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Common Terms
Terms you are likely to encounter when exploring TMDLs include:
Background Loading (BL):
Loads
of naturally occurring materials that would have entered the water body prior to disturbance of the watershed by human activities.
For example, phosphorus derived from the natural rocks in a watershed provides the native
vegetation with their nutrient P requirement. Background loads must be taken into consideration in
setting TMDLs.
Beneficial uses:
Uses
of water bodies that include drinking water supplies, fishing, swimming,
boating, wildlife habitat, and shellfish harvesting. Aesthetic
factors may be considered as well, such as appearance and odor. Beneficial uses for water bodies are
determined by state and tribal environmental agencies.
Best Management Practices (BMPs):
Practices
that are designed to lessen environmental damage from nonpoint sources. Examples include erosion control from
urban developments, agriculture and forestry sites; fertilizer and animal waste
management on farms; riparian zone installation on agricultural lands, rangelands and forested lands;
and runoff management in urban systems.
Concentration:
The
amount – usually mass or weight – of a material in a given volume of
water. Units include milligrams per liter (mg/L, parts per million or
ppm); micrograms per liter (mg/L,
parts per billion or ppb); sometimes as ounces per gallon (oz/gal). For
bacteria, it is frequently expressed as number of cells per 100 cm3.
Impaired Waters: Water
bodies either partially supporting designated uses or not supporting designated uses such as fishing, swimming, recreational and drinking
water.
Load Allocation (LA): Dividing
up and allocating the total quantity of pollutant entering a water body daily (originating from
nonpoint sources)
among all the nonpoint sources.
Margin of Safety (MOS): In
essence, every TMDL implementation is a full-scale experiment in how water bodies will respond to changes in watershed management. For
this reason, an additional allocation based on uncertainty about the response of the water body
to decrease in the load of a pollutant is set. For example, if a water body containing 0.15
ppm total P generates excessive algal growth, the best science and professional judgment may predict
that total P levels should be at 0.10 ppm in order for algal growth not to threaten aquatic life;
however, an additional Margin of Safety may be applied, because we are unsure of how the water body
will respond, and the Target Load then set at 0.08 ppm total P.
Nonpoint Source Pollution: Pollution
originating from diffuse sources on the landscape, from runoff or groundwater. Examples include runoff from fields receiving
manure applications, stormwater runoff from urban landscapes, or roadbed erosion in
forestry.
Parameters: Following
is a list of the most typical individual pollutants or water quality
conditions.
· Total
phosphorus (TP)
· Ammonia/ammonium
· Total
suspended solids 1 (TSS-
total particles suspended in the water)
· Total
dissolved solids (TDS – salts dissolved in water). See Suspended
Sediments/Turbidity in
TMDL Parameters Section for explanation describing difference between
TDS and TSS.
· Temperature
· Pathogens
(fecal coliform and bacteria)
· Pesticides
· Nitrate
· Habitat
Alteration/Modification – turbidity and TSS pollution due to dams,
dredging, channelization, ditching, housing developments, highway construction
and maintenance, and county operations
· BOD
· Low
DO
· Metals
· pH
· Sulfates
· Legacy
pollutants such as chlordane, mercury, and dieldrin. Legacy pollutants
hopefully are no longer being generated or discharged directly into water bodies
although some legacy pollutants are being deposited via the atmosphere Land use and land cover will certainly have an impact on which
parameters or pollutants appear in any given water body segment or lake; the list of pollutant
parameters may also vary from state to state or region to region. For example, naturally occurring
arsenic is on Nebraska’s list, and naturally occurring chloride and selenium, possibly from return
irrigation water high in salts, are on Kansas’ list.
Point Source Pollution: Pollution
originating at a point, such as an industrial plant, wastewater treatment plant, or feedlots. Frequently the waste stream enters the
water body through a pipe or ditch, making sampling and flow monitoring relatively
straightforward.
Target Loads: The
quantity of a pollutant that can enter a water body per day without
degradation
Total Maximum Daily Load (TMDL): The
maximum quantity of a pollutant that can enter a water body without adversely affecting the beneficial uses of the
water body.
TMDL = Sum of all WLA +
Sum of all LA + BL + MOS.
Waste Load Allocation (WLA): Dividing
up and allocating the total quantity of pollutant entering a water body (originating daily from
point sources)
among all the point sources.
Note: Some people
refer to Total Suspended Solids (TSS) as Total Suspended
Sediments. A few states differentiate total suspended solids,
siltation and sediments. Many states do not make these
distinctions. Siltation is a function of Total Suspended Solids
settling out and sedimentation is a product of Total
Suspended Solids (TSS). TSS is the parameter that can be regulated by
load.
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