Bedrock Zone Research Articles

Características de la conductividad hidráulica en montañas e implicancias para conceptualizar el flujo del agua subterránea en el basamento

Influences of hydraulic conductivity (K) heterogeneities on bedrock groundwater (BG) flow systems in mountainous topography are investigated using a conceptual 2D numerical modelling approach. A conceptual model for K heterogeneity in crystalline bedrock mountainous environments is developed based on a review of previous research, and represents heterogeneities due to weathering profile, bedrock fracture characteristics, and catchment-scale (∼0.1–1 km) structural features. Numerical groundwater modelling of K scenarios for hypothetical mountain catchment topography indicates that general characteristics of the BG flow directions are dominated by prominent topographic features. Within the modelled saturated BG flow system, ∼90 % or more of total BG flux is focussed within a fractured bedrock zone, extending to depths of ∼100–200 m below the ground surface, overlying lower-K bedrock. Structural features and heterogeneities, represented as discrete zones of higher or lower K relative to surrounding bedrock, locally influence BG flow, but do not influence general BG flow patterns or general positions of BG flow divides. This result is supported by similar BG transit-time distribution shapes and statistics for systems with and without structural features. The results support the development of topography-based methods for predicting general locations of BG flow-system boundaries in mountain regions.

Hydrothermal alteration, mass transfer and magnetite mineralization in dextral shear zones, western Hudson Highlands, New York, United States

Massive magnetite veins formed during hydrothermal mineralization within northeast striking dextral shear zones in Proterozoic-age crystalline bedrock of the western Hudson Highlands. The veins formed in an open fracture system in right step-over dilational jogs during the late stages of movement. Acidic metamorphic fluids derived from metavolcanic country rock and saturated with iron, flushed through fractures, reacted with wall rock, and exchanged chemical species. Buffered by the composition of the local country rock, fluids migrated and mixed along the fault during ‘seismic pumping’ events. The fluids deposited mineral assemblages in the fractures that reflect the changing flux, fluid buffering, and/or physical conditions.This process produced three zones, outward from the unaltered country rock which are: 1) a bleached zone of altered wall rock adjacent to the vein, 2) an outer banded zone in the vein, of ferromagnesian-rich bands, and 3) a core of massive magnetite ore and gangue minerals. Bleached zones are dominated by amphibole and/or pyroxene, with scapolite, biotite, and apatite, within metavolcanic and quartzofeldspathic gneiss, or phlogopite and calcite within calc-silicate country rock. Calc-silicate banded and massive assemblages contain clinopyroxene, calcite, amphibole, and/or biotite or phlogopite. Quartzofeldspathic and metavolcanic banded and massive assemblages are dominated by amphibole and/or orthopyroxene, with quartz and/or sulfides locally. Both assemblages contain magnetite central to the deposits.Geochemical modeling of the bleached zone shows overall gains in volume (2.5–20.3%) and mass (3.1–18.1g relative to 100g of wall rock). In all cases, iron (2.4–5.3g), magnesium (1.0–2.8g), and calcium (0.6–6.5g) were gained, especially adjacent to calc-silicates. Deposits adjacent to quartzofeldspathic country rock had large gains in silica (4.4–7.4g), whereas deposits in mafic metavolcanic rock lost silica (1.4–3.8g). Based on the mobility of silica, fluid fluxes were calculated between 5.3×105 and 6.6×106cm3/cm2 for bleached zone alteration. Elements in abundance in the country rock contributed to the composition of the deposits, dominantly silica, carbonate, and sulfides.

