Mass Balance Results Research Articles

A Game Changer in Scale-Squeeze Technology

Background Field trials using a new scale-inhibitor technology that improves treatment lifetime of scale squeezes have been successfully performed in the Gulf of Mexico. Tomson Technologies, in partnership with Shell, developed proprietary nanoparticle carriers that enhance scale-inhibitor adsorption to the reservoir and control the return rate for extended periods of time. This technology results in less chemical bleed off in the initial flowback and increases the chemical retained in the reservoir, allowing for more effective squeeze treatments. Both nanoparticle-enabled phosphonate and polymer inhibitors have now been developed and successfully squeezed in the field. Phosphonate inhibitors are widely used for squeeze treatment due to their desirable adsorption and release properties in carbonate and sandstone reservoirs. Minor changes have been made to the chemistry, but overall, the fundamentals have remained unchanged for decades. Polymeric scale inhibitors have also been developed for cases in which phosphonates are not applicable. The nano-enhanced technology provides a large improvement of treatment lifetime of 2 to 4 times (200-400%) when compared to incumbents, making this technology advancement attractive even in cases where current squeezes are considered successful. The well selected for this case study is an offshore formation with a predominantly sandstone mineralogy (approximately 80% quartz) with 25-30% porosity and bottomhole temperature of 183°F (83°C). Technology From the Lab to Field A sandpack sample from the trial well was used in the laboratory to deter-mine the adsorption and desorption properties of the nano-enabled inhibitor in realistic rock conditions. Multiple conditioning steps were used before product was injected in a sequence that mimicked field squeeze treatments. Mass-balance results from the sandpack experiment show adsorption of approximately 8 mg of polymer retained per gram of crushed reservoir rock used in the experiment. A typical rule of thumb for phosphonate-scale inhibitors (only as a comparison since this is a polymeric scale inhibitor) is 1-2 mg of inhibitor retained per gram of rock. Therefore, this is considered a large improvement on adsorption. There are challenges associated with measuring polymers in brine as residuals; however, multiple methods, both in-house and external, were com-pared to ensure accuracy. The results using the nano-enhanced scale inhibitor show concentrations higher than 1 mg/L of active polymer for over 7,000 pore volume of return in the sandpack experiment. Complete intact core experiments were also conducted with reservoir fluids and showed no formation damage during the injection of the product with regained oil permeability of 96%. Oil permeability was in the 150-200 mD range for the intact core experiments. Third-party coreflood testing was performed with nitrified and foamed stages to ensure compatibility with the nano-enabled chemistry. No formation damage was observed with the nitrification of the stages containing the nano-enabled chemistry. Field Application Case Study After extensive lab validation of the product and supporting corefloods to de-risk the technology, Well A was selected by Shell to be the first well treated with the new nano-enabled extended-lifetime inhibitor.

Supergene Mass-Balance Study Assuming Zero Lateral Copper Flux Using Geostatistics to Recognize Metal Source Zones in Exotic Copper Deposits

Recognition of effective factors that influence the spatial extension of supergene weathering zones is important both for the identification of high potential areas of exotic deposits and for the cost-effective planning of mining. In particular, recognition of exotic mineralization around porphyry copper deposits early in mine development prevents them from being buried beneath mine infrastructures such as waste dump and tailing structures. Mass-balance modeling, a practical method for determining high potential areas of undiscovered exotic mineralization, investigates important factors in forming exotic deposits. Mass-balance modeling is a two-phase methodology that becomes progressively more detailed. An initial result, presented here as phase 1, is based solely on Cu assays. Phase 2 incorporates relict sulfide mineral studies to improve phase 1 modeling results and computes actual fluxes of copper that escaped vertically downward from the leached cap to form the enrichment blanket and then flowed laterally away to form exotic mineralization. In addition, geostatistical approaches, especially sequential Gaussian simulation, are useful tools for investigating the spatial relationships and modeling of mass-balance results in phase 1 studies. This paper introduces a method for interpolation and downscaling of the preliminary mass-balance analysis (phase 1) to highlight the role of geological features in the evolution of the supergene process. Using only copper assays without any need for relict sulfide mineralogy, this approach can be used to approximately identify the geographic direction of metal movement in exotic copper deposits, and thus serve as an initial exploration guide in prospecting for exotic deposits. For this, a vertical columnar block model was constructed for each of the supergene weathering zones and preliminary analysis of mass balance was conducted to reconstruct the apparent total leached zone column height assuming zero lateral flux. This analysis was applied to each of the vertical block model columns. The results of mass balance were interpolated in a 5 × 5 m grid by sequential Gaussian simulation method, and the simulated surface of the total leached zone was conflated with geological features. The roles of topography, argillic alteration and linear structures were identified in the transport of supergene solutions in the Miduk porphyry copper deposit of Iran. In the northern section of the deposit, which is in accordance with the topography gradient and the presence of advanced argillic alteration zone, the computed top total of leaching is below the actual surface topography, whereas the hypogene isograd curves confirm the expansion of primary copper in these areas. The northern section of the deposit was introduced as a susceptible area for the removal of copper-bearing solutions from the supergene enrichment system.

