Agrochemical Usage and Water Pollution

Return to: Main page / Water Resources / Environmental Stresses

Introduction

Fertiliser Usage / Pesticide Usage / Nitrates In The Water / Fate Of Nitrate Fertilisers In The Soil / Herbicides In The Water /

Bibliography

Introduction

The high yields of modern agriculture in Mauritius is only possible with considerable inputs of fertilisers, insecticides and pesticides. This intensive use of agro chemicals has attracted criticisms from various quarters such as local environmental groups and the press.

The aim of this section is to try to clarify this somehow confused situation.

The area under cultivation is of the order of 90,100 Ha and that represents 48% of the size of the island. It is indeed a very high proportion. Of that land surface, 93% is under sugar cane cultivation. The remainder is under tea, tobacco, fruit and vegetable cultivation. It is clear that sugar cane cultivation virtually determines the use of agro-chemicals on the island.

Fertiliser Usage

On average, the island uses up about 57,500 tonnes of chemical fertilisers per year. A government study in 1988 estimated that the average annual level of fertiliser application was very high, of the order of 600 kg per hectare which is 3 times that of Western Europe and 60 times that of Africa.

With such high rates of application, it is understandable that some concern is expressed about fertiliser usage in Mauritius.

Pesticide Usage

Over the 1977-1986 period, the State of The Environment report of 1991 indicated that on average the country imported 1100 tonnes of pesticides annually. Herbicides represent 59% of the total, insecticides 31%, fungicides 10%. This translates, on the field into rates of application of between 1.0 to 50 kg ( active ingredient ) per hectare per year.

Impact of Agrochemical Usage on the Environment and Public Health.

Nitrates In The Water...

The State Of The Environment report of 1991 clearly indicates ( P 186 ) that the Central Water Authority had analysed groundwater and had found Nitrates levels approaching 45 mg / litre which is the maximum internationally acceptable in potable water. (Unfortunately no detailed data as to when and where such figures were recorded is offered.) The same report on P 173 tend to suggest that the impact of high nitrogen usage could already be visible but does not elaborate sufficiently to substanciate that view.

In The Geology And Water Resources Of Mauritius (Ch 22 P 574), a couple of graphs are presented which show the concentrations of nitrates in groundwater measured over the 1980-1988 period. In the north, values measured from bore holes No 1 (Choisy) and 123 (Poudre D'Or) were systematically above the 10 mg per litre limit and very often around 20 mg per litre with occasional peaks exceeding 40 mg per litre. At bore holes No 117 ( Morcellement St Andre) and No 12 ( Plaine Des Papayes), nitrates concentrations tended to hover around 10 mg per litre over the 1980-1988 period.

More recent figures for nitrate concentrations from bore holes in the East are available in the same book. At Caroline and Cluny from January 1992 to December 1993 the concentrations measured were low, around 10 mg per litre for the former and close to nil for Cluny. Whereas at Haute Rive, Fond Du Sac, Riche Terre and Petite Riviere concentrations measured oscillated between 20 and 40 mg per litre. ( Fig 22.3 P 576)

The M.S.I.R.I. ( Mauritius Sugar Industry Research Institute ) on the other hand has from 1991 to 1995 measured nitrate concentrations in potable water at regular intervals at different localities such as Camp Diable, Curepipe, Goodlands and Albion. Over that five years period, they never measured concentrations of nitrates in excess of 10 mg per litre, though wide fluctuations were observed.

Without casting any doubt on the above figures, it is clear that concentrations of nitrates in groundwater and potable water show remarkable variations from one place to the other. Though the MSIRI does not believe (see below) that chemical fertilisers contribute to the presence of nitrates in water, there still remains the fact that at certain locations, at certain times relatively high values of nitrate concentrations have been measured. Good, sound reasons should be brought forward as to their presence.

Taking into account that only 20% of the population is connected to the sewage network the 80% remainder discharging their sewage into pits, is it possible that sewage is an important source of nitrates in water?

