Water Testing

During work in the aft cargo hold in 1989, divers found a flat wooden platform six feet below the main deck which stopped further progress. Since the vessel had a 10.6 ft. depth of hold, the platform could not be the bilge ceiling. Divers also noted a considerable drop in the water temperature at the bottom of the excavation. Several holes in the platform appeared to be expelling water under slight positive pressure that was cooler than the ambient river temperature. This phenomena could not be explained but speculation suggested the vessel settled on top of a fresh water spring that breached the hull. Proper sampling and testing equipment was not available at the time to positively determine the source (Cantelas 1992:29).

On April 3 and 4, 1992 an attempt was made to find the source of the cooler water and determine its impact on the site. The physical and chemical properties could help explain preservation conditions on the wreck. Rick Speckler, a geologist with the United States Geological Survey, Water Resources Division in Altamonte Springs, Florida, provided the expertise and sampling equipment.

The sediment covering the access hole in the deck and filling the hold was removed with a dredge. A diver then carried probes down to the wreck to measure specific conductance, dissolved oxygen (DO), temperature and pH. He also took water samples. Measurements were made and samples taken in the river's water column and the hold interior. Following the first set of tests in the interior, the diver cleared the bottom of the hold again to provide better access to the small holes in the flat wooden platform. The second set of measurements proved useful to compare to the first set.

Ultimately, using a diver to conduct testing and sampling procedures compromised the results. The diver's movements suspended so much sediment that water samples could not be adequately filtered for chemical analysis. The samples would have provided the best means to determine whether the source is ground water or water derived from an aquifer via a spring. Contaminants introduced into the site environment from the river skewed other tests monitored on the surface. The results of these tests are suspect but may show general trends relevant to the site environment (Table 1).

Two sets of data taken inside the hold reflect variations in measurements taken at the same location. Specific conductance shows the greatest variation resulting from the diver suspending sediment. This measurement would be useful to determine chloride levels in the water that directly affect metal preservation on the site. The variations, however, indicate these levels cannot be accurately determined with the data recovered.

The pH measured at deck level and below the wooden platform was 7.18 and 6.58 respectively. This indicates a change from a slightly alkaline environment outside the vessel to slightly acidic inside the hold. The pH of sediment filling the interior is a major factor in determining what material types are preserved. Many organic materials such as cotton and paper have a low tolerance to acidity and may not survive well on the site.

Depth Specific Conductance Dissolved Oxygen Temperature 0C pH
Surface 3000 9.20 17.9
Increasing Depth 3110 8.80 17.6
" 3320 8.71 17.3
" 3360 8.30 17.2
22 Feet River Bottom 3320 7.44  

 

27.3 Feet

Deck

3000 .83 17.9 7.18
2 Feet Below Deck 3000 End of probe umbilical 17.9
Descending 3020 20.2
33.4 Feet Bottom of Excavation 3610 22.0
After second dredging
27.5 Feet Deck 4170 17.7
33.6 Feet Bottom of Excavation 3920 21.7
Below Wooden Platform 4540 22.1 6.58

Table 1. Water Testing Results From the Aft Cargo Hold. (Speckler 1992).

The amount of DO in the water column slowly declines from the surface to the river bottom. Between the river bottom and the main deck, there is a dramatic and sudden drop in DO, from 7.44 mg/l to .83 mg/l. This drop demonstrates the general anaerobic conditions on the site. The sensor cable linked to the surface monitor did not reach below deck level and interior measurements were not taken. Low DO levels generally account for the excellent preservation conditions found on the site.

Water temperature remained stable from the surface to the main deck, varying only .70C. A striking rise of 4.40C was noted between the main deck and the area below the wooden platform at the bottom of the hold where it reached a maximum of 22.10C. This temperature is very close to ground water temperature of 22.70C recorded in the Duval County surficial sand aquifer. The temperature remains stable throughout the year, regardless of river temperature (Leve and Goolsby 1969:7-8). In May 1989, divers noted cooler water in the bottom of the excavation because the ambient river temperature was warmer than the ground water. The water temperature in the hold suggests that the site may rest in fresh ground water below brackish river levels.

Generally, tests conducted in April 1992 were inconclusive regarding the water source found at the hold bottom. The positive pressure flow noted in 1989 was not found during this investigation. This attempt did demonstrate that future testing inside the hold must be tailored to the site conditions. A remotely monitored testing station would avoid the obvious problems introduced by using a diver.

Future work should address several specific questions on site conditions. The presence of freshwater directly affects the physical condition of the site and the conservation needs of recovered artifacts. Most of the wreck is buried well below the river bottom, possibly in the surficial ground water table, effectively isolating it from the brackish river water. This condition would lower the chlorides in the site and simplify conservation needs. Other factors, such as dissolved oxygen and pH, affect preservation conditions and would help explain differential preservation among various material types.