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| STATEMENT OF PROBLEM As part of a remedial investigation, a chemical company was required to have a karst investigation performed. SPECIFICS OF PROJECT Dye tracing and mapping of the potentiometric surface was used to determine groundwater flow at the site. Work began in December 1995 and was completed in May 1997. SITE DESCRIPTION AND SCOPE OF WORK The boundaries of the study area were controlled by the water table elevations in the area and ground water discharge points. The study area around the facility is bounded by a creek to the north, east, and south and the topographically higher area to the west. The site is underlain by up to 40 feet of residual soils. The soils consist of brown, stiff, silty clay with occasional high angle fractures and are the result of in-place weathering of the underlying rock. The underlying rock for most of the site consists of hundreds of feet of gently dipping Paleozoic limestones. The uppermost ground water at the site occurs in unconfined conditions in the soil overlying the fractured rock. Faults in the bedrock were not expected to significantly affect aquifer properties at the site. A potentiometric surface map for the site was developed. The water level data used to develop the potentiometric surface map consisted of ground water measurements of the site monitoring wells and ground-surface elevations obtained from the city Sewer Index Map which had 2-foot topographic contour intervals. The sewer maps were used to estimate elevations of the stream and spring monitoring points. The potentiometric surface map indicates ground-water flow in four general directions from the site: north, northeast, southeast, and south. General dye injection locations were chosen based on potential source areas with respect to ground-water flow directions from the site to the north, northeast, southeast, and south. Four different types of dye were used during the Dye Tracer Study: After the dye was injected, the wells were flushed with potable water to push or flush the dye out of the well and into the formation. Due to the low percolation rates at each location, potable water was continually flushed into the wells for several weeks to ensure the dyes were pushed out of the wells. Locations were monitored for over one year for dye. CONCLUSIONS AND RECOMMENDATIONS Although there are large areas within the study area that are obviously well-developed (mature) karst, the karst hydrogeologic inventory did not reveal any of the features usually associated with well-developed (mature) karst in the vicinity of the site. The inventory did not locate any sinkholes, sinking streams, caves or springs flowing from bedrock conduits. Instead, the area has surface streams, many soil seeps, small springs, ponds and swamps, all indicative of a high water table and poor subsurface drainage. The water table in most of the monitoring wells on site is well above bedrock, often indicative of slow drainage in the underlying bedrock. All dye injection wells drilled had very low capacities for injecting water into the aquifer. Although, the site does not have the appearance of well-developed (mature) karst, this research did indicate some ground water flow through conduits in the underlying carbonate bedrock. Estimates of the velocity of ground water beneath the site based on slug tests and evaluated by the Bouwer and Rice Method indicated the groundwater velocities were in the range of feet per year. The following results of the Dye Tracer Study indicate that ground water velocity is much faster (feet per day) but still relatively slow when compared to most karst aquifers. The results of the Dye Tracer Study, support the historical interpretation of ground-water flow from the site. |
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