Nonpoint Source Water Pollution 235water-soluble pesticides, and nitrogen-based plant nutrients (ammonia, nitrite, and
nitrate) are a function of the contact time between the pollutant and water. Longer
contact times allow more pollutants to be dissolved in the water. SolubL pollutants are
often more concentrated in groundwater than surface runoff, particularly in agricultural
regions where pesticide and nitrogen concentrations in local wells may exceed safe
levels for drinking water. Insoluble pollutants include suspended sediments, along with
most metals, microbial pathogens, most forms of phosphorus, and many pesticides and
organics that are insoluble in water or are physically or chemically bound to sediment
particles.
SEDIMENT EROSION AND THE POLLUTANTTRANSPORT PROCESSSediment erosion is the result of a complex interaction between the timing, intensity,
and duration of precipitation and the structure of the landscape. Because many nonpoint
source pollutants are transported with suspended sediments, the factors that contribute
to soil erosion also contribute to nonpoint source pollution.
Soil erosion can be separated into four categories: rainsplash, sheetwash, gullying,
and sfreandchannel erosion (Table 11-2). Rainsplash erosion results when rain strikes
the ground with enough force to splatter and dislodge soil particles. The energy in a
raindrop is determined by droplet size, velocity of fall, and the intensity characteristics
of the particular storm. Soil stability (size, shape, composition, and strength of soil
aggregates and soil clods) and slope are also important in determining how readily
the raindrop dislodges soil particles. Vegetation cover greatly reduces or eliminates
rainsplash erosion by preventing raindrops from landing on exposed soil surfaces and
by reducing evaporation, which keeps the soil moist and makes the soil particles less
susceptible to detachment.
Sheetwash erosion can occur when the rate of precipitation exceeds the rate at
which water can infiltrate into the soil. Under these conditions water accumulates at
the soil surface and forms irregular sheets that flow downhill, carrying sediment along
with the surface runoff. The intensity and duration of precipitation are important factors
Table 11-2. Types of Erosion That May Contribute to Nonpoint Source Pollution
Type of erosion Description
Rainsplash
Sheetwash
Gullying
Stredchannel
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From direct impact of falling raindrops; detached soil and
chemicals can be transported downhill.
From raindrop splash and runoff in wide, thin layers of
surface water; wall of water tsansports pollutants downhill.
From concentrated rivulets of water cutting 5- to 10-cm rills
in soil; rills grow into gullys.
From confluence of rills and gullys; increased volumes and
velocities cause erosion along streams and river beds.
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