Glacier variations at Aru Co in western Tibet from 1971 to 2016 derived from remote-sensing data

ABSTRACTTwin glaciers collapsed in 2016 near Aru Co, western Tibet and caused extreme loss to human beings. In this study, we attempted to track the dynamics of glaciers in the region, for example the glacier area and mass changes in Aru Co for the period 1971–2016, which were determined using topographic maps and Landsat images and ASTER-derived DEMs (2011–16), the Shuttle Radar Terrain Mission DEM (2000) and topographic maps (1971). Our results showed that the glacier area of Aru Co decreased by −0.4 ± 4.1% during 1971–2016. The geodetic mass-balance results showed that the glaciers in Aru Co lost mass at a rate of −0.15 ± 0.30 m w.e. a−1 during 1971–99, while they gained mass at a rate of 0.33 ± 0.61 m w.e. a−1 for the period 1999–2016. The twin glaciers experienced a larger negative mass budget than the others in the region before 1999. This process produced large amounts of meltwater, followed by a sustained increase in the meltwater on the pressure melting point, possibly in response to a period of positive mass balance (1999–2016) and then, transferred to the glacier bed until the glaciers collapsed.

Estimating fluxes in anthropogenic lead using alluvial soil mass-balance geochemistry, geochronology and archaeology in eastern USA

Predicting human impact on Earth's surface requires a quantitative understanding of past human–environmental interactions. This study uses mass-balance geochemistry of buried alluvial soils , geochronology and archaeology to constrain soil age and quantify the anthropogenic addition of lead (Pb) to alluvial soils from the Delaware River valley in northeastern USA. Mass addition of Pb was calculated for two alluvial terrace landforms. The results show that for buried soils that formed during the middle and late Holocene , the addition of Pb was estimated at 1.71 ± 0.02 μg cm −2 and −39 ± 0.43 μg cm −2 , respectively. The corresponding Pb addition rates were estimated at 0.04 μg cm −2 y −1 and −0.28 μg cm −2 y −1 , where the negative rate documents Pb loss from the soil. Conversely, the Pb addition to buried alluvial soils that formed <100 years ago were estimated at 486 μg cm −2 and 526 μg cm −2 . The corresponding mean Pb addition rates were estimated to be 10.06 ± 2.13 μg cm −2 y −1 and 8.91 ± 4.66 μg cm −2 y −1 . The recent Pb addition rates documented in this study are over 2 orders of magnitude higher than their middle and late Holocene counterparts. The >200 times increase in Pb and rapid deposition rates in the 19th–20th century alluvial soils coincide with the presence of historic Euro-American artifacts, providing support for human-induced Pb addition. Furthermore, the soil stratigraphic context of a tooth-crown bottle cap (maximum age = 58 y BP) and other archaeological age-based estimates allowed us to constrain the residence time of a historic alluvial soil to 58–39 y. The mass-balance results show that 19th–20th century Pb additions to these two alluvial profiles are unlike anything documented in the buried alluvial soils prior to the Industrial Era. Although human-induced mass influx of Pb had been previously documented in nearby lake cores and upland soils, quantification of Pb flux in buried alluvial soils, especially those with archaeological deposits, will further constrain the magnitude, extent and timing of anthropogenic Pb impact on Earth's surface.

