Evaluation of the presence of Aminomethylphosphonic acid (AMPA) in surface water


Back to Index

Monday 01st November 2004

Author: M-A Reding, Monsanto Europe SA

1. Background

Aminomethylphosphonic acid commonly known as AMPA, the major degradate of glyphosate in the environment, is widely included in the surface water monitoring program for plant protection products, particularly because it can be determined by the same analytical procedure as glyphosate. The results show that AMPA is more frequently found in surface waters than glyphosate, and very often at higher levels.

This document explains the different origins of AMPA in the environment, provides the relevant environmental fate, toxicology and aquatic ecotoxicological data for AMPA, and evaluates its relevance as a metabolite of a plant protection product. It also provides a view on the attempt of some authors to quantify the origin of AMPA in surface waters.

 

2. Sources of AMPA in the environment

Aminomethylphosphonic acid is the simplest member of a broad class of chemicals containing an N-CH2-PO3H2 moiety that are known as aminomethylenephosphonates. Members of this class of chemicals include detergent additives and sequestering agents to prevent scaling in boilers and cooling water facilities, as well as the herbicide N-phosphonomethylglycine (glyphosate). Since AMPA is the primary degradate of glyphosate in the environment, and since glyphosate only reaches surface water through secondary mechanisms, such as run-off, drainage or spray drift, agricultural applications are considered as a diffuse source of AMPA. Detergent additives and sequestering agents are directly discharged into the environment via drains, so they are considered as point sources of AMPA.

Formation of AMPA from glyphosate

The fate of glyphosate in the terrestrial and aquatic environment has been extensively studied. Glyphosate is degraded over time by microorganisms in soil and water to carbon dioxide and other naturally occurring products. AMPA is the only metabolite observed in significant amounts during the degradation process. One molecule of AMPA is formed by the degradation of one molecule of glyphosate.

In laboratory soil degradation (1) and water/sediment (2) studies conducted according to current guidelines, AMPA reached a maximum levelof 42.4% of applied glyphosate in soil and 10.3 % of applied glyphosate in water. The figure of 42.4% is the highest found in any soil degradation study in the laboratory or the field, a more typical figure from these studies would be 25%.

Degradation of other aminomethylenephosphonates to AMPA

Four other aminomethylenephosphonates widely used in Europe are possible sources of AMPA in the environment . The table below lists their common names, their abbreviations, their respective main use areas in Europe (3), and the number of AMPA molecules that can potentially be formed from one molecule of each compound.

Name of compound

Abbreviation

Main use

(in Europe)

N° of AMPA molecules

Amino tris(methylenephosphonate)

ATMP

Industrial boilers/cooling

1

Ethylenediamine tetra(methylenephosphonate)

EDTMP

Laundry detergents

2

Hexamethylenediamine tetra(methylenephosphonate)

HDTMP

Industrial boilers/cooling

2

Diethylenetriamine penta(methylenephosphonate)

DTPMP

Detergents

3

As the European legislation for industrial chemicals is currently much less demanding than for pesticides, there is typically only very limited information available, if any, on the environmental fate of such products. However, evidence for the formation of AMPA from ATMP was found in a study conducted on a mineral test medium and on river water (4). In this study, the degradation to AMPA reached a maximum of 30-33% of the initial ATMP in non-sterile mineral medium and approximately 20% of the starting ATMP in the river water. The degradation mechanism of EDTMP to AMPA was found to be similar to that of ATMP but the degradation of DTPMP and HDTMP to AMPA has not been experimentally established. Nonetheless, it is reasonable to assume that these compounds will form AMPA as well, based on their structural similarities to ATMP and EDTMP.

