Effects of an Extreme Flood on Trace Elements in River WaterFrom Urban Stream to Major River Basin

Major floods adversely affect water
quality through surface runoff,
groundwater discharge, and damage to municipal water infrastructure.
Despite their importance, it can be difficult to assess the effects
of floods on streamwater chemistry because of challenges collecting
samples and the absence of baseline data. This study documents water
quality during the September 2013 extreme flood in the South Platte
River, Colorado, USA. Weekly time-series water samples were collected
from 3 urban source waters (municipal tap water, streamwater, and
wastewater treatment facility effluent) under normal-flow and flood
conditions. In addition, water samples were collected during the flood
at 5 locations along the South Platte River and from 7 tributaries
along the Colorado Front Range. Samples were analyzed for 54 major
and trace elements. Specific chemical tracers, representing different
natural and anthropogenic sources and geochemical behaviors, were
used to compare streamwater composition before and during the flood.
The results differentiate hydrological processes that affected water
quality: (1) in the upper watershed, runoff diluted most dissolved
constituents, (2) in the urban corridor and lower watershed, runoff
mobilized soluble constituents accumulated on the landscape and contributed
to stream loading, and (3) flood-induced groundwater discharge mobilized
soluble constituents stored in the vadose zone.