Modeling the response of groundwater levels in wells to changes in barometric pressure

In many cases, water levels in wells are observed to fluctuate significantly in response to changes in barometric pressure. In this study, a physically based conceptual model for the influence of barometric pressure on groundwater wells was developed and tested;It is proposed that water level fluctuations in response to barometric pressure are due, in large part, to the different manner in which the pressure is propagated through the water column in the well and the porous media outside the well. Changes in pressure transmit through the water column in the well to the screened region with essentially negligible loss in pressure. On the contrary, pressure changes transferred through the porous media to the screened elevation outside the well undergo an irreversible transformation of fluid potential (head loss). Consequently, the loss in pressure head through the porous medium causes a lateral hydraulic head gradient to be developed around the well-screen region, as well as a vertical one through the porous medium. In response to the head gradient developed due to changes in barometric pressure, groundwater flows are induced through the well screen, with subsequent changes in well-casing storage. In the proposed model the well itself is an essential element. The well-water flux across the screen and the consequent change in wellcasing storage were appropriatety linked with groundwater flow in the surrounding porous medium and estimated through an iteration technique. This approach incorporates the traditional governing theories on groundwater flow: conservation of mass and Darcy\u27s Law; Groundwater was modeled as two-dimensional (radial and vertical) unsteady flow, and solved by using finite element approximations. The basic concept of the model was successfully applied to the modeling of slug tests and furthermore it was demonstrated that a series of slug/bad tests and effects of barometric pressure on wells are theoretically related to each other in physical and numerical senses through the principle of super position;The results suggest that the physically based model in this study is very effective in estimating the water level fluctuations in a well due to changes in barometric pressure. The magnitude and behavior of the well response varies with the hydraulic properties (hydraulic conductivity and specific storage) and well geometry (casing radius, screened length, and depth of well)

Hydraulic properties of Quaternary stratigraphic units in the Walnut Creek watershed

Accurate estimates of the hydraulic properties of fine-grained units are necessary to predict the transport of contaminants in groundwater and assess the feasibility of remediation in these units. There are few data available for these aquitard units in comparison to their aquifer counterparts. The objective of this study was to determine the hydraulic properties, specifically transmissivity (T), hydraulic conductivity (K), and storativity (S), of some Quaternary stratigraphic units in the Walnut Creek watershed of central Iowa. Piezometers were installed by the overcored Shelby Tube method and some were oriented at a 45 degree angle to the ground surface in order to investigate the effects of fractures. Estimates of hydraulic conductivity from slug tests ranged from 6 x 10⁻¹¹ m/s in Wisconsinan colluvium to 7 x 10⁻⁵ m/s in Holocene alluvium. Oxidized till units showed a range of K values from 4 x 10⁻⁷ m/s for Pre-Illinoian till (N = 2) to a geometric mean of 2 x 10⁻⁶ m/s (N = 12; log transformed standard deviation (ltsd) = 1.4) in the late Wisconsinan till. A pumping test performed in the oxidized late Wisconsinan till and analyzed with the Theis method (with Jacob correction; Jacob, 1963) showed K values that were higher than those estimated from slug tests. For the pumping tests, T = 3 x 10⁻⁵ m²/s (N = 4; ltsd = 0.062), S = 0.12 (N = 4; ltsd = 0.06), and K = 1 x 10⁻⁵ m/s (N = 4; ltsd = 0.15). Results from the Neuman analysis (Neuman, 1972) were similar and estimates of specific yield (Sᵧ) values were nearly identical to the S values of both methods. No significant difference was observed between vertical and angled piezometers, which suggests that till may behave as an equivalent porous medium at this testing scale. However, vertical anisotropy (KᵣKᶻ) for the oxidized till ranged from 80 to 125 and these values may reflect the fractured nature of the material. A second pumping test was performed in the oxidized Pre-Illinoian till that is confined by a thin layer of unoxidized late Wisconsinan till and overlain by an unconfined aquifer in oxidized late Wisconsinan till. Using the Hantush and Jacob (1955) analysis, the pumping test yielded values of T = 6 x 10⁻⁷ m²/s, S = 3 x 10⁻⁵, and K = 1 x 10⁻⁷ m/s. This K value is nearly identical to that estimated from the slug test (4 x 10⁻⁷ mis), but is lower than in oxidized till units from the first pumping test. The vertical K' of the aquitard was estimated to be 8 x 10⁻⁹ m/s, which is a value similar to that estimated from unoxidized late Wisconsinan till (1 x 10⁻⁹ m/s). The "Ratio Method" (Neuman and Witherspoon, 1972) was used to estimate the aquitard diffusivity (α'), which was determined to be 2.5 x 10⁻⁷ m²/s. The results of this study suggest that pumping tests and slug tests using angled and vertical piezometers will provide valuable data for groundwater contamination and remediation studies in these fine-grained units in Iowa