Concerns raised as brown water drains into marine park

Images are circulating on social media once again of brown floodwaters pouring out of a culvert on South Sound Road and into the sea, in an area designated as a marine replenishment zone.

While the brown colour of the water has some residents concerned that the water may contain sewage, Timothy Austin, deputy director of the Department of the Environment, explained that the primary cause of both the colour and the accompanying odour is related to the breakdown of leaves in the mangrove wetlands.

“Due to the weir system on the inland side of the culvert, the water only flows when rainwater reaches a certain level within the catchment basin,” Austin explained. “As a result, during the drier periods, the standing water tends to collect a lot of debris, stagnate and becomes rather foul, including increasing its mangrove tannin levels, leading to very dark red water.”

He added that that low oxygen and high hydrogen sulfide levels in floodwaters can result in a strong odour, similar to that of sewage.

The Department of Environment believes the situation is likely to get worse as more of the South Sound mangrove wetland is filled in for development, and rainwater-storage capacity is reduced.

Austin said that the DOE, Water Authority and other agencies have “repeatedly raised the issue with the National Roads Authority and recommended that money must be invested in a long-term drainage solution, but, to date, no such study has been undertaken”.

While the DoE confirmed it has not conducted any water-quality tests as yet, it is “confident that raw sewage is not a contributing factor at this time,” Austin added.

The large volume of water exiting from a single location by the culvert is causing the water clarity of the sea in the area also to be affected. This is likely due to the prevailing winds and currents pulling the water down along the shoreline towards Sand Quay island, off Southwest Point.

According to the DoE, given the volume of water, fish kills “would not be unexpected” and can stem from two potential sources – low oxygen and the salinity of the flush water which “can impact marine species resident in the area, although, theoretically, they can move off to better water conditions further offshore”.

The environmental agency explained that the relatively low salinity and the high levels of nutrients in the water can also be problematic, because they may result in algae blooms that can affect corals reefs. At the present time, the DoE said, it is monitoring the situation and will provide updates if there are any further developments.