Chemical and Water Quality Kinetic Characteristics and Processes - Hydro-Environmental Analysis: Freshwater Environments - page 375

Following iron, the next most thermodynamically favorable TEA is sulfate (SO 4 ). Sulfate reduc-
tion typically occurs following extended periods of anoxia with the ORP becoming negative at
about -150 mV (Gordon and Higgins 2007). Sulfate reduction is recognized as an important TEA
in estuarine and marine sediments, but it is often assumed to have a less important role in fresh-
water systems (Capone and Kiene 1988), partially due to its relatively lower sulfate concentrations.
However, perhaps even in cases where the magnitude of the sulfate reduction is small compared to
other TEAs, it still remains important because of the end product of that reduction, hydrogen sulide
(H 2 S), commonly referred to as “rotten egg” gas.
Sulfate reduction also reduces the pH and the ORP to a point where fermentation begins to occur,
where organics are reduced to organic acids and alcohols, which accumulate as end products. These
end products produce an immediate oxygen demand when released to an aerobic environment and
typically have objectionable odors. These reduced organics are also precursors to methane forma-
tion (Gordon and Higgins 2007).
When anaerobic conditions and organic acids are present, CO 2 will serve as a TEA and form
methane, which may accumulate in bottom waters or sediments. For methanogenesis, the ORP
remains at about -200 mV and the pH increases due to the destruction of organic acids (Gordon and
Higgins 2007). The methane is relatively insoluble, forming a gas phase that may escape from the
sediments and bottom waters as bubbles (Di Toro 2001).
14.4 OXYGEN
14.4.1 S tandardS and c rIterIa
DO is vital to aquatic life (for aerobic organisms) and is used by most regulatory agencies as an
indicator of aquatic health and as a surrogate enforcement parameter. The typical minimum DO
criteria for the protection of aquatic life are 5 mg L −1 for warmwater ecosystems and 6 mg L −1 for
cold-water ecosystems. However, criteria vary as a function of aquatic life and life stages and the
exposure periods (e.g., using instantaneous, daily, weekly, or monthly values). Therefore, the pro-
tection of aquatic health in the implementation of the Clean Water Act (e.g., the national pollutant
discharge elimination system [NPDES] permitting and total maximum daily load [TMDL] studies)
is commonly based, in part, on maintaining adequate DO concentrations in lakes and reservoirs.
While the concept for oxygen criteria in lakes and reservoirs is similar to that used in rivers and
streams, its implementation is problematic since, unlike rivers and streams, lakes and reservoirs
commonly stratify so that there are often large spatial as well as temporal variations in the DO con-
centrations. In the case of stratiied lakes, hypolimnetic oxygen depletion may be naturally occur-
ring or enhanced due to excess loads of nutrients and organic materials (e.g., eutrophication, which
will be discussed in Chapter 17).
In reservoirs, which are not natural systems, stratiication is commonly the result of what is
called “hydromodiication.” The U.S. Environmental Protection Agency (USEPA 1993) deined
hydromodiication as the “alteration of the hydrologic characteristics of coastal and non-coastal
waters, which in turn could cause degradation of water resources,” which includes the impacts of
dams. For example, if a dam were not there then perhaps stratiication (and hence hypolimnetic
oxygen depletion) would not have occurred. For this reason, the U.S. EPA concluded that TMDL
evaluations were not required for systems that are impaired (not meeting water quality standards)
due to hydromodiication.
The Kansas water quality standards illustrate the dilemma. Kansas established universal (appli-
cable streams, lakes, etc.) numeric criteria of 5 mg L −1 for DO and included the following condi-
tion: “the concentration of dissolved oxygen in surface waters shall not be lowered by the inluence
of artiicial sources of pollution.” However, as noted by the Kansas Department of Health and
Environment (KDHE 2011), lakes “with moderate levels of nutrient or algal content have suficient
levels of dissolved oxygen, but see dissolved oxygen diminish at their lower depths,” such as Clinton