Resistivity mapping as a tool for identification and characterisation of weakness zones in crystalline bedrock: definition and testing of an interpretational model

In recent years, the focus on feasibility studies for tunnels has increased in Norway. Traditionally, the refraction seismic method and the very low-frequency electromagnetic method (VLF-EM) have been used. The Geological Survey of Norway introduced the electrical resistivity traversing method (ERT) in feasibility studies for tunnel construction purposes. Resistivity modelling shows that the method has the potential to characterise fracture zones geometrically; i.e., the thickness, dip, and depth extent. Based on previous studies, a model for mineralogical characterisation is proposed. This model, and the possibility for geometrical characterisation, is critically tested with success on three tunnel projects. The results of the comparison study, with regards to weakness zones, show that VLF-EM is a method that is capable of locating fracture zones, while refraction seismic is capable of locating and indicating the width of the zone, and can be used to imply the thickness of the soil cover above bedrock. The 2D resistivity method is able to locate the weakness zone, indicate the width, depth extent, and the dip of the zone, and in addition, characterise the zone with respect to stability or water problems. The crystalline bedrock characterisation is divided into three groups: resistivity values above 3,000 Ω m indicating good rock quality, values between 3,000 and 500 Ω m indicating bedrock with mainly water problems, while values <500 Ω m indicate clay-bearing, unstable rock with fewer water problems. From our investigations, we conclude that the 2D resistivity method is a very good supplement to traditional methods for feasibility studies on tunnelling purposes in crystalline rock.

국내 원전 부지 내 암석의 광물학적 특성 규명 및 풍화에 따른 방사성 세슘(137Cs)의 흡착 평가

원자력 발전소 중대사고에 의해 방사성 세슘(<TEX>$^{137}Cs$</TEX>)이 지하수계로 유출될 경우를 가정하여, 연구 지역의 깊이에 따른 암석매질의 특성을 규명하고 세슘의 흡착계수를 정량적으로 평가하였다. 대상지역인 신고리 원전 3, 4호기의 지하 암석매질은 주로 석영 및 장석류로 이루어진 화강암 계열이며, 운모류를 10~20% 함유하고 있다. 비교적 얕은 심도(6.3~7.4 m)의 파쇄대에서 2차 광물인 녹니석이 일부 포함되어 있었지만, 기반암에서는 거의 발견되지 않았다. <TEX>$^{137}Cs$</TEX>의 흡착분배계수(<TEX>$K_d$</TEX>)는 파쇄대 지역에서 약 880~960 mL/g로 기반암 지역에서의 820~840 mL/g보다 비교적 높게 나타났으며, 이는 파쇄대에 포함되어 있는 풍화생성물인 2차 광물들에 의한 영향으로 판단된다. 따라서 <TEX>$^{137}Cs$</TEX>이 지하 매질로 유출될 경우 대부분은 천부 지역에 흡착되어 세슘에 의한 오염 확산 속도가 지연될 것이라고 예상되며, 이러한 결과는 원자력 발전소 안정성 평가인자 자료로 활용될 것으로 기대된다. This study was to characterize the minerals in fractured and bedrock zone, and determine quantitatively sorption for radioactive cesium (<TEX>$^{137}Cs$</TEX>) at the Korean nuclear facility site. The rock samples were granite group that mainly consists of quartz and feldspar with 10~20% mica minerals. Chlorite was observed as secondary mineral for the rock samples collected from fractured zone, but not for bedrock samples. The <TEX>$^{137}Cs$</TEX> sorption distribution coefficients increased to <TEX>$K_d$</TEX> = 880~960 mL/g in the fractured zone because of the presence of secondary minerals formed by weathering processes, compared to the bedrock zone (<TEX>$K_d$</TEX> = 820~840 mL/g). These results suggest that the released <TEX>$^{137}Cs$</TEX> to groundwater environment could be significantly retarded in the fractured zone in the case of severe nuclear accident at the study site.

