AQUATIC ECOLOGY BASELINE REPORT 2020
With all the debate recently about the use of PACl (Polyaluminium Chloride) we thought to repost this article of the ecology study conducted for the Te Mato Vai Project and let the science speak…
Last year a field study on the 10 streams where water intakes have been upgraded was conducted. At most of these 10 intakes, additional storage has been created, treatment plants constructed, and trunk mains replaced.
Alongside the field study a report that described the existing aquatic ecological values of the 10 intake and discharge sites was produced. As part of this study, we assessed the potential impacts of the water treatment plant operations on these values.
We used published data from the Cook Islands Government and information gathered from surveying the streams to inform the report. Samples from both upstream and downstream were collected to survey the overall aquatic habitat and wildlife populations in the streams.
Among the key findings from this ecological report are:
Water quality (bacteria, nutrients, dissolved oxygen) of the streams on Rarotonga has been consistently poor for several years. Bacteria (enterococci) concentrations are extremely high. The high concentrations are likely due to adjacent land use practices, where waste from wild animals and small-scale pig, poultry and/or goat farms stretching up the valleys to the intake locations are washed into the streams during farming processes and/or heavy rain.
Nutrient concentrations have been consistently elevated in most streams. This is likely due to farming and the naturally high concentrations of phosphorus from the volcanic geology of Rarotonga.
There are no threatened freshwater aquatic plant species recorded on Rarotonga.
There are no pollution sensitive aquatic macroinvertebrates taxa (such as koura) currently known on Rarotonga. We collected samples of the animals from the stream bed and edge and assigned them a sensitivity score (i.e. how tolerant they are to pollution). The animals we collected scored between two and five, which are considered tolerant to pollution.
Due to the remoteness of Rarotonga, the diversity of freshwater fish species is very low and there are no threatened freshwater fish species on Rarotonga. Our ecologists did not observe any keystone species during the field survey or desktop review, so we would not expect the food web to collapse.
Any aluminium released to the waterways from the scour ponds is likely to have low toxicity. Further, at normal pH levels for Rarotonga streams there is a low risk of aluminium accumulating over time because the level of ionic aluminium is extremely low, even though there are instances of naturally occurring high levels of aluminium in the stream water at times following heavy rains.