 

3. Environmental fate of AMPA

Table 1 : Summary of environmental fate of AMPA

Type of study

Endpoint

Degradation in soil

Rate of degradation – laboratory (1)

Rate of degradation – field (5)

Mean DT50 = 120 days

Mean DT50 = 150 days

Mobility in soil

Adsorption/desorption study (5)

Koc = 1200 - 25000

Dissipation in water

Water sediment study (6)

DT50 water = 2-5 days

DT50 total system = 19-45 days

AMPA degrades rather slowly in soil. Degradation is primarily microbial in both soil and water sediment. It is tightly bound to soil and has thus a very low potential to leach to deeper soil layers. It dissipates very rapidly from surface water.

 

4. Aquatic ecotoxicology

Table 2 : Summary of aquatic toxicity of AMPA

Test organism

Exposure period

Endpoint

Result (mg/L)

Reference

Rainbow trout

96 h

LC50

NOEC

520

32

(7)

Daphnia

48 h

EC50

NOEC

690

320

(8)

Algae
(Scenedesmus)

72 h

ErC50*

NOErC

452

0.96**

(9)

* The EC50 based on the growth rate is used, according to the recommendation of OECD

** As noted in the Monograph

The Water Framework Directive (2000/60/EC) provides a procedure for setting environmental quality standards (EQS) for the protection of aquatic biota and against which the monitoring data should be assessed. Using this procedure and applying a safety factor of 1000 (given in Annex V, section 1.2.6.), the EQS (annual average) for AMPA in surface water would be 450 µg/L. This figure is in line with the rounded guide value for surface water proposed by the Swedish authorities of 500 µg/L for AMPA.

 

5. The relevance of AMPA as a metabolite of glyphosate

Document SANCO/221/200 – rev.10 of 25/02/2003 provides guidance for the assessment of the relevance of metabolites (of plant protection products) in groundwater. It outlines a procedure, a stepwise scheme, to identify 'relevant metabolites' for which the 0.1 μg/L limit of the Drinking Water directive should apply. Although AMPA is unlikely to be found in groundwater, the compound has been assessed following the outlined procedure, as the general approach may be applicable for regional management of surface water resources intended for the abstraction of drinking water.

Biological relevance

Active substances of plant protection products are defined on the basis of their biological activity against plants or harmful organisms. The same criterion is used in the 'relevant metabolite guidance document' to identify those breakdown products which, from a regulatory perspective, should be treated in the same way as the active substance.

The herbicidal activity of AMPA has been tested at 5.6 kg/ha, a rate well above the amount at which this metabolite might be found in the environment following normal agricultural practice and no visual injury was observed in any of the eleven weed species tested.. This rate is very excessive, as it even exceeds the maximum recommended application rate for glyphosate of 4.32 kg/ha.

Thus, AMPA has no herbicidal activity at the levels at which it is expected to be found in the environment.

Toxicological relevance

Glyphosate and AMPA have very similar chemical structures. Studies of the metabolism of glyphosate in laboratory animals indicate that glyphosate is not biotransformed into AMPA. Studies of the metabolism of AMPA in laboratory animals indicate that AMPA is only moderately absorbed from the gastrointestinal tract, and is rapidly excreted un-metabolized in the urine. In the 1997 toxicological evaluation of AMPA (http://www.inchem.org/documents/jmpr/jmpmono/v097pr04.htm) the JMPR concluded that the results of the toxicology studies with AMPA showed little toxicity and that AMPA was considered to be of no greater toxicological concern than glyphosate. The more recent review for re-registration in Europe came to the same conclusion and glyphosate is not considered to be a carcinogen, mutagen, teratogen , a reproductive or neuro-toxicant. (http://europa.eu.int/comm/food/fs/sfp/ph_ps/pro/eva/existing/list1_glyphosate_en.pdf)

Screening for genotoxicity

The mutagenicity potential of AMPA has been tested in four separate assays (Table 1). These included the Ames mutagenicity test and recombination assay (10), an unscheduled DNA synthesis test (11) and the mouse micronucleus assay (12). All these results support the weight of evidence conclusion that AMPA is non-genotoxic.

