A PRELIMINARY INVESTIGATION OF THE GROUND WATER GEOCHEMISTRY AND MICROBIOLOGY AT THE LINE HOLE AND AIRPORT WELL FIELDS, SAN SALVADOR, BAHAMAS

Recent studies by Moore and Martin (2006) and Schwabe et al. (2008) have suggested that the traditional model of karstification by acids generated through meteoric carbon dioxide solution and by mixing solution in the subsurface may have to be modified. They present strong evidence, from South Andros Black Hole (Schwabe and Herbert, 2004) which shows that solution in these environments is dominated by strong acids (H₂SO₄,), generated by bacteria, and weak acids from the solution of CO₂, generated by decay of organic matter. The bacteria “float” on the more dense marine water at the halocline and consist mostly of sulfate reducing and sulfide producing varieties. Their studies, however, were all conducted in environments with large-scale (meter or more) openings. They have hypothesized that these bacterially-dominated processes also control solution at the small (centimeter) scale, but this hypothesis has not yet been tested. As a preliminary test, we conducted a study of solution at small scales on San Salvador Island, Bahamas. We examined three abandoned wells that penetrated the halocline. We measured conductivity, dissolved oxygen, pH, temperature, and sulfide at 10-25 cm intervals within the well water columns. In addition, we sampled and cultured bacteria from above, at, and below the halocline. While Schwabe et al. found large changes in temperature caused by bacterial mats dissipating light energy as heat, in pH caused by the oxidation of H₂S to H₂SO₄ in oxygen rich waters, and in dissolved oxygen caused by organic decay near the halocline, we found no changes in temperature and pH throughout the water column. Dissolved oxygen did decrease substantially at the halocline and we did find sulfate-reducing/sulfide-producing bacteria. These results suggest that, while microbial processes probably are contributing to solution at the centimeter scale, they to do not appear to dominate it in same way that they appear to do in large scale openings such as those examined by Schwabe. It may be that the small space prevents the build up of large enough bacterial populations to produce the pH and temperature changes that thy observed. In addition, the larger surface area to volume ratios found at small scales may allow buffering to keep the pH high.