Groundwater Nutrient Transport on Atolls
Research this year showed that nitrate levels varied with bird populations and size of motu with very high levels on Tahuna Rahi and Iti as expected. We have also found geochemical conditions to be extremely stratified within the motu sediments with anoxic and denitrified conditions at very shallow depths below the water table.
Another accomplishment in 2023 was that we equipped current monitoring wells on Tahuna Iti, Honuea, Tahuna Rahi and Onetahi with real time water quality sensors to record - at an hourly frequency - changes in salinity, temperature and water level. These data could provide key information on the effects of seasonal weather changes and extreme events such as swells on the groundwater system. Our sensors could also provide useful data on the groundwater response to other, longer-term forcings such as rat eradication and bird population rebound, changing vegetation, etc.
In December we successfully drilled a 4 m deep boring through a substantial rock layer and set a monitoring well at 3 meter depth. This was done using a portable mud rotary drilling rig. This monitoring well will be used to sample water chemistry and level below the rock confining layer which we believe has a significant impact on water flow and nutrient flux to the reef.