Chemical and Biological Reduction of PCE by Pneumatic Fracturing and Injection of ZVI in Saprolite

AbstractThis article presents a case study of the source‐area treatment of tetrachloroethene (PCE) in a low‐permeability formation using zero‐valent iron (ZVI). Evidence of the stimulation of biological reduction processes within the treatment zone occurred. Pneumatic fracturing and injection of microscale ZVI slurry in the overburden and weathered bedrock zones was performed at a commercial brownfields redevelopment site in Maryland. A 20,000‐square‐foot source area impacted with PCE at concentrations greater than 15,000 µg/L was treated at depths ranging from 10 to 70 feet bgs. An average ZVI dosage of 0.0024 iron‐to‐soil mass ratio within the overburden zone led to a 75 percent decrease in PCE mass in less than one year. For the weathered bedrock zone, an average 0.0045 iron‐to‐soil mass ratio resulted in a 92 percent decrease in PCE mass during the same period. The reducing environment and hydrogen generated by the ZVI may have stimulated Dehalobacter populations, as evidenced by concentrations up to 104 cells per milliliter measured within the treatment area despite a groundwater pH as high as 9. The biological reductive dechlorination of the chlorinated ethenes explains the temporary increase in trichloroethene and cis‐1,2‐dichloroethene concentrations. © 2013 Wiley Periodicals, Inc.

Dissecting the deep biosphere: retrieving authentic microbial communities from packer-isolated deep crystalline bedrock fracture zones

Deep fracture zones in Finnish crystalline bedrock have been isolated for long, the oldest fluids being tens of millions of years old. To accurately measure the native microbial diversity in fracture-zone fluids, water samples were obtained by isolating the borehole fraction spanning a deep subsurface aquifer fracture zone with inflatable packers (500 and 967 m) or by pumping fluids directly from the fracture zone. Sampling frequency was examined to establish the time required for the space between packers to be flushed and replaced by indigenous fracture fluids. Chemical parameters of the fluid were monitored continuously, and samples were taken at three points during the flushing process. Microbial communities were characterized by comparison of 16S ribosomal genes and transcripts and quantification of dsrB (dissimilatory sulfate reduction) gene. Results suggest that fracture-zones host microbial communities with fewer cells and lower diversity than those in the drill hole prior to flushing. In addition, each fracture zone showed a community composition distinct from that inhabiting the drill hole at corresponding depth. The highest diversity was detected from the 967-m fracture zone. We conclude that the applied packer method can successfully isolate and sample authentic microbial fracture-zone communities of deep bedrock environments.

Distribution and properties of soft weathered bedrock at ≤ 1 m depth in the contiguous United States

ABSTRACTThe weathered bedrock zone is increasingly recognized as an important part of ecological and hydrologic systems, but its distribution is poorly known in the contiguous United States. We used spatial and laboratory characterization data from the US Department of Agriculture (USDA)‐Natural Resources Conservation Service to assess the distribution and soil‐like properties of soft weathered bedrock (saprock and saprolite) within the 48 contiguous United States. Because USDA soil inventories generally do not extend below 2 m, and because the upper 1 m is clearly involved in ecosystem function and vadose zone hydrology, we restricted our inquiry to soft weathered bedrock within 1 m of the land surface. Soft weathered bedrock within 1 m of the land surface is widespread throughout the contiguous United States, underlying at least 6% of the land area. In‐depth analysis of three states showed that soft weathered bedrock at the ≤ 1‐m depth underlies 22% of the total land area in California, 33% in Wyoming, and 18% in North Carolina. Soft weathered bedrock hosts pedogenic activity, as indicated by morphological features such as roots, clay films, and iron (Fe)‐/manganese (Mn)‐oxide concretions recorded in pedon descriptions in the database. The physical and chemical properties of soft weathered bedrock are often similar to those of the overlying soil, suggesting that in many respects soft weathered bedrock behaves like soil. It supplies water and nutrients to plants whose roots penetrate into it and it modulates throughflow runoff to streams. For a more complete understanding of soft weathered bedrock, systematic data are needed on its thickness across landscapes and a consistent terminology for its various forms needs to be adopted and widely used. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Rain, rock moisture dynamics, and the rapid response of perched groundwater in weathered, fractured argillite underlying a steep hillslope

