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CHAPTER 1. PHOSPHORUS IN RUNOFF AND STREAMS
J. C. RYDEN, J. K. SYERS, AND R. F. HARRIS
the solid phases with which it is in contact in runoff and streams, as
pointed out by Taylor ( 1967) and Biggar and Corey (1969).
Critical concentration limits have been suggested for P in surface waters
which, if exceeded, will lead to excessive biological productivity (Sawyer,
1947; Mackenthun, 1968). In this review, however, rather than emphasizing critical concentrations, P in runoff and streams will be discussed
mainly from the standpoint that any P load constitutes a potential increase
in the P fertility of surface waters.
This review will use essentially the definitions proposed by Langbein
and Iseri ( 1960).
Watershed (drainage basin; catchment area). A part of the surface of
the earth that is occupied by a drainage system, which consists of a surface
stream, or a body of standing (impounded) surface water, together with
all tributary surface streams and bodies of standing surface water.
Stream. A general term for a body of flowing water. In hydrology the
term is usually applied to the water flowing in a natural channel.
Stream flow. The discharge (of water) that occurs in a natural channel.
Runoff.That part of precipitation that falls on land and ultimately appears in surface streams and lakes. Runoff may be classified further according to its source.
Surface runoff (overland flow). That part of rainwater or snowmelt
which flows over the land surface to stream channels. Surface runoff may
also enter standing waters directly or be consolidated into artificial channels, e.g., storm sewers in urban areas (urban runoff), before entering a
stream or body of standing water.
Subsurface runoff (storm seepage). That part of precipitation which infiltrates the surface soil and moves toward streams as ephemeral, shallow,
perched groundwater above the main groundwater level. In many agricultural areas subsurface runoff may be intercepted by artificial drainage systems, e.g., tile drains, accelerating its movement to streams.
Groundwater run08 (base runoff). That part of precipitation that has
passed into the ground, has become ground water, and is subsequently
discharged into a stream channel or lake as spring or seepage water.
In addition to runoff, the other potential contributors to streams and
standing waters are precipitation incident on the water surface and industrial and sewage effluents (Fig. 1 ) .
PHOSPHORUS IN RUNOFF AND STREAMS
McCarty (1967) and Vollenweider ( 1968) have made a useful division
of sources of P to surface waters based on the ease of quantification.
Point sources enter at discrete and identifiable locations and are therefore amenable to direct quantification and measurement of their impact
on the receiving water. Major point sources include effluents from indus-
FIG. 1. Schematic representation of the relationships between phosphorus sources
and runoff, streams, and standing waters.
trial and sewage-treatment plants (Fig. 1) . Diffuse .wurces may be defined
as those which at present can be only partially estimated on a quantitative
basis and which are probably amenable only to attenuation rather than
to elimination. Diffuse sources require the most investigative attention.
Vollenweider ( 1968) further divided diffuse sources into:
1. Natural sources such as eolian loading, and eroded material from
virgin lands, mountains and forests.
2. Artificial or semiartificial sources which are directly related to human
activities, such as fertilizers, eroded soil materials from agricultural and
urban areas, and wastes from intensive animal rearing operations.
The loads of P imparted to runoff and streams from natural diffuse
sources provide a datum line against which the magnitude of P loads from
artificial sources may be compared.
J. C. RYDEN, J. K.
R. F. HARRIS
In natural systems, P occurs as the orthophosphate anion (Pod3-)which
may exist in a purely inorganic form (H2P0,- and HP0,2-) or be incorporated into an organic species (organic P ) . Under certain circumstances
inorganic orthophosphate may exist as a poly- or condensed phosphate.
A secondary distinction is made between particulate and dissolved forms
of P, the split conventionally being made at 0.45 pm.
Other terminology used is as follows:
Total P . All forms of P in a runoff or stream sample (dissolved and
particulates in suspension) as measured by an acid-oxidation treatment
(e.g., acid ammonium persulfate).
Dissolved inorganic P . P in the filtrate after 0.45 pm separation determined by an analytical procedure for inorganic orthophosphate.
Organic P . P that may be determined within the dissolved and particulate fractions by the difference between total P and inorganic P.
Factors Affecting the Dynamics of Phosphorus in Runoff and Streams
Before evaluating the magnitude of various P sources in terms of the
loads of P in runoff and streams, and the extent to which previous studies
of P loadings enable an adequate definition of P sources, it is important
to understand the physical and chemical factors affecting the dynamics of
P in runoff and streams. These factors determine not only the movement
of P into runoff and streams, but also its distribution between the aqueous
and particulate phases.
All terrestrially derived diffuse sources of P are associated with the
movement of water in contact with a solid phase. The solid phase may
be stationary with respect to water flow, or may move in the flow at some
speed equal to or less than the flow. Precipitation disposed of as subsurface
or groundwater runoff is primarily in contact with a stationary solid phase,
namely the soil profile and, in the case of groundwater runoff, possibly
the bedrock. Consequently, the amounts and concentrations of P carried
in subsurface and groundwater runoff will be influenced by the time of
contact with any component in the soil profile capable of interacting with
dissolved P in the percolating water and by the concentration of dissolved
P that the soil components maintain in the soil solution. Time of contact
between the percolating solution and any soil component will in turn depend on the rates of infiltration and percolation into and through the soil.
PHOSPHORUS IN RUNOFF AND STREAMS
Some of the theories developed to describe water movcment in soils can
be applied to evaluate the potential loss of P from various soil types as
a result of subsurface runoff. Gardner (1965) developed equations to describe the movement of nitrate in the soil profile due to leaching. The
chemical interactions that occur between dissolved inorganic P and soil
components (discussed later), when water percolates through the soil,
must also be taken into consideration. Inclusion of a term in the equations
developed by Gardner (1965) to describe the relationship between P in
particulate and aqueous phases is therefore necessary. This could take the
form of a linear adsorption isotherm relevant to the concentrations of dissolved inorganic P maintained in the solution of a particular soil. Biggar
and Corey (1969) have also reviewed the literature on infiltration and
percolation of water in agricultural soils as it pertains to nutrient
The movement of solid phase material in contact with natural waters
occurs during surface runoff and in streams. The amounts of solid material
capable of entering surface runoff will depend on the intensity of rainfall,
physical and chemical attachment between various solid components, and
the amounts and energy of runoff waters (Guy, 1970). It is the energy
of surface runoff or stream water, however, that governs the amounts of
a specific size fraction of particulate materials which will remain in suspension during water flow.
The primary source of particulate material to surface runoff and streams
is eroding soil (Guy and Ferguson, 1970), although in urban areas with
little ongoing development, particulates may be dominated by specifically
urban detrital material (e.g., street litter and dust) and organics derived
from urban vegetation.
The total on-site losses of soil due to sheet and rill erosion are not necessarily delivered to streams. The amount of sediment that travels from a
point of erosion to another point in the watershed is termed the sediment
yield (Johnson and Moldenhauer, 1970). Consequently the Universal Soil
Loss Equation used to predict field soil losses on an average annual basis
(Wischmeier and Smith, 1965) must be corrected when used to predict
sediment loads in streams because deposition of particulates may occur
on the land surface as a result of slope variations before surface runoff
reaches a stream. It is for this reason that estimates of soil loss in surface
runoff from sites within a particular watershed cannot be translated into
total P losses through a knowledge of the total P content of the soil, if
the P loss is to be related to P enrichment of surface waters.
An associated complication arises from the fact that soil P is primarily
associated with the solid phase. As soil erosion is a selective process with
respect to particle size, selectivity has been observed for P loss in surface