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Development and Application of Numerical Models to Estimate Fluxes through the Regional Aquifer beneath the Pajarito Plateau

Los Alamos Natl. Lab., Earth and Environmental Sciences Division, T003, Los Alamos, NM 87544

View larger version (48K): [in a new window]   Fig. 1. The Española Basin and vicinity, with basin-scale numerical model outline shown in red, site-scale model outline shown in green. Black arrows are generalized groundwater flow directions, based on regional water level data (Keating et al., 2003). Striped arrows indicate groundwater flow between this basin and adjacent basins. Circled numbers refer to USGS stream gages: 1, Rio Chama at Chamita; 2, Rio Grande at San Juan; 3, Santa Cruz River; 4, Santa Clara Creek; 5, Rio Grande at Otowi; 6, Rio Frijoles; 7, Rio Grande at Cochiti. Circled "A" indicates the mouth of the Pojoaque Creek (see Table 4).

View larger version (126K): [in a new window]   Fig. 2. The Pajarito Plateau, with major well fields indicated.

View larger version (16K): [in a new window]   Fig. 3. Storm volume-weighted means in oxygen isotope values from 3 yr of precipitation, plotted as a function of precipitation station elevation, derived from Adams et al. (1995) and Anderholm (1994).

View larger version (55K): [in a new window]   Fig. 4. Elevation of the potentiometric surface (meters) beneath the Pajarito Plateau. Elevations are derived principally from wells with short screens near the top of the aquifer, either indicated by a red circle (data collected post-2000) or a yellow circle (pre-2000). Further descriptions of the older measurements are discussed in Fig. 9 legend.

View larger version (34K): [in a new window]   Fig. 5. Head data from cross section through southern portion of the plateau. (Note: PM-2 is a Los Alamos County water supply well.)

View larger version (65K): [in a new window]   Fig. 6. Top view of the site-scale model grid. The LANL boundary is indicated, as well as trace of hypothetical vertical plane (green line) used for flux analysis.

View larger version (92K): [in a new window]   Fig. 7. Site-scale model grid, colored according to major hydrostratigraphic units.

View larger version (26K): [in a new window]   Fig. 8. Comparison of simulated and measured hydrographs for representative wells on the plateau. Model parameters shown in Table 2.

View larger version (29K): [in a new window]   Fig. 9. Comparison of measured and simulated present-day heads at the top of the saturated zone along two east–west transects. *These data were collected in wells that are no longer accessible for measuring. LA4 and LA1B are in a well field that has been shut down since the early 1990s; all wells have shown significant recovery since they were shut down. The lower circle shows the most recent measurement available. The upper circle shows the water level measured during the initial drilling, which would approximate current water levels if the wells had fully recovered. H-19 has not been measured since 1949. Measured drawdowns in this portion of the aquifer have been very small. In 1997 the furthest western well (TW-4), closer to water supply wells than H-19, had declined <1 m since 1950.

View larger version (41K): [in a new window]   Fig. 10. Estimates of total recharge and flux through a vertical plane east of LANL, according to four sets of model parameters.

View larger version (20K): [in a new window]   Fig. 11. Simulated discharge to the Rio Grande and estimated proportion of production in local well fields that originates as storage and as captured recharge.

View larger version (13K): [in a new window]   Fig. 12. Sensitivity of predicted percentages of production in 2004 coming from aquifer storage to values of Ss (in parentheses). The vertical axis is the sum of squared errors x 10–3, in meters x 10–3, for 929 water level measurements in 75 wells, from 1946 to 2004. Numbers indicate predicted percentages.

View larger version (22K): [in a new window]   Fig. 13. Measured January flow at the Otowi Gage, compared with (a) contributing flow at Rio Chama, Rio Grande at San Juan, minor tributaries (Table 4), and estimated baseflow, and (b) measured January flow at the Cochiti gage. Numbers refer to reaches in Table 4.