An estuary in dire need
By Matthieu Taymans (PCFML resident environmental engineer)
On Thursday we have investigated the hydrolgy of the Gangaya river from where it enters the ocean, all the way up to the spring from which it flows. The hydrologic network, while quite simple in the lower most urbanized part of the watershed, extends upward into a maze of small channels and dikes diverting the main stream to cultivated alluvial plains. The spring appears to be a basin that overflow into the river most of the year, however it also consists of underground streams which fuel the river through a sandy aquifer that sits on impermeable layers.
Until 1990 the upper part of the watershed was host to over 2000 acres of rice fields, however as populations increased in the area an increasing number of people diverted water to their own fields upsteam, which caused a collapse in rice production due to water shortages. Rice has now been replaced by banana and tapioca crops, along with a few leafy vegetables.
Fertilisation across the watershed is undertaken in a largely organic manner and uses mainly chicken manure. Further investigation will enable us to assess whether the quantities applied have an impact on the quality of the water, which does show signs of eutrophication all along the waterway, however increasingly toward the estuary mouth.
Hydrologically speaking the complexity of the network in the upper non-urbanised section of the watershed acts as a pollution buffer, as when it rains the flow smothers the debit variations at the estuary mouth. The lower part of the estuary is channelled into a relatively narrow stream and fuelled by multiple sewage channels. This means that rain events have a rapid impact on flushing this waste material out to sea. Furthermore, several rubbish dumping sites are present on the banks of the river in the lower basin and the river seems to be the main channel through which sewage is disposed.
Increased eutrophication can be observed as we follow the stream towards the estuary mouth. Thankfully, in some areas aquatic vegetation acts as a buffer as it absorbs nutrients that are leached into the river. Continuous oxygenation through steadily flowing water also contributes to creating this buffer effect. Despite this, the last kilometre from the outlet displays signs of heavy pollution and eutrophication. Further investigation through sampling will allow us to determine the most appropriate solutions.
Having a better understanding of the hydrology of Gangaya river and estuary is crucial to designing the most appropriate solution for this waterway. The variation in debris in the last part of the river will have an effect on its remediation, and those effects as well as seasonal trends in the debit curve, will be taken into account as we further explore potential solutions.