Potassium dichromate (K 2 Cr 2 O 7 ) used as an anticorrosion agent and a biocide was discharged from a cooling tower from 1956 to 1972 to upper Sandia Canyon at Los Alamos National Laboratory (LANL), New Mexico.Between 31,000 to 72,000 kg of this chemical were discharged directly to surface water dominantly sourced from the cooling tower and another outfall releasing treated sewage effluent.Chromate (CrO 4 2-) migrated through the 300-meter thick vadose zone reaching the regional aquifer and created an extensive groundwater plume approximately 2.74 km in length and 1.13 km in width.Groundwater in the regional aquifer at LANL is aerobic and is characterized by a mixed calcium-sodium bicarbonate composition and a circumneutral pH.Chromate is mobile under these geochemical conditions and does not adsorb significantly onto ferric (oxy)hydroxide and clay minerals, with the highest concentration of dissolved chromium exceeding 800 μg/L or 0.80 mg/L at the site.The New Mexico Water Quality Control Commission (WQCC) groundwater standard for total dissolved chromium is 0.050 mg/L (50 μg/L).Chromium contamination occurs in the upper 33 meters of the saturated zone in the regional aquifer and is migrating to the east and southeast nearly at the same rate of groundwater flow, averaging approximately 42 meters per year.The chromium plume in the regional aquifer beneath Mortandad Canyon mixes with (1) a treated sewage effluent plume, containing boron, chloride, nitrate, sulfate, and other chemicals, released from Sandia Canyon near the cooling tower and (2) a groundwater plume consisting of treated-industrial effluent that contains 1,4-dioxane, fluoride, nitrate, perchlorate, tritium, uranium, and other chemicals and radionuclides.The centroid of the chromium plume has greater than 400 μg/L of dissolved chromium, between 150 and 240 pCi/L of tritium, from 4 to 8 mg/L of nitrate plus nitrite(N), and concentrations of chloride and sulfate typically exceeding 30 and 60 mg/L, respectively.Initial phases of aquifer remediation of the chromium plume are currently being evaluated by LANL and NMED, consisting of (1) pump and treat followed by reinjection and (2) injection of sodium dithionite and molasses to bio(geo)chemically reduce chromate to chromium(III).Injection of these two reductants result in precipitation of amorphous chromium hydroxide with dissolved concentrations of chromium(VI) typically less than 5 μg/L in zones of application.Geochemical modeling using PHREEQC was conducted to evaluate chromate reduction, reaction products, and aqueous speciation in the presence of 0.059 molar sodium dithionite and 0.057 molar sodium sulfite (pH buffer) at regional aquifer well R-42.Results of the PHREEQC simulations confirm that sodium dithionite initially promotes reductive dissolution of manganese dioxide (pyrolusite) and ferric hydroxide under reducing conditions, as sodium dithionite disproportionates ultimately to sulfate and hydrogen sulfide.Mackinawite and iron sulfide ppt, stable as transient intermediate phases, are calculated to approach equilibrium under reducing and acidic conditions prior to chromium(III) precipitation.Dissolved ferrous iron enhances precipitation of chromium(III) hydroxide under slightly oxidizing and circumneutral pH conditions with dissolved Mn(II) stable in groundwater.Manganese(II) is a redox buffer for chromium(III) hydroxide inhibiting reoxidation to chromium(VI).

