It is clear from several kinds of evidence that the dike soils and pond sediments at Gelang Patah contain substantial amounts of pyrite dispersed throughout the system. Some indication of the total burden of pyrite in the pond network can be derived from the peroxide oxidation experiments. The dike soil sample analysed from Pond 29 had 10 meq/100 g of potential acidity. The dike soils analysed by Rosly ben Hassan in November 1981 had some samples with about an order of magnitude greater potential acidity. Using this range of values as representative of dike soils at Gelang Patah, the total quantity of acid available in a 1 m thickness of soil with an area of 10 ha is 107-108 moles (Table 19).
From the XRF analysis of sulphur and iron, the sample of Pond 29 dike soil analysed probably has at least 1 percent pyrite by weight. Using a 1 m soil layer of 10 ha area, the total pyrite burden is at least 103 t. If 4 moles of H+ are produced per mole of pyrite, the total acid available is 3×107 moles (Table 19).
Removal of acidity from the dike soils and pond sediments is presently occurring by runoff from precipitation and exchange of brackish water in the ponds with the source stream. Exchange of brackish water with the present management practice of draining about half of the volume of the ponds every two days should remove about 2 × 106 moles of acid per year, assuming the alkalinity data the consultants collected in November-December 1981 are representative of brackish water leaching (very little rain fell during the period of their sampling) and neutralization of the acid with bicarbonate from the source water (Table 20). Leaching by precipitation runoff probably extracts a comparable amount of acid each year, assuming the precipitation runoff acidities they measured following one rain shower are representative (Table 20).
Filling of the entire pond system to the top of the dikes with brackish water could possible remove a substantial quantity of pyrite acidity. Some indication of the maximum amount of acid a single filling episode could remove can be made by extrapolating the acidity achieved in the short-term laboratory leaching experiments (Table 20). If the pond system were filled to a mean depth of @2 m (water above or near the top of the interior dikes), and the water drained through the dike soils during draw-down, it might be possible to remove @106 moles of acid. This is comparable to the estimates made above for the amount of acid removed during normal operations in 2 to 4 months. This would suggest that many cycles of filling would be required to significantly reduce the total pyrite burden in the dike soils.
If neutralization of all of the pyrite acidity were to be attempted by direct application of limestone, magnesium limestone or ash, very large quantities of material would be required (Table 20). The amounts of crushed carbonate rock would be comparable to the total pyrite mass (103 t), and the ash would require orders of magnitude more material.
The purpose of the above budget calculations is to provide first order indications of the rates that pyrite acidity at Gelang Patah is now being or could be alleviated. All of the calculations have large uncertainties (at least a factor of two), and especially the one involving purging of the dike soils with brackish water, because of a lack of knowledge of how efficiently water would flow through and equilibrate with the dike soils in an episode of flooding. With these limitations in mind, simply dividing the estimated rates of leaching (Table 20) into the estimated total pyrite burden (Table 19) gives removal times of: (1) 5 to 50 years for brackish water leaching by present water exchange practices; (2) 3 to 30 years for precipitation runoff leaching, and (3) 1 to 10 years for flooding to near the top of the dikes with brackish water, assuming 10 flooding episodes per year. Each of these processes should help to leach acidity from dike soils, but accomplishing sufficient pyrite removal to approximate the very low acidity potential the consultants measured for the soils of aquaculture systems in Thailand will clearly require years, even if the most optimistic assumptions proved to be valid.
