Bandelier National Monument (BNM) was created to protect an extraordinary inventory of archaeological resources carved in the Tshirege Member of the Bandelier Tuff. These include more than one thousand excavated chambers, called cavates, used for dwelling, storage, and textile production. The glass-rich tuffs at the base of the Tshirege Member are poorly consolidated and susceptible to erosion by wind, rain, and mechanical abrasion, with resultant loss of cultural material. However, rock surfaces develop protective weathering rinds that are resistant to erosion. Using optical microscopy, SEM-EDS, XRD, and electron microprobe analysis, we determined that this rind consists of clay and silt sediments colonized by lichens and other surface biota, accompanied by the precipitation of secondary minerals in the near-surface pore space. Scoping experiments focused on glass-organic acid interactions indicate that oxalic acid excreted by microbial crust constituents catalyzes biogeochemical reactions that lead to the preferential dissolution of Si, Al, and Fe components of the volcanic glass; these cations become available for precipitation of opal, and smectite and sepiolite clays. Enzyme assays that quantify biological activity at outcrop surfaces indicate that microbial populations initially thrive as they derive nutrients from the dissolution reactions of the glass, but activity starts to decline as precipitation of secondary minerals limits access to new sources of nutrients, so that alteration processes are self-limiting. As case hardening progresses, imbibition rates at the surface decrease, and the erosion resistance of the altered surfaces is substantially improved. This article presents summary results of research conducted over a period of five years to characterize the roles of lichens and other microflora in rind formation, and the resulting contributions to tuff stability. The interaction of lichens and other microflora with rock surfaces in archaeological sites and monuments is usually explored in terms of biodeterioration and consequent damage. However, this study shows that, under some circumstances, lichens and microflora provide a level of erosion protection to relatively porous and unconsolidated rock strata that outweighs their biodeteriorative effects.

This report consists of four major sections, including this introductory section. Section 2 provides an overview of previous investigations related to the development of the current sitescale model. The methods and data used to develop the 3-D groundwater model and the techniques used to distill that model into a form suitable for use in the GoldSim models are discussed in Section 3. Section 4 presents the results of the model development effort and discusses some of the uncertainties involved. Eight attachments that provide details about the components and data used in this groundwater pathway model are also included with this report. The groundwater modeling effort reported here is a revision of the work that was conducted in 2005 (Stauffer et al., 2005a) in support of the 2008 Area G performance assessment and composite analysis (LANL, 2008). The revision effort was undertaken primarily to incorporate new geologic information that has been collected since 2003 at, and in the vicinity of, Area G. The new data were used to create a more accurate geologic framework model (GFM) that forms the basis of the numerical modeling of the site’s long-term performance. The groundwater modeling uses mean hydrologic properties of the geologic strata underlying Area G; this revision includes an evaluation of the impacts that natural variability in these properties may have on the model projections.

The Cerro Toledo Formation (CTF), a series of intracaldera rhyolitic dome complexes and their associated extracaldera tephras and epiclastic sedimentary deposits, records the dynamic interplay between volcanic, tectonic, and geomorphic processes that were occurring along the western margin of the Rio Grande rift between major caldera-forming eruptions of the Bandelier Tuff 1.65–1.26Ma. The Alamo Canyon and Pueblo Canyon Members differ significantly despite deposition within a few kilometers of each other on the Pajarito Plateau. These differences highlight spatial distinctions in vent sources, eruptive styles, and depositional environments along the eastern side of the Jemez Mountains volcanic field during this ca. 400,000year interval. Intercalated pyroclastic fall deposits and sandstones of the Pueblo Canyon Member reflect deposition with a basin. Thick Alamo Canyon Member deposits of block-and-ash-flow tuff and pyroclastic fall deposits fill a paleovalley carved into coarse grained sedimentary units reflecting deposition along the mountain front. Chemistry and ages of glass from fall deposits together with clast lithologies of sedimentary units, allow correlation of outcrops, subsurface units, and sources. Dates on pyroclastic fall deposits from Alamo Canyon record deep incision into the underlying Otowi Member in the southern part of the Pajarito Plateau within 100k.y. of the Toledo caldera-forming eruption. Reconstruction of the CTF surface shows that this period of rapid incision was followed by aggradation where sediments largely filled pre-existing paleocanyons. Complex sequences within the upper portion of the Otowi Member in outcrop and in the subsurface record changes in the style of eruptive activity during the waning stages of the Toledo caldera-forming eruption.

