Lisa Shevenell

Travertine deposits of Soda Dam, New Mexico, and their implications for the age and evolution of the Valles caldera hydrothermal system

Uranium-thorium disequilibrium dates and stable isotope analyses of travertine deposits at Soda Dam, New Mexico, have been used to determine the age and to document the evolution of the Valles caldera hydrothermal system. Soda Dam discharges from Paleozoic and Precambrian rocks in San Diego Canyon southwest of the caldera, and the canyon was filled with several hundred metres of Upper Bandelier Tuff during formation of Valles caldera, 1.12 Ma. According to the dates, the Valles hydrothermal system is at least 106 yr old or nearly as old as the caldera. The basic hydrology of the system has thus remained virtually unchanged during this time. Travertine ages indicate pulses in travertine deposition from ∼1.0 to 0.48 Ma, from 0.107 to 0.058 Ma, and from 0.005 Ma to present. The volume of various travertine deposits suggests that the hot-spring system at Soda Dam was once larger than it is today, possibly discharging as much as 5 to 10 times more fluid of Na-HCO3-Cl composition. Stable isotopes of the travertines indicate, however, that the hot-spring system has probably never been more than about 10 °C hotter than the present maximum temperature of 48 °C. Stable isotopes also show that the δ13C of CO2 and δ18O of H2O in hot-spring waters has remained relatively constant for 106 yr. The similar ages between caldera formation and the onset of large-scale hydrothermal circulation and travertine deposition on pre-Bandelier Tuff rocks imply that incision rates by the Jemez River in Bandelier Tuff were relatively rapid. As much as 400 m of tuff were cut by the ancestral Jemez River in 105 yr or less.

Geochemical modeling approach to predicting arsenic concentrations in a mine pit lake

Abstract Between 1968 and 1983, the North pit at the Getchell Mine, Humboldt County, NV, filled with water to form a lake. In 1983, water quality data were collected with the following results: As concentrations of 0.29 to 0.59 mg/L, pH of 7.1 to 7.9, SO 4 concentrations of 1490 to 1640 mg/L, and TDS of 2394 to 2500 mg/L. Using geochemical modeling techniques presented here, pit lake waters have been theoretically allowed to react for 8.5 a, the approximate time that the North pit had been completely full by 1983. Modeling results predict pH of 7.9 to 8.2, SO 4 concentrations of 1503 to 1644 mg/L, TDS of 2054 to 2366 mg/L, and As concentrations ranging from 0.57 in the hypolimnion to 96 mg/L in the epilimnion. In the epilimnion, model results do not match observed As concentrations, suggesting that mechanisms, such as precipitation of arsenate salts or adsorption to mineral surfaces, may control As levels in an actual pit lake system. Adsorption to Fe oxyhydroxide surfaces is questioned by the authors because of the low Fe content in the Getchell system, but adsorption to Al(OH) 3 (gibbsite) and clay mineral surfaces may be important in controlling natural As concentrations.

Analysis of well hydrographs in a karst aquifer: estimates of specific yields and continuum transmissivities

Hydrograph analysis techniques have been well developed for hydrographs obtained from streams and springs, where data are cast in terms of total discharge. The data obtained from well hydrographs provide water level versus time; hence, a method of hydrograph analysis is required for situations in which only water level data are available. It is assumed here that three segments on a recession curve from wells in a karst aquifer represent drainage from three types of storage: conduit (C), fracture (F) and matrix (M). Hydrographs from several wells in a karst aquifer are used to estimate the specific yields (Sy) associated with each portion of the aquifer (C, F, M), as well as continuum transmissivities (T). Data from three short injection tests at one well indicate continuum T at this well bore is approximately 5 m2 day−1, and tests at numerous other wells in the aquifer yield results between 1 and 7 m2 day−1. The T estimated with well hydrographs from two storms indicates a T of 9.8 m2 day−1. Well-developed conduit systems in which water levels in wells show a flashy response typically show Sys of 1 × 10−4, 1 × 10−3, and 3 × 10−3 for C, F, and M, respectively. Less well-developed conduit areas show more nearly equal Sys (8.6 × 10−4, 1.3 × 10−3, 3 × 10−3). Areas with no evidence for the presence of conduits have only one, or in some cases two, slopes on the recession curve. In these cases, water-level responses are slow. Recession curves with a single slope represent drainage from only the lower T matrix. Those with two slopes have an additional, more rapid response segment on the recession curve which represents drainage from the higher T, lower Sy, fractures in the system.

