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

STATE OF THE WATERSHED
Dissolved Oxygen
Dissolved oxygen, normally reported in milligrams of
oxygen per liter of water (mg/l), is a traditional measure
of aquatic health because aquatic organisms depend
on it for survival. If DO concentrations become too
low in a water body, large-scale die-offs of aquatic
life (i.e., fi sh kills) can occur. Fish kills typically result
when concentrations of DO fall below 2 mg/l. High
DO concentrations (greater than 10 mg/L) during the
daytime can also be indicative of an unhealthy aquatic
environment because they typically result from high
photosynthetic activity caused by excessive algal
growth. At night, the same algal cells that produce DO
during the daytime can consume large amounts of DO,
which causes the water body to become DO-depleted,
making the water column unsuitable for aquatic life.
The Texas Surface Water Quality Standards (30
TAC §§307.1-307.10) specify the dissolved oxygen
criteria that must be met for limited, intermediate, high
and exceptional aquatic life uses in water bodies of
the State of Texas. Having received a High Aquatic
Life use designation by the State of Texas, the Tidal
segment of the Arroyo Colorado has an associated 24-
hour average DO criterion of 4.0 mg/l and a 24-hour
DO minimum criterion of 3.0 mg/l (Table 7). The Texas
Surface Water Quality Standards also state that, in tidal
streams, under conditions of density stratifi cation, the
DO criteria must be met in the mixed surface layer of
the water column, which is defi ned by the TCEQ as the
vertical portion of the water column located between the
surface and the depth at which the conductivity of the
water is 6,000 umhos higher than the conductivity at the
surface.
DO concentrations are largely dependent on the
Water sampling in the Arroyo Colorado Tidal
Dissolved oxygen probes
temperature and salinity of water. The amount of airmixing
or aeration from wind and water turbulence
is also a key factor determining DO concentrations,
as is the presence of oxygen-demanding substances
and living organisms in the water. Dissolved oxygen
levels typically fl uctuate during the daily cycle. Higher
DO levels are typically observed in the afternoon, at
the height of photosynthetic activity, while the lowest
DO levels typically occur in the early morning, when
algal respiration (i.e., oxygen consumption) is at its
maximum.
Elevated nutrient levels in the tidal portion of
the Arroyo Colorado also contribute to periodic
low DO levels. The wide diurnal fl uctuations in DO
observed in the tidal segment of the Arroyo Colorado
(DO concentrations ranging from 0 to 12/mg/l) are
characteristic of a eutrophic (i.e., high algal productivity)
water body (APAI 2006).
During the Phase I TMDL study, data from 48
stations were used to assess water quality in the
tidal segment of the Arroyo Colorado (Figure 12). Of
the 13 monitoring stations representing the upper
portion of the Arroyo Colorado tidal segment, six
showed DO values below the criteria yielding a 64%
compliance rate (eight non-compliant samples out of
22 measurements). In the lower portion of the tidal
segment, only one of the 35 remaining downstream
stations produced DO values below the assessment
criteria yielding a 99% compliance rate (one noncompliance
sample out of 109 measurements). For
most of these stations, however, compliance with
the DO criteria was based on instantaneous DO
measurements, which shows an incomplete picture of
DO dynamics at specifi c locations within a water body.
Subsequent water quality assessments have made use
Arroyo Colorado Watershed Protection Plan 40 January 2007