David A. Stonestrom

CHEMICAL WEATHERING IN A TROPICAL WATERSHED, LUQUILLO MOUNTAINS, PUERTO RICO : I. LONG-TERM VERSUS SHORT-TERM WEATHERING FLUXES

Abstract The pristine Rio Icacos watershed in the Luquillo Mountains in eastern Puerto Rico has the fastest documented weathering rate of silicate rocks on the Earth’s surface. A regolith propagation rate of 58 m Ma−1, calculated from iso-volumetric saprolite formation from quartz diorite, is comparable to the estimated denudation rate (25–50 Ma−1) but is an order of magnitude faster than the global average weathering rate (6 Ma−1). Weathering occurs in two distinct environments; plagioclase and hornblende react at the saprock interface and biotite and quartz weather in the overlying thick saprolitic regolith. These environments produce distinctly different water chemistries, with K, Mg, and Si increasing linearly with depth in saprolite porewaters and with stream waters dominated by Ca, Na, and Si. Such differences are atypical of less intense weathering in temperate watersheds. Porewater chemistry in the shallow regolith is controlled by closed-system recycling of inorganic nutrients such as K. Long-term elemental fluxes through the regolith (e.g., Si = 1.7 × 10−8 moles m−2 s−1) are calculated from mass losses based on changes in porosity and chemistry between the regolith and bedrock and from the age of the regolith surface (200 Ma). Mass losses attributed to solute fluxes are determined using a step-wise infiltration model which calculates mineral inputs to the shallow and deep saprolite porewaters and to stream water. Pressure heads decrease with depth in the shallow regolith (−2.03 m H2O m−1), indicating that both increasing capillary tension and graviometric potential control porewater infiltration. Interpolation of experimental hydraulic conductivities produces an infiltration rate of 1 m yr−1 at average field moisture saturation which is comparable with LiBr tracer tests and with base discharge from the watershed. Short term weathering fluxes calculated from solute chemistries and infiltration rates (e.g., Si = 1.4 × 10−8 moles m−2 s−1) are compared to watershed flux rates (e.g., Si = 2.7 × 10−8 moles m−2 s−1). Consistency between three independently determined sets of weathering fluxes imply that possible changes in precipitation, temperature, and vegetation over the last several hundred thousand years have not significantly impacted weathering rates in the Luquillo Mountains of Puerto Rico. This has important ramifications for tropical environments and global climate change.

High CO2 emissions through porous media: transport mechanisms and implications for flux measurement and fractionation

Abstract Diffuse emissions of CO2 are known to be large around some volcanoes and hydrothermal areas. Accumulation-chamber measurements of CO2 flux are increasingly used to estimate the total magmatic or metamorphic CO2 released from such areas. To assess the performance of accumulation chamber systems at fluxes one to three orders of magnitude higher than normally encountered in soil respiration studies, a test system was constructed in the laboratory where known fluxes could be maintained through dry sand. Steady-state gas concentration profiles and fractionation effects observed in the 30-cm sand column nearly match those predicted by the Stefan-Maxwell equations, indicating that the test system was functioning successfully as a uniform porous medium. Eight groups of investigators tested their accumulation chamber equipment, all configured with continuous infrared gas analyzers (IRGA), in this system. Over a flux range of ∼200–12,000 g m−2 day−1, 90% of their 203 flux measurements were 0–25% lower than the imposed flux with a mean difference of −12.5%. Although this difference would seem to be within the range of acceptability for many geologic investigations, some potential sources for larger errors were discovered. A steady-state pressure gradient of −20 Pa/m was measured in the sand column at a flux of 11,200 g m−2 day−1. The derived permeability (50 darcies) was used in the dusty-gas model (DGM) of transport to quantify various diffusive and viscous flux components. These calculations were used to demonstrate that accumulation chambers, in addition to reducing the underlying diffusive gradient, severely disrupt the steady-state pressure gradient. The resultant diversion of the net gas flow is probably responsible for the systematically low flux measurements. It was also shown that the fractionating effects of a viscous CO2 efflux against a diffusive influx of air will have a major impact on some important geochemical indicators, such as N2/Ar, δ15N–N2, and 4He/22Ne.

