Coral Cam Web Site

Where's the Salt?                               Mr. Cesare Filice
(Limestone Filtration) 

OBJECTIVE:   To show students how porous limestone filters saltwater.


In Bermuda, when wells are drilled, the salinity of the water obtained is variable.  The amount of salt present in the well water is dependent upon its proximity to the ocean, the height above sea level and the depth of the well. 
Saltwater penetrates the limestone along the shoreline.  The further away from the shoreline the lower the concentration of salt.  In addition, freshwater enters the lens when rain is absorbed by the ground.  Within the ground the water reorganizes itself into different lenses depending upon its density (salt concentration). 
Here we will demonstrate how limestone filters saltwater. 


    Conductivity meter (Keltec Soluble Salts Analyser)

    2  250 mL beakers                         saltwater of known concentration

    drill                                                1/4 inch drill bit

    eye dropper                                   2 pieces of limestone (approx. 2" X 2" X 10")


1.Pour 100 mL of tap water into a 250 mL beaker and 100 mL saltwater into the other beaker. 

2.Drill a hole into the pieces of limestone, lengthwise starting at one end.  The hole should end 1 cm from the other end. 

3.Place a piece of drilled limestone into each beaker containing the saltwater, with the hole facing upwards. 

4.Allow the limestone to stand in the beakers overnight. 

5.If after a few hours, the limestone has soaked up most of the saltwater, add more saltwater to the beaker.  Do the same for the other beaker.  Make sure that you do not pour any water into the drilled hole. 

6.Next day, using the eye dropper, remove a sample of water from the hole and squirt the sample into the conductivity meter. 

7.Allow a few minutes for the sediment to settle. 

8.Take a conductivity reading and determine the salt concentration. 

9.Repeat steps 6 through 8 for the beaker containing tap water. 

10.Calculate the percent salt filtered for each sample. 

     conc. of salt        conc. of salt
     in beaker        -     in sample
     ____________________________________    X    100   = 
          conc. of salt in beaker 

11.You may have a negative number indicating that some salt has leached from the limestone.

BBSR and TCOE Coral Web Site Team 1999
Funded by a grant from the Goldman Foundation