Various field studies have concluded that shallow groundwater in weathered bedrock underlying hillslopes can contribute to both base and stormflow and thus dominate runoff. The processes associated with recharge from the ground surface, through this unsaturated zone, have received little study, yet they influence runoff dynamics, the chemical evolution of water, and moisture availability. Here we use five measurement systems to document soil and rock moisture dynamics within a 4000 m2zero‐order basin in which all runoff occurs through weathered argillite. At this site, the weathered bedrock zone (in which the groundwater fluctuates by 8 m seasonally) varies in depth from ∼4 m at the base of the hillslope to nearly 19 m near the hill top. An aggregate‐rich, porous, 0.5 m thick soil overlies the weathered bedrock. We find that during the first rains of the wet season, water rapidly travels meters into the weathered bedrock zone. Consistently, however, groundwater at some places responds quickly to the first major storm, well before the wetting front has been detected much beyond about 1 m. Furthermore, throughout the wet season, the lower portion of the unsaturated weathered bedrock shows little or no moisture change. These observations suggest a fracture‐dominated flow path, leading to a highly variably groundwater response across the hillslope for a given storm. Seasonal changes in rock moisture content are greatest in the first 5 to 10 m depth and may exceed the magnitude of moisture changes in the soil, suggesting that it could constitute a significant unmapped moisture reservoir.

Multi-channel Analysis of Surface Waves (MASW) in Karst Terrain, Southwest Georgia: Implications for Detecting Anomalous Features and Fracture Zones

Multi-channel analysis of surface waves (MASW) was used to map bedrock topography, image anomalies in the soil overburden, and detect variations in the degree of limestone weathering in covered, karst terrain near Albany, Georgia. Surface-wave data were acquired along seven parallel lines and two perpendicular cross-lines. Two surveys using a hammer and moving van-source were conducted to address the trade-off between lateral resolution and depth penetration. Resolution matrices show that the depth ranges most effectively imaged are 0.5–4 m for half-spread hammer data, 3–12 m for full-spread hammer data, and 8–20 m for van-source data. A steep S-velocity gradient marked by the 350 m/s contour at depths of 9–12 m is interpreted as the bedrock surface, in agreement with coincident borehole data. The models indicate variations within limestone bedrock (S-velocity: 350–700 m/s) and anomalous zones within the sandy-clay overburden. Standard penetration tests at two locations 45-m apart confirm variations in limestone rigidity suggested by the velocity models. At one location, a dissolution feature (N-value: 0) within highly weathered, saturated limestone (N-values: 12 and 19) correlates with low S-velocities (&lt;350 m/s). Borehole data from the second location shows more rigid limestone (N-values: 25 and 34) that correlates with S-velocities between 350–400 m/s. A zone of weathered rock at depths of 12–20 m imaged along a suspected fracture zone is consistent with borehole data and sinkhole formation at the ground surface. An anomaly associated with a burn pit at the intersection of two perpendicular profiles was imaged at two different scales. S-velocity models and distinct changes in apparent velocity of direct P-waves and surface waves across shot gathers constrain the lateral and vertical extent of the anomaly, and borehole data verify changes in soil composition and depth (5 m) to the base of the anomaly.

Spatial discontinuity and temporal evolution of channel morphology along a mixed bedrock-alluvial river, upper Drôme River, southeast France: Contingent responses to external and internal controls