Table 1. Genotoxicity Tests with AMPA

 

TEST SYSTEM

TEST SPECIES

AMPA CONCENTRATION

RESULTS

Ames (with and without activation)

S. typhimurium TA 1535, TA 1537, TA 1538, TA 98 and TA 100;

E. coli WP2 hcr

10 to 5000 µg/plate

Negative

Recombination Assay

B. subtilis H17 and M45 strains

20 to 2000 µg/filter

Negative

Mouse Micronucleus

CD-1 male and female Mice

100 to 1000 mg/kg body weight i.p. single dose

Negative

Unscheduled DNA synthesis

Isolated rat hepatocytes

5 to 5000 µg/ml

Negative

Screening for toxicity

Under the scheme, this step has only to be performed if the parent compound is classified as acutely or chronically toxic or very toxic, and based on the toxicological profile of glyphosate, this does not apply to AMPA. However, both a 90-day subchronic rat (13) and dog (14) study are available and show that the subchronic toxicity of AMPA is low, like that of the parent molecule, glyphosate.

The potential for AMPA to produce developmental toxicity in rats has also been investigated. The maternal and developmental NOEL was 400 mg/kg/day (15). There was no evidence of birth defects at any dose level

Conclusion

AMPA

  • is unlikely to leach to groundwater
  • has no herbicidal activity at the level it is expected to be found in surface water
  • has low toxicity to aquatic life
  • is not genotoxic
  • has low subchronic toxicity
  • is not teratogenic

The toxicology data on AMPA indicating low toxicity and the close structural similarity to the parent, glyphosate, which has also been shown to have very low toxicity in a vast number of studies, show that AMPA is not of toxicological concern.

AMPA can be considered as a non-relevant metabolite with respect to health risk for mammals and humans. The limit of 0.1 µg/L does thus not apply to this compound. The same conclusion was reached by of the Dutch authorities when reviewing the data package available for AMPA (15).

 

6. Assessment of the origin of AMPA in surface water

Some authors have attempted to quantify the origin of AMPA found in surface water (16) (17). As a metabolite of a plant protection product, the compound is assessed according to more stringent legislation than as a metabolite of an industrial chemical for which the limit values are generally much higher.

Pesticides are the most thoroughly tested chemicals in terms of environmental fate and impact. Numerous studies have to be performed according to agreed guidelines and scientific data must be available to assess their presence and behaviour in the environment before any product can be sold.

Other chemicals, such as detergents, are not submitted to the same scrutiny; the available data to assess their environmental fate is far less extensive and the studies are conducted according to a design selected by the researcher, and not to a standard set of guidelines for methodology with the additional requirement for compliance with the Principlesof Good Laboratory Practice.

The difference in the degree of rigour to which the studies are conducted make it virtually impossible to objectively compare the rate and route of degradation of glyphosate with the other industrial precursors of AMPA for which the existing environmental data differ so greatly in quantity and quality.

  • In order to assign proportionate amounts of AMPA in surface water to their respective sources, it is important to know the concentration of the parent compounds in the same surface water over time. A large set of surface water monitoring data is available for glyphosate. For other aminomethylenephosphonates no monitoring data are available; it is thus not possible to conduct a reality check on the calculated concentrations for those compounds.
  • The metabolism of glyphosate has been extensively studied in water/sediment systems according to OECD and EU standard guidelines (120 days study). The rate and route of degradation of glyphosate in water and sediment is well understood. For other phosphonates, one single study (with one type of aminophosphonate) has been conducted on river water over a period of 96 hrs and is generally used to define the degradation rate of those other aminomethylenephosphonates and the subsequent maximum formation of AMPA.

Therefore, attempting to quantify the different origins of AMPA, besides being a scientifically debatable objective, is a near impossible task with the current state of knowledge of the environmental concentration and fate of aminomethylenephosphonates other than glyphosate.

