Phosphorus

Phosphorus is the eleventh-most abundant mineral in the earth's crust and does not exist in a gaseous state. Natural inorganic phosphorus deposits occur primarily as phosphate in the mineral apatite. Apatite is defined as a natural, variously colored calcium fluoride phosphate (Ca5F(PO4)3) with chlorine, hydroxyl, and carbonate sometimes replacing the fluoride. Apatite is found in igneous and metamorphic rocks, and sedimentary rocks. When released into the environment, phosphate will speciate as orthophosphate according to the pH of the surrounding soil.

Phosphate is usually not readily available for uptake in soils. Phosphate is only freely soluble in acid solutions and under reducing conditions. In the soil it is rapidly immobilized as calcium or iron phosphates. Most of the phosphorus in soils is adsorbed to soil particles or incorporated into organic matter (Smith, 1990; Craig et al., 1988; Holtan et al., 1988).

Phosphorus in freshwater and marine systems exists in either a particulate phase or a dissolved phase. Particulate matter includes living and dead plankton, precipitates of phosphorus, phosphorus adsorbed to particulates, and amorphous phosphorus. The dissolved phase includes inorganic phosphorus (generally in the soluble orthophosphate form), organic phosphorus excreted by organisms, and macromolecular colloidal phosphorus.

The organic and inorganic particulate and soluble forms of phosphorus undergo continuous transformations. The dissolved phosphorus (usually as orthophosphate) is assimilated by phytoplankton and altered to organic phosphorus. The phytoplankton are then ingested by detritivores or zooplankton. Over half of the organic phosphorus taken up by zooplankton is excreted as inorganic P. Continuing the cycle, the inorganic P is rapidly assimilated by phytoplankton (Smith, 1990; Holtan et al., 1988).

Lakes and reservoir sediments serve as phosphorus sinks. Phosphorus-containing particles settle to the substrate and are rapidly covered by sediment. Continuous accumulation of sediment will leave some phosphorus too deep within the substrate to be reintroduced to the water column. Thus, some phosphorus is removed permanently from biocirculation (Smith, 1990; Holtan et al., 1988).

A portion of the phosphorus in the substrate may be reintroduced to the water column. Phosphorus stored in the uppermost layers of the bottom sediments of lakes and reservoirs is subject to bioturbation by benthic invertebrates and chemical transformations by water chemistry changes. For example, the reducing conditions of a hypolimnion often experienced during the summer months may stimulate the release of phosphorus from the benthos. Recycling of phosphorus often stimulates blooms of phytoplankton. Because of this phenomenon, a reduction in phosphorus loading may not be effective in reducing algal blooms for a number of years (Maki et al., 1983).

Criteria for phosphorus:

The EPA water quality criteria state that phosphates should not exceed .05 mg/l if streams discharge into lakes or reservoirs, .025 mg/l within a lake or reservoir, and .1 mg/l in streams or flowing waters not discharging into lakes or reservoirs to control algal growth (USEPA, 1986). Surface waters that are maintained at .01 to .03 mg/l of total phosphorus tend to remain uncontaminated by algal blooms.

Numerical Categories:

Designated Use                                              Limit

Freshwater
Aesthetics
Federal criteria
	streams/rivers:	.1 mg/l
	streams entering lakes:	.05 mg/l
	lakes/reservoirs:	.025 mg/l
(USEPA, 1986)
example State criteria used:
Reservoirs (CO)	chlorophyll a	15 ug/l
	Total P	.035 mg/l
(Minn.)	Total P	.015 mg/l
Impoundments (EPA Region 4)
water supply	Total P	.015 mg/l
aquatic life	Total P	.025 mg/l
Lakes (NC)	chlorophyll a	40 ug/l
	Total P	.05 mg/l
mountain lakes	.02 mg/l
(VT)	Total P	.014 mg/l
(USEPA, 1994d)
Estuaries (recommended)
Aquatic life support		0.1 ug/l elemental phosphorus (USEPA, 1994d)
maximum diversity		0.01* total phosphorus (and nitrogen < 0.1) mg/l
moderate diversity		0.1*   (and nitrogen < 1.0)  mg/l

