More Than a First Flush: Urban Creek Storm Hydrographs Demonstrate Broad Contaminant Pollutographs

Stormwater runoff clearly impacts
water quality and ecological
health of urban receiving waters. Subsequent management efforts are
often guided by conceptual models of contaminant “first flushes”,
defined by disproportionate concentrations or mass loads early in
the storm hydrograph. However, studies examining the dynamics of contaminant
transport and receiving water hydrology have primarily focused on
“traditional” stormwater contaminants and point sources,
with less evaluation of chemically complex nonpoint pollution sources.
Accordingly, we conducted baseflow and storm sampling in Miller Creek,
a representative small, urban watershed in the Puget Sound region
(WA, USA). We comprehensively characterized organic contaminant profiles
and dynamics via targeted quantification of 35 stormwater-derived
chemicals, complementary nontarget HRMS analyses, and surrogate chemical
metrics of ecological health. For quantified analytes, total daily
baseflow loads were 0.8–3.4 g/day and storm event loads were
∼80–320 g/storm (∼48 h interval), with nine contaminants
detected during storms at >500 ng/L. Notably, urban creek “pollutographs”
were much broader than relatively sharp storm hydrographs and exhibited
transport-limited (rather than mass-limited) source dynamics, with
immediate water quality degradation during low-intensity precipitation
and continued mobilization of contaminant mass across the entire hydrograph.
Study outcomes support prioritization of source identification and
focused stormwater management efforts to improve water quality and
promote ecosystem function in small urban receiving waters.