Uranium is an actinide of considerable environmental interest present in aquifer systems worldwide.Dissolved concentrations of natural (background) uranium vary from less than 8.40e-09 M (2.0 µg/L) to 7.65e-06 M (1.82 mg/L) in groundwater east of the Rio Grande within the Española Basin, New Mexico.Total dissolved concentrations of natural uranium range from 5.04e-10 M to 5.76e-09 M (0.12 µg/L to 1.37 µg/L) in the regional aquifer beneath the Pajarito Plateau west of the Rio Grande.Dominant uranyl aqueous complexes consisting of UO 2 (CO 3 ) 2 2-, Ca(UO 2 ) 2 (CO 3 ) 3 0 , and UO 2 (CO 3 ) 3 4-are mobile under circumneutral pH and oxidizing conditions characteristic of the Santa Fe Group (Tesuque Formation).Oxidative dissolution of uranium(IV) minerals and hydrolysis of uranium(IV)-bearing silicates and oxides associated with Proterozoic granitic rocks in the Sangre de Cristo Mountains enhanced mobilization of uranium from source material.Hydrolysis of soluble uranium-bearing volcanic ash and granitic detritus present in the Tesuque Formation also contribute to highly variable uranium concentrations occurring in Santa Fe Group groundwater.Uranium(IV, VI) is associated with clay galls, opal, chert, fossil bone, carbonaceous material, smectite, and ferric (oxy)hydroxide in the San Jose mining district (Arroyo Seco and Oxide Butte) exposed as outcrops in the present-day vadose zone.The dominant uranium(VI) minerals identified in this mining district include carnotite (K 2 (UO 2 ) 2 V 2 O 8 •3H 2 O), metaautunite (Ca(UO 2 ) 2 (PO 4 ) 2 •2-6H 2 O), and schröckingerite (NaCa 3 (UO 2 )(CO 3 ) 3 (SO 4 )F•10H 2 O).Evapoconcentration of porewater in the Santa Fe Group likely was a critical process that enhanced solute saturation leading to precipitation of uranium(VI) minerals that have higher aqueous solubilities compared to uranium(IV) minerals, including uraninite and coffinite.Results of deionized (DI) water-leach tests and EPA 3050 partial digestions (pH1) performed on oxidized sediments collected from the San Jose mining district show that concentrations of dissolved uranium range from 3.21 to 52.21 µg/g and from 8.48 to 107.8 µg/g, respectively, using a ratio of 150 mL DI and acid to 100 g solid.Based on X-ray diffraction analyses, smectite varies from 45 to 68 weight percent in two samples collected from Oxide Butte, whereas this clay mineral is only present up to 1-2 weight percent in two samples collected from Arroyo Seco.Lower leachate concentrations of dissolved uranium suggest that this metal is strongly adsorbed and/or precipitated as uranyl phases on smectite surfaces abundant in the Oxide Butte samples.Higher concentrations of uranium were leached from the smectite-poor Arroyo Seco samples, suggesting weak adsorption of this actinide onto mineral surfaces.Desorption/dissolution coefficients for uranium based on ratios of 3050 digestion acid-to DI-leach results range from 3.10 to 5.74 mL/g and from 3.16 to 3.96 mL/g, for the Arroyo Seco and Oxide Butte samples, respectively.Oxyanions of arsenic, selenium, and vanadium represent competing adsorbates that may limit uranium(VI) adsorption onto smectite and ferric (oxy)hydroxide present at Oxide Butte.Higher concentrations of dissolved uranium(VI) occurring in oxidizing groundwater in other areas of the Española Basin are associated with increasing concentrations of dissolved sodium and decreasing concentrations of dissolved calcium.This suggests that exchange reactions result in adsorption of Ca 2+ onto exchange sites and release of Na + and UO 2 2+ to groundwater.

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.

