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

primarily agricultural) and decreased baseflow because of groundwater production. For example, in 2002 the Before recent drilling and characterization efforts in the vicinity State of New Mexico was unable to honor interstate of Los Alamos National Laboratory (LANL), conceptual models had been developed for recharge and discharge in the regional aquifer on stream compacts for surface water delivery to Texas and the basis of sparse data. By integrating site-wide data into a numerical incurred substantial penalties. There are also concerns model of the aquifer beneath the plateau we provide new insight into about water quality at the regional scale because of a large-scale aquifer properties and fluxes. This model is useful for variety of contaminants, both nonanthropogenic and anunderstanding hydrologic mechanisms, assessing the magnitudes of thropogenic. Beneath the Pajarito Plateau, there is subdifferent terms in the overall water budget, and, through sampling, for stantial contamination from various LANL sources in interpreting contaminant migration velocities in the overlying vadose shallow groundwaters in some locations (primarily alluzone. Modeling results suggest that the majority of water produced vial aquifers), and it is unclear what the ultimate impact in well fields on the plateau, extracted at rates approaching 70% of of this contamination will be on the regional aquifer in total annual recharge, is derived from storage. This result is insensitive the future. Some of the LANL-derived contamination to assumptions about the percentage of total recharge that occurs in the near vicinity of water supply wells, because of strong anisotropy has been observed in the regional aquifer at trace conin the aquifer that prevents fast transport of local recharge to deeper centrations much below the EPA drinking water stanunits from which production occurs. Robust estimates of fluxes in the dards. To assess the future water quality and quantity shallow portion of the aquifer immediately down gradient of LANL issues, 21 deep characterization wells have been drilled are important for contaminant transport simulations. Our model calsince 1995, and flow and transport models have been culations show that these fluxes have decreased in the past 50 years developed both at the siteand basin-scale. by approximately 10% because of production in water supply wells. Historical liquid effluent discharges in canyons are To explore the role of parameter uncertainty in flux prediction, a the most likely sources of this deep groundwater conpredictive analysis method was applied. Results showed that predicted tamination. These contaminants must migrate through flux through older basalts in the aquifer can vary by a factor of three the unsaturated rocks of the vadose zone before reachbecause of uncertainty in aquifer properties and total recharge. We explored the impact of model parameter uncertainty on these results; ing the regional aquifer. Some of the most convenient however, the true uncertainty of our predictions, including the impact sampling locations for groundwater contamination are of possible conceptual model errors, is likely to be larger and is difficult the wells drilled to the regional aquifer, where samples to quantify. can be obtained by pumping screened intervals. These samples provide important information on the rates of movement of water and contaminants in the vadose zone. G beneath the Pajarito Plateau is part However, to interpret the results, a basic understanding of a regional aquifer that extends throughout the of the flow conditions in the regional aquifer is required. Española Basin (an area roughly 6000 km2; Fig. 1). This Therefore, studies of the regional aquifer serve the puraquifer is the primary source of water for the LANL; pose of providing a stronger basis for evaluating the the communities of Santa Fe, Española, Los Alamos; vadose zone travel times and contaminant transport beand numerous pueblos. Four water supply well fields havior. exist on the plateau (Fig. 2). One additional well field Beneath the Pajarito Plateau, the aquifer is very deep that supplies the City of Santa Fe (Buckman) sits just (up to 360 m below ground surface). The thick vadose to the east of Rio Grande close to the plateau. As is zone is quite complex hydrologically (Birdsell et al., the case for many aquifers in the semiarid southwest, 2005) and includes perched aquifers in some locations. there is concern that current withdrawal rates may not One emphasis of the recent groundwater characterizabe sustainable for long periods of time, and current tion efforts has been to provide more quantitative estidrought conditions might have significant impacts on mates of recharge through the vadose zone (Birdsell et both surface water and groundwater quantity and qualal., 2005; Kwicklis et al., 2005). The most obvious ratioity. Of particular concern is surface water flow in the nale for doing so has been to identify likely pathways Rio Grande, which is reduced both by direct diversions and fluxes for contaminant transport through the vadose zone. A second, perhaps less obvious rationale, has been E. Keating, B. Robinson, and V. Vesselinov, Los Alamos Natl. Lab., to determine fluxes through the regional aquifer, which Earth and Environmental Sciences Division, T003, Los Alamos, NM in turn allow better estimation of aquifer properties, 87544. Received 30 June 2004. *Corresponding author (ekeating@lanl. groundwater velocities, contaminant fate and transport, gov). and water quantity issues. The importance of estimating Published in Vadose Zone Journal 4:653–671 (2005). recharge for water resource evaluation has been quesSpecial Section: Los Alamos National Laboratory doi:10.2136/vzj2004.0101 © Soil Science Society of America Abbreviations: LANL, Los Alamos National Laboratory; PM, Pajarito Mesa. 677 S. Segoe Rd., Madison, WI 53711 USA 653 Published online August 16, 2005