Selected Water Quality Processes in Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 145

Anaerobic environments are common in aquatic sediments, below the thin oxidized surface
layer, but anaerobic conditions in the water column are very undesirable and often symptomatic of
pollution. Anaerobic environments are also referred to as anoxic, and water with a diminished sup-
ply is referred to as hypoxic. Another phrase in common usage today to describe hypoxic waters is
dead zones . Dead zones are becoming increasingly common in a variety of aquatic habitats.
So, how much oxygen is necessary for a healthy aquatic environment? The amount would vary
with the type of environment and what could be considered healthy. One measure is the regulatory
standards and criteria proposed by the U.S. EPA and adapted by state agencies. The ambient dis-
solved standards included in the U.S. EPA's quality criteria for water (USEPA 1986) are listed in
Table 5.3. For cold-water environments and early life stages where intergravel (the hyporheic zone)
concentrations are critical, the ambient criteria are those that would be expected to result in the
intergravel concentrations given in parentheses. To relect the diel variations that may occur, some
states have adapted standards for both daily averages (such as 5 mg L -1 ) and instantaneous mini-
mums (such as 4 mg L -1 ).
DO standards and criteria are also used to determine how much material may be discharged into
a stream or river without violating those standards, used as a basis for establishing discharge per-
mits (wasteload allocations). The typical result of a discharge of organic materials into a stream is a
decrease in DO concentrations. However, the impact of the deoxygenation is not only a function of
the amount of material discharged, but also the combined function of all of the instream processes
impacting DO concentrations. The typical result is a longitudinal variation in DO concentrations,
as illustrated by the idealized DO sag curve illustrated in Figure 5.20.

TABLE 5.3
Ambient Dissolved Oxygen Standards (mg L -1 )

Cold-Water Criteria Warmwater Criteria

Early Life Stages 1,2 Other Life Stages Early Life Stages 2 Other Stages

30-day mean NA 6.5 NA 5.5

7-day mean 9.5 (6.5) NA 6.0 NA

7-day minimum NA 5.0 NA 4.0

1-day minimum 8.0 (5.0) 4.0 5.0 3.0

Source: USEPA, Quality criteria for water 1986, United States Environmental Protection Agency, Ofice
of Water Regulations and Standards, Washington, DC, 1986.

River

DO
(mg L -1 )

Saturation

Recovery zone
(plants)

Critical concentration
(decomposition = reaeration)

Distance

Decomposition
dominates

Reaeration
dominates

FIGURE 5.20 Dissolved oxygen sag. (From Chapra, S.C., Surface Water-Quality Modeling , WCB McGraw-
Hill, New York, 1997.)