The rehabilitation of degraded river channels is often guided by assumptions of continuity, yet in response to spatial and temporal variations in controlling conditions rivers typically display discontinuous response in space and time. This study examines the development of a 5km reach of the Drôme River, S.E. France, characterised by alternating alluvial and bedrock zones that are separated by abrupt downstream transitions. This reach is representative of the Drôme River as a whole, and other rivers in the European Alps where braided channel planforms have been replaced by more complex, discontinuous morphologies. The primary aims are to understand how this spatial complexity has developed on the Drôme; evaluate how temporal channel changes have been affected by local factors, particularly bedrock exposures, and by long-term, catchment-scale changes in sediment supply and the flood activity; and consider the implications of this discontinuous geomorphology for reach management. The development of geomorphological zonation is examined by documenting sequential changes in channel planform between seven periods, using aerial photography (1948–2006) and by analysing change in bed elevation from profiles surveyed in 1928, 2003 and 2005. Between 1948 and 2001 bedrock exposed in the channel bed and along the floodplain margins defined discontinuities in sediment connectivity that were largely responsible for the configuration of channel zones. The impact of floods on this system was not proportional to flood magnitude. A modest flood in 1978 was an important event that, by incision and avulsion at key locations, defined a pattern of zonation that persisted until the end of the study in 2006. During the final 5years of the study, alluvial zones that previously responded to large floods by widening underwent narrowing, despite the occurrence of a large flood, and led to an overall reduction in width variance. This resulted from progressive incision beneath and disconnection from formerly active channel areas, in response to long-term, catchment-scale reductions in sediment supply and flood frequency. In 2006 the pattern of zonation remains distinct, disguising this recent change in channel response and underlining the need for long-term and sequential perspectives of channel development to fully understand the processes in operation; contemporary snapshots of channel form may be misleading. Understanding interactions between inherent channel complexity and prevailing flow and sediment conditions, and how this shapes channel response to individual floods, is essential when interpreting future trajectories of channel change and likely response to management intervention.

Groundwater hydrology of boreal peatlands above a bedrock tunnel – Drainage impacts and surface water groundwater interactions

Summary Little attention has been given to the pattern of hydrological connectivity between peatlands and water bearing fracture zones in crystalline bedrock. The construction of the railway tunnel Romeriksporten provided an opportunity for studying impacts of bedrock tunnelling on peatland hydrology and the hydraulic connectivity between peat deposits and deeper layers in an area with fractured Precambrian gneisses, exposed bedrock, and surficial covers of thin till deposits and peatsoils. Above the tunnel the water level in peat wells fluctuated with maximum depths up to 3 m, and water that otherwise would have generated surface runoff infiltrated in the peat. Drawdowns of the groundwater table in peatlands were observed as far as 340 m away from the tunnel trace. The deep drainage base provided prolonged water table drawdowns in peatlands in dry periods, and differential settlements in drained peatsoils resulted in secondary changes in patterns of surface water storage and flow. The groundwater drawdowns were influenced by the balance between tunnel leakage and water supplies from catchments and the wetland position within the groundwater flow system. Deep and simultaneous lowering and fluctuations of hydraulic heads in wells in the peat, the subpeat sediments and the bedrock above the tunnel demonstrated hydraulic connectivity between the peat layers and the bedrock, and revealed vertical flow even through highly humified peat layers in the catotelm. This shows that in peatlands with a subsurface of a relatively thin till cover above fractured crystalline bedrock, favourable conditions for groundwater flow from peatsoils to rock are not always restricted to a few specific sites, and indicates that attention should be given to the influence of peat water–bedrock water connectivity on impacts of groundwater exploitation, droughts and climate changes in such areas.

Persistence of hydrologic variables and reactive stream solute concentrations in an east Tennessee watershed

Time and frequency domain analyses were conducted on weekly time series of water chemistry (nitrate, sulfate and calcium concentrations) collected from November 1995 to December 2005 at the West Fork of Walker Branch in Oak Ridge, Tennessee to evaluate the extent of their persistence and the relationship of this persistence to discharge and rainfall. In this study, spectral and wavelet analyses provided a theoretical basis for insights into long-term water chemistry behavior. All water chemistry parameters showed some level of persistence that was influenced by rainfall and/or discharge. Short-term persistence (less than a year) was related to the persistence of rainfall and discharge, whereas long-term persistence (more than a year) was related to the persistence of discharge. The Walker Branch conceptual hydrology model is augmented by these results that relate characteristic periodicities with flowpaths through different zones: the vadose zone (<20 week period), saturated zone (20–50 week period) and bedrock zone (>50 week period) with implications for reactive chemistries within the watershed.

