Characterizing the Chemical Profile of Biological Decline in Stormwater-Impacted Urban Watersheds

Chemical contamination is an increasingly
important conservation
issue in urban runoff-impacted watersheds. Regulatory and restoration
efforts typically evaluate limited conventional parameters and pollutants.
However, complex urban chemical mixtures contain hundreds to thousands
of organic contaminants that remain unidentified, unregulated, and
poorly understood. This study aimed to develop broadly representative
metrics of water quality impairment corresponding to previously documented
biological degradation along gradients of human impacts. Stream samples
(n = 65, baseflow/rainfall conditions, 2017–2018)
were collected from 15 regional watersheds (Puget Sound, WA, USA)
across an urbanization gradient defined by landscape characteristics.
Surface water chemical composition characterized via non-targeted
high-resolution mass spectrometry (7068 detections) was highly correlated
with landscape-based urbanization gradient (p <
0.01) and season (p < 0.01). Landscape-scale changes
in chemical composition closely aligned with two anchors of biological
decline: coho salmon (Oncorhynchus kisutch) mortality risk (p < 0.001) and loss of stream
macroinvertebrate diversity and abundance (p <
0.001). We isolated and identified 32 indicators for urban runoff
impacts and corresponding receiving water ecological health, including
well-known anthropogenic contaminants (e.g., caffeine, organophosphates,
vehicle-derived chemicals), two related environmental transformation
products, and a novel (methoxymethyl)­melamine compound. Outcomes support
data-directed selection of next-generation water quality indicators
for prioritization and evaluation of watershed management efforts
intended to protect aquatic ecosystems.