The Gelang Patah Coastal Aquaculture Demonstration and Training Project facilities include 10–12 ha of brackishwater grow-out ponds, which presently have about half of the water in each pond renewed at two-day intervals, from the tidal stream which bounds the main external dike on the north through manual operation of a set of gates to effect the water exchange. Salinities of 24–25‰ in the network of 33 ponds (Figure 1) and the source stream were essentially constant throughout the period of the consultants' field work. Total precipitation over a period of two-three weeks prior to and including the field work was less than 4 cm (Table 1).
The chemical properties of primary interest in this work were those related
to development of acidic sulfate solutions from leaching of pyrite-bearing soils
in the dike network through precipitation runoff and by brackish water penetrating
fissures in the dike soil during water level changes associated with renewal of
water in the ponds. One of the most reliable indications of the total amount of
acid which has reacted with brackish water is the alkalinity (primarily due to
bicarbonate, HCO3-) of the water. Surface sea water has a salinity of about 35‰
and an alkalinity of @ 2.3 milliequivalents per kg of water. Assuming the fresh
water, which reduces the salinity to about 25‰ (70 percent of open sea water
salinity of 35 percent, has zero alkalinity, one would expect the tidal stream
source water at Gelang Patah to be about 1.6 meq/
(0.7×2.3). All of the stream
water samples measured during the filed work had alkalinities very close to this
expected value. Thus, no appreciable strong acid had been added to the brackish
waters in their transit of approximately 10–15 km from the coast to the Gelang
Patah ponds.
A large number of pond water samples were analysed for alkalinity and iron
during the consultants' field work (Table 2). Alkalinity was measured by sequential
additions of acid followed by pH measurements between pH 4 and pH 3 to provide a
very precise value of titration alkalinity which is not dependent upon the absolute
accuracy of the pH data. Iron was measured by colorimetry using ferrozine to form
a coloured complex with Fe+2. Approximately half of the samples had alkalinities
between 1.5 and 1.8 meq/, indicating little or no acid impact (Fig. 2). One of
the ponds (Pond 23), which had been left for a number of weeks without water
exchange, consistently had an alkalinity of about 0.6 meq/, indicating that two
thirds of the alkalinity of water entering the pond network had been lost by reaction
with acid. About half of the other ponds (Ponds 1,3,4,5,6,7,8,9,10,16,21,24,25,
27,28,30,32) and some areas of the discharge canals had alkalinities ranging
between 1.0 and 1.5 meq/ for at least one sample, indicating 10–40 percent of the
alkalinity has been lost by reaction with acid during transit and storage in the
pond network.
Almost all of the pond and canal samples had total iron concentrations greater than the source stream water (Fig. 3), but there was not a strong positive correlation with reduced alkalinity. Even Pond 23, which had substantially more visible iron precipitate near both the water surface and sediment surface than most of the other ponds, did not have unusually high total iron concentration compared with the other ponds except within those discrete layers of relatively large particles.
All of the pond waters had easily measurable NH+4, but very little NO-2, PO4≡, HS- or Mn++ (Table 3), and the rust-coloured particulate layers were predominantly made of Fe (oxides and oxyhydroxides?).
Interstitial waters in the sediments of Pond 29 had substantially higher dissolved Fe+2, HS- and NH+4 than in the overlying water, and no good evidence of significant sulfate reduction in the sediments (Tables 4, 5). (Sulfate values reported are only approximate; there was a systematic calibration difference between the two sets of interstitial water samples.) The interstitial water data were obtained by equilibration for almost a week of a series of chambers inserted into the sediment and covered by a membrane permeable to dissolved ions, but not to particles greater than a few tenths of a micron. Elevated total iron and sulfide occurred well below the sediment-water interface, indicating relative little molecular diffusion or other transport of iron from the sediments into the water.
