The relationship between the ground water sample and the surrounding ground water environment is controlled by wellborn flow and mixing that carry ground water to the sampling device. Controlled laboratory conditions and uniform inflow and inflowing concentration allowed detailed investigations of wellbore concentration responses to pump‐induced flow and mixing independent of external complicating influences. During pumping from the top of the screen, the results agreed with wellbore flow theory except for high‐frequency, high‐amplitude concentration fluctuations and mixing with the blank casing due to slight density contrasts (∼0.005%). Concentration fluctuations indicated partial mixing of ground water with well water and declining concentrations in the blank casing indicated that convection due to the slight density contrast overcame pump‐induced velocities. During pumping from the bottom of the well, opposing forces of buoyancy and pumping produced small‐scale cumulative mixing as the ground water from different portions of the screen approached the pump. Even during low‐flow pumping (i.e., 220 mL/min), 5% of the water was prepurge well water after five well volumes were pumped. In the field the density contrasts that resulted in these mixing processes might be caused by a slight cooling of the wellhead or a slight increase in temperature with depth (>0.2°C/m). If the temperature gradient was more pronounced, the convective mixing of screen water with the potentially altered casing water would be more thorough and sampling without mixing would not be possible. If the temperature gradient is stable or if the water in the blank casing is not significantly altered, casing water mixing will not present a problem.
Mathematical models that simulate common monitoring well sampling demonstrate the distortions that vertical concentration averaging causes during the mapping and modeling of an idealized, three‐dimensional contaminant plume emanating from a simple source of constant solute concentration. The apparent extent of the plume, mapped using simulations of a regular grid of screened monitoring wells, ranged from a worst case of 0% of the original plume area for long screens (4 m) in a low‐permeability formation to 90% for short screens (1 m) in a high‐permeability formation. When well design and purging procedure were inconsistent among wells, the mapped plume exhibited spurious directional skewing, bifurcation, zones of low concentration, intermittent sources, or multiple sources. Although the study plume was not retarded, calibrating a transport model to the monitoring well data resulted in retardation factors of up to 23. If first‐order decay was assumed, the apparent decay constant was found to be as much as 1.8 × 10‐7 sec‐1 (T1/2= 45 days). Apparent retardation or decay was inconsistent from well to well, depending on the saturated screen length, the degree of screen desaturation during purging, and the distance from the source. The study indicates that the quantitative assessment of contaminant distributions and transport processes requires discrete vertical sampling in the common situation where concentrations vary sharply with depth, even in the most ideal hydrogeologic environment. If screened monitoring wells are used, screen lengths and placements should be appropriate to the contamination situation being assessed and inherent biases must be considered. Even so, vertical concentration averaging biases and the resulting inconsistencies can result in highly misleading evaluations of ground‐water contamination problems.
Ground water data variability and sample repsentativeness ness are controlled by internal monitoring‐well flow and mixing, processes that require further investigation. By defining a representative sample using flow weighted average concentrations and modeling laminar wellbore flow, this study uses deviations from those representative concentrations as a measure of data variability. The laminar wellbore flow model relates ground water concentration and permeability distributions to the concentration response at a pump during purging or sampling. Given laminar wellbore flow, even if the concentrations within the well are not chemically altered, transient responses of concentrations to pumping lead to various amounts of variability depending on the screen length and position relative to hydraulic and chemical heterogeneities. In the simplest case, uniform inflow and inflowing concentrations, the concentration response to pumping is the same as the concentration response with thorough mixing. In more complex situations involving ground water concentration and permeability heterogeneities, pumped concentrations are controlled by those heterogeneities and wellbore flow. A model that describes concentration responses in the presence of linear heterogeneities allows a first‐order approximation of variability during pumping. Using field screening of concentration and permeability heterogeneities, this first‐order approximation can be used to design screen lengths that will limit the variability to study‐specific tolerances. In the case of existing wells, measurement of inflows and concentration variability will allow the assessment of variability during pumping. This investigation demonstrates that, in order to decipher sources of data variability, investigations must examine internal wellbore processes in detail as well as the relationships to external conditions characterized in the field.
The current understanding of Lake Warren as a proglacial lake stage in the Lake Erie basin during the last deglaciation is based on limited stratigraphic information from strandlines and a wide range of radiocarbon ages. The purpose of this study is to use ground-penetrating radar (GPR) and optically stimulated luminescence (OSL) dating to reconstruct the stratigraphy, depositional environment, and age of the Oak Openings Ridge (OOR), a former strandline of Lake Warren in northwestern Ohio. Both sedimentary exposures and >4 km of GPR data were used to demonstrate that the OOR is a barrier spit that migrated from the northeast to the southwest, and is currently blanketed by an aeolian sheet. Sediments observed in exposures show a shallowing-up sequence attributed to the retreat of proglacial lakes from the area. Corresponding GPR data reveal three distinct GPR facies. The lowermost radar facies 1 (RF1) is a sandy barrier spit platform of a lower beach face prograding across finergrained lacustrine mud or till. Eroded into RF1 is an upper beach face of RF2, the top of which is visible in cutbank and borrow pit exposures. Overlying the RF2 beach face is uppermost unit RF3, consisting of low-relief, aeolian parabolic dunes and sand sheets. Four OSL ages from a climbing ripple sequence in RF2 average 14.2 ± 0.5 ka, consistent with an earlier published OSL age of 14.1 ± 1.0 ka (Campbell et al. in 2011) from the same unit. From earlier work, the overlying aeolian dunes (RF3) record westerly winds after formation of the OOR, and OSL dating records episodic activity from the Younger Dryas chronozone to ϳ8000 years ago. The results suggest that the OOR formed in two phases. First, a barrier spit prograded into Lake Warren from the northeast. Second, parabolic sand dunes and a sand sheet formed episodically for ϳ5000 years thereafter. The sediment source for the sand body is from southeast Michigan, but it is of uncertain origin.Résumé : Notre compréhension actuelle du lac Warren en tant qu'une étape de lac proglaciaire dans le bassin du lac Érié durant la dernière déglaciation est basée sur de l'information stratigraphique limitée d'anciennes lignes de rivage et d'une vaste plage d'âges 14 C. Le but de la présente étude est d'utiliser des données géoradar et de datation par luminescence stimulée optiquement (OSL) pour reconstruire la stratigraphie, l'environnement de déposition et l'âge de la crête Oak Openings (OOR), une ancienne ligne de rivage du lac Warren dans le nord-ouest de l'Ohio. Des affleurements de roches sédimentaires et plus de 4 km de données géoradar ont été utilisés pour démontrer que l'OOR est une flèche littorale qui a migré du nord-est au sud-ouest et qui est actuellement recouverte d'une couche de dépôts éoliens. Les sédiments observés dans les affleurements montrent une séquence moins profonde vers le haut attribuée au retrait des lacs proglaciaires de la région. Les données géoradar correspondantes révèlent trois faciès géoradar distincts. Le faciès géoradar inférieur (RF1) est une plateforme de flèc...
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.
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