Failing to account for barometric pressure effects in water level measurements can introduce errors by misestimating the total head and by adding noise to water level measurements. For determining the total head in an aquifer, we assert that the air pressure head at the water surface in the well must be added to measured water levels (equivalent to using an absolute pressure transducer) even though the resulting values may have larger temporal and spatial variability than the original water level measurements. At the Savannah River Site in South Carolina, the average barometric pressure variation is 6 to 7 cm, with a range of over 30 cm. Failure to account for barometric pressure variability could result in misestimation of the direction and magnitude of the hydraulic gradient at the site. We also demonstrate procedures for removing barometric effects, such as to reduce noise during an aquifer pumping test, and to identify mechanisms by which barometric pressure affects water levels. Three mechanisms are summarized including: an instantaneous response for confined aquifers; a delayed response due to borehole storage in confined and unconfined aquifers; and a delayed response in unconfined aquifers due to the passage of barometric pressure changes through the unsaturated zone. Using data from the Savannah River Site, barometric efficiencies are estimated using linear regression and a modification of Clark's Method. Delayed responses are estimated using regression deconvolution. The type of barometric effect provides diagnostic information about whether the aquifer is confined or not, the presence of borehole storage or skin effects, and the air diffusivity coefficient within the unsaturated zone. We also show how removal of barometric pressure effects improves the ability to observe otherwise unnoticeable effects.
breakthrough curves are often used to estimate model parameters, including effective porosity, finger width, Saprolite is a form of weathered bedrock that is commonly used as mobile zone porosity, and the retardation factor. Each the host material at waste disposal sites in the Southeastern Piedmont. However, estimating the unsaturated hydraulic and transport proper-approach provides valuable information about the beties of saprolite is difficult due to saprolite's low permeability. We havior of solute transport through unsaturated media. demonstrate the use of short-duration fluid irrigation pulses for main-Water potential responses are also used to estimate taining unsaturated conditions in intact saprolite columns. Concomihydraulic parameters (Inoue et al., 1998). In these cases, tant Cl Ϫ tracer experiments demonstrate that irrigated waters moved perturbations at the surface, from precipitation and irrithrough an effective volumetric porosity (0.038-0.108 cm 3 cm Ϫ3) subgation, and within the subsurface, from fluid injections stantially less than the ambient water-filled porosity (0.44 cm 3 cm Ϫ3). or removals, are monitored and used for parameter esti-We observed the unexpected result that irrigation-induced pressure mation. The unsaturated hydraulic conductivity, hywave velocities (1983-3670 cm d Ϫ1) were ≈ 1000 times faster than draulic diffusivity, or sorptivity are determined in these tracer velocities (2.04-6.00 cm d Ϫ1). The relationship between pressure wave velocities and fluid velocities is described using kinematic wave cases. Perturbations due to fluid injection or extraction, theory, presented for four parametric representations (Brooks-Corey, or the modification of the pressure head at a surface or van Genuchten-Mualem, Broadbridge-White, and the Galileo Numpoint, induce a step or spike change in fluid pressure ber), that predicts fluid pressure velocities to be from approximately that is transmitted through the unsaturated zone. The two to fifteen times faster than saprolite tracer velocities. None of pressure wave response, also called the kinematic the kinematic models was able to reproduce observed rapid pressure model, has been used to model subsurface stormflow wave velocities. A hydraulic form of the advection-diffusion equation (Beven, 1982), vertical flow through unsaturated soils based on Richards' equation is presented that favorably predicts the
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