THE IMPACT OF HUMAN VISITS ON WATER QUALITY IN LIGHTHOUSE CAVE, SAN SALVADOR, THE BAHAMAS

Many of the caves on San Salvador Island, the Bahamas are flank margin caves, which form at the margins of the fresh-water lens of the carbonate island. Generally, these caves are less than 150,000 years old and are approximately two to seven meters above current sea level. Humans can have a devastating impact on caves just by entering into the cave system. These threats can come from: 1) disturbance of water, which can increase turbidity and particle suspension; 2) the introduction of ‘foreign’ materials such as organics carried in through our clothes and skin; and 3) sediment compaction. Additionally, sunscreen, bug spray, perfume, sweat, and skin flakes can alter the water chemistry of the cave. Several groups of 10-20 students and/or tourists, mostly from the Gerace Research Centre and Club Med San Salvador, enter and tour Lighthouse Cave daily. Our research objectives were to determine whether or not chemical changes occur in the water after these groups enter the cave, and, since the caves are tidally influenced, whether or not the water chemistry changes with the tide. We collected water samples and field parameters, such as conductivity, salinity, dissolved oxygen (DO), pH, and water turbidity from Lighthouse Cave and two control caves. Cations, anions, and trace metals were analyzed to determine if the water chemistry changed after group visits and with the tides.

Slight changes occurred after visitors entered the water in Lighthouse Cave. The pH increased from 6.66 to 7.19, conductivity increased from 54mS to 54.4mS, salinity increased from 35.7ppt to 35.9ppt, and the DO decreased from 26.5% to 19.2%. Zinc dropped from 0.05 mg/L to 0.03 mg/L and iron decreased from 0.1 mg/L to 0 mg/L. The cations, sodium, potassium, magnesium, and calcium, increased between 1-8 mg/L after the cave was visited. The tidal changes in parameters and cations observed were the of same magnitude as the cave tour, with the exception of a 400 mg/L decrease in calcium from low to high tide.

The preliminary results revealed that humans and tides do have an impact on the water quality inside caves. These changes could be due to visitors mixing up the water column and suspending the sediment in the water as they tour the cave, along with the addition of chemicals on the skin of the visitors and tidal changes.