Residence times of shallow groundwater in West Africa: implications for hydrogeology and resilience to future changes in climate

Although shallow groundwater (<50 mbgl) sustains the vast majority of improved drinking-water supplies in rural Africa, there is little information on how resilient this resource may be to future changes in climate. This study presents results of a groundwater survey using stable isotopes, CFCs, SF6, and 3H across different climatic zones (annual rainfall 400–2,000 mm/year) in West Africa. The purpose was to quantify the residence times of shallow groundwaters in sedimentary and basement aquifers, and investigate the relationship between groundwater resources and climate. Stable-isotope results indicate that most shallow groundwaters are recharged rapidly following rainfall, showing little evidence of evaporation prior to recharge. Chloride mass-balance results indicate that within the arid areas (<400 mm annual rainfall) there is recharge of up to 20 mm/year. Age tracers show that most groundwaters have mean residence times (MRTs) of 32–65 years, with comparable MRTs in the different climate zones. Similar MRTs measured in both the sedimentary and basement aquifers suggest similar hydraulic diffusivity and significant groundwater storage within the shallow basement. This suggests there is considerable resilience to short-term inter-annual variation in rainfall and recharge, and rural groundwater resources are likely to sustain diffuse, low volume abstraction.

Tidal exchange between a freshwater tidal marsh and an impacted estuary: the Scheldt estuary, Belgium

Tidal marsh exchange studies are relatively simple tools to investigate the interaction between tidal marshes and estuaries. They have mostly been confined to only a few elements and to saltwater or brackish systems. This study presents mass-balance results of an integrated one year campaign in a freshwater tidal marsh along the Scheldt estuary (Belgium), covering oxygen, nutrients (N, P and Si), carbon, chlorophyll, suspended matter, chloride and sulfate. The role of seepage from the marsh was also investigated. A ranking between the parameters revealed that oxygenation was the strongest effect of the marsh on the estuarine water. Particulate parameters showed overall import. Export of dissolved silica (DSi) was more important than exchange of any other nutrient form. Export of DSi and import of total dissolved nitrogen (DIN) nevertheless contributed about equally to the increase of the Si:N ratio in the seepage water. The marsh had a counteracting effect on the long term trend of nutrient ratios in the estuary.

The influence of drainage boundaries on specific mass-balance results: a case study of Engabreen, Norway

AbstractMass-balance measurements were initiated on Engabreen, an outlet glacier from the Svartisen ice cap, Norway, in 1970. The glacier boundary was defined based on where meltwater drained, as the interest in Engabreen was mainly hydrological. However, the apparent discrepancy between the calculated cumulative glacier mass balance since 1970 and changes in glacier geometry prompted a re-examination of the glacier boundary. The glaciological drainage boundary is defined by studying whether ice flow physically contributes to Engabreen tongue and corresponds to a glacier with an area of 27.2 km2, significantly smaller than that defined by the hydrological drainage boundary at 39.6 km2. This glaciological drainage boundary is here named the ice-flow perimeter. The area difference between this and the hydrological drainage boundary is largest for the altitudinal range 1300–1400ma.s.l. Generally, the ‘glaciological’ glacier is lower in mean altitude than the ‘hydrological’ glacier, and this affects the calculated specific mass balance. Using the glaciological boundary leads to reductions in mean annual winter and summer balance (when spatial differences are ignored) of 0.12 mw.e. (from 2.92 to 2.80mw.e.) and 0.15 mw.e. (from –2.32 to –2.47mw.e.), respectively. The reduction in mean net balance for the period 1970–2006 is 0.27mw.e. (from +0.59 to +0.32mw.e.) which is about 50% of the calculated mass surplus in this period. This illustrates that the choice of glacier outline can significantly influence the long-term cumulative mass balance and that results from outlet glaciers must be interpreted with care when used for regional estimates of glacier change.