Soil-gas helium and surface-waves detection of fault zones in granitic bedrock

Fracture and fault networks are conduits that facilitate groundwater movement in hard-rock terrains. Soil-gas helium emanometry has been utilized in Wailapally watershed, near Hyderabad in southern India, for the detection of fracture and fault zones in a granite basement terrain having a thin regolith. Based on satellite imagery and geologic mapping, three sites were selected for detailed investigation. High spatial resolution soil-gas samples were collected at every one meter at a depth of >1.5m along 100m long profiles (3 in number). In addition, deep shear-wave images were also obtained using the multichannel analysis of surface waves. The study clearly indicates several soil-gas helium anomalies (above 200 ppb) along the profiles, where the shear-wave velocity images also show many near-surface vertical low velocity zones. We thus interpret that the soil-gas helium anomalous zones and the vertical low-velocity zones are probable traces of fault/fracture zones that could be efficient natural recharge zones and potential groundwater conduits. The result obtained from this study demonstrates the efficacy of an integrated approach of soil-gas helium and the seismic methods for mapping groundwater resource zones in granite/gneiss provinces.

Fractured bedrock and saprolite hydrogeologic controls on groundwater/surface-water interaction: a conceptual model (Australia)

Hydrologic conceptual models of groundwater/surface-water interaction in a saprolite-fractured bedrock geological setting often assume that the saprolite zone is hydraulically more active than the deeper bedrock system and ignore the contribution of deeper groundwater from the fractured bedrock aquifer. A hydraulic, hydrochemical, and tracer-based study was conducted at Scott Creek, Mount Lofty Ranges, South Australia, to explore the importance of both the deeper fractured bedrock aquifer system and the shallow saprolite layer on groundwater/surface-water interaction. The results of this study suggest that groundwater flow in the deeper fractured bedrock zone is highly dynamic and is an important groundwater flow pathway along the hillslope. Deep groundwater is therefore a contributing component in streamflow generation at Scott Creek. The findings of this study suggest that hydrologic conceptual models, which treat the saprolite-fractured bedrock interface as a no-flow boundary and do not consider the deeper fractured bedrock in hydrologic analyses, may be overly simplistic and inherently misleading in some groundwater/surface-water interaction analyses. The results emphasise the need to understand the relative importance of subsurface flow activity in both of these shallow saprolite and deeper bedrock compartments as a basis for developing reliable conceptual hydrologic models of these systems.

Plume and lithologic profiling with surface resistivity and seismic tomography

Improved surface-based geophysical technologies that are commercially available provide a new level of detail that can be used to guide ground water remediation. Surface-based multielectrode resistivity methods and tomographic seismic refraction techniques were used to image to a depth of approximately 30 m below the surface at the Natural and Accelerated Bioremediation Research Field Research Center. The U.S. Department of Energy (DOE) established the research center on the DOE Oak Ridge Reservation in Oak Ridge, Tennessee, to conduct in situ field-scale studies on bioremediation of metals and radionuclides. Bioremediation studies are being conducted on the saprolite, shale bedrock, and ground water at the site that have been contaminated with nitrate, uranium, technetium, tetrachloroethylene, and other contaminants (U.S. DOE 1997). Geophysical methods were effective in imaging the high-ionic strength plume and in defining the transition zone between saprolite and bedrock zones that appears to have a significant influence on contaminant transport. The geophysical data were used to help select the location and depth of investigation for field research plots. Drilling, borehole geophysics, and ground water sampling were used to verify the surface geophysical studies.

