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