Reduction of toxic Cr(VI) to less toxic Cr(III) is an important process for attenuating Cr(VI) transport in groundwater. This process results in immobilization of chromium as Cr(III) and effectively decreases the overall mobility of the chromium inventory. During both abiotic and biotic reduction of Cr(VI) to Cr(III), a kinetic isotope effect occurs in which the lighter isotope, 52Cr, reacts preferentially, leaving the remaining dissolved Cr(VI) enriched in the heavier isotope, 53Cr. Cr isotopes have proven to be a useful tool for estimating the magnitude of Cr(VI) reduction and for determining where in a hydrologic system reduction is occurring. In this paper, we discuss patterns of reduction in perched-intermediate and regional aquifer systems contaminated with Cr(VI) related to historical use of potassium dichromate as an anticorrosion agent in cooling towers at a power plant at the Los Alamos National Laboratory in northern New Mexico. We utilize Cr isotopes to assess the relative effects of mixing and reduction on measured δ53Cr in groundwater, with an emphasis on where in the system reduction occurs. Chromium isotope measurements provide strong evidence for reduction of Cr(VI) in vadose zone basalts.

New Ar-40/Ar-39 results from drill-hole cuttings of basaltic and basaltic andesite flows from the Guaje well field of the Pajarito Plateau along the western part of the Espanola Basin in north-central New Mexico yielded Middle Miocene ages (11.5-13.2 Ma). The volcanic eruptions were closely associated with intense faulting, subsidence, and sedimentation, and the results provide age constraints for the volcanic and tectonic processes along the western margin of the Espanola Basin. The Middle Miocene volcanic rocks are interbedded within the Santa Fe Group, which is divided into the Hernandez and Vallito Members of the Chamita Formation and the Chama El Rito Member of the Tesuque Formation, in descending stratigraphic order. New and published geochemical results from the Guaje well field and from other surface and subsurface mafic and intermediate lava flows within the Pajarito Plateau suggest that the volcanic rocks erupted from different magmatic sources and centers close to the Pajarito fault zone. Multiple pulses of volcanic eruptions mostly confined to the hanging wall of the Pajarito fault zone, which represents the current western boundary bull of the Espanola Basin, suggest that the Pajarito fault system has been sporadically reactivated several times, beginning at least in the Middle Miocene and continuing to the Plio-Pleistocene. Moreover, the volcanic, tectonic, and sedimentary records in the Pajarito Plateau suggest that there is no evidence for eastward migration of tectonic and volcanic activities from the Canada de Cochiti fault zone in the southern part of the Jemez Mountains to the Pajarito fault zone during the early Pliocene (4-5 Ma).

A series of site investigations and decision-support analyses have been performed related to a chromium plume in the regional aquifer beneath the Los Alamos National Laboratory (LANL). Based on the collected data and site information, alternative conceptual and numerical models representing governing subsurface processes with different complexity and resolution have been developed. The current conceptual model is supported by multiple lines of evidence based on comprehensive analyses of the available data and modeling results. The model is applied for decision-support analyses related to estimation of contaminant-arrival locations and chromium mass flux reaching the regional aquifer, and to optimization of a site monitoring-well network. Plume characterization is a challenging and nonunique problem because multiple models and contamination scenarios are consistent with the site data and conceptual knowledge. To solve this complex problem, an advanced methodology based on model calibration and uncertainty quantification has been developed within the computational framework MADS (http://mads.lanl.gov). This work implements high-performance computing and novel, efficient and robust model analysis techniques for optimization and uncertainty quantification (ABAGUS, Squads, multi-try (multi-start) techniques), which allow for solving problems with large degrees of freedom.