Processes of oscillatory basin filling and excavation in a tectonically active orogen: Quebrada del Toro Basin, NW Argentina

Intramontane basins may act as important sediment storage areas, serve as recorders of the history of deformation, record syntectonic deposition, and document the evolution of climatic conditions during deposition. We document the timing, cyclicity, and processes that led to the filling and reexcavation of the intramontane Quebrada del Toro basin in NW Argentina. Geomorphic and geologic observations indicate that the basin was filled with sediment that has been subsequently excavated at least two times in the last ∼8 m.y. The last filling and excavation cycle occurred within the last 0.98 m.y. and has led to the deposition and removal of ∼61.4 km 3 of material from the basin, leading to a basin-wide averaged minimum denudation rate of 0.16 mm/yr. Aggradation within the basin took place due to channel steepening of the downstream fluvial system that connects the intramontane basin to the foreland. This portion of the fluvial system is actively incising through an uplifting bedrock zone. We use observations within the Toro to test a quasi-physically based model of channel aggradation behind a rising base level that rises due to downstream channel steepening. Our work shows that the bedrock incision rate constant required to reproduce conditions observed within the Toro basin is consistent with values measured independently in similar rock types. Therefore, in intramontane basins that experience similar processes of filling and evacuation, this model may be used to assess the relative importance of tectonic rock uplift, bedrock resistance to fluvial incision, and climate in determining the geomorphic and sedimentologic history of these basins.

Water chemistry of mires in relation to the poor-rich vegetation gradient and contrasting geochemical zones of the north-eastern fennoscandian Shield

The relationship of the poor-rich vegetation gradient with water chemistry and geochemical bedrock zones was studied with a data set from 36 pristine mire sites in the middle boreal zone, Finland. The bipartition of bedrock zones into a granitoid zone and three other, more alkaline bedrock zones was clearly connected to the poor-rich gradient in the study area. Mires in the granitoid bedrock zone were predominantly bogs and poor fens, while most of the rich fen sites were located in other bedrock zones. In fens with strongly minerogenous hydrology, the geochemically distinct bedrock zones showed significant differences in concentrations of calcium, manganese, phosphorus, as well as in the total equivalent charge of base cations and the ratio of calcium and magnesium. In light of the indicator species baseda priori poor-rich classification anda posteriori correlations with vegetation ordinations and a poor-rich indicator cover index, pH showed the strongest relationship of the water chemical variables with the poor-rich gradient. High calcium concentrations only separated extremely rich fens from other categories. The poor-rich gradient of mires is considered as a primarily pH-related vegetation continuum. A more general use of pH-classes along with hydrological and vegetation based classification is suggested for mire classification instead of the “poor-rich” terminology.

Contributions of water supply from the weathered bedrock zone to forest soil quality

One measure of forest soil quality is the ability of the soil to support tree growth. In mediterranean-type ecosystems, such as most of California's forests, there is virtually no rainfall during the summer growing season, so trees must rely on water stored within the substrate. Water is the primary limitation to productivity in these forests. Many forest soils in California are relatively thin, but are underlain by thick zones of weathered bedrock. Weathered granitic bedrock, the most prevalent lithology, has available water capacities (0.12-cm water/cm rock) that approach those of soils (0.2-cm water/cm soil) and, because the weathered rock zone is usually so much thicker (several meters) than the soil (<1 m), it almost always constitutes the larger storage reservoir for plant-available water. The weathered bedrock retains the original rock fabric and is friable and easily excavated, but the primary minerals are not thoroughly altered to clay minerals, so it is not considered saprolite. Roots of ponderosa pine ( Pinus ponderosa) seedlings penetrate through the soil and encounter weathered bedrock within the first 2 years on many sites. Thus, the influence of the weathered bedrock zone on plant growth begins early. Root access to the weathered bedrock is restricted to fractures, which are often spaced about 50 cm apart. Water is extracted from the intervening rock matrix through unsaturated flow toward the root mat in the fractures and by mycorrhizal fungal hyphae that penetrate the rock matrix. At one site in the Sierra Nevada, 30-year-old Jeffrey pine ( P. jeffreyi) depleted the soil-held water by mid-June and relied on water stored in the weathered bedrock until the rainy season began in October. In this case, the weathered bedrock supplied at least 70% of the water used by the trees during the growing season. In the same area, we found that thin soils are not a detriment to pine productivity when they are underlain by a thick zone of weathered bedrock. In mediterranean-type ecosystems, the weathered bedrock zone is an essential component of the plant water supply system and is thus an important contributor to forest soil quality.