LANL) has conducted multidisciplinary research on uranium since the mid-1940s.Treated and non-treated industrial aqueous discharges, mainly containing isotopically natural uranium with a 238 U/ 235 U atom ratio = 137.8813,have been discharged to Acid, Pueblo, Los Alamos, and Mortandad Canyons.These discharges provide recharge to shallow alluvial and perched-intermediate depth groundwater and the regional aquifer at typical depths of <24 meters (m), 183 m, and 296 m, respectively.Background water chemistry in the regional aquifer varies from an oxidizing (median Eh = 332 mV), calcium-sodium-bicarbonate to a sodium-calcium-bicarbonate composition.Small amounts of enriched uranium, containing a 238 U/ 235 U atom ratio <137.8813,have been locally measured in alluvial groundwater within Mortandad Canyon.The Environmental Protection Agency drinking water standard for total uranium and the New Mexico Water Quality Control Commission standard for dissolved uranium is 0.126 µM (0.030 mg/L).Upper tolerance limits have been calculated by the New Mexico Environment Department and LANL for numerous solutes naturally present in the regional aquifer.These include total dissolved uranium (5.336 nM, 0.00127 mg/L), dissolved oxygen (0.253 mM, 8.10 mg/L), nitrate(N) (0.056 mM, 0.78 mg/L), chloride (0.084 mM, 2.98 mg/L), perchlorate (4.324 nM, 0.00043 mg/L), and sulfate (0.061 mM, 5.84 mg/L).Background concentrations of dissolved uranium in the regional aquifer increase with average groundwater age, especially east of the Rio Grande.Concentrations of dissolved uranium(VI) elevated above background, with a maximum value of 0.0504 µM (0.012 mg/L), have been detected in several regional aquifer monitoring wells installed in Pueblo, Los Alamos, and Mortandad Canyons.Concentrations of nitrate, perchlorate, and/or tritium coreleased with aqueous uranium(VI) species are elevated above background in the regional aquifer at several monitoring wells.Thermochemical calculations suggest that uranyl carbonate-carbonato complexes, including UO 2 CO 3 0 , UO 2 (CO 3 ) 2 2-, Ca(UO 2 ) 2 (CO 3 ) 3 0 , and UO 2 (CO 3 ) 3 4-, dominate in the regional aquifer.These uranyl complexes are mobile under oxidizing and circumneutral pH conditions characteristic of the regional aquifer at Los Alamos.Concentrations of natural reductants, including dissolved hydrogen sulfide, dissolved ferrous iron, dissolved organic carbon, and solid organic matter are not sufficient to enhance reduction of uranium(VI) to uranium(IV) aqueous complexes (UOH 3+ and U(OH) 4 0 ).The regional aquifer is undersaturated with respect to amorphous UO 2 , uraninite, and coffinite.Background concentrations of dissolved uranium(VI) in upper sections of the regional aquifer are initially controlled by partial dissolution of soluble volcanic glass followed by specific adsorption of uranium(VI) complexes onto hydrous ferric oxide and cation exchange of uranyl cation with calcium on smectite surfaces.Upper sections of the regional aquifer beneath the Pajarito Plateau are enriched in silica and groundwater shows variable saturation with respect to uranophane (Ca(H 3 O) 2 (UO 2 ) 2 (SiO 4 ) 2 .3H 2 O) and is oversaturated with respect to haiweeite (Ca(UO 2 ) 2 Si 6 O 15 •5H2O) depending on pH and calcium and silica activities.

Analysis of groundwater chemistry can yield important insights about subsurface conditions, and provide an alternative and complementary method for characterizing basin hydrogeology, especially in areas where hydraulic data are limited. More specifically, hydrochemical facies have been used for decades to help understand basin flow and transport, and a set of facies were developed for the Roswell Artesian Basin (RAB) in a semi-arid part of New Mexico, USA. The RAB is an important agricultural water source, and is an excellent example of a rechargeable artesian system. However, substantial uncertainties about the RAB hydrogeology and groundwater chemistry exist. The RAB was a great opportunity to explore hydrochemcial facies definition. A set of facies, derived from fingerprint diagrams (graphical approach), existed as a basis for testing and for comparison to principal components, factor analysis, and cluster analyses (statistical approaches). Geochemical data from over 300 RAB wells in the central basin were examined. The statistical testing of fingerprint-diagram-based facies was useful in terms of quantitatively evaluating differences between facies, and for understanding potential controls on basin groundwater chemistry. This study suggests the presence of three hydrochemical facies in the shallower part of the RAB (mostly unconfined conditions) and three in the deeper artesian system of the RAB. These facies reflect significant spatial differences in chemistry in the basin that are associated with specific stratigraphic intervals as well as structural features. Substantial chemical variability across faults and within fault blocks was also observed.

