In this study, an electromagnetic induction survey was conducted with the EM-31 to assess the spatial variability of ground conductivity, a proxy for groundwater salinity, in a mangrove on North Hutchinson Island, Florida, a carbonate barrier island. A previous study established a relationship between ground conductivities and pore-water salinities, but data points are not spaced closely enough to properly observe the potential effect of mangrove vegetation on ground conductivities. We present here apparent conductivities measured along five profiles that traverse the field site; data were inverted to obtain ground conductivity for the vadose and saturated zones. The vegetation types are dense black mangrove, scrubby black mangrove and salt pan (little or no vegetation). At this site, average water-table levels were 0.2 m below ground level. The mangrove roots systems extend to .6 m to 1 m below the ground surface. Sampled pore-water conductivities range from near freshwater to hypersaline. Effective depth measurements range from 2 m to 5.5 m for the EM31. The average vadose-zone ground conductivities derived from inversion of the data are 1400 mS/m, but range from 75 mS/m to 12,000mS/m. The average saturated -zone ground conductivities are 1900 mS/m, and range more narrowly from 820 mS/m to 2400 mS/m. These large conductivity values mean the low-induction number assumption is not satisfied so true conductivity values are larger than what is measured, but spatial distribution and variability is still observable. There is a larger degree of variability observed in the vadose zone than the saturated zone, but the saturated zone generally has higher conductivity values associated with it; which is controlled by saline-hypersaline groundwater. The density of mangrove vegetation shows a strong correlation with ground conductivity variability in both zones— vegetated areas have more variability than salt pan areas. This is due to root systems removing salt and water from the ground but excreting the salts at the leaves, which eventually returns to the ground. Salt pans, lacking vegetation, have evenly distributed ground conductivities.
Presented at the AGU Fall Meeting on December 9-13, San Francisco, CA
Available at: http://works.bepress.com/christine-downs/7/