Testing the In‐Well Horizontal Laminar Flow Assumption with a Sand‐Tank Well Model

Abstract

AbstractThe assumption of horizontal laminar flow within a monitoring well is a commonly cited basis for interval sampling using low‐flow or no‐purge sampling techniques. A few studies have shown horizontal flow over short distances within the well for short periods of time. Others have demonstrated specific circumstances under which the assumption fails. But surprisingly, little focus has been given to confirming 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 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 largely mimic real world conditions of a submerged well in a moderately high‐permeability sand. To observe flow behavior in the simulated well, a dye “stringer” was introduced into an injection port upgradient of the simulated well. In all tests, regardless of flow rate or small density differences, the dye stringer eventually mixed throughout the model monitoring well. Since the model approximates a section of an at‐scale well subjected to real world bulk flow rates, mixing appears to be the rule rather than the exception for near–neutrally buoyant contaminant stringers in homogeneous flow fields. Despite additional heterogeneities introduced by field conditions, there are several clear and important implications of this study: (1) some degree of in‐well mixing and flow‐weighted concentration averaging may occur 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; and (5) vertical stratification within an aquifer may not be accurately monitored by sampling multiple intervals within an open well.