The Pajarito Plateau is an important source of abundant potable groundwater for Los Alamos National Laboratory (LANL) and the communities of Los Alamos and White Rock. Geologic investigations were undertaken as part of a plateau-wide hydrogeological investigation to develop conceptual models of the groundwater system as a framework for numerical simulations of groundwater flow. The Pajarito Plateau is located in the western part of the Espanola basin where rocks of the Jemez and Cerros del Rio volcanic fields overlie and interfinger with Neogene basin-fill sedimentary rocks. The vadose zone is about 200 m (600 ft) thick beneath mesas on the east side of the plateau and more than 375 m (1245 ft) thick on the west side. Groundwater occurs as shallow groundwater in canyon-floor alluvium, moderately deep groundwater perched in bedrock units of the vadose zone, and groundwater associated with the regional saturated zone. The most productive rocks of the regional aquifer occur in a westward-thickening wedge of coarse-grained, Miocene and Pliocene volcaniclastic rocks derived from the Jemez volcanic field. Eastern aquifer rocks consist of fine-grained, somewhat less productive Miocene sedimentary deposits derived from highland sources to the east and north. Intermediate and mafic lavas interbedded with the Miocene and Pliocene sedimentary deposits are components of the regional aquifer locally. The hydrogeology of the Pajarito Plateau is probably typical of groundwater systems along the margins of the Rio Grande rift where arid to semiarid, sediment-filled basins receive most of their recharge from adjacent mountainous areas.

Geologic Framework of a Groundwater System on the Margin of a Rift Basin, Pajarito Plateau, North‐Central New Mexico

Large-scale models are frequently used to estimate fluxes to small-scale models. The uncertainty associated with these flux estimates, however, is rarely addressed. We present a case study from the Espanola Basin, northern New Mexico, where we use a basin-scale model coupled with a high-resolution, nested site-scale model. Both models are three-dimensional and are analyzed by codes FEHM and PEST. Using constrained nonlinear optimization, we examine the effect of parameter uncertainty in the basin-scale model on the nonlinear confidence limits of predicted fluxes to the site-scale model. We find that some of the fluxes are very well constrained, while for others there is fairly large uncertainty. Site-scale transport simulation results, however, are relatively insensitive to the estimated uncertainty in the fluxes. We also compare parameter estimates obtained by the basin- and site-scale inverse models. Differences in the model grid resolution (scale of parameter estimation) result in differing delineation of hydrostratigraphic units, so the two models produce different estimates for some units. The effect is similar to the observed scale effect in medium properties owing to differences in tested volume. More important, estimation uncertainty of model parameters is quite different at the two scales. Overall, the basin inverse model resulted in significantly lower estimates of uncertainty, because of the larger calibration dataset available. This suggests that the basin-scale model contributes not only important boundary condition information but also improved parameter identification for some units. Our results demonstrate that caution is warranted when applying parameter estimates inferred from a large-scale model to small-scale simulations, and vice versa.

Coupling basin- and site-scale inverse models of the Española aquifer.

Before recent drilling and characterization efforts in the vicinity of Los Alamos National Laboratory (LANL), conceptual models had been developed for recharge and discharge in the regional aquifer on the basis of sparse data. By integrating site-wide data into a numerical model of the aquifer beneath the plateau we provide new insight into large-scale aquifer properties and fluxes. This model is useful for understanding hydrologic mechanisms, assessing the magnitudes of different terms in the overall water budget, and, through sampling, for interpreting contaminant migration velocities in the overlying vadose zone. Modeling results suggest that the majority of water produced in well fields on the plateau, extracted at rates approaching 70% of total annual recharge, is derived from storage. This result is insensitive to assumptions about the percentage of total recharge that occurs in the near vicinity of water supply wells, because of strong anisotropy in the aquifer that prevents fast transport of local recharge to deeper units from which production occurs. Robust estimates of fluxes in the shallow portion of the aquifer immediately down gradient of LANL are important for contaminant transport simulations. Our model calculations show that these fluxes have decreased in the past 50 years by approximately 10% because of production in water supply wells. To explore the role of parameter uncertainty in flux prediction, a predictive analysis method was applied. Results showed that predicted flux through older basalts in the aquifer can vary by a factor of three because of uncertainty in aquifer properties and total recharge. We explored the impact of model parameter uncertainty on these results; however, the true uncertainty of our predictions, including the impact of possible conceptual model errors, is likely to be larger and is difficult to quantify.