*These figures are recommended; eutrophication is also dependent on freshwater influx, nutrient cycling, dilution, and flushing of a pollutant load in a particular estuary. (NOAA/EPA, 1988)

Health Effects:

1. Phosphate: Phosphate itself does not have notable adverse health effects. However, phosphate levels greater than 1.0 may interfere with coagulation in water treatment plants. As a result, organic particles that harbor microorganisms may not be completely removed before distribution.
   (See http://www.chemsoc.org/exemplarchem/entries/2001/duncan/page-3.htm)

Environmental Effects:

The growth of macrophytes and phytoplankton is stimulated principally by nutrients such as phosphorus and nitrogen. Nutrient-stimulated primary production is of most concern in lakes and estuaries, because primary production in flowing water is thought to be controlled by physical factors, such as light penetration, timing of flow, and type of substrate available, instead of by nutrients (McCabe et al., 1985).

• Freshwater system impacts: Generally, phosphorus (as orthophosphate) is the limiting nutrient in freshwater aquatic systems. That is, if all phosphorus is used, plant growth will cease, no matter how much nitrogen is available. The natural background levels of total phosphorus are generally less than 0.03 mg/l. The natural levels of orthophosphate usually range from 0.005 to 0.05 mg/l (Dunne and Leopold, 1978).

  Many bodies of freshwater are currently experiencing influxes of phosphorus and nitrogen from outside sources. The increasing concentration of available phosphorus allows plants to assimilate more nitrogen before the phosphorus is depleted. Thus, if sufficient phosphorus is available, elevated concentrations of nitrates will lead to algal blooms. Although levels of 0.08 to 0.10 mg/l orthophosphate may trigger periodic blooms, long-term eutrophication will usually be prevented if total phosphorus levels and orthophosphate levels are below 0.5 mg/l and 0.05 mg/l, respectively (Dunne and Leopold, 1978).

  

• Estuarine system impacts: In contrast to freshwater, nitrogen is generally the primary limiting nutrient in the seaward portions of estuarine systems (Paerl, 1993). Here, nitrogen levels control the rate of primary production. If the system is supplied with high levels of nitrogen, algal blooms will occur. Systems may be phosphorus limited, however, or become so when nitrogen concentrations are high and N:P>16:1 (Jaworski, 1981). In such cases, excess phosphorus will trigger eutrophic conditions. The recommended level of total phosphorus in estuaries and coastal ecosystems to avoid algal blooms is 0.01 to .1 mg/l and 0.1 to 1 mg/l of nitrogen (a 10:1 ratio of N:P). The higher concentrations support less diversity (NOAA/EPA, 1988).

• Freshwater and estuarine systems: Nutrient-induced production of aquatic plants in both freshwater and estuaries has several detrimental consequences:
  1. Algal mats, decaying algal clumps, odors and discoloration of the water will interfere with recreational and aesthetic water uses.
  2. Extensive growth of rooted aquatic macrophytes will interfere with navigation, aeration, and channel capacity.
  3. Dead macrophytes and phytoplankton settle to the bottom of a water body, stimulating microbial breakdown processes that require oxygen. Eventually, oxygen will be depleted.
  4. Aquatic life uses may be hampered when the entire water body experiences daily fluctuations in dissolved oxygen levels as a result of plant respiration at night. Extreme oxygen depletion can lead to death of desirable fish species.
     
  5. Siliceous diatoms and filamentous algae may clog water treatment plant filters and result in reduced time between backwashing (process of reversing water flow through the water filter in order to remove debris).
  6. Toxic algae (occurrence of "red tide") have been associated with eutrophication in coastal regions and may result in paralytic shellfish poisoning (Mueller et al., 1987).
     
  7. Algal blooms shade submersed aquatic vegetation, reducing or eliminating photosynthesis and productivity (Dennison et al., 1993; Batiuk et al., 1992)

Sources:

1. Nonpoint sources:
   • Natural: Phosphate deposits and phosphate-rich rocks release phosphorus during weathering, erosion, and leaching (Smith, 1990). Phosphorus may be released from lake and reservoir bottom sediments during seasonal overturns.
     

