Tucked away on the third floor of Global House is a small office with a big responsibility -- monitoring the water lenses on an island without rivers.

Government hydrogeologist Mark P. Rowe is responsible for the monitoring and management of the fresh ground water in the subterranean reservoirs of Bermuda.

American Leonard Vacher was the first hydrogeologist here and Mr. Rowe was once his teenage assistant. But now the student has taken a role alongside the teacher as co-author and replaced him as advisor to Government.

The 43-year-old Bermudian holds a degree in geology from the University of London, School of Mines and a Masters in hydrogeology.

A specialised field within geology, hydrogeology is the study of the currents of underground and surface water, and Mr. Rowe's office is the caretaker of studies into the size of fresh water lenses deep underground and amount of pollution in the water.

"There isn't a textbook. It's not like bridge making,'' Mr. Rowe said about hydrogeology.

He explained that every location and circumstance is different and with the small number of islands similar to Bermuda geologically, there are only about a dozen hydrogeologists worldwide with the same relative experience.

Bermuda of course, is a limestone (calcium carbonate) crown on a volcanic base without any natural surface water that is potable.

The formation of the rock from sand dunes occurs over time when water percolates through the sand, causing calcium to dissolve, collect and reharden.

About a quarter of our rainwater eventually collects between the pores of limestone sand grains in the rock deep underground.

The lighter density of fresh water allows it to sit on top of a layer of rock saturated with brackish ground water while seawater saturated rock is below.

Since the 1970s the lens system has been the primary source of water for non residential use.

"Bermuda has an advantage with Watlington Waterworks sea water reverse osmosis plant,'' Mr. Rowe said, adding that this will lessen any drought problems that the Island might have in the future.

Mr. Rowe spent early years with Dr. Vacher testing the salinity of private wells and establishing where the boundaries of the lens were.

This, he explained, would help educate them on the nature and behaviour of the ground water.

"I still do monitoring of the wells through 120 observation bore holes throughout the Island,'' Mr. Rowe said.

"Every three months we spend a week or so in the field to test the levels of salinity and get a rough idea of volume of water.'' The office also tests for 250 pollutants, although the alkalinity of the Bermuda limestone acts as a sand filter.

"We do check at trouble spots, particularly at the old Pembroke dump (now known as the Pembroke landfill)'', Mr. Rowe explained.

And certain sites have to be monitored, like fuel storage facilities, farms, and places were paint is manufactured and furniture is stripped and refinished.

Government spends between $10,000 and $20,000 a year to study ground water quality to ensure there is no threat to public health.

Mr. Rowe was one of the publishers of the detailed "Geological Map of Bermuda in 1989'', and in 1991 wrote the follow-up book "An Explanation of the Geology of Bermuda''.

Dr. Vacher and Mr. Rowe have also collaborated to produce chapters on Bermuda in "The Geology and Hydrogeology of Carbonate Islands'', and "Terrestrial and Shallow Marine Geology of the Bahamas and Bermuda''.