An existing model was modified in recognition of new geohydrologic interpretations and adjusted to simulate hydrographs in well fields in the Los Alamos area. Hydraulic-head drawdowns at the Buckman well field resulting from two projected ground-water-withdrawal alternatives were estimated with the modified model. The Chaquehui formation (informal usage) is the main new feature of recent hydrologic interpretations for the Los Alamos area. The Chaquehui occupies a 'channel' that was eroded or faulted into the Tesuque Formation, and the Chaquehui is more permeable than the Tesuque. The Chaquehui is a major producing zone in the Pajarito Mesa well field and to a lesser extent in the Guaje well field. Model modification included splitting the four layers of the McAda-Wasiolek model (McAda, D.P., and Wasiolek, Maryann, 1988, Simulation of the regional geohydrology of the Tesuque aquifer system near Santa Fe, New Mexico: U.S. Geological Survey Water- Resources Investigations Report 87-4056, 71 p.) into eight layers to better simulate vertical ground-water movement. Other model modifications were limited as much as possible to the area of interest near Los Alamos and consisted mainly of adjusting hydraulic-conductivity values representing the Tesuque Formation, Chaquehui formation (informal usage), and Puye Formation, and adjusting simulated recharge along the Pajarito Fault Zone west of Los Alamos. Adjustments were based mainly on simulation of fluctuations in measured hydraulic heads near Los Alamos. Two possible alternative plans for replacing Guaje well field production were suggested by Los Alamos National Laboratory. In the first plan (Guaje alternative), the Guaje field would be renewed with four new wells replacing the existing production wells in the Guaje field. In the second plan (Pajarito-Otowi alternative), the Guaje well field would be retired and its former production would be made up by additional withdrawals from the Pajarito Mesa and Otowi well fields. A projection for each of these alternatives was made through 2012 using the new eight- layer model. In the Guaje field, projected hydraulic heads at the end of 2012 were as much as 50 feet lower with the Guaje alternative; in the Pajarito Mesa field, hydraulic heads were as much as 12 feet higher with the Guaje alternative. At the western end of the Los Alamos well field, projected hydraulic heads were about 20 feet higher with the Guaje alternative; at the eastern end of the Los Alamos field, the difference between alternatives was much less. At the Buckman field, projected hydraulic heads were about 2 feet higher with the Guaje alternative because the Buckman field is closer to the Pajarito Mesa field than to the Guaje field. Ways of improving the understanding of the flow system include developing a more accurate representation of the structure and extent of the Tesuque Formation, Chaquehui formation, and Puye Formation of the Santa Fe Group and obtaining more detailed geologic and hydrologic data for the Chaquehui and Puye. Data that describe water chemistry, hydraulic head, and degree of saturation would be valuable for determining the location and quantity of recharge on the Pajarito Plateau, especially along the west side of the Plateau and in canyon bottoms. Chloride concentrations in soil at the top of the Bandelier Tuff could be used to verify the concept that evapotranspiration accounts for nearly all precipitation over a large area of the plateau.