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Pilot Project to Test Natural Water Treatment Capacity of Wetland And Tailing Sand Filtration On Mined Phosphate Lands

03-136-209Final

This project involves the treatment of flood surface waters by and reclaimed water treatment through natural processes on lands previously mined by phosphate mining companies. As a result of the mining process, the phosphate companies produce open mine pits, clay settling areas (CSA) and tailing sand deposits, which the companies are required to reclaim as land and lakes, wetlands, pastures, and agricultural lands. The basis for this project was the premise that the natural systems, in particular, wetlands created on CSAs followed by tailing sand filtration, will remove the organic, inorganic and microbiological contaminants from the waters, resulting in water that will meet drinking water standards. After collecting and analyzing a total of 725 water samples from the end point of the natural treatment system at the tailing sand filter basin, all EPA and State of Florida mandated Primary and Secondary Drinking Water Standards (PDWS/SDWS) were met except for a few parameters. The parameters that exceeded SDWS were iron, manganese, fluoride, color, and odor, which are parameters that commonly occur in natural groundwater at concentrations exceeding the secondary drinking water standards. There were two exceedances of chloroform, which is found in the Group 2 Unregulated drinking water standards, but all other parameters found in PDWS, Volatile Organic Compounds, Synthetic Organic Contaminants (pesticides and herbicides), Group 1 Unregulated, Group 3 Unregulated, and Radionuclide parameters were either undetected in the laboratory analyses or were detected, but at concentrations lower than the drinking water standard for that parameter. During the study, Cryptosporidium and Giardia were never found present in the filter basin, but both microorganisms were found present in the wetland and in the water from the cooling pond and effluent discharge. In varying concentrations, fecal and total coliform were found present in the wetland, cooling pond, and effluent discharge on a regular basis; however, total coliform concentrations exceeded the recommended limit of 4 colonies per unit of 100 milliliters less than 30 percent of the time in the water pumped from the filter basin. SI attributes the presence of total coliform to high water levels within the filter basin because of several, re-occurring operational impacts and very rainy periods. There is also a hypothesis that the very low nutrient and very low total dissolved solids concentration in the water pumped from the treatment wetland in combination with the very high vertical hydraulic conductivity of the filter bed tailing sands prevented the formation of a biologically active layer at the sand/water interface commonly referred to is (among others) to remove coliform bacteria. In addition to these constraints, the periodic nature of the wetland pumpage did not help to promote the development of this biofilm.

An additional important finding is a reduction in surface water temperature averaging 5.4° C with a maximum of 8.5° C while flowing through the wetland. Additionally, during filtration through the tailing sand filter the temperature increased by an average of 1.3° C. The average net difference in temperature of the water flowing into the wetland and the treated and filtered water flowing from the filter basin is 3.9° C, with a maximum of 9.8° C.

Another significant discovery is that the concentrations of sulfate were reduced in the surface water more than could be accounted for by dilution of rainfall. In addition, the pH of the cooling pond water was reduced by approximately two units during the flow through the wetland, indicating a reducing environment. This observation, in combination with the observation of the hydrogen sulfide odor coming from the wetland water as it was delivered to the surface of the filter basin, leads to a qualitative observation that sulfate concentrations in the water flowing into the wetland are being reduced. No field data were collected to determine the sulfide concentrations in water pumped from the wetland or water pumped from the basin. It is reasonable to infer a correlation between hydrogen sulfide smell and the odor measurements. If this correlation holds then the odor data suggests that the water pumped from the basin may also be in a reducing state. Recharging water into the aquifer that is low or depleted in oxygen and in a reducing state could reduce the probability or prevent the dissolution of metals from the limestone matrix.

The project envisions the treated water from the basin will be used to be recharged to the underlying Floridan Aquifer through a recharge well that will be installed on-site as part of the overall feasibility test of the Aquifer Recharge and Recovery Project (ARRP) concept. This concept envisions recharging naturally treated surface, storm and waste waters to the underlying Floridan Aquifer, an extensive confined groundwater system capable of storing and transmitting large quantities of water, for later retrieval through another (pumping) well at some distance away and at a different time.

The Florida Institute of Phosphate Research (FIPR) and the Southwest Florida Water Management District (SWFWMD) have funded this pilot project to assess the natural treatment capacity of previously mined phosphate lands in support of the ARRP concept. The installation of the recharge well is funded cooperatively by the SWFWMD and Progress Energy Florida.