IDENTIFICATION AND CONTROL OF AMBIENT FLOWS IN OBSERVATION WELLS

It is well known that vertical head gradients exist in natural aquifer systems, and borehole flow-meter data have shown that such gradients commonly set up spontaneous vertical flows in monitoring wells, often called ambient flows. Using a measured ambient flow distribution at the Savannah River Site, Elci et al. (Ground Water,36(6), 853-862, 2001) showed that even after full purging or micro-purging, data from such wells will yield misleading and ambiguous information concerning solute concentrations, location of a contaminant source and plume geometry. For a given horizontal hydraulic conductivity (K) distribution and vertical head gradient, ambient flows increase with aquifer thickness, and they may be suppressed by increasing the well-bore hydraulic resistance and/or reducing screen penetration. On a first order basis, the results for homogeneous aquifers may be used to estimate the behavior of heterogeneous aquifers by computing a power-law average of the heterogeneous K(z). From the analysis of vertical gradients on well water levels (Elci et al., Journal of Hydrology, in review, 2002), it is shown also that in the presence of high vertical hydraulic gradients (Grad(h) >0.5), it is physically possible for a well screen to be fully submerged below the water table, and yet have an internal water level below the top of the screen. Contrary to common perceptions, water levels in fully-penetrating wells can deviate significantly from the elevation of the formation water table when the local vertical hydraulic gradient exceeds about 0.01.