Evaluating Preferential Recharge in Blue Ridge Aquifer Systems Using Saline Tracers

Multiple saline tracers were used to explore the role of geologic structure on groundwater recharge at the Fractured Rock Research Site in Floyd County, Virginia. Tracer migration was monitored through soil, saprolite, and fractured crystalline bedrock for a period of 3 months with chemical, physical, and geophysical techniques. Potassium chloride (KCl) and potassium bromide (KBr) tracers were applied at specific locations on the ground surface to directly test flow pathways in a shallow saprolite and deep fractured rock aquifer.

Previous work at the Fractured Rock Research Site have identified an ancient thrust fault complex that is present in the otherwise competent metamorphic bedrock; fracturing along this fault plane has resulted in a highly transmissive aquifer that receives recharge along the vertically oriented portion of the fault zone. A shallow aquifer has been located above the thrust fault aquifer in a heterogeneous saprolite layer that rapidly transmits precipitation to a downgradient spring.

Tracer monitoring was accomplished with differential electrical resistivity, chemical sampling, and physical monitoring of water levels and spring discharge. Tracer concentrations were monitored quantitatively with ion chromatography and qualitatively with differential resistivity surveys. KCl, applied at a concentration of 10,000 mg/L, traveled 160 meters downgradient through the thrust fault aquifer to a spring outlet in 24 days. KBr, applied at a concentration of 5,000 mg/L, traveled 90m downgradient through the saprolite aquifer in 19 days. KCl and KBr were present at the sampled springheads for 30 days and 33 days, respectively. Tracer breakthrough curves indicate diffuse flow through the saprolite aquifer and fracture flow through the crystalline thrust fault aquifer. Heterogeneities in the saprolite aquifer had a large effect on tracer transport, with breakthrough peaks varying several days over vertical distances of several meters.

Monitoring saline tracer migration through soil, saprolite, and fractured rock provided data on groundwater recharge that would not have been available using other traditional hydrologic methods. Travel times and flowpaths observed during this study support preferential groundwater recharge controlled by geologic structure. Geologic structure, which is not currently considered an important factor in current models of Blue Ridge hydrogeology, should be evaluated on a local or regional scale for any water resources investigation, wellhead protection plan, or groundwater remediation project.