The 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.

A new downhole groundwater sampler reduces bias and error due to sample handling and exposure while introducing minimal disturbance to natural flow conditions in the formation and well. This "In Situ Sealed", "ISS", or "Snap" sampling device includes removable/lab-ready sample bottles, a sampler device to hold double end-opening sample bottles in an open position, and a line for lowering the sampler system and triggering closure of the bottles downhole. Before deployment, each bottle is set open at both ends to allow flow-through during installation and equilibration downhole. Bottles are triggered to close downhole without well purging; the method is therefore "passive" or "nonpurge". The sample is retrieved in a sealed condition and remains unexposed until analysis. Data from six field studies comparing ISS sampling with traditional methods indicate ISS samples typically yield higher volatile organic compound (VOC) concentrations; in one case, significant chemical-specific differentials between sampling methods were discernible. For arsenic, filtered and unfiltered purge results were negatively and positively biased, respectively, compared to ISS results. Inorganic constituents showed parity with traditional methods. Overall, the ISS is versatile, avoids low VOC recovery bias, and enhances reproducibility while avoiding sampling complexity and purge water disposal.

Where well water and formation water are compositionally different or heterogeneous, pump effluent composition will vary due to partial mixing and transport induced by pumping. Investigating influences of purging and sampling methodology on composition variability requires quantification of wellbore flow regimes and mixing. As a basis for this quantification, analytical models simulating Poiseuille flow were developed to calculate flow paths and travel times. Finite element modeling was used to incorporate influences of mixing. Parabolic velocity distributions within the screened interval accelerate with cumulative inflow approaching the pump intake while an annulus of inflowing formation water contracts uniformly to displace an axial cylinder of pre-pumping well water as pumping proceeds. Increased dispersive mixing forms a more diffuse formation water annulus and the contribution of formation water to pump effluent increases more rapidly. Models incorporating viscous flow and diffusion scale mixing show that initially pump effluent is predominantly pre-pumping well water and compositions vary most rapidly. After two screen volumes of pumping, 94% of pump effluent is inflowing formation water. Where the composition of formation water and pre-pumping well water are likely to be similar, pump effluent compositions will not vary significantly and may be collected during early purging or with passive sampling. However, where these compositions are expected to be considerably different or heterogeneous, compositions would be most variable during early pumping, that is, when samples are collected during low-flow sampling. Purging of two screen volumes would be required to stabilize the content and collect a sample consisting of 94% formation water.

This field study was conducted to examine whether the method or the flow rate (fill rate) used to fill a sample vial affects recovery of volatile organics (VOCs). To our knowledge, there have not been any systematic studies that have examined this issue. For this field study, three fill rates (50 mL/min, 250 mL/min, and ~1 L/min) and three filling methods (top‐pour, side‐pour, and bottom‐fill) were used to fill sample vials. We found that the bottom‐fill method, with the tubing submerged in the sample as it fills, yielded the greatest recovery (i.e., highest concentrations) of VOCs. Little improvement was observed by pouring down the side of a vial vs. simply pouring straight down from the top. We also found that filling the vials at the fastest fill rate (~1 L/min) yielded higher recovery than the slowest fill rate (50 mL/min) using all three filling methods. These results are counter to prevailing guidance and conventional wisdom that slower filling is preferable to faster filling and that pouring down the side of a sample vial is the best practice for VOC sampling. However, because we were unable to randomize the order the samples were collected with respect to fill rate, we recommend a follow‐on study be conducted that will allow us to confirm our findings and better determine which fill rates minimize losses of VOCs

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