Hundreds of domestic wells in northern New Mexico, have concentrations of U, As, and NO 3 − that exceed the Environmental Protection Agency’s (EPA) maximum contaminant level (MCL) for drinking water consumption. As part of a case study in groundwater quality, we collected groundwater samples from 749 domestic wells throughout the eastern half of the Española Basin. All water samples were analyzed for major ions, trace metals, and alkalinity. Selected samples were also analyzed for stable isotopes of O, H, and N. Of the wells we measured, 15, 173, and 99 had respective NO 3 − , U, and As concentrations that exceeded the EPA’s MCL. Total dissolved solids (TDS), U, and HCO 3 − were elevated in the Sangre de Cristo mountain block and around the town of Nambé. Our findings suggest that roll-front U deposits and devitrification of volcanic ash result in elevated U near Nambé, while weathering of granitic rocks accounts for high U in the mountain block. Arsenic concentrations were high in much of the study area with the exception of the Santa Fe metro region and the mountain block. Elevated As concentrations can be explained by devitrification of volcanic ash, anion exchange with clays, and mixing with hydrothermal fluids. In wells with high NO 3 − concentrations, analysis of N isotopes are consistent with contamination from domestic wastewater effluent. Our findings suggest that the geochemistry of the region is largely influenced by local geology while groundwater contamination from domestic water treatment and wastewater effluent is an emerging issue.

Endocrine disruptor chemicals (EDC), including pharmaceutical and personal care products (PPCP), are contaminants of significant concern found in aquatic environments worldwide.Numerous EDC and PPCP have been measured in treated and non-treated sanitary sewage effluent throughout the United States and Europe during the past two decades.Since the mid-1940s, Los Alamos National Laboratory (LANL) and Los Alamos County, New Mexico have released sewage effluent to three watersheds that provide recharge to groundwater beneath the Pajarito Plateau.The regional aquifer at Los Alamos is a sole-source aquifer providing high-quality drinking water to the public and LANL.Sampling stations located across the Pajarito Plateau were analyzed for 32 EDC and PPCP with detection limits in the low nanogram/liter range.These include wells screened in alluvial and perched intermediate-depth groundwater and wells and springs pumping or discharging from the regional aquifer.Surface-water sampling locations were selected in two watersheds currently receiving treated sewage and cooling-tower effluent from NPDES outfalls.Analytical results showed EDC and/or PPCP at most of the sampling locations, with variable types and frequency of detections.Commonly detected pharmaceuticals include acetaminophen, caffeine, carbamazepine, DEET, Dilantin, meprobamate, methadone, salicylic acid, and sulfamethoxazole.Acetaminophen, caffeine, and sulfamethoxazole were detected at the highest frequency.Several of the wells containing pharmaceuticals also contain contaminants associated with treated sewage, cooling tower, and industrial-derived effluents such as nitrate, boron, chromate, and tritium.Presence of EDC and PPCP in groundwater at Los Alamos refines the conceptual model for fate and transport of mobile contaminants migrating through the deep vadose zone (305 meters thick) to the regional aquifer.

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.