Over-parameterised, uncertain ‘mathematical marionettes’ — How can we best use catchment water quality models? An example of an 80-year catchment-scale nutrient balance

The Integrated Catchment Model of Nitrogen (INCA-N) was applied to the River Lambourn, a Chalk river-system in southern England. The model's abilities to simulate the long-term trend and seasonal patterns in observed stream water nitrate concentrations from 1920 to 2003 were tested. This is the first time a semi-distributed, daily time-step model has been applied to simulate such a long time period and then used to calculate detailed catchment nutrient budgets which span the conversion of pasture to arable during the late 1930s and 1940s. Thus, this work goes beyond source apportionment and looks to demonstrate how such simulations can be used to assess the state of the catchment and develop an understanding of system behaviour. The mass-balance results from 1921, 1922, 1991, 2001 and 2002 are presented and those for 1991 are compared to other modelled and literature values of loads associated with nitrogen soil processes and export. The variations highlighted the problem of comparing modelled fluxes with point measurements but proved useful for identifying the most poorly understood inputs and processes thereby providing an assessment of input data and model structural uncertainty. The modelled terrestrial and instream mass-balances also highlight the importance of the hydrological conditions in pollutant transport. Between 1922 and 2002, increased inputs of nitrogen from fertiliser, livestock and deposition have altered the nitrogen balance with a shift from possible reduction in soil fertility but little environmental impact in 1922, to a situation of nitrogen accumulation in the soil, groundwater and instream biota in 2002. In 1922 and 2002 it was estimated that approximately 2 and 18 kg N ha − 1 yr − 1 respectively were exported from the land to the stream. The utility of the approach and further considerations for the best use of models are discussed.

Mineralogic controls on the composition of natural waters dominated by silicate hydrolysis

A detailed mineral-solute mass-balance spreadsheet model has been developed to improve understanding of mineralogic controls on water composition in surface waters and groundwater systems dominated by silicate lithologies. The spreadsheet-based approach is used to investigate mass-balance solutions over a range of mineral compositions for predominant, multi-element phases such as plagioclase, biotite, chlorite, amphibole, and dioctahedral smectite. The mass-balance technique has been applied to a variety of hydrogeologic environments including calibrated watersheds, springs and groundwater systems. Lithologic settings include plutonic, metamorphic, minor volcanic, and sedimentary (alluvial and glacial deposits) materials. Sites investigated, most of which allowed comparison with earlier work, include ephemeral and perennial springs of the central Sierra Nevada Mountains, California; the Loch Vale, Colorado high-altitude watershed; four catchments in the Inyo Mountains, California and Nevada; watersheds in the Catoctin Mountain, Maryland area; and ground-water flow systems in Vekol Valley, Arizona and in the Trout Lake area, Wisconsin. Analytical mass balances were derived with a novel approach that permits graphic analysis of the functional dependency of mineral mass-transfer coefficients on variations in major silicate composition. A 10 x 10 matrix of mineral-solute reaction equations was used to solve for the best mass balances. Alternative methods were developed to investigate cases where the number of mineral phases exceeds the number of solute constraints (10) that are commonly found in clastic, silicate-dominated systems with diverse sources and/or phase mixtures of variable chemistry (such as an assortment of plagioclase compositions). Techniques were also developed to allow for important structurally controlled compositional variation in clay minerals. Final mass-balance choices were tempered by thermodynamic stability calculations for various minerals under near-surface weathering conditions. Mineral mass-transfer ratios were also found to be useful in restricting mass balances. Additional constraints were provided by establishing consistent sets of similar mineral assemblages and mineral compositions for a series of wells along a ground-water flow path, or through comparison of multiple nearby watersheds subject to similar climatic and vegetative conditions. Whereas unique analytical solutions from spreadsheet mass balances were not always possible, a surprisingly limited range was obtained for mineral assemblages and compositions satisfying observed mineral occurrences and thermodynamic stability conditions. Our results demonstrate the importance of having well-characterized mineral compositions for both reactant and product phases. The mass-balance results are particularly sensitive to the compositions of plagioclase, dioctahedral smectite, and a few primary mafic mineral groups (trioctahedral mica, amphibole, and pyroxene). In addition to mineral controls, the specific examples studied demonstrate effects of dolomite and calcite overprints on silicate weathering, biomass fluctuations, road salt contamination, and on buried evaporite salts. A comparison of the mass-balance results from surface- and groundwater systems indicates strong similarities, suggesting that similar processes dominate the mineral controls in both settings, and that, under soil weathering conditions ion exchange might have been overemphasized, except under conditions where pre-existing waters are displaced by solutions of significantly different composition.

