Processes affecting dissolved-oxygen concentrations in the lower reaches of Middle Fork and South Fork Beargrass Creek, Jefferson County, Kentucky

This report provides data on dissolved-oxygen
(DO) concentrations and identifies the environmental
processes that most affect DO concentrations
during base-flow periods in the lower
reaches of Middle Fork and South Fork Beargrass
Creek in Jefferson County, Kentucky. These
reaches are affected by inputs from combined-sewer
overflows. Sections of the lower reaches of
the two streams run through single-family residential
areas and public parks that are used extensively
by local residents during the summer.
Recreational fishing and wading also are common
in the Middle Fork reach.
Continuous-record data collected during the
summer and early fall (July-September 1996 on
the Middle Fork and July-October 1995 on the
South Fork) at three monitoring sites along each
reach indicate generally decreasing DO concentrations
in the downstream direction except for
the South Fork Beargrass Creek at Winter Avenue
site where channel modifications have resulted in
higher velocities along with shallower depths during
low-flow conditions. The channel modifications
at this site increased the reaeration-rate
coefficient (a measure of the capacity of the
stream to absorb oxygen through the air-water
interface), increased the potential for algae to
attach to the rough concrete surface, and
increased algal exposure to sunlight.
Synoptic data available for selected constituent
concentrations were used to calibrate and verify
a computer model (U.S. Environmental
Protection Agency QUAL2E model) capable of
simulating processes that affect DO concentrations
in streams. The results of the study indicate
that streamflow, reaeration, and sediment-oxygen
demand (SOD) are the factors that most affect net
production and depletion of DO in the lower
reaches of Middle Fork and South Fork Beargrass
Creek. For the QUAL2E model, streamflow is
used in the determination of depth, which in tum
is used to estimate the consumption of oxygen by
SOD. Streamflow also is used in the determination
of the reaeration-rate coefficient. From the
QUAL2E simulations, DO concentrations (in the
mass balance) attributed to reaeration and SOD
were at least an order of magnitude greater than
any of the other factors that can affect
DO concentrations. Large diurnal variability in
DO concentrations resulted at the monitoring sites
located at upstream and downstream ends of the
Middle Fork and South Fork reaches, but as indicated
in model simulation, the net effect of photosynthesis
and respiration on DO concentration
was small. Nitrogen, ammonia, and carbonaceous
biochemical-oxygen demand were present at low
concentrations in each of the study reaches; the
model results indicate these constituents did not
have a substantial effect on DO concentrations.
Model simulations indicated that lowering the
SOD rate by 50 percent would result in a substantial improvement in DO concentrations in the
Middle Fork Beargrass Creek reach for extremely
low base-flow conditions but would result in only
limited improvement in DO concentrations in the
South Fork Beargrass Creek reach. However, no
simulations for extremely low base-flow conditions
were conducted for the South Fork Beargrass
Creek reach. More information on SOD is
needed for stream reaches affected by periodic
inputs of effluent. In such stream systems, the
temporal and spatial variability of SOD needs to
be better defined.