export from a residential stormwater pond

Highlights: • Organic carbon (C) forms in a residential stormwater pond discharge were investigated. • Elevated DOC concentration and flux were found in pond discharges. • DOC and POC concentrations peaked at the beginning of storm events. • Ratios of C/N indicated in situ aquatic sources of DOC and POC in pond discharges. • Controls on terrestrial organic sources are needed to reduce DOC loads in downstream waters. 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.