QUANTIFYING TIME-VARYING GROUND-WATER DISCHARGE AND RECHARGE IN WETLANDS OF THE NORTHERN FLORIDA EVERGLADES

Developing a more thorough understanding of water and chemical budgets in wetlands depends in part on our ability to quantify time-varying interactions between ground water and surface water. We used a combined water and solute mass balance approach to estimate time-varying ground-water discharge and recharge in the Everglades Nutrient Removal project (ENR), a relatively large constructed wetland (1544 hectare) built for removing nutrients from agricultural drainage in the northern Everglades in South Florida, USA. Over a 4-year period (1994 through 1998), ground-water recharge averaged 13.4 hectare-meter per day (ha-m/day) or 0.9 cm/day, which is approximately 31% of surface water pumped into the ENR for treatment. In contrast, ground-water discharge was much smaller (1.4 ha-m/day, or 0.09 cm/day, or 2.8% of water input to ENR for treatment). Using a water-balance approach alone only allowed net ground-water exchange (discharge 2 recharge) to be estimated (212 6 2.4 ha-m/day). Discharge and recharge were individually determined by combining a chloride mass balance with the water balance. For a variety of reasons, the ground-water discharge estimated by the combined mass balance approach was not reliable (1.4 6 37 ha-m/day). As a result, ground-water interactions could only be reliably estimated by comparing the mass-balance results with other independent approaches, including direct seepage-meter measurements and previous estimates using ground-water modeling. All three independent approaches provided similar esti- mates of average ground-water recharge, ranging from 13 to 14 ha-m/day. There was also relatively good agreement between ground-water discharge estimates for the mass balance and seepage meter methods, 1.4 and 0.9 ha-m/day, respectively. However, ground-water-flow modeling provided an average discharge esti- mate that was approximately a factor of four higher (5.4 ha-m/day) than the other two methods. Our study developed an initial understanding of how the design and operation of the ENR increases interactions between ground water and surface water. A considerable portion of recharged ground water (73%) was collected and returned to the ENR by a seepage canal. Additional recharge that was not captured by the seepage canal only occurred when pumped inflow rates to ENR (and ENR water levels) were relatively high. Management of surface water in the northern Everglades therefore clearly has the potential to increase interactions with ground water.

Evaluation of an Osmonics® spiral-wound cross-flow filtration system for sub-μm sampling of Cd, Cu and Ni in seawater

Abstract A 1-kDa (1 kDa = 1000 daltons) Osmonics spiral-wound cross-flow filtration (CFF) membrane has been evaluated for sampling Cd, Cu and Ni in 1-kDa fractions of prefiltered seawater. Size-fractionated samples were collected as part of a CFF intercomparison experiment (Buesseler et al., 1996-this issue) and in the Gulf of Maine. Mass-balance results averaged 93 ± 6% (Cd), 81 ± 19% (Cu), and 99 ± 18% (Ni) for the intercomparison experiment and 56 ± 17% (Cd), 119 ± 38% (Cu), and 91 ± 6% (Ni) for the Gulf of Maine samples. Differences in mass balance between these studies may be related, in part, to removal of sub-μm particles and associated trace metals during prefiltration. An Amicon CFF system showed higher (2–12 ×) colloidal concentrations for Cu and Ni compared to Filtron and Osmonics systems; all CFF systems showed similar Cd, Cu and Ni mass-balance results. A model based on particle impaction with a filter shows the potential for adsorption of sub-μm particles by a 1-kDa Osmonics CFF membrane and conventional (Nuclepore) filters.