Numerous wells in addition to stratigraphic sections of surface exposures provide a robust data set to examine Santa Fe Group stratigraphic relations near the White Rock and Buckman areas of the Española Basin. Here, wells penetrate Santa Fe Group strata, ranging in age from ca. 13.5 to 8.5 Ma, that locally underlie Plio-Pleistocene strata. Surface exposures of Santa Fe group strata range in age from ca. 12 to 8.5 Ma. Santa Fe Group strata are characterized by fluvial deposits of sandy to gravelly channel-fills intercalated with floodplain deposits of clay, silt, very fineto fine-grained sand, and silty sand. Most of the aquifer under the Buckman well field consists of ancestral Rio Grande fluvial deposits belonging to the Vallito Member of the Chamita Formation, which overlies finer-grained, basin-floor deposits of the Pojoaque Member of the Tesuque Formation. The Vallito Member is a very pale brown to pink to light gray unit dominated by subrounded (minor rounded and subangular), relatively clean, chertand volcanic-bearing, quartz-dominated sand that is locally frosted. Vallito Member gravels consist of very fine to coarse pebbles. The Vallito Member interfingers westward with light gray volcaniclastic sediment of the Hernandez Member (Chamita Formation) west of Buckman, deposited by an ancestral Rio Chama derived from the northwest that flowed alongside and merged with the ancestral Rio Grande. A high degree of mixing occurs between the Vallito and Hernandez Members within 1-4 km of their interfingering zone, which extends about 7 km to the west of the Rio Grande at Buckman. The gravel fraction of the Hernandez Member in the study area includes very coarse pebbles and cobbles, and is dominated by subrounded to rounded, dark gray to greenish dacites-andesites with less than 15% quartzite. Locally, the Vallito Member overlies, and interfingers eastward with, fluvial deposits of the Cejita Member of the Tesuque Formation. However, it appears that the northeast-derived river associated with the Cejita Member merged with the ancestral Rio Grande north of Buckman. More commonly, the Vallito Member interfingers eastward with granite-bearing alluvial-slope deposits of the Cuarteles Member of the Chamita and Tesuque Formations. As is the case throughout the Española Basin, the Cuarteles Member here progressively prograded westward in the middle to late Miocene. A probable angular unconformity and a general down-section increase of dips indicate that the Santa Fe Group in the study area was deposited during active west-tilting of the Española Basin halfgraben. We interpret a westward increase of stratal tilts as related to subsidence-related flexure on the eastern side of an intrabasin half-graben, marked by a pronounced low Bouguer gravity anomaly. The lack of significant playa or lacustrine deposits in these strata indicates that closed basin conditions did not exist during 8.5 to 13.5 Ma. 210 KONING, BROXTON, SAWYER, VANIMAN & SHOMAKER west of the Rio Grande in the study area, with only minor remnants on the east side of the river along White Rock Canyon. Although the stratigraphic relations of the Pliocene and Pleistocene units are generally obvious from surface exposures, those of the underlying Santa Fe Group cannot be understood without the aid of subsurface data. Understanding these relations is important for interpreting how groundwater flows in the aquifer – both in the cone of depression surrounding the Buckman well field and also beneath the Pajarito Plateau to the west, where movement of contaminants from Los Alamos National Laboratory pose a concern. Numerous site-specific studies at Buckman and White Rock document subsurface geology and hydrogeologic conditions at particular well sites. However, a comprehensive stratigraphic framework for the area has not been published previously. Furthermore, the existing geologic map of the White Rock Canyon quadrangle (Dethier, 1997) does not subdivide the Santa Fe Group. In this study, we examine the sedimentologic properties of various lithostratigraphic units in the Santa Fe Group for the area, and then correlate these to the stratigraphic nomenclature for the Santa Fe Group in the Española Basin established by Galusha and Blick (1971), Cavazza (1986), and Koning and Aby (2005). Also, we relate how the sedimentologic properties of these units result in certain patterns or signatures of down-hole geophysical data. Lastly, we discuss tectonic and hydrogeologic implications interpreted from our stratigraphic data. METHODS AND STRATIGRAPHIC FENCE DIAGRAMS The lead author revised the White Rock quadrangle geologic map of Dethier (1997) by differentiating the Santa Fe Group into the lithostratigraphic units discussed in this paper (Figs. 2, 8; Dethier and Koning, 2007). He also measured and described strata in three stratigraphic sections illustrating the exposed lithologic units (Fig. 2; see also Dethier and Koning, 2007). We then compiled subsurface data from the wells drilled at Buckman and White Rock (Figs. 1, 2). At Buckman, various well data are available for the Buckman-1 through -9 water supply wells, the old-Buckman-#6 well, the Skillet observation well, the OSE-USGS Buckman monitoring well, and the Nuclear Dynamic #34 exploratory borehole (ND-34). Under the Pajarito Plateau near White Rock, pertinent wells include R-9, R-10, R-12, R-13, R-16, R-22, R-34, and PM-1. This subsurface data set includes cuttings logs of various quality, borehole geophysical logs, and samples of cuttings from Buckman-9, R-10, and R-16. Cuttings from R-10 and R-16 were collected at 5-ft intervals and examined with a hand lens and petrographic scope. Cuttings from Buckman-9 were collected at 10-ft intervals and examined with a hand lens. In addition, we examined detailed written descriptions of the cuttings from the Buckman-1 and -2 wells (courtesy of John Shomaker and Associates, Inc.), which were sampled at 5 ft-intervals. We cannot rely solely on cuttings for complete textural characterizations because of mixing and possible incomplete returns as cuttings are carried up the borehole by muddy drilling fluid. Rather, cuttings were utilized primarily for compositional characterizations and used together with standard geophysical logs to document relative changes in grain sizes with depth. For the Buckman well field, geophysical logs include resistivity, spontaneous potential, and gamma measurements – with some wells having neutron and sonic data (e.g., Buckman-9 and the OSE-USGS Buckman monitoring well). Wells R-16 and R10 near White Rock (Figs. 3, 4) contain these standard logs in addition to relatively recent innovations to borehole geophysical logging, such as the Formation Micro-Imager (electrical conductivity images and bedding orientations); Triple Detector Litho-Density (bulk density and photoelectric factor); Natural Gamma Spectroscopy (gross natural gamma and potassium, thorium, and uranium concentrations); Combinable Magnetic Resonance (porosity and pore-size distributions); and Elemental Capture Spectroscopy (neutron-induced gamma spectroscopy for eight rock-forming elements and hydrogen). Higher resistivity values typically indicate lower clay content, relatively clean sand, or cementation. Varying borehole diameters between wells may influence the data obtained from these logs, particularly the intensity or magnitude of a reading. Using these geophysical borehole tools in conjunction with outcrop and cuttings lithologic data, we then constructed three FIGURE 1. Location map of study area relative to the cities of Santa Fe and White Rock, New Mexico. Locations of the two cross-section lines (A-A’ and B-B’) together with relevant wells are also depicted. 211 SURFACE AND SUBSURFACE STRATIGRAPHY OF THE SANTA FE GROUP west-east stratigraphic fence diagrams through the study area. The first correlates the outcrop stratigraphic sections across the river and into well R-16 (Fig. 5). The second correlates subsurface strata between Buckman wells 1, 2, 3a, 7, 8, and 9-most (1, 2, 3a, 9) of which have adequate cuttings descriptions (courtesy of John Shomaker and Associates; Fig. 6). The third west-east fence diagram depicts permeability-related stratigraphic relations between Buckman well 4, Nuclear Dynamic #34 exploratory borehole, and old Buckman #6 (Fig. 7). Stratigraphic relations in these diagrams were then compared with borehole geophysical and cuttings data from the other wells to produce a west-east cross-section (Fig. 8). We also scrutinized the borehole geophysical tool data to ascertain any patterns between the assorted geophysical data and the lithologic units.

