Assessing Groundwater Flow Velocity at Multiple Measurement Scales

Characterizing groundwater flow velocity in heterogeneous aquifers is one of the major uncertainties in designing and implementing in situ remedial technologies.  Here we describe the measurement of groundwater flow velocity at multiple length scales to assess the influence of spatial hydrogeological heterogeneity on the design of an in-situ bioremediation system.  Standard hydraulic testing methods, including pumping tests, slug tests and point dilution tracer tests, are compared to a novel in-situ groundwater velocity probe. The Point Velocity Probe (Labaky et al., 2009), or PVP, is a small scale measurement device for determining groundwater flow velocity at the borehole scale, which is typically in the range 2 4 inches.

A hydraulic testing array was installed to a depth of approximately 75 feet in a fining downward sequence of stratified glacial sands without distinct lithological contacts.  Previous investigations indicate an upper sand unit of high to moderate hydraulic conductivity and a lower silty unit with lower hydraulic conductivity.  Pumping tests and a borehole dilution test, using potassium bromide as the conservative tracer, were undertaken in well completions that were screened in the upper and lower aquifer units.  The PVP was installed with multiple measurement ports in both the upper and lower units of the aquifer.  Estimates of groundwater flow velocity at the scale of 10s feet were inferred from a combination of pumping test hydraulic conductivity data and local hydraulic gradient measurements. The borehole dilution test measured flow velocity at the 1ft scale and the PVP measured flow velocity at the scale of inches.

We discuss the outcomes of the study, which focus on the measurement of groundwater flow velocity at different measurement scales. The implications of the findings for the design of an in-situ enhanced bioremediation system are discussed and recommendations for future hydraulic testing are proposed.