Principios básicos de la modelización geoquímica directa e inversa

Geochemical modeling consists in the application of thermodynamic and physicochemical principles in the hydrogeochemical systems interpretation. It has been developed following two different approaches: a) inverse modeling (or mass balance calculations), which uses observed chemical and isotopic data from waters and rocks to identify geochemical reactions responsible of them, in a quantitative way; and b) forward modeling, which attempts to predict water compositions and mass transfer that can result from hypothesized reactions, from observed initial conditions on water-rock system compositions. Both of them have intrinsic uses and limitations which drive to their use in specific problems. For systems with adequate chemical, isotopic, and mineralogic data, the inversemodeling approach of speciation and mass-balance modeling provides the most direct means of determining quantitative geochemical reaction models. In contrast, for systems with missing or inadequate data, reaction-path modeling provides an a priori method of predicting geochemical reactions. In some cases it is useful to combine forward modeling with the results from inverse models. The mass-balance results determine the net mass transfer along the flow path, but these results are only partially constrained by thermodynamics. The forward modeling can be used both, to prove thermodynamic consistency for them, and to predict water quality at points where there are no enough data. Recent advances in geochemical modeling are focused on finding the most efficient numerical procedures for coupling geochemical reaclions (both equilibrium and kinetic) with the hydrodynamic transport equations in compositionally-complex systems, on uncertainty analysis, and on model validation for actual geochemical systems.

Transport of calcium, magnesium and SO 4 in the Floridan aquifer, west-central Florida: implications to cementation rates

Prior hydrogeochemical studies have documented a down-gradient increase in dissolved calcium, magnesium and SO 4 in the confined, upper Floridan aquifer and have attributed the changes to dissolution of gypsum and dolomite and precipitation of calcite. In this study, hydrochemical data (mol l −1) are coupled with calculated discharges (1 year −1) to calculate the down-gradient changes in ion fluxes (mol year −1) along two predevelopment stream tubes in west-central Florida. Evaluation of ion inflows (including leakage from above) and outflows to and from well-to-well sectors of the stream tubes enables isolation of the changes in ion fluxes due to the previously identified reactions and, hence, quantification of the geological work done by those reactions in the sectors. Over about half of the stream tubes, the amount of calcite precipitation is small to non-existent (0 ±0.25% of the bulk volume per million years), which is consistent with geological evidence. Significant cementation rates are calculated for areas influenced by upward flow, such as at the end of the stream tubes and along the Peace River. Half of these significantly large values are impossibly large, apparently reflecting an inherited signal from a flow history below the depth of the upper Floridan wells. Similarly, mass-balance results that suggest significant dissolution of dolomite and gypsum probably reflect upwelling, not diagenesis at the depths of the sampled wells.

Geochemical investigation of salt-water intrusion into a coastal carbonate aquifer: Mallorca, Spain

Geochemical processes occurring within the mixing zone on Mallorca, Spain, were investigated in relation to the diagenesis of carbonate minerals and to the development of porosity and permeability within the Pleistocene limestone aquifer. Rock core and ground-water samples were obtained from the fresh-water zone, the mixing zone and the top of the sea-water zone at two sites near the coast. Ground-water withdrawals in the vicinity of one study site depressed the water table to below sea level, resulting in sea-water encroachment and a thin mixing zone. Laboratory tests conducted on the rock core indicated that porosity and hydraulic conductivity were not enhanced within the transitory location of the present-day mixing zone. Vuggy zones in the core correspond to the approximate location of the water table. Because the ground water was supersaturated with respect to the carbonate minerals, dissolution of the limestone aquifer is no predicted for the present-day mixing zone. Results of mass-balance and source-rock calculations indicated an excess of calcium and strontium in the mixing-zone waters in relation to what was expected from conservative mixing. These elevated concentrations are hypothesized to result from limestone dissolution in the vadose zone.

