The primary mechanism which leads to decreasing alkalinity and pH in the ponds is probably abiotic pyrite oxidation and acid export from dike soil during rainstorms. Experiments were performed to simulate this process in the laboratory, using dike soil, pond sediment and pond water from Pond 29. In view of the documented ability of bacteria to catalyze pyrite oxidation, the consultants also examined pond waters, pond surface scum, and their experimental materials for filamentous bacterial forms which could signify the presence and growth of iron-oxidizing bacteria. They performed time-course measurements of alkalinity and pH in their experiments to follow the oxidation process after the initial leaching event. This stimulates pond conditions in the days following a rainstorm and could indicate bacterial activity.
Rainwater leaching of dike soil was simulated by making 1:1 (wt:wt) extractions of dike soil with distilled water. This primary leachate was then used at 100 percent strength and diluted to 20 and 50 percent strength with Pond 29 water. A 100 percent pond water (0 percent leachate) control was also employed. These leachate/pond water mixtures were incubated in triplicate for eight days, both alone and over pond sediment. Alkalinity, pH, and detailed microscopic examinations were made on days 0, 3, and 8 following leachate preparations.
Experiment results are shown in Fig. 7. Initial (day 0) results show the alkalinity and pH levels for the freshly prepared 0, 20, 50 and 100 percent strength leachate solutions. As expected, a straightforward negative correlation between leachate strength, pH, and alkalinity was observed.
No changes were observed after three days at the 20 and 50 percent leachate dilutions but there was a considerable disappearance of acid (or production of alkalinity) in the 100 percent leachate sample by day 3. After another 5 days, no further changes were observed at the 20, 50, or 100 percent leachate strengths. These are puzzling results and the consultants have no good explanation for the large changes they observed at day 3 in 100 percent leachate and at day 8 in 0 percent leachate. However since these are extreme cases which would not be encountered in the ponds, they focus on the intermediate dilutions. These results suggest that most or all of the alkalinity and pH decrease happens during or directly after a rainstrom, and that further biologically mediated processes did not contribute further alkalinity destruction processes in the absence of sediments to any significant extent. By day 8, the full strength pond water had lost all its alkalinity and showed a precipitous pH decline to 3.21. The cause of this surprising result is unclear. It does indicate the poor buffering capacity of the pond water and the fragile poising of acceptable water quality in the pond. This sample did not develop a prominent surface mineral slick or yellowish-brown granular precipitates on the sides, so pyrite oxidation is not suspected as a cause of the alkalinity destruction in this “unpolluted” sample. Fine yellow biofilms precipitates did develop in the 20, 50 and 100 percent strength leachate samples by day 2–3. These were examined microscopically (see below). The films all contained refractile gold or brown granules of amorphous ferric hydroxide, suggesting that biological pyrite oxidation was occurring in these samples.
These results from more realistic experiments show a very different pattern
from the water-only results. At 0 and 20 percent leachate strengths, marked
deterioration from the initial conditions are apparent by day 3. No changes were
noted after day 3, indicating that the process was rapid and complete by that time.
Since alkalinity destruction also took place in untreated pond water without sediment
by day 8, not all of this effect can be ascribed to the sediment alone. However,
the alkalinity decrease in the 20 percent leachate shows that the sediment does
contribute to pyrite oxidation, on biological time scales (days rather than minutes
to hours). At leachate concentrations of 50 and 100 percent some ameliorating effect
of the sediment is apparent. Alkalinity values are higher at days 3 and 8 than
on day 0. Apparently the sediment can buffer the water system near 0.2 meg
-1 and
pH 4–6, which is still quite far from optimal conditions. All the sediment-containing
samples developed heavy mineral slicks on the water surface, reddish-brown
precipitates on the container sides and filamentous, flocculent, filmy deposits on
the sediment surfaces (see below). The sediment water interface, in these ponds
as in many other aquatic systems, is a zone of intensive biological activity.
In this particular case, it appears to be a zone of complex pyrite oxidation which
contributes to water quality deterioration.
These examinations were carried out using transmitted light Hoffman modulation interference contrast microscopy. This optical system facilitates visualization of delicate microorganisms in mineral matrices such as iron oxidizing filamentous bacteria surrounded by amorphous ferric hydroxide precipitates. Refractile brownish-gold granules were ubiquitous in surface slicks and sediment-water interfaces from ponds and experimental samples and also in the discoloured gills from dead shrimps. These granules indicate active pyrite oxidation in the ponds. Shrimp gills accumulate and may be clogged by these iron precipitates.
Heavy reddish granular deposits from Pond 23 (30 November 1981) contained abundant filamentous forms in intimate association with the granules. These organisms could generally only be seen following treatment with IN HCl, which dissolved the Fe(OH)3. Although the iron-oxidizing bacteria cannot be identified on morphological grounds alone, the clear association of these filamentous forms with iron hydroxides deposited over several days, strongly suggests that they did precipitate the iron through pyrite oxidation. Similar filamentous forms were also seen in the yellow biofilms covering the walls of the experimental vessels and in all other samples examined. The sediment surfaces in the experimental systems all developed heavy rust-red flocculent deposits with mucus-laden, thread-like projections, which contained abundant filamentous organisms. No filamentous bacteria were observed in any shrimp gills examined.
In conclusion the consultants' experiments indicated a clear capability for continued alkalinity destruction, probably from bacterial pyrite oxidation in the water and at the sediment-water interface. The most rapid and significant process of water quality deterioration, however, was simple leaching of dike soil with distilled water (or rainwater) and probably also with brackish water flushed through fissures in the dikes during water stage changes.