http://nuclearactive.org/wp-content/uploads/2014/06/Keating-et-al..pdf

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

For the period 1998 through 2001, the total water used at Los Alamos from all sources ranged from 1325 million gallons (Mg) in 1999 to 1515 Mg in 2000. Groundwater production ranged from 1323 Mg in 1999 to 1506 Mg in 2000 from the Guaje, Pajarito, and Otowi fields. Nonpotable surface water used from Los Alamos reservoir ranged from zero gallons in 2001 to 9.3 Mg in 2000. For years 1998 through 2001, over 99% of all water used at Los Alamos was groundwater. Water use by Los Alamos National Laboratory (LANL) between 1998 and 2001 ranged from 379 Mg in 2000 to 461 Mg in 1998. The LANL water use in 2001 was 393 Mg or 27% of the total water use at Los Alamos. Water use by Los Alamos County ranged from 872 Mg in 1999 to 1137 Mg in 2000, and averaged 1006 Mg/yr. Four new replacement wells in the Guaje field (G-2A, G-3A, G-4A, and G-5A) were drilled in 1998 and began production in 1999; with existing well G-1A, the Guaje field currently has five producing wells. Five of the old Guaje wells (G-1, G-2, G-4, G-5, and G-6) were plugged and abandoned in 1999, and one well (G-3) was abandoned but remains as an observation well for the Guaje field. The long-term water level observations in production and observation (test) wells at Los Alamos are consistent with the formation of a cone of depression in response to water production. The water level decline is gradual and at most has been about 0.7 to 2 ft per year for production wells and from 0.4 to 0.9 ft/yr for observation (test) wells. The largest water level declines have been in the Guaje field where nonpumping water levels were about 91 ft lower in 2001 than in 1951. The initial water levels of the Guaje replacement wells were 32 to 57 ft lower than the initial water levels of adjacent original Guaje wells. When production wells are taken off-line for pump replacement or repair, water levels have returned to within about 25 ft of initial static levels within 6 to 12 months. Thus, the water-level trends suggest no adverse impacts by production on long-term water supply sustainability at Los Alamos. This report summarizes production data and aquifer conditions for water production and monitor wells in the Los Alamos, New Mexico, and Los Alamos National Laboratory (LANL) area (Figure 1). Water production wells are grouped within the Guaje, Pajarito, and Otowi fields, the locations of which are shown on Figure 1. Wells from these fields supply all the potable water used for municipal and most industrial purposes in Los Alamos County (LAC), at LANL, and at Bandelier National Monument. This report has three primary objectives: (1) Provide a continuing historical record of metered well production and overall water usage; (2) Provide data to the Department of Energy (DOE) and LANL management, and Los Alamos County planners for operation of the water supply system and for long-range water resource planning; and (3) Provide water-level data from regional aquifer production wells, test wells, and monitoring wells.

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Water Supply at Los Alamos 1998-2001

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Water supply at Los Alamos during 1973

This report summarizes production and aquifer conditions for water wells in the Los Alamos, Guaje, and Pajarito Well Fields . The wells supply all of the potable water used for municipal and some industrial purposes in Los Alamos County and the Los Alamos National Laboratory. The spring gallery in Water Canyon supplies nonpotable water for industrial use while the rest of the nonpotable water supply used for irrigation is surface water from the Guaje and Los Alamos Reservoirs. Included is a section on the chemical and radiochemical quality of water from the supply wells, gallery in Water Canyon and the reservoirs in Guaje and Los Alamos Canyons. A section on the quality of water with reference to compliance with state and federal regulations is included in the report.