Mass Balance Along Two Transects of the Greenland Ice Sheet (Abstract)

The mass balance and sensitivity of calculated mass balance to uncertainties in the data and in the model for the variation of ice velocity with depth are addressed, using data from the EGIG transect and from the OSU transect in south Greenland (Drew and Whillans, 1984). The calculation uses a non-steady continuity model with allowances for three-dimensional flow and horizontal velocity variation with depth. Depth variation in horizontal velocity is obtained, using the constitutive relation for ice with calculated temperature profiles and with full allowance for longitudinal stresses and enhancement of flow due to ice anistrophy and texture. Separate calculations are made for different thickening or thinning rates, until a match between observed and calculated surface velocities is obtained. For the EGIG transect, our mass-balance results are in the range reported by Mälzer and Seckel (1975) and Bindschadler (1984). Results for the OSU transect, just south of the Arctic Circle on the south dome, are also reported. Of particular interest is the sensitivity study, which is designed to determine which aspects of the data and flow behavior are most critical to the calculations. Data that are comparatively well-constrained are flow-line definition, surface velocity and thickness. The flow lines and lateral spreading are obtained from satellite radar altimetry (Zwally and others, 1983) and checked against velocity data along the OSU profile. Thicknesses are from aerial radar sounding (Overgaard, personal communication). Surface velocities along the EGIG transect are corrected in a manner similar to that suggested by Robin (1983, figure 2.17b) and possible errors are not critical. Along the OSU transect the surface velocities are well-determined by Doppler satellite tracking in short-arc translocation mode. Accumulation rates are not well-determined and show substantial scatter for reasons that are not understood. Along the OSU transect, accumulation rates were obtained by augering for the nuclear bomb horizons. As reported earlier (Mock, 1967), the accumulation rates in this region show no clear pattern with elevation or slope. Similar, but less severe, problems exist in the EGIG area. The lack of a good model describing the geographic variation in accumulation rate results in calculated surface velocity uncertainties of 9% for the OSU and 6% for the EGIG transect. The calculated results are comparatively insensitive to parameters affecting the temperature profile. This is because, in this model, the mean velocity is determined by continuity and temperature affects only the ratio of horizontal velocity at a given depth to the mean velocity for the entire profile. Longitudinal stresses are included because they affect the viscosity through the effective shear stress. Neglecting this would lead to a 30% error in calculated surface velocity near the ice divide and about 6% away from the divide. Provided longitudinal stresses are included, uncertainties in these stresses are not critical. Enhancement factors are, however, important. Most authors believe that ice fabric, texture, and impurity content can affect strain rates by 100% to 1200% (Shoji and Langway, 1984). As with the temperature profile, this problem does not affect calculated mean velocities, which are based on continuity, but it does affect the ratio of surface to mean velocity. Enhancement factors are more critical than temperature uncertainties and therefore failure to include depth-variable flow enhancement alters calculated surface velocities by 5 to 10%. In summary, the mass balance along two transects has been investigated by comparing measured and calculated surface velocities. The most critical problems are (1) understanding the geographic distribution in accumulation rate and (2) correctly allowing for enhanced shearing due to structural variations with depth.

Mass Balance Along Two Transects of the Greenland Ice Sheet (Abstract)

