WATERSHEDS - Water Quality Engineering in Natural Systems - page 224

TABLE 6.7. Pollutant Concentration from Animal Operations

BOD 5 COD Total N Total P

(mg/L) (mg/L) (mg/L) (mg/L) Source

Runoff from feedlot 800-11,000 3000-30,000 100-2,100 10-500 Miner et al. (2000)

Runoff from grazed pasture - - 4.5 7 Robins (1985)

Untreated sewage 160 235 30 10 Novotny et al. (1989)

increases the erosion potential of croplands. With the
exception of arid lands, soil loss by erosion from fields
is at least an order of magnitude higher than back-
ground loads. Soil erosion is a major cause of diffuse
pollution in many agricultural areas, and sediment is
also the most visible pollutant. It has been reported that
of nutrient (N and P) losses from cropland, about 90%
is associated with soil loss. Losses of nutrients from
cropland represent a relatively small portion of applied
fertilizer; however, the concentrations in the runoff
almost always exceed the criteria for preventing accel-
erated eutrophication of receiving water bodies.
Soils have the capacity to retain many pollutants in
their particulate form, which are far less environmen-
tally damaging than in the dissolved form. This is par-
ticularly relevant to the soil retention of phosphates,
hydrophobic organic chemicals, ammonium, and metals.
The capacity of soils to retain and absorb pollutants
depends on its composition and redox status. The most
important component is the soil organic matter, fol-
lowed by ph, clay content, soil moisture, and cation-
exchange capacity (CEC). Typically, at some point, the
soil becomes saturated by the pollutant and larger quan-
tities are released in dissolved form into groundwater
and baseflow of surface waters. The first indication that
the soil retention capacity has been exhausted for some
pollutants is the dramatic increase in nitrate pollution
in ground and surface waters. Well-aerated agricultural
soils have a lower retention capacity for nitrogen that is
readily nitrified into mobile nitrate forms. For other
pollutants, as long as the soil retention capacity is not
exhausted, the result is a net accumulation of pollutants
in the soil. Typically, most of the phosphorus applied is
retained in the soil, and saturation is typically reached
within a few years of excess phosphorus application.
Pollution from animal operations can be divided into
that from pastures and that from concentrated animal
feeding operations (feedlots). It is interesting to note
that phosphorus production by one dairy cow or heifer
is on the order of 18 kg/yr (40 lb/yr), and of that amount,
a significant portion may reach the receiving water body,
depending on the proximity of the farm to the water-
course and on the degree of pollutant attenuation
during overland flow. The phosphorus load by one cow

is equivalent to 18-20 humans. The BOD concentration
of barnyard runoff exceeds that of sewage by two orders
of magnitude, and therefore barnyard runoff can cause
significant oxygen depletion in receiving waters. Typical
concentrations of BOD 5 , COD, total nitrogen, and TP
in feedlot runoff, runoff from grazed pasture, and
untreated sewage are shown in Table 6.7. Barnyard
runoff also carries pathogenic microbes, such as the pro-
tozoan Cryptosporidium , and these microbes can trans-
mit diseases from livestock to humans, including such
diseases as salmonellosis, staphylococcus, tetanus, foot-
and-mouth disease, mad cow disease, and tuberculosis.
The majority of feedlot wastes reaching surface waters
are transported by surface runoff. Biosolids disposal
from domestic (urban) wastewater treatment plants on
agricultural land can also be a significant source of pol-
lution if these biosolids are not treated to reduce the
levels of pathogenic microorganisms. Pathogenic micro-
organisms are typically partitioned between aqueous
and (suspended) solid phases in surface runoff, and
areas with clay soils are more likely than sandy soils to
contribute pathogens to surface waters. Enteric viruses
are often concentrated in sewage sludge because they
have a tendency to combine with solids, and alkaline
conditions resulting from the addition of lime (CaO) to
biosolids is effective in inactivating viruses.
In pastures , animals roam and feed on natural vegeta-
tion; rangeland typically refers to large-scale but low-
density animal operations. Pastureland is a significant
source of diffuse pollution when proper erosion control
practices are not in place or when grazing livestock are
allowed to approach or enter surface waters. Overgraz-
ing and allowing livestock to approach and enter water-
courses are the major pollution activities on pastureland
and rangeland. If these activities are controlled, pollu-
tion from such land can be minimal.
Undisturbed forests or woodland represent the best
protection of lands from sediment and pollution losses.
Woodlands and forests have high resistance to surface
runoff due to ground mulch and terrain roughness.
Even lowland forests with high water tables absorb
large amounts of precipitation and actively retain water
and contaminants. Uncontrolled logging operations
(clearcutting) disturb the forest resistance to erosion,