(95c) Influence of Subsurface Storage on the Microbiological and Physicochemical Quality of Surface Water

In land-scarce countries such as Singapore, storage of surface water in underground caverns is being considered as a potential solution to ensure water security for the future. However, long-term subsurface storage of the surface water is expected to undergo quality changes, as the conditions in underground caverns would differ markedly from those in surface water reservoirs. The objective of this study was to investigate the effect of subsurface storage conditions on the quality of surface water in model underground caverns.

Methods

Bench-top simulated underground caverns were constructed by drilling cylindrical wells of diameter 150 mm and depth 50 mm in a granite slab and half of the wells were covered with opaque black acrylic sheets. The remaining wells were used as control and covered with transparent acrylic sheets for the light from a white LED source (Wavelength - 6000 K, Power - 10 W, Intensity ~ 1 W/m²) to pass through. Raw water from three local reservoirs was mixed in equal proportion and 600 ml of the mixed water was added to each well. Microbiological water quality parameters were monitored periodically for six months by CFU counts to estimate bacteria and fungi, and by chlorophyll-A measurement to estimate total algae. Physicochemical water quality parameters including pH, conductivity, redox potential, chloride, total organic carbon (TOC), total nitrogen (TN), nitrate, ammonia and phosphate were measured either using water quality test kits or by water quality sensor probes. Leaching of heavy metal ions from granite was quantified using inductively coupled plasma coupled with mass spectrometry (ICP-MS). When needed, sodium tungstate at a final concentration of 10 mM was used to inhibit denitrification. Total RNA extracted from the water stored in cavern condition and control was sequenced by 2×300 bp paired end sequencing on MiSeq platform. Data were processed and analyzed using open source tools.

Results

Heterotrophic plate count (HPC) in the planktonic phase reduced significantly (p<0.05) in the control (2.5±0.9 ×10⁴ CFU/ml) as well as in the cavern condition (1.1±0.8 ×10⁵ CFU/ml) as compared to the initial HPC (3.0±0.3 ×10⁵ CFU/ml) of the raw water. On the contrary, total algae increased in the light and decreased in the dark. There was no appreciable change in the pH, conductivity, redox potential, chloride concentration and TOC of the water. In addition, leaching of metal ions from granite was found to be highly dependent on the source of the granite. Under the simulated bench cavern condition used in this study, arsenic and cadmium leached out from the granite and reached a concentration of 8±1 mg/L and 41±2 mg/L, respectively, within a month. Intriguingly, the concentration of TN in the water under the cavern condition increased steadily from 0.8±0.1 mg/L to 2.4±0.2 mg/L over 6 months, which was not observed in the control. Further analyses showed that most of the N accumulated in the cavern condition was in the form of nitrate. In the presence of sodium tungstate (a denitrification inhibitor), nitrate accumulation was observed in the control with light, suggesting that the nitrate accumulation in the cavern condition was likely due to the inhibition of denitrification. Taxonomic and functional analyses of the RNA sequencing data support the theory that denitrification is inhibited in the cavern condition.

Conclusions

Storage of surface water in the dark under cavern condition inhibits denitrification and leads to the accumulation of nitrate in the water. Leaching of metal ions from the rocks interacting with the water can also be a concern depending on the geochemical property of the storage system. The findings draw our attention to the potential areas of concern that should be considered in the design and operation of underground caverns for water storage.