Atmospheric effects on the classification of surface minerals in an arid region using Short-Wave Infrared (SWIR) hyperspectral imagery and a spectral unmixing technique

This study focuses on the comparison of spectral unmixing results from at-sensor radiance data and atmospherically corrected data, i.e., surface reflectance. The airborne Short-Wave Infrared (SWIR) Full Spectrum Imager (SFSI) and Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) were used to acquire data over Cuprite, Nevada, in June 1995 and 1996, respectively. This is an arid region that is well known for its exposed bedrock and alteration zones. For this project, a portion of the SWIR-2 spectral range covering the atmospheric window between 2050 and 2350 nm and a linear spectral unmixing technique were used to map surface minerals, including products of hydrothermal alteration. In total, eight end-members were extracted from the imagery using a new automatic procedure called Iterative Error Analysis (IEA). The atmospheric corrections were applied using a look-up-table procedure implemented in the Imaging Spectrometer Data Analysis System (ISDAS) and created with the radiative transfer model MODTRAN3. The fraction, or abundance, maps derived from the two types of data were compared using the coefficient of determination (R2) and the Average Euclidean Distance Coefficient (AEDC). Very good unmixing agreement was found between the results from the at-sensor radiance data and those from the surface reflectance data. For the SFSI data, the R2 values range from 0.72 to 0.95 and the AEDC values range from 0.098 to 0.023, whereas for the AVIRIS data, the R2 values range from 0.92 to 0.99 and the AEDC values range from 0.021 to 0.008. This suggests that, for the spectral range considered, atmospheric corrections are not necessary for mineral mapping in an arid region and for desert atmospheric conditions.

Evaluation of two-dimensional resistivity methods in a fractured crystalline-rock terrane

A series of two-dimensional resistivity profiles collected in the Blue Ridge Province of southwest Virginia and results from numerical modeling of synthetic data reveal substantial differences in depth of investigation, resolution, and sensitivity using Wenner, Wenner–Schlumberger, dipole–dipole, and pole–pole data collection techniques. Resistivity profiles were collected using short (2-m electrode spacing, 48-m profile length), intermediate (6-m electrode spacing, 144-m profile length), and long (10-m electrode spacing, 240-m profile length) arrays over shallow unconsolidated soils and regolith overlying crystalline bedrock. Pole–pole data were only collected with the short array. Numerical modeling was used to simulate both vertical and horizontal structures similar to subsurface conditions in the study area. All of the apparent resistivity data were inverted into earth models using a computer program that uses an l 1 norm smoothness constrained inversion technique. Earth models generated from both field data and numerical modeling acquired by the dipole–dipole technique consistently indicated more detail and greater depth of investigation than the other techniques. The dipole–dipole method uniquely imaged thin saturated sands and isolated high resistivity bodies beneath the 48-m-length array, significant horizontal and vertical resistivity variations including a thick transitional resistivity zone in the 144-m-length array, and anomalous low resistivity zones in crystalline bedrock in the 240-m-length array. Earth models created from the Wenner and Wenner–Schlumberger apparent resistivity data had shallower depths of investigation and revealed significantly less geologic detail than the profiles generated from the dipole–dipole surveys. The earth model from the pole–pole data had the greatest depth of investigation but low resolution and limited geologic detail when compared to the dipole–dipole survey.