There’s so many projects that we are doing here it’s difficult to keep track of what field work relates to each and who is doing the project. There are all the never ending areas of KT’s PhD, Fiona’s various pet projects, George’s Masters, Didi’s pet projects and eventually my Masters. So it was nice yesterday to do some work that directly related to my project. In truth its overlap between mine and KT’s projects, but all the same it was good to see the data your collecting and knowing you will be using it later.

We were using our YSI logger to profile boreholes around the wellfield. The wellfield is a large wooded area to the West of where we are staying. It is situated on the area with the most freshwater and a system of trenches and pumping stations extract water slowly for the use of local towns and settlements. Within this rather remote and wild area are a scattering of boreholes. They range in date back to the 1970’s and go as deep as 40m down. Within these is where we get direct access to the water under the ground, Groundwater.

Under the island rain percolates through the soil and rock and forms the fresh groundwater of the island as the water table. Below the freshwater however is salty sea water which, as Andros is an island, comes in from the oceans at depth. The fresh water, being less dense then the salty sea water, floats on the salt water in the form of a lens. Like a contact lens shaped piece of ice floating in water. This freshwater lens is physically and chemically highly responsive to changes in rain, tides and water removal by pumping. KT is interested in how the chemistry of the freshwater changes with depth and time and I am modelling how the fresh water flows underground. Getting access to the groundwater is problematic as, annoyingly, it is underground.

It is here that the lovely boreholes across North Andros come in handy. Using the YSI we can lower it down the boreholes and it records pH, Specific Electrical Conductivity (SEC, which is a proxy for salinity), Temperature and Depth every second! This gives us a highly detailed insight into the structure of the water below ground. This also gave me and Didi the lovely job of spending a day lowering the 5kg logger down 30m boreholes at a rate of 2-3cm every 5-10s.

The logger is lowered on a metal cable which has to be rotated and held manually at the top, whilst in the midday sun. Understandably this resulted in a lot of hotness. We persisted and with the help of classic 80’s pop and homemade Palmbrellas we profiled 6 boreholes before our car broke down. Didi and I now been in about 8 car breakdowns. In 3 different cars. Garages make a lot of money on Andros. We spend a lot of time sitting in the middle of nowhere waiting for rescue.

This is highly interesting for my project as you can see the top and bottom of the freshwater lens and flow areas within it. As water sits in the rock it dissolves the rock around it increasing its dissolved content, thus having a higher SEC. The lens therefore has SEC variations within it depending on how long the water has been in the rock and where it came from. It is not a simple distribution however. The many caves, tunnels and fractures within the limestone create zones of faster flow. Waters that flow faster have spent less time in the rock and therefore have a difference in conductivity. The YSI profile picks out these conduits of flow like a vegetarian picking chorizo out of their risotto, nom nom nom, and I use this flow data to model the rock and the flow within it.