The Albion Research centre carried out chemical analysis of sea water to determine, amongst other parameters, the concentration of nitrate-nitrogen. At ten different locations through out the lagoon, the presence of nitrate-nitrogen was either not detected or the concentration was well within normal values ( 0.1 - 0.2 mg/l )

Fate of Nitrate Fertiliser in the Soil...

In 1976, the MSIRI used isotopic Nitrogen to differentiate between nitrates already found in the soil and nitrates applied to the soil in the form of fertilisers.

This research gave the following results:

(1) less than 5% of nitrogen from fertilisers was washed away from the soil.

(2) This nitrogen was to be found mainly in the upper 15 cm of the soil, very little nitrogen was present beyond 60 cm in depth.

(3) Very rapidly, fertiliser nitrogen in the form of nitrates or ammonia was converted into organic forms. This form being fixed in the soil, cannot be washed away.

This research has enabled the MSIRI to conclude that nitrates from fertilisers cannot find its way to the different water bodies and pollute them.

But, in an interview published in the University Of Mauritius Science festival magazine of the 8th and 10th of February 1996, scientists of the MSIRI did acknowledge that 4000 tonnes of Nitrogen used yearly is still unaccounted for, though it is most probable that the greater part is denitrified to nitrous oxide and nitrogen gas.

Herbicides in the water...

The MSIRI carried out further research on the fate of herbicides sprayed in sugar cane fields. Over the 1995-1996 period they took samples from 25 different points on the river systems of the Grand River North West (GRNW) catchment area. This river is fed by numerous smaller rivers carrying waters drained from neighbouring sugar cane fields.

They tested samples for the following herbicides : Atrazine, Diuron, Hexazinone, 2-4 D, Acetochlor. These are the most widely used herbicides in Mauritius. The period of application is from July to December. It is believed that 75% of herbicides are applied during that interval.

Their research indicated that in ground water over 75% of the samples were tested negative and for river waters 50% of the samples were tested negative. In the samples which were tested positive, the concentration measured was between 0.05 to 0.5 parts per Billion (ppb), very few samples contained more than 2 ppb and no samples went beyond the authorised limits which are as follows for a few herbicides:

Atrazine: 3 ppb

2,4 D : 70 ppb

Hexazinone: 14 ppb

Diuron: 210 ppb

The minute quantities detected can be explained by the fact that herbicides are rapidly degraded by bacteriological activity. For example over 80% of 2-4 D applied is dissipated within a week and 50% of ioxynil ( an active component in some herbicides) is degraded within 2 weeks.

Though the research was carried out within one river catchment area, the usage of herbicides being essentially the same throughout the island, it is reasonable to think that similar results would be obtained elsewhere ( though that argument does not invalidate the need to actually go out and measure those levels).

In 1993, the concentration of hexazinone and ioxynil (applied at 0.75 and 0.186 kg a.i. per hectare) in water percolationg at 1 m depth in lysismetres at Belle Rive and Reduit was found to be low, never exceeding 10 ppb.

Furthermore, after one year it was estimated that only 0.32% and 3.20% of the hexazinone and ioxynil, respectively, were leached at Belle Rive and 0.45% and 1.53%, respectively, at Reduit.

At both Belle Rive and Reduit leachate concentration of Hexazinone rose with increased rainfall but never exceeded 10 ppb.

The above research results have enabled the MSIRI to conclude that

Bibliography

(1) Government Of Mauritius, State Of The Environment Report 1991

(2) Proag, V , The Geology and Water Resources Of Mauritius, Mahatma Gandhi Institute 1995

(3) La Pollution Des Eaux Par Les Produits Agrochimiques-PROSI Magazine Septembre 1995 No 320

(4) Les Engrais Azotés, La pollution de l'eau et la Recherche- PROSI Magazine July 1989 No 246

(Note PROSI: Public Relation Office of the Sugar Industry)

  1. Annual Report 1994 of the MSIRI.


    Last Update: Thursday, January 22, 1998