Controls on pit lake water quality at sixteen open-pit mines in Nevada

Thirty-five mines in Nevada currently have, or will likely have, a pit lake. The large bulk mineable deposits in Nevada mined below the water table are of several types, including Carlin-type Au, quartz-adularia precious metal, quartz-alunite precious metal and porphyry-Cu (-Mo) deposits. Of the 16 past or existing pit lakes at 12 different Nevada mines, most had near neutral pH and low metal concentrations, yet most had at least one constituent (e.g., SO4) which exceeded drinking water standards for at least one sampling event. Water quality data indicate that, in general, poor water quality will not develop in Carlin-type Au deposits. Wall rocks in the geologic environment typical of these deposits, and in the specific pits sampled, contain substantial amounts of carbonate, which buffers the pH at slightly basic conditions and thereby limits the solubility of most metals. Similarly, the quartz-adularia precious metal deposits generally have geologic conditions that buffer pH and naturally prevent the development of poor water quality. In both of these deposit types, certain elements such as As and Se that are mobile in neutral to basic waters may accumulate to levels near or exceeding drinking water standards. Pit lakes forming in quartz-alunite precious metal deposits hosted in volcanic rocks or in porphyry-Cu (-Mo) deposits in plutonic rocks are of greatest environmental concern in Nevada, as both deposit types have relatively high acid-generating potential and low buffering capacity. However, the sampled Nevada pits in these deposit types indicate that the water may not be of poor quality. In addition, water quality in some pits may actually improve with time due to the increased water-rock ratio as the pit fills with water, as suggested by pit waters at one mine in a Carlin-type deposit (Getchell) that improved between 1968 and 1982. Although water quality in pits in each deposit type is generally good, local, site specific conditions (e.g., surface water inflow) and variations (e.g., evaporation rates) result in some pit lakes (e.g., Boss) in the quartz-adularia deposit type being of substantially poorer water quality than other lakes (e.g., Tuscarora) in the same deposit type. Despite underlying geologic controls based on deposit type, site specific variations in hydrogeologic conditions and surface geologic features can result in differing water quality in pit lakes in the same deposit types, and these factors may, in some cases, provide an overriding control on the geochemical evolution of specific pit lakes.

Effects of precipitation events on colloids in a karst aquifer

The effects of precipitation events on colloid mobilization were evaluated during several storms from six wells in a karstic aquifer at the Oak Ridge Y-12 Plant in eastern Tennessee (USA). Turbidity increases and rapidly recedes following rain events. Although the magnitude of the turbidity increases are relatively small (≤4.78 NTU), the increased turbidity suggests transient increases in colloid abundance during storm versus non-storm periods. During the larger storms (>19 mm), the increased turbidity is associated with increases in pH, total organic carbon (TOC) and temperature, and with decreases in dissolved oxygen (DO). These larger storms result in flushing of a greater proportion of higher pH, TOC (and lower DO) soil or matrix waters into the fractures and conduits than occurs during smaller storms. Smaller storms also result in increases in turbidity, but show increases in DO and decreases in pH reflecting less influence on the water chemistry from the longer residence time epikarst or and matrix waters, and greater impact from the more dilute, newly recharged waters. Due to the complexity of karst flow and temporal variations in flow and chemistry, controls on turbidity are not consistent through time and space at the wells. During smaller storms, recharge by lower ionic strength waters may promote colloid release and thus contribute to observed increases in turbidity. During larger storms, elevated turbidity may be more related to pH increases resulting from greater influx of matrix and soil waters into fractures and conduits. Chemical factors alone cannot account for the changes in turbidity observed during the various storms. Because of the complicated nature of flow and particle transport in karst aquifers, the presence of colloids during precipitation events is dictated by a complex interplay of chemical reactions and the effects of physical perturbations due to increased flow through the conduits and fractures. Simple trends in water quality parameters could not be identified, and broad generalizations cannot easily be made in karst settings, and some of the expected correlations between chemical parameters during the storms were not observed in this work.

