The Dye Tracing Pages

Hydrogeologic Investigation of the Saad-Trousdale Superfund Site, Nashville, Tennessee

STATEMENT OF PROBLEM Potential karst groundwater flow with possible rapid flow to distant springs. Due to the proximity of the three sites, determination of groundwater flow under any one required an investigation of the other two. SPECIFICS OF PROJECT The investigation included the following: a) hydrogeologic research, b) mapping of the potentiometric surface of the uppermost aquifer and c) dye traces of groundwater flow. HYDROGEOLOGIC INVENTORY All streams and their tributaries were walked and all springs and tributaries were marked on a topographic map. A water well investigation was made but only three accessible water wells were found in the area. A Geoguard TUBER pressure transducer and data logger was installed at Croft Spring at Grassmere Wildlife Park (the suspected groundwater discharge point for the Saad and Radnor Yard Sites) to record stage and temperature. Figure 1 is a plot of stage vs. precipitation at Croft Spring. POTENTIOMETRIC SURFACE INVESTIGATION The following information was used in constructing the potentiometric surface map: a) Water levels in monitoring wells, b) Water levels at springs (surveyed from benchmarks), c) Water levels along perennial streams estimated from a five-foot contour interval topographic map, d) Water levels in the storm sewer system at Radnor Yard, measured at manholes, and e) Spring locations and topography. A potentiometric surface map was prepared for both the wet season and the dry season in order to determine whether the groundwater drainage divides moved throughout the season. Figure 2 is a map of the potentiometric surface during the dry season and Figure 3 is a map of the potentiometric surface during the wet season. DYE TRACE INVESTIGATION Passive dye receptors were placed in springs, streams, wells, drainpipes, and storm sewers identified during the hydrogeologic inventory. ISCO Automatic Water Samplers were installed at Croft Spring and the Radnor Yard Storm Sewer Outfall (the two suspected discharge points for groundwater from the Saad and Radnor sites). Six dyes were injected into existing monitor wells, pits and one drilled dye injection well. Figure 4 shows the dye flow routes for each particular dye along with the potentiometric surface and the groundwater drainage divides. HYDROGEOLOGIC CONCEPTUAL MODEL The hydrogeologic profile along line ABCDEFGH (Figures 5 & 6) indicates that Croft Spring is located at the top of the Hermitage Formation along its contact with the Bigby-Cannon Limestone. Many of the springs in the area are located along or just below this contact. The argillaceous limestone and shale beds within the Hermitage appear to be responsible for diverting the downward movement of groundwater forcing it to flow laterally to springs. Although small amounts of leakage may be sinking through the Hermitage confining layer into the underlying Carters Limestone, the hydrogeologic evidence indicates that most of the groundwater recharge into the uppermost aquifer in the fill/residuum and Bigby-Cannon Limestone discharges from springs along or near the top of the Hermitage. However, dye traces were needed to test this hypothesis.