Nonmonotonic matric pressure histories during constant flux infiltration into homogeneous profiles

Constant flux infiltration experiments were performed at rates less than saturated hydraulic conductivity (Ksat) on uniform columns of soils and glass beads with low initial water contents. Tensiometers measured matric pressure (ψ) histories at various depths in the 0.05-m diameter, 0.73-m-tall columns. One of two devices applied the steady fluxes: one was a ceramic plate applicator that allowed monitoring of pressures above the plate, and the other was a rain applicator that sprinkled water uniformly over the surface. Two columns were used. One allowed destructive sampling for water content in a way that prevented postrun changes in water content. The other allowed prerun replacement of pore air with gases of different solubilities. Contrary to predictions based on the Richards equation, transmission zone ψ values passed through maxima, then decreased continuously as wetting fronts moved down columns. Such nonideal behavior was observed in Aiken silt loam, Hanfoird sandy loam, Delhi sand, Oakley sand, and glass beads. At an infiltration rate equal to 70% of Ksat, values of ψ at 2, 5, and 8 cm depths in the glass beads column decreased an average of 52% from their respective maxima in about 3 hours. In the soil columns the nonmonotonic behavior was less pronounced but statistically significant at all rates examined (3–59% of Ksat). Matric pressure reversals were larger, and corresponding maxima higher, when a given rate was preceded by a series of step wise increasing rates. Tests of possible explanations provided no evidence to support hypotheses involving trapped-air dynamics, convective air flow, particle rearrangement, or experimental artifacts. The results cast doubt on the ability of the Richards equation to predict the course of constant flux infiltration including the initiation of runoff.

Soil, Plant, and Terrain Effects on Natural Perchlorate Distribution in a Desert Landscape

Perchlorate (ClO) is a contaminant that occurs naturally throughout the world, but little is known about its distribution and interactions in terrestrial ecosystems. The objectives of this Amargosa Desert, Nevada study were to determine (i) the local-scale distribution of shallow-soil (0-30 cm) ClO with respect to shrub proximity (far and near) in three geomorphic settings (shoulder slope, footslope, and valley floor); (ii) the importance of soil, plant, and terrain variables on the hillslope-distribution of shallow-soil and creosote bush [ (Sessé & Moc. ex DC.) Coville] ClO; and (iii) atmospheric (wet plus dry, including dust) deposition of ClO in relation to soil and plant reservoirs and cycling. Soil ClO ranged from 0.3 to 5.0 μg kg. Within settings, valley floor ClO was 17× less near shrubs due in part to enhanced leaching, whereas shoulder and footslope values were ∼2× greater near shrubs. Hillslope regression models (soil, = 0.42; leaf, = 0.74) identified topographic and soil effects on ClO deposition, transport, and cycling. Selective plant uptake, bioaccumulation, and soil enrichment were evidenced by leaf ClO concentrations and Cl/ClO molar ratios that were ∼8000× greater and 40× less, respectively, than soil values. Atmospheric deposition ClO flux was 343 mg ha yr, ∼10× that for published southwestern wet-deposition fluxes. Creosote bush canopy ClO (1310 mg ha) was identified as a previously unrecognized but important and active reservoir. Nitrate δO analyses of atmospheric deposition and soil supported the leaf-cycled-ClO input hypothesis. This study provides basic data on ClO distribution and cycling that are pertinent to the assessment of environmental impacts in desert ecosystems and broadly transferable to anthropogenically contaminated systems.