A BSTRACT — Boreholes drilled for groundwater characterization at Los Alamos National Laboratory (LANL) encountered four petrographically and chemically distinct dacite lavas beneath the Pajarito Plateau. Coarsely porphyritic lavas with 17-38% phenocrysts (42-74% plagioclase, up to 28% pyroxene, 15-43% amphibole) and 64.9-66.3% SiO 2 occur in boreholes in the southwest part of LANL. Modal mineralogy and chemistry links these lavas to Cerro Grande, a Sierra de los Valles dacite dome west of the study area. Boreholes in the west-central part of LANL encountered a thin dacite lava with 22% phenocrysts (81-83% plagioclase, 12-15% pyroxene, and 1% relict amphibole) and relatively high SiO 2 (67.4%) overlying equally thin basaltic lavas of the Cerros del Rio volcanic field. Boreholes in the northern part of LANL encountered thicker fine-grained dacitic lava with 2-5% phenocrysts (up to 35% plagioclase, 61-88% pyroxene, absent or minor amphibole) and relatively low SiO 2 (~63.5%). One borehole in the east-central part of LANL encountered a sequence of three crystal-poor (4-6% pheno- crysts) dacite lavas, with ~66% SiO 2 , 4-6% resorbed quartz and trace amounts of resorbed olivine, intercalated with basalt. Pajarito Plateau dacite lavas erupted over a relatively short time interval (2.3-3.6 Ma) during which both the Jemez Mountains and Cerros del Rio volcanic fields were active. The southwestern group of dacites probably represents a thick lobe of Cerro Grande lava that flowed ~3.5 km eastward into the subsiding Española Basin. Other plateau dacites, which do not correlate with dacites exposed in the Sierra de los Valles, probably represent local eruptions within the western part of the basin. Thickness variations and spatial distribution indicate the northern group of dacites probably erupted from a buried vent near the Pajarito fault zone. The dacites encountered in the west-central and eastern parts of LANL represent minor pulses of dacite volcanism in areas dominated by basaltic volcanism of the Cerros del Rio volcanic field. The Pajarito Plateau dacites overlap spatially and temporally with intermediate volcanic rocks in the eastern part of the Jemez volcanic field and with mafic volcanic rocks in the western part of the Cerros del Rio volcanic field. The distinctive compositional and petrographic characteristics of the Plateau dacites probably reflect a transitional style of magmatism that developed in the narrow region between these adjacent, concurrently active volcanic fields.

Boreholes drilled for groundwater characterization at Los Alamos National Laboratory encountered at least three petrographically and chemically distinct intermediate lavas in the west-central Pajarito Plateau.These lavas represent a narrow time interval (2.3-3.6 Ma) during which both the Jemez Mountains volcanic field and Cerros del Rio volcanic field were active.