Addition of magmatic volatiles into the hot spring waters of Loowit Canyon, Mount St. Helens, Washington, USA

Geochemical studies on cold meteoric waters, post-1980 hot spring waters, fumarole emissions from the dacite dome, and volcanic rocks at Mount St. Helens (MSH) from 1985 to 1989 show that magmatic volatiles are involved in the formation of a new hydrothermal system. Hot spring waters are enriched in δ18O by as much as 2‰ and display enrichments in δD relative to cold waters. A well-defined isotopic trend is displayed by the isotopic composition of a>400°C fumarole condensate collected from the central crater in 1980 (-33‰ δD, +6‰ δ18O), of condensate samples collected on the dome, and of cold meteoric and hot spring waters. The trend indicates that mixing occurs between local meteoric water and magmatic water degassing from the dacite dome. Between 30 and 70% magmatic water is present in the dome fumarole discharges and ≈10% magnatic water has been added to the waters of the hydrothermal system. Relations between Cl, SO4 and HCO3 indicate that the hot spring waters are immature volcanic waters formed by reaction of rocks with waters generated by absorption of acidic volcanic fluids. In addition, the B/Cl ratios of the spring waters are similar to the B/Cl ratios of the fumarole condensates (≈0.02), values of δ13C in the HCO3 of the hot springs (-9.5 to-13.5‰) are similar to the magmatic value at MSH (-10.5‰), and the 3He/4He ratio, relative to air, in a hot spring water is 5.7, suggesting a magmatic origin for this component.

Processes controlling colloid composition in a fractured and karstic aquifer in eastern Tennessee, USA

Abstract Groundwater was sampled from a number of wells along recharge pathways between fractured shale and karstic formations to evaluate the chemical and hydrologic mechanisms controlling the nature and abundance of groundwater colloids. The colloids recovered using low flow rate purging and sampling exhibited a composition and abundance consistent with lithology, flow paths, and effects of hydrology and aqueous chemistry on colloid mobilization and stability. In general, the larger-size colloids and Ca-containing colloids were more abundant in the karstic lithologies, while Na-containing colloids were more important in the shales. The composition of the colloids reflected recharge pathways from the fractured shale and dolomite formations on the ridges into the limestone in the valley floor. The Mg-colloids in the limestone reflect the possible contributions from the dolamite, while the Na, K, and Si reflect possible contributions from the shale. However, it was not possible to use the colloid composition as a signature to demonstrate colloid transport from one lithology to another. Mixing of recharge water from the shale with groundwater within the limestone formation and precipitation/dissolution reactions could account for the colloids present in the limestone without invoking transport of specific shale-derived colloids into the limestone formation. The abundance of colloids in groundwater appears to be controlled by both chemical factors affecting colloid stability, as well as physical factors related to hydrology (storm-driven recharge and water velocities). In general, colloids were more abundant in wells with low ionic strength, such as shallow wells in water table aquifers near sources of recharge at the top of the ridges. Increases in cation concentrations due to dissolution reactions along flow paths were associated with decreases in colloid abundance. However, in spite of elevated ionic strength, colloid concentrations tended to be unexpectedly high in karstic wells that were completed in cavities or water-bearing fractures. The higher levels of colloids appear to be related to storm-driven changes in chemistry or flow rates that causes resuspension of colloids settled within cavities and fractures.