Groundwater exchanges near a channelized versus unmodified stream mouth discharging to a subalpine lake

The terminus of a stream flowing into a larger river, pond, lake, or reservoir is referred to as the stream-mouth reach or simply the stream mouth. The terminus is often characterized by rapidly changing thermal and hydraulic conditions that result in abrupt shifts in surface water/groundwater (sw/gw) exchange patterns, creating the potential for unique biogeochemical processes and ecosystems. Worldwide shoreline development is changing stream-lake interfaces through channelization of stream mouths, i.e., channel straightening and bank stabilization to prevent natural meandering at the shoreline. In the central Sierra Nevada (USA), Lake Tahoes shoreline has an abundance of both “unmodified” (i.e., not engineered though potentially impacted by broader watershed engineering) and channelized stream mouths. Two representative stream mouths along the lakes north shore, one channelized and one unmodified, were selected to compare and contrast water and heat exchanges. Hydraulic and thermal properties were monitored during separate campaigns in September 2012 and 2013 and sw/gw exchanges were estimated within the stream mouth-shoreline continuum. Heat-flow and water-flow patterns indicated clear differences in the channelized versus the unmodified stream mouth. For the channelized stream mouth, relatively modulated, cool-temperature, low-velocity longitudinal streambed flows discharged offshore beneath warmer buoyant lakeshore water. In contrast, a seasonal barrier bar formed across the unmodified stream mouth, creating higher-velocity subsurface flow paths and higher diurnal temperature variations relative to shoreline water. As a consequence, channelization altered sw/gw exchanges potentially altering biogeochemical processing and ecological systems in and near the stream mouth.

Volatile organic compounds in the unsaturated zone from radioactive wastes

Volatile organic compounds (VOCs) are often comingled with low-level radioactive wastes (LLRW), but little is known about subsurface VOC emanations from LLRW landfills. The current study systematically quantified VOCs associated with LLRW over an 11-yr period at the USGS Amargosa Desert Research Site (ADRS) in southwestern Nevada. Unsaturated-zone gas samples of VOCs were collected by adsorption on resin cartridges and analyzed by thermal desorption and GC/MS. Sixty of 87 VOC method analytes were detected in the 110-m-thick unsaturated zone surrounding a LLRW disposal facility. Chlorofluorocarbons (CFCs) were detected in 100% of samples collected. Chlorofluorocarbons are powerful greenhouse gases, deplete stratospheric ozone, and are likely released from LLRW facilities worldwide. Soil-gas samples collected from a depth of 24 m and a horizontal distance 100 m south of the nearest waste-disposal trench contained >60,000 ppbv total VOCs, including >37,000 ppbv CFCs. Extensive sampling in the shallow unsaturated zone (0-2 m deep) identified areas where total VOC concentrations exceeded 5000 ppbv at the 1.5-m depth. Volatile organic compound concentrations exceeded background levels up to 300 m from the facility. Maximum vertical diffusive fluxes of total VOCs were estimated to be 1 g m yr. Volatile organic compound distributions were similar but not identical to those previously determined for tritium and elemental mercury. To our knowledge, this study is the first to characterize the unsaturated zone distribution of VOCs emanating from a LLRW landfill. Our results may help explain anomalous transport of radionuclides at the ADRS and elsewhere.

Solute profiles in soils, weathering gradients and exchange equilibrium/disequilibrium

Abstract The spatial and temporal changes in hydrology and pore water elemental and 87/86Sr compositions were used to determine contemporary weathering rates in a 65 to 226 ky old soil chronosequence formed from granitic sediments deposited on marine terraces along coastal California. Cl-corrected Na, K and Si increased with depth denoting inputs from the weathering of plagioclase and K-feldspar. Solute 87/86Sr exhibited progressive mixing of sea water-dominated precipitation with inputs from less radiogenic plagioclase. Linear approximations to these weathering gradients were used to determine plagioclase weathering rates of between 0.38 and 8.9 × 10−15 moles m−2 s−1. The lack of corresponding weathering gradients for Ca and Sr indicated short-term equilibrium with the clay ion exchange pool which requires periodic resetting by natural perturbations to maintain continuity, in spite of soil composition changes reflecting the effects of long-term weathering.

