Mnw2 also offers an improved capability to simulate

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Unformatted text preview: lated to changes in lift for a given pump, and intraborehole flows with a pump intake located at any specified depth within the well. MNW2 also offers an improved capability to simulate nonvertical wells. A new output option allows selected multi-node wells to be designated as “observation wells” for which changes in selected variables with time will be written to separate output files to facilitate postprocessing. MNW2 is compatible with the MODFLOW–2000 and MODFLOW–2005 versions of MODFLOW and with the version of MODFLOW that includes the Ground-Water Transport process (MODFLOW–GWT). Introduction Wells that have relatively long open (uncased) intervals or screens may be hydraulically connected to multiple aquifers or to different parts of a single aquifer where the hydraulic heads differ. Such wells (or boreholes) can provide preferential pathways to flow and solute transport that short-circuit natural predevelopment fluid flowlines, whether the well is pumped or unpumped (such as a long-screen observation well). Although the hydraulic head in the aquifer can vary along the length of the borehole, there would only be a single water level within the well itself. Flow can occur within these wells in response to the varying differences in hydraulic head between the well and the aquifer along the length of the borehole in addition to flow caused by pumping stresses. Although the net pumpage from a well may be known, the flow between the aquifer and the borehole can vary substantially along the length of the borehole, and that distribution is generally unknown. If a nonpumping well has a relatively long open interval or well screen and (or) is open to multiple aquifers, then the borehole may be adjacent to materials having a range of hydraulic heads. This can (and will) induce flow within a nonpumping well. This phenomenon has long been recognized, and much effort has been directed towards the development and use of geophysical methods to measure the flow in the borehole (for example, see Izbicki and others, 1999; Paillet and others, 2002) and methods to simulate and characterize the flow (for example, see Giddings, 1987; Kaleris, 1989; Reilly and others, 1989; Hanson and Nishikawa, 1996). It is also recognized that long-screen wells or long open boreholes with intraborehole flow potentially provide pathways for contaminants to move from one location to another in a ground-water flow system (Konikow and Hornberger, 2006a). Examples of flow through long boreholes are illustrated in figure 1, which shows a two-aquifer system in which the transmissivities of the two aquifers are identical but their head distributions are different. Well A is a nonpumping well open to both the upper and lower aquifers. Because the head in the lower aquifer is higher than the head in the upper aquifer, the head in the well will equilibrate to some intermediate value between the two aquifer heads at the location of the well. Consequently, the head in the well will be lower than that in the lower aquifer, and there will be flow into the well at that horizon. Likewise, because the head in the well will be higher than that in the upper aquifer, there will be flow out of the well and into the upper aquifer. Even though it is a nonpumping well, there is upwards flow through the borehole and this flow transfers water between the two aquifers. Well B in figure 1 is a pumping well that is also open to both the upper and lower aquifers. Because of the pumpage, the head in the well is lower than in either aquifer. However, more flow enters the well in the lower aquifer than in the upper aquifer, even though the transmissivities are identical, because the head gradient between the aquifer and the well is greater in the lower aquifer than in the upper aquifer. 2 Revised Multi-Node Well (MNW2) Package for MODFLOW Ground-Water Flow Model Figure 1. Schematic cross section showing flow patterns that can be induced by a multiaquifer well and simulated by the MNW2 Package (modified from Halford and Hanson, 2002). Well A is a nonpumping well, well B is a pumping well, T is transmissivity, hWELL is the water level in the well, and Q is discharge rate from the well (in MODFLOW convention, discharge has a negative sign). In a numerical ground-water simulation model, long wells, which are connected to more than one node of the grid, must be represented and simulated accurately to understand and predict their effects on the local and regional flow fields. Traditional approaches for simulating wells in numerical models, such as the Well (WEL) Package of MODFLOW (McDonald and Harbaugh, 1988; Harbaugh and McDonald, 1996; Harbaugh and others, 2000; Harbaugh, 2005) assume that the well is connected to a single node of the grid representing the ground-water flow system and that the water level in the well is identical to the head at the connected node. For a long well, however, flow rates between the well and each node are unknown, and the numerical method must be able to calculate the single water level in the well, whether the well is pumping or nonpumping and calculate the contribution of...
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This document was uploaded on 01/20/2014.

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