Watershed Protection Plan - Lower Rio Grande Valley Development ...

STATE OF THE WATERSHED
Figure 12. Location of Water Quality Monitoring Stations on the Arroyo Colorado Used in the Phase I TMDL Study and
Arroyo Colorado Sub-basins
of 24-hour DO data collected between 2000 and 2005
at several stations in the tidal segment and two stations
in the above-tidal segment of the Arroyo Colorado.
The most recent DO data for the Arroyo Colorado
shows modest but encouraging improvements in the
24-hour average DO at stations located in the Zone of
Impairment. Although the minimum DO concentrations
over a 24-hour period are occasionally below the
criteria at these stations, the 24-hour average DO
concentration was recorded below the criteria only once
since 2002 (Figure 13).
USDA personnel preparing for water quality sampling on the
Arroyo Colorado
Biochemical Oxygen Demand
Biochemical oxygen demand (BOD) is a collective
measure of all oxygen-demanding substances
dissolved and suspended in a particular sample of
water. BOD is, essentially, a measure of the organic
matter that is prone to decomposition in water. Like
many other organisms, bacteria use organic matter
as food and, in order to use the food for energy and
cell growth, bacteria must fi rst break down the organic
matter into more simple chemical structures. To do
this, most bacteria use a process called respiration,
in which oxygen is combined with the organic matter
to break down complex molecules into simple ones.
The respiration process can consume large amounts
of dissolved oxygen in the water column. To measure
only the dissolved fraction of BOD, water samples are
sometimes fi ltered to remove suspended particles.
The most signifi cant contributors of BOD to the
Arroyo Colorado are nonpoint sources including rainfall
runoff and irrigation return fl ows from agricultural
land, urban runoff, nonpoint source wastewater from
colonias and improperly functioning septic systems. A
sizable loading of BOD to the Arroyo Colorado (23%)
also comes from permitted wastewater outfalls (TCEQ
2003).
In general, in-stream BOD levels are not considered
to be very high in the Arroyo Colorado. Based on
January 2007 41 Arroyo Colorado Watershed Protection Plan