Multimodel analysis of anisotropic diffusive tracer‐gas transport in a deep arid unsaturated zone

Gas transport in the unsaturated zone affects contaminant flux and remediation, interpretation of groundwater travel times from atmospheric tracers, and mass budgets of environmentally important gases. Although unsaturated zone transport of gases is commonly treated as dominated by diffusion, the characteristics of transport in deep layered sediments remain uncertain. In this study, we use a multimodel approach to analyze results of a gas-tracer (SF6) test to clarify characteristics of gas transport in deep unsaturated alluvium. Thirty-five separate models with distinct diffusivity structures were calibrated to the tracer-test data and were compared on the basis of Akaike Information Criteria estimates of posterior model probability. Models included analytical and numerical solutions. Analytical models provided estimates of bulk-scale apparent diffusivities at the scale of tens of meters. Numerical models provided information on local-scale diffusivities and feasible lithological features producing the observed tracer breakthrough curves. The combined approaches indicate significant anisotropy of bulk-scale diffusivity, likely associated with high-diffusivity layers. Both approaches indicated that diffusivities in some intervals were greater than expected from standard models relating porosity to diffusivity. High apparent diffusivities and anisotropic diffusivity structures were consistent with previous observations at the study site of rapid lateral transport and limited vertical spreading of gas-phase contaminants. Additional processes such as advective oscillations may be involved. These results indicate that gases in deep, layered unsaturated zone sediments can spread laterally more quickly, and produce higher peak concentrations, than predicted by homogeneous, isotropic diffusion models.

On the conversion of tritium units to mass fractions for hydrologic applications

We develop a general equation for converting laboratory-reported tritium levels, expressed either as concentrations (tritium isotope number fractions) or mass-based specific activities, to mass fractions in aqueous systems. Assuming that all tritium is in the form of monotritiated water simplifies the derivation and is shown to be reasonable for most environmental settings encountered in practice. The general equation is nonlinear. For tritium concentrations c less than 4.5×1012 tritium units (TU) – i.e. specific tritium activities<5.3×1011 Bq kg−1 – the mass fraction w of tritiated water is approximated to within 1 part per million by w ≈ c×2.22293×10−18, i.e. the conversion is linear for all practical purposes. Terrestrial abundances serve as a proxy for non-tritium isotopes in the absence of sample-specific data. Variation in the relative abundances of non-tritium isotopes in the terrestrial hydrosphere produces a minimum range for the mantissa of the conversion factor of [2.22287; 2.22300].

Nutrient processes at the stream‐lake interface for a channelized versus unmodified stream mouth

Inorganic forms of nitrogen and phosphorous impact freshwater lakes by stimulating primary production and affecting water quality and ecosystem health. Communities around the world are motivated to sustain and restore freshwater resources and are interested in processes controlling nutrient inputs. We studied the environment where streams flow into lakes, referred to as the stream-lake interface (SLI), for a channelized and unmodified stream outlet. Channelization is done to protect infrastructure or recreational beach areas. We collected hydraulic and nutrient data for surface water and shallow groundwater in two SLIs to develop conceptual models that describe characteristics that are representative of these hydrologic features. Water, heat, and solute transport models were used to evaluate hydrologic conceptualizations and estimate mean residence times of water in the sediment. A nutrient mass balance model is developed to estimate net rates of adsorption and desorption, mineralization, and nitrification along subsurface flow paths. Results indicate that SLIs are dynamic sources of nutrients to lakes and that the common practice of channelizing the stream at the SLI decreases nutrient concentrations in pore water discharging along the lakeshore. This is in contrast to the unmodified SLI that forms a barrier beach that disconnects the stream from the lake and results in higher nutrient concentrations in pore water discharging to the lake. These results are significant because nutrient delivery through pore water seepage at the lakebed from the natural SLI contributes to nearshore algal communities and produces elevated concentrations of inorganic nutrients in the benthic zone where attached algae grow.