Regional potential evapotranspiration in arid climates based on temperature, topography and calculated solar radiation

Values of evapotranspiration are required for a variety of water planning activities in arid and semi-arid climates, yet data requirements are often large, and it is costly to obtain this information. This work presents a method where a few, readily available data (temperature, elevation) are required to estimate potential evapotranspiration (PET). A method using measured temperature and the calculated ratio of total to vertical radiation (after the work of Behnke and Maxey, 1969) to estimate monthly PET was applied for the months of April–October and compared with pan evaporation measurements. The test area used in this work was in Nevada, which has 124 weather stations that record sufficient amounts of temperature data. The calculated PET values were found to be well correlated (R2=0·940–0·983, slopes near 1·0) with mean monthly pan evaporation measurements at eight weather stations.In order to extrapolate these calculated PET values to areas without temperature measurements and to sites at differing elevations, the state was divided into five regions based on latitude, and linear regressions of PET versus elevation were calculated for each of these regions. These extrapolated PET values generally compare well with the pan evaporation measurements (R2=0·926–0·988, slopes near 1·0). The estimated values are generally somewhat lower than the pan measurements, in part because the effects of wind are not explicitly considered in the calculations, and near-freezing temperatures result in a calculated PET of zero at higher elevations in the spring months. The calculated PET values for April–October are 84–100% of the measured pan evaporation values. Using digital elevation models in a geographical information system, calculated values were adjusted for slope and aspect, and the data were used to construct a series of maps of monthly PET. The resultant maps show a realistic distribution of regional variations in PET throughout Nevada which inversely mimics topography. The general methods described here could be used to estimate regional PET in other arid western states (e.g. New Mexico, Arizona, Utah) and arid regions world-wide (e.g. parts of Africa). Copyright

EVOLUTION OF HYDROTHERMAL WATERS AT MOUNT ST. HELENS, WASHINGTON, USA

Abstract Hydrothermal water samples at Mount St. Helens collected between 1985 and 1989 and in 1994 are used to identify water types and describe their evolution through time. Two types of low temperature hydrothermal systems are associated with the 1980 eruptions and were initiated soon after emplacement of shallow magma and pyroclastic flows. The Loowit hot spring system is located in the breach zone and is associated with the magma conduit and nearby avalanche deposits, whereas the Pumice Plain (PP) system is associated with pyroclastic flows and avalanche deposits ≈ 3 to 5 km north of the volcano. The PP waters first discharged at the surface in 1981, whereas the Loowit waters began to issue at the surface in 1983. δD, δ18O and 3H indicated all thermal waters are dominantly derived from post-1980 recharge. Fluids flow through, and are restricted to, the shallow 1980 avalanche and pyroclastic deposits. All water cooled with time (up to 43 °C on the PP and up to 20 °C in Loowit in 5 years), and chemical compositions have changed rapidly. All waters have highly variable, and unreliable geothermometer temperatures with maximum indicated temperatures

Exploration drilling and reservoir model of the Platanares geothermal system, Honduras, Central America

Abstract Results of drilling, logging, and testing of three exploration core holes, combined with results of geologic and hydrogeochemical investigations, have been used to present a reservoir model of the Platanares geothermal system, Honduras. Geothermal fluids circulate at depths ≥ 1.5 km in a region of active tectonism devoid of Quaternary volcanism. Large, artesian water entries of 160 to 165°C geothermal fluid in two core holes at 625 to 644 m and 460 to 635 m depth have maximum flow rates of roughly 355 and 560 l/min, respectively, which are equivalent to power outputs of about 3.1 and 5.1 MW(thermal). Dilute, alkali-chloride reservoir fluids (TDS ≤ 1200 mg/kg) are produced from fractured Miocene andesite and Cretaceous to Eocene redbeds that are hydrothermally altered. Fracture permeabillity in producing horizons is locally greater than 1500 and bulk porosity is ≤ 6%. A simple, fracture-dominated, volume-impedance model assuming turbulent flow indicates that the calculated reservoir storage capacity of each flowing hole is approximately 9.7 × 106 l/(kg cm−2), Tritium data indicate a mean residence time of 450 yr for water in the reservoir. Multiplying the natural fluid discharge rate by the mean residence time gives an estimated water volume of the Platanares system of ≥ 0.78 km3. Downward continuation of a 139°C/km “conductive” gradient at a depth of 400 m in a third core hole implies that the depth to a 225°C source reservoir (predicted from chemical geothermometers) is at least 1.5 km. Uranium-thorium disequilibrium ages on calcite veins at the surface and in the core holes indicate that the present Platanares hydrothermal system has been active for the last 0.25 m.y.