the Committee and carried out effectively by the District scientists. The following sections summarize key findings of the environmental workgroups and describe the major issues that the Committee had regarding the approach and/or results of each workgroup. Other detailed criticisms are found in the body of the report (primarily in Chapters 3 and 4).
The wetlands workgroup was tasked with assessing the potential effects of surface water withdrawals on floodplain wetlands, specifically changes to vegetation communities that might result from altered hydrology and/or changing salinity regimes. To accomplish this they first identified river segments that have the highest likelihood of change. These then became the focus of subsequent analyses. The workgroup assessed existing MFL transect data on wetland plant community types across the elevational/hydrological gradient of the floodplain in those river segments to determine how the communities might change with withdrawals. LiDAR data were acquired for some areas of the watershed to create a digital elevation model (DEM), which was then subjected to a GIS analysis to predict hydroperiod changes in wetlands. The goal of this analysis was to determine whether water withdrawals have the potential to (1) alter the species composition of floodplain wetland communities, (2) alter the extent of wetlands or various wetland communities found there, and/or (3) lead to a shift in the location of boundaries between wetland types. The workgroup focused on river segment 8, where impacts to river stage were predicted to be greatest, and segment 2, where changes in the salinity regime were predicted to be highest. The wetlands workgroup found “moderate” impacts to wetlands in segments 2 and 8 under the most extreme future withdrawal scenario.
The wetlands workgroup produced a solid analysis of potential impacts of water withdrawals to the St. Johns River. Their integration of a LiDAR-based DEM with floodplain stage exceedence curves to assess the spatial extent of hydrological impacts is a novel approach, resulting in a robust picture of the spatial extent of dewatering and shifting boundaries between wetland types. The salinity analysis strategically made use of the Ortega River tributary as a model system from which results could be translated to the larger St. Johns River. Because the Committee is confident that the methods developed by the workgroup will be adaptable to other river segments and be useful to analyze potential changes in river flow in the future, it recommends expanding the analysis as more data and resources become available.
The biogeochemistry workgroup identified several potential effects of water withdrawals on biogeochemical processes in the St. Johns River and its drainage basin, all related to the possibility that soil accretion would be reduced or oxidation of organic soils (histosols) would be enhanced in riparian wetlands of the river as a consequence of changes in stage induced by water withdrawals. The workgroup concluded that two effects of water withdrawal had potentially high significance: reduced nutrient sequestration and increased release of colored dissolved organic matter. The workgroup considered how much additional release of the constituents of interest would occur from soil organic matter as a result of water withdrawals, how much of the additional material would be exported from the wetlands to lakes, and what effects could result