Site Characterization by Direct Mass Flux in Central Florida

G360 PROJECT TEAM: Dr. Beth Parker, Dr. Emmanuelle Arnaud, Dr. John Cherry, Dr. Carlos Maldaner, Steven Chapman & Sam Jacobson (MSc Candidate)

Several sites are possible contributors of trace 1,4‐dioxane detections in a municipal supply well pumping from the karstic Upper Floridan aquifer. Efforts are underway in partnership with a site owner and its consultants, University of Florida and HSW Engineering, Inc., to assess potential contributions from one nearby industrial site; however, the horizontal hydraulic gradient and its direction are so small as to be indeterminate because of the high karste transmissivity and small site dimensions, thus stripping confidence from conventional Darcy’s law‐based mass flux calculations. To avoid multiple measurement uncertainties, we advanced direct mass flux measurement techniques in two cored holes to characterize this site. Depth-discrete, high resolution rock core contaminant profiles were developed, and borehole geophysical and hydrophysical logs were collected. Physical caliper profiles captured the significant variability in borehole diameter, locating zones that could be sealed using oversized FLUTe™ borehole liners. Hydraulically active features under natural hydraulic conditions were qualitatively identified using active line heating deployed with fiber optic cables for distributed temperature sensing (A-DTS) (Maldaner et al. 2019, WRR).

These datasets informed placement of modified, fractured rock passive flux meters (PFM) in multiple, depth-discrete zones (1-2 metres long), external to the FLUTeliners, to quantify both water and contaminant fluxes. Vertical arrays of pressure and temperature sensors were co-deployed with the PFMs to capture transient hydraulic conditions. A  presentation will be given at the NGWA Conference on Fractured Rock and Groundwater in Burlington, Vermont, held September 23-24, 2019, to further discuss challenges and adjustments made for karstic boreholes as well as in-situ groundwater and contaminant flux distributions used to estimate mass discharge.

Sam Jacobson, MSc Candidate at G360, logging a rotasonic limestone rock core.
Digital images of borehole sections. Depth is listed in bottom-right corner of each image in feet below ground surface. Top left image is in the borehole casing, and  represents a known 6-inch diameter. There is a wide variety in the regularity, diameter, and coherence of the borehole.
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