DYE “STRINGER” INTRODUCED IN SAND-TANK WELL-MODEL SHOWS IN-WELL DILUTION AND HOMOGENIZATION, NOT HORIZONTAL LAMINAR FLOW

Continual horizontal flow of formation water through a monitoring well screen is commonly cited as a basis for interval sampling using low-flow or no-purge sampling techniques. A few studies have shown horizontal flow for short periods of time and others have demonstrated specific circumstances under which the assumption fails. But surprisingly, this investigator could not find direct testing of the underlying concept—that under “normal” conditions (i.e. no vertical hydraulic gradient) water enters one side of a well and exits the other side of the well at roughly the same elevation.

To test the horizontal flow assumption, a physical sand tank model was constructed to observe flow-through in a simulated monitoring well. The well, filter pack, and aquifer on either side of the well largely mimics real-world conditions of a submerged well in a moderately high permeability sand. Behavior of a dye “stringer” was recorded using digital time-lapse photography at real-world ground water seepage velocities. Neutrally-buoyant and different-density dye “stringers” were observed entering the well over a period of several days for each test. In all tests, regardless of flow rate or small density differences, the dye stringer eventually diluted and mixed in the monitoring well.

Since the well model closely approximates a cross-section of an actual-size well subjected to real-world flow rates, mixing appears to be the rule rather than the exception for near-neutrally-buoyant contaminant stringers as they enter monitoring wells. There are several clear and important implications. 1) In-well mixing and flow-weighted concentration averaging occurs in a well before any purge or sampling efforts are made. 2) In-well mixing may mask low to moderate contaminant stratification in an aquifer. 3) Contaminant stratification, if present inside a well, implies strong contaminant stratification outside the well. 4) Contaminant stratification inside a well may not correspond to stratification at the same interval outside the well. 5) Vertical stratification within an aquifer may not be accurately monitored by sampling multiple intervals within an open well. 6) In-well barriers may limit mixing and assist in defining intervals of aquifer contaminant stratification.