Foul-smelling, dark red water currently flowing into the sea from a culvert near the South Sound boardwalk is not pollution but mangrove run-off, Department of Environment Deputy Director Tim Austin has confirmed.
The run-off came from the nearby wetlands, caused by increased rainfall from recent storms, Austin said.
However, Austin said the department is still keeping an eye on the discoloured water as he responded to queries from the Cayman Compass on Tuesday.
The DoE was “well aware” of the situation, he said.
While the brackish water is colouring the coastline in the surrounding area, he said, unlike in the past, no dead fish have been spotted.
Back in 2017, a resident reported the problem to the DoE, and at that time he said, in his response to the complaint, that it was an ongoing issue that is likely to get worse as South Sound mangrove wetland is filled in and rainwater storage capacity is reduced.
The DoE, Water Authority and other agencies, he said in his 2017 response, had repeatedly raised the issue with the National Roads Authority and recommended investment in a long-term drainage solution, but no such study had been undertaken.
“We have been back several times since then and this is actually part of much bigger South Sound mangrove drainage basin/wetland development issue that the several government agencies have been trying to bring attention to,” Austin told the Compass Tuesday in his emailed response.
Austin said there is renewed interest in storm water management from the NRA.
“They are coordinating an interdepartmental group to look at the wider issues but South Sound hasn’t been singled out as yet,” Austin told the Compass.
As for the environmental impacts of the run-off, Austin said the area has already been hit particularly hard by sargassum which had caused nearshore seagrass to die off.
However, the DoE has not “looked quantitatively at the impacts and it is not something that we are likely to have the immediate capacity given all of the other priorities we are faced with”, Austin said.
As he explained in his 2017 statement, the culvert in question is connecting the drainage canals in the South Sound wetland to South Sound lagoon.
“Due to the weir system on the inland side of the culvert the water only flows when rain water reaches a certain level within the catchment basin. 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 said at the time.
When the rains raise the level above the weir system, as was the case with the recent passage of storms, “the ‘first flush’ is rather intense and the low oxygen and high hydrogen sulfide levels can result in a strong odour, similar to sewage”.
In his 2017 report, Austin said, there had been some development in the area surrounding the culvert, including clearing substantial amounts of mangroves and reconfiguring drainage ditches, which may also have contributed to reductions in quality of the drainage water.
Tests previously conducted
Austin said no tests have been conducted recently on the run-off, but back in 2012, Water Authority investigations into the water quality found “that the test results do not provide conclusive evidence that the water that is discharged into the Sound is contaminated with wastewater”.
According to that 2012 report, “The pipeline and culvert were designed to discharge excessive stormwater from the South Sound wetland area and the low lying adjacent development, therefore it is most likely that the discharge in the Sound is mainly stormwater. There is a possibility that some of the illegally discharged wastewater from the Randyke Gardens apartments is mixed with the discharge, but this is not quantified.”
It is unclear if any further water quality tests will be conducted.
As for the strong stench, Austin explained in his 2017 report that the smell of sewage in the area could also be influenced by the fact that just offshore, but very close to the culvert exit point into South Sound, there are a couple of sink holes on the sea floor that have collapsed over the years.
“These areas are subject to ground water upwelling when the rains cause the water table to raise higher on land relative to the sea. This means ground water naturally rich in hydrogen sulphide due to the lack of oxygen is pushed out of these holes and is clearly visible as an upwelling event. The hydrogen sulfide rapidly comes out of solution and gases off which is why we can smell it. It can be smelt in very low concentrations,” he said.
The prevailing wind direction and currents in South Sound, Austin explained in his previous report, mean that the water draining from the culverts is kept close to shore as it moves westward to flush out of the South Sound by Sand Quay.
“This is very evident due to the red tannins present in the water. Given that this water is relatively fresh compared to normal sea water there is a considerable drop in salinity. This can result in shifts in algae composition particularly favouring the fast growing slimy green algaes that thrive in lower salinity,” he added.