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Water supply at Los Alamos during 1991

This report summarizes production and aquifer conditions for water wells in the Guaje, Pajarito, and Otowi Well Fields. These wells supplied all of the potable water used for municipal and some industrial purposes in Los Alamos County and the Los Alamos National Laboratory during 1993. The wells in the Los Alamos Well Field were transferred to San Ildefonso Pueblo in 1992. Four of the wells in the Los Alamos Well Field were plugged in 1993. One of the two new wells in the Otowi Well Field became operational in 1993. The spring gallery in Water Canyon supplied nonpotable water for industrial use, while surface water from the Los Alamos Reservoir was diverted for irrigation. In 1993 no water was used from the Guaje Reservoir. Due to the maintenance and operating cost of diverting water from the reservoirs, it is not economically feasible to continue their use for irrigation. This report fulfills some of the requirements of the Los Alamos Groundwater Protection Management Program by documenting use of the groundwater for water supply and providing information hydrologic characteristics of the main aquifer. This report is a joint effort between the Laboratory Water Quality and Hydrology Group and the Utilities Department of Johnson Controls World Services Inc. (JCI). The purpose of this report is to ensure a continuing historical record and to provide guidance for management of water resources in long-range planning for the water supply system. We have issued one summary report for the period of 1947 to 1971 and 22 annual reports that contain the results of our studies of these water supplies. An additional report summarized the hydrology of the main aquifer with reference to future development of groundwater supplies. A report was issued in 1988 that examined the status of wells and future water supply.

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Water supply at Los Alamos during 1993. Progress report

Production of potable municipal water supplies during 1994 totaled about 1,426.6 million gallons of wells in the Guaje, Pajarito, and Otowi Fields. The non-potable water supply for industrial use was about 11.6 million gallons from the spring gallery in Water Canyon. There was no water used for irrigation from Guaje or Los Alamos Reservoirs; thus, the total water usage in 1994 was about 1,438.2 million gallons. Pumps in Guaje Well 5 and Otowi Well 4 failed during the year and were not operational by the end of 1994. Water production resumed in Pajarito Well 3 in June. Wells in the Los Alamos Field, on Pueblo Land, were plugged and abandoned in 1992, or were transferred to San Ildefonso Pueblo. This report fulfills requirements which require the Laboratory to monitor and document groundwater conditions below Pajarito Plateau, and to protect the main aquifer from contamination associated with Laboratory operations by providing information on hydrologic characteristics of the main aquifer, including operating conditions of the municipal water supply system.

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Water supply at Los Alamos during 1994. Progress report

Municipal potable water supply during 1992 was 1,516 {times} 10{sup 6} gallons from wells in the Guaje and Pajarito well fields. About 13 {times} 10{sup 6} gallons were pumped from the Los Alamos Well Field and used in the construction of State Road 501 adjacent to the Field. The last year the Las Alamos Field was used for municipal supply was 1991. The nonpotable water supply used for steam plant support was about 0.12 {times} 10{sup 6} gallons from the spring gallery in Water Canyon. No nonpotable water was used for irrigation from Guaje and Los Alamos Reservoirs. Thus, the total water usage in 1992 was about 1,529 {times} 10{sup 6} gallons. Neither of the two new wells in the Otowi Well Field were operational in 1992.

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Water supply at Los Alamos during 1992

Municipal potable water supply during 1990 was 1,642 [times] 10[sup 6] gallons from wells in three fields. The nonpotable water supply used exclusively for industry was about 9 [times] 10[sup 6] gallons from the spring gallery in Water Canyon. ne nonpotable water used for irrigation was 2.2 [times] 10[sup 6] gallons from the Guaje Reservoir and 4.6 [times] 106 gallons from the Los Alamos Reservoir. Thus, the total water usage in 1990 was about 1,658 [times] 10[sup 6] gallons. Water supply was satisfactory in that production met demand, and water quality in the distribution system was in compliance with State and Federal regulations. A new well, Otowi-1, was completed in August 1990 at a depth of 2,497 ft with a water level of 675 ft. Step and aquifer tests indicated that the well will have a yield of from 800 to 1,000 gallons per minute when construction is completed.

A.K. Stoker

Papers

Water supply at Los Alamos during 1991

Water supply at Los Alamos during 1993. Progress report

Water supply at Los Alamos during 1994. Progress report

Water supply at Los Alamos during 1992

Water supply at Los Alamos during 1990