 

7. Conclusion

AMPA is of no particular concern under the Water Framework Directive or the pesticide legislation

  • There are several sources of AMPA: glyphosate is a diffuse source; other aminomethylenephosphonates are point sources of AMPA in surface waters.
  • Environmental fate studies demonstrate that AMPA is tightly adsorbed to soil and has thus a low potential to leach. It dissipates rapidly from surface water.
  • If assessed according to the procedure given in the Water Framework Directive, the Environmental Quality Standard would be 450 µg/L. If the annual average concentrations in surface water, derived from monitoring data, are below this value then no ecologically significant effects are predicted to occur.,
  • AMPA is a non-relevant metabolite of glyphosate, is not of toxicological concern, and therefore the 0.1 µg/L limit for its presence in drinking water does not apply.

 

8. References

  1. Galicia H., Flückiger, J. (1993) Degradation of 14C Glyphosate in three soils incubated under aerobic conditions. Cheminova Report n° RCC 271618. (DT50 for AMPA were calculated from the AMPA soil residue data of this report).
  2. Heintze A. (1996) Degradation and metabolism of glyphosate in two water/sediment systems under aerobic conditions. Monsanto unpublished report MLL 31186.
  3. Gledhill W.E., Feijtel T.C.J. (1992) Environmental properties and safety assessment of organic phosphonates used for detergent and water treatment applications. The Handbook of Environmental Chemistry, Vol 3, Part F, Springer Verlag, 261-285.
  4. Streber J., Wierich P. (1987) Properties of aminotris(methylenephosphonate) affecting its environmental fate: degradability, sludge adsorption, mobility in soils and bioconcentation. Chemosphere, 16, 1323-1337.
  5. Endpoints list of the glyphosate review report : http://europa.eu.int/comm/food/plant/protection/evaluation/existactive/list1_glyphosate_en.pdf
  6. Knoch E., Spirlet M. (1999) Aminomethylphosphonic acid : Water/sediment metabolism. Monsanto unpublished report MSL-19217.
  7. Bowman J.H. (1991) Acute toxicity of AMPA to rainbow trout. Monsanto unpublished report number AB-90-402.
  8. Burges (1991) Acute toxicity of AMPA to Daphnia magna. Monsanto unpublished report number AB-90-401.
  9. Dengler D., Mende P. (1994) Testing of toxic effects of AMPA on the single cell green algae S. subspicatus. Monsanto study number XX-93-271; Monsanto unpublished report MSL-10854.
  10. Shirasu Y. (1980) CP50435: Microbial Mutagenicity Study. Monsanto report number ET-80-402.
  11. Bakke J.P. (1991) Evolution of the potential of AMPA to induce unscheduled DNA synthesis in the in vitro hepatocyte DNA repair assay using the male F-344 rat. Monsanto unpublished report number SR-91-234.
  12. Kier L.D., Stegeman S.D. (1993) Mouse micronucleus study of AMPA. Monsanto study number ML-90-404; Monsanto unpublished report MSL-13243.
  13. Estes F.L. (1979) 90-Day subacute rat toxicity study with CP50435. Monsanto unpublished report number IRD-78-174.
  14. Tompkins E.C. (1991) 90-Day oral (capsule) toxicity study in dogs with AMPA. Monsanto unpublished report number WI-90-354.
  15. Holson, J. F. (1991) A Developmental Toxicity Study of AMPA in Rats. Monsanto unpublished report number WI-90-266.
  16. Information published on the CTB website : http://www.ctb.wageningen.nl/ in the databank under Roundup Dry, registration number 11229N.
  17. Staats N., Faasen R., Kalf D.B. (2002) AMPA; inventarisatie van bronnen in Nederlands oppervlaktewater – RIZA/IVAM B.V. Report ISBN 90 3695 4282
  18. Kalf D.F., Berbee R.P.M. (2002) Bronnen van AMPA op rij gezet. RIZA document 02.162x

Related Attachments


Back to Index