The mass balance and sensitivity of calculated mass balance to uncertainties in the data and in the model for the variation of ice velocity with depth are addressed, using data from the EGIG transect and from the OSU transect in south Greenland (Drew and Whillans, 1984). The calculation uses a non-steady continuity model with allowances for three-dimensional flow and horizontal velocity variation with depth. Depth variation in horizontal velocity is obtained, using the constitutive relation for ice with calculated temperature profiles and with full allowance for longitudinal stresses and enhancement of flow due to ice anistrophy and texture. Separate calculations are made for different thickening or thinning rates, until a match between observed and calculated surface velocities is obtained.For the EGIG transect, our mass-balance results are in the range reported by Mälzer and Seckel (1975) and Bindschadler (1984). Results for the OSU transect, just south of the Arctic Circle on the south dome, are also reported.Of particular interest is the sensitivity study, which is designed to determine which aspects of the data and flow behavior are most critical to the calculations. Data that are comparatively well-constrained are flow-line definition, surface velocity and thickness. The flow lines and lateral spreading are obtained from satellite radar altimetry (Zwally and others, 1983) and checked against velocity data along the OSU profile. Thicknesses are from aerial radar sounding (Overgaard, personal communication). Surface velocities along the EGIG transect are corrected in a manner similar to that suggested by Robin (1983, figure 2.17b) and possible errors are not critical. Along the OSU transect the surface velocities are well-determined by Doppler satellite tracking in short-arc translocation mode.Accumulation rates are not well-determined and show substantial scatter for reasons that are not understood. Along the OSU transect, accumulation rates were obtained by augering for the nuclear bomb horizons. As reported earlier (Mock, 1967), the accumulation rates in this region show no clear pattern with elevation or slope. Similar, but less severe, problems exist in the EGIG area. The lack of a good model describing the geographic variation in accumulation rate results in calculated surface velocity uncertainties of 9% for the OSU and 6% for the EGIG transect.The calculated results are comparatively insensitive to parameters affecting the temperature profile. This is because, in this model, the mean velocity is determined by continuity and temperature affects only the ratio of horizontal velocity at a given depth to the mean velocity for the entire profile.Longitudinal stresses are included because they affect the viscosity through the effective shear stress. Neglecting this would lead to a 30% error in calculated surface velocity near the ice divide and about 6% away from the divide. Provided longitudinal stresses are included, uncertainties in these stresses are not critical.Enhancement factors are, however, important. Most authors believe that ice fabric, texture, and impurity content can affect strain rates by 100% to 1200% (Shoji and Langway, 1984). As with the temperature profile, this problem does not affect calculated mean velocities, which are based on continuity, but it does affect the ratio of surface to mean velocity. Enhancement factors are more critical than temperature uncertainties and therefore failure to include depth-variable flow enhancement alters calculated surface velocities by 5 to 10%.In summary, the mass balance along two transects has been investigated by comparing measured and calculated surface velocities. The most critical problems are (1) understanding the geographic distribution in accumulation rate and (2) correctly allowing for enhanced shearing due to structural variations with depth.

Present-Day Glaciation in the South Shetland Islands

This paper presents mass-balance results from Deception Island for 1968–69 to 1973–74, from King George Island for the balance years 1969–70 and 1970–71, and from Livingston Island from 1971–72 to 1973–74.The accumulation areas of all localities are in the soaked fades, with a firn/ice transition at King George Island at 12 to 20 m depth. Of the glaciers studied, only “Gl” on Deception Island terminates wholly on land and has a relatively large ablation area.The mass-balance curves are similar for King George Island and Livingston Island, with equilibrium lines at around 150 m elevation. “Gl“ on Deception Island has more negative summer balances, and the equilibrium line ranged from 275 to 370 m during the six balance years. Here, there were no years of positive net mass balance, and large negative net values during the 1970–71 to 1972–73 balance years. This resulted from a lowered albedo caused by ash from the August 1970 eruption. Ash layers from the Deception Island eruptions are also observed on Livingston Island and King George Island, where they form stratigraphic markers in the accumulation areas of the glaciers.Annual balance variations from 1957–58 to 1970–71, based on stratigraphic studies at Deception Island and King George Island, show good correlations, indicating that the variations reflect changes in regional climate.

Present-Day Glaciation in the South Shetland Islands

This paper presents mass-balance results from Deception Island for 1968–69 to 1973–74, from King George Island for the balance years 1969–70 and 1970–71, and from Livingston Island from 1971–72 to 1973–74. The accumulation areas of all localities are in the soaked fades, with a firn/ice transition at King George Island at 12 to 20 m depth. Of the glaciers studied, only “Gl” on Deception Island terminates wholly on land and has a relatively large ablation area. The mass-balance curves are similar for King George Island and Livingston Island, with equilibrium lines at around 150 m elevation. “Gl“ on Deception Island has more negative summer balances, and the equilibrium line ranged from 275 to 370 m during the six balance years. Here, there were no years of positive net mass balance, and large negative net values during the 1970–71 to 1972–73 balance years. This resulted from a lowered albedo caused by ash from the August 1970 eruption. Ash layers from the Deception Island eruptions are also observed on Livingston Island and King George Island, where they form stratigraphic markers in the accumulation areas of the glaciers. Annual balance variations from 1957–58 to 1970–71, based on stratigraphic studies at Deception Island and King George Island, show good correlations, indicating that the variations reflect changes in regional climate.