   • Anthropogenic: The primary anthropogenic nonpoint sources of phosphorus include runoff from 1) land areas being mined for phosphate deposits, 2) agricultural areas, and 3) urban/residential areas. Because phosphorus has a strong affinity for soil, little dissolved phosphorus will be transported in runoff. Instead, the eroded sediments from mining and agricultural areas carry the adsorbed phosphorus to the water body. An additional source is the overboard discharge of phosphorus-containing sewage by boats.
     

2. Point sources: Sewage treatment plants provide most of the available phosphorus to surface water bodies. A normal adult excretes 1.3 - 1.5 g of phosphorus per day. Additional phosphorus originates from the use of industrial products, such as toothpaste, detergents, pharmaceuticals, and food-treating compounds. Primary treatment removes only 10% of the phosphorus in the waste stream; secondary treatment removes only 30%. The remainder is discharged to the water body (Smith, 1990). Tertiary treatment is required to remove additional phosphorus from the water. The amount of additional phosphorus that can be removed varies with the success of the treatment technologies used. Available technologies include biologicall removal and chemical precipitation (Tchobanoglous 1991).
   

Analytical techniques:

A. Total Phosphorus and Orthophosphate: Analysis involves two procedural steps: 1) conversion of the phosphorus form into dissolved orthophosphate by a digestion method, and 2) colorimetric evaluation of the dissolved orthophosphate concentration. (APHA, 1992)

Step 1: Digestion methods

1. Perchloric Acid Digestion: Recommended only for extremely difficult-to-analyze samples, such as sediments.

2. Nitric Acid-Sulfuric Acid Method Recommended for most samples.

3. Persulfate Oxidation Method This simple method should be cross-checked with one or more thorough techniques and adopted if results are identical.

1. Ascorbic Acid Method: Ammonium molybdate and potassium antimonyl tartrate react with orthophosphate to form a heteropoly acid that is reduced to molybdenum blue by ascorbic acid. See also The Ascorbic Acid Method at a Glance
   • Detection limits: Ranges change with light path used.
      

     Range (mg/l P)	Path (cm)
     0.3 - 2.0	0.5
     0.15 - 1.3	1.0
     0.01 - 0.25	5.0

   • Interferences: Arsenates react with the molybdate to form a similar blue color. Nitrite and hexavalent chromium interfere to yield results 3% less than actual at 1 mg/l and 10% to 15% less than actual at 10 mg/l.

2. Automated Ascorbic Acid Reduction Method: Ammonium molybdate and potassium antimonyl tartrate react with orthophosphate in an acid medium to form an antimony- phosphomolybdate complex that forms a blue color suitable for photometric measurements when reduced by ascorbic acid.
   • Detection limits: 0.001 to 10.0 mg/l P when photometric measurements are performed at 650 to 600 in a 15mm tubular flow cell, or 880 nm in a 50mm tubular flow cell.

   • Interferences: >50 mg/l Fe(3+), 10 mg/l Cu, and 10 mg/l SiO2. Turbidity, color may interfere. Arsenate provides a positive interference.

3. Vanadomolybdophosphoric Acid Colorimetric Method: Ammomium molybdate reacts under acid conditions to form a heteropolyacid. In the presence of vanadium, yellow vanadomolybdophosphoric acid is formed, the intensity of which indicates the amount of orthophosphate present.
   • Detection limits: 1 to 20 mg/l P. This method is not good for water samples - best for soils.

   • Interferences: Silica and arsenate interfere in heated samples. Blue color is formed by ferrous iron, but does not interfere if iron concentration is < 100 mg/l.

4. Stannous Chloride Method: Molybdophosphoric acid is formed and reduced by stannous chloride, forming an intensely colored molybdenum blue.
   • Detection limits: 0.001 to 6 mg/l P

   • Interferences: Silica and arsenate interfere in heated samples. Blue color is formed by ferrous iron, but does not interfere if iron concentration is < 100 mg/l.