Lava Tube spring is a subaqueous spring that discharges from the Servilleta Basalt directly beneath the Rio Grande 21 km northwest of Questa, New Mexico.Lava Tube spring is the largest single active spring in New Mexico, with a discharge rate of approximately 22,680 L/min (6000 gal/min).Additional springs discharging within the study area are characterized by low flow rates (<150 L/min, <40 gal/min).Water samples collected upstream, at Lava Tube spring, and downstream along the Rio Grande on July 12, 2012 were analyzed for major ions, trace metals, tritium, and stable isotopes of hydrogen and oxygen.The upstream and downstream stations are 273 m north and 182 m south of Lava Tube spring, respectively.Surface-water flow rates average 1.90 cms (67 cfs) upstream of Lava Tube spring and 2.69 cms (95 cfs) downstream of the spring during sampling.Surface-water flow velocities averaged 0.16 and 0.27 m/sec (0.38 and 0.87 ft/sec) upstream and downstream of Lava Tube spring, respectively.Groundwater discharging from Lava Tube spring mixes with the Rio Grande surface water and constitutes between 32 and 38 percent of chemical tracers measured in the Rio Grande below the spring in the study area.Mixing calculations are based on concentration differences of chloride, sulfate, and uranium, tritium activity, and δ 18 O and δ 2 H values. Lava Tube spring and the Rio Grande are oxidizing with respect to Fe and dissolved oxygen concentrations range from 8.84 to 9.77 mg/L during sampling.Rio Grande surface water and groundwater discharging from Lava Tube spring are characterized by a Na-Ca-HCO 3 composition, with higher concentrations of major ions and selected trace elements (As, B, Mn, Mo, and Sr) and tritium activities measured in surface water.Lava Tube spring δ 18 O and δ 2 H values are more negative than Rio Grande surface water, possibly reflecting recharge at higher elevation.Lava Tube spring and the Rio Grande are undersaturated with respect to calcite and amorphous silica and oversaturated with respect to albite, gibbsite, and chalcedony, based on geochemical calculations using the computer program PHREEQC.Lava Tube spring approaches equilibrium with respect to Na-and K-saponite and analcime most likely produced from hydrolysis reactions with Servilleta basaltic glass.Concentrations of dissolved U(VI) below 10 -8 molal are stable as UO 2 (CO 3 ) 2 2-and UO 2 (CO 3 ) 3 4-based on geochemical modeling simulations.Speciation calculations suggest that concentrations of dissolved As(V) below 10 -7 molal are dominantly stable as HAsO 4 2-.

Cooling tower water ranging between 7 to 18mg/L of Cr(VI) was released to a stream channel entering a wetland at Los Alamos National Laboratory, New Mexico, USA from 1956 to 1972. The wetland contains between 5700 and 27,000kg total Cr mainly as Cr(III), with a median mass of 11,000kg total Cr, representing 21 to 49% of the total Cr. Chemical analyses conducted on wetland sediments show that 97.3 to 99.9% of the Cr is stable as Cr(III). Mass balance calculations confirm that there are sufficient concentrations of Fe(II) along with the organic-rich sediments to maintain Cr(III) stability within the wetland.

present time. However, many private water wells in the region produce water with concentrations of uranium (up to 1,820 g/L (ppb)) that exceed the safe drinking water standard of 30 g/L. Therefore, it is important to understand the source of the uranium in the groundwater and the processes involved. Potential sources for uranium in the groundwater include 1) uranium occurrences in the Tesuque Formation (San Jose mining district), 2) rhyolitic volcanic ash beds and sandstones with volcanic detritus found interbedded within the Tesuque Formation, 3) veins, replacements, and pegmatites in Proterozoic rocks (San Jose and Nambe mining districts), and 4) Proterozoic granitic rocks in the Sangre de Cristo Mountains. The sandstone uranium occurrences in the Tesuque Formation represent natural precipitation and concentration from uraniferous groundwaters, likely derived from 1) rhyolitic volcanic ash beds within the Tesuque Formation, 2) the alteration of granitic and/or volcanic detritus within the sedimentary host rocks, and 3) Proterozoic rocks in the Sangre de Cristo Mountains to the east. One property, the San Jose No. 13 (NMSF0033), produced 12 lbs (5 kg) of U3O8 at a grade of 0.05% U3O8 in 1957. Uranium in modern groundwaters likely was derived from the same sources, as well as from leaching and oxidation of older uranium occurrences in the Tesuque Formation. Uranium then precipitated from the waters to form the geochemical anomalies found in the prospects.