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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 ) .



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.





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.



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

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