Freeze-core Samples Research Articles

Controlled Reservoir Drawdown—Challenges for Sediment Management and Integrative Monitoring: An Austrian Case Study—Part B: Local Scale

The present case study deals with a controlled drawdown beyond the operational level of the Gepatsch reservoir (Austria). Based on the awareness of potential ecological consequences, an advanced set of measures was conducted and an integrative monitoring design was implemented. This pre- and post-event monitoring included measurements regarding the cross sectional variability and habitat-related turbidity, freeze-core sampling to obtain knowledge on fine sediment infiltration and an evaluation of the macroinvertebrate communities as well as fish egg development (salmonid incubation). The results of the sedimentological as well as biological investigations show a negligible impact on the downstream located aquatic system due to the controlled drawdown of the Gepatsch reservoir. In addition, recommendations based on the findings from this study regarding possible methods for local scale monitoring can be given.

A conceptual model for unbiased calculations of invertebrate abundances from freeze core samples

Freeze coring is a commonly used method for the investigation of the bed sediment fauna of rivers. It is considered to produce quantitative numbers of invertebrate abundance in different depth layers. Calculations of abundance use total volume of the freeze core sample as spatial reference. This definition of sample volume is incorrect. In the present, study freeze core samples are shown to consist of two parts: (1) a cylindrical inner core in which the pore water has turned into ice and (2) all parts of sediment, which protrude from this inner core. Invertebrates are fixed only within the inner core since the protruding parts contain no pores and therefore no invertebrate habitat. In samples from gravel rivers, the protruding parts form a considerable bias, which depends on the size of the core and the coarseness of the sediment. In the present study, total volume of individual core segments varied between 97 and 248% of actual sample volume. The inner core can be measured directly to avoid the bias. The procedure is proposed for future studies to produce unbiased, comparable values of invertebrate abundance and consequently reliable data on vertical distribution.

Laboratory and field investigations on freeze and gravity core sampling and assessment of coring disturbances with implications on gas bubble characterization

AbstractThe quantification of greenhouse gas emissions from aquatic ecosystems requires knowledge about the spatial and temporal dynamics of free gas in sediments. Freezing the sediment in situ offers a promising method for obtaining gas‐bearing sediment samples, unaffected by changes in hydrostatic pressure and sample temperature during core withdrawal and subsequent analysis. This article presents a novel freeze coring technique to preserve the in situ stratigraphy and gas bubble characteristics. Nondestructive X‐ray computed tomography (CT) scans were used to identify and characterize coring disturbances of gravity and freeze cores associated with gassy sediment, as well as the effect of the freezing process on the gas bubble characteristics. Real‐time X‐ray CT scans were conducted to visualize the progression of the freezing process. Additional experiments were conducted to determine the freezing rate to assess the probability of sediment particle/bubble migration, and gas bubble nucleation at the phase transition of pore water to ice. The performance of the freeze coring technique was evaluated under field conditions in Olsberg and Urft Reservoir (Germany). The results demonstrate the capability of the freeze coring technique for the preservation of gas‐bearing sediments and the analysis of gas bubble distribution pattern in both reservoirs. Nevertheless, the obtained cores showed that nearly all gravity and freeze cores show some degree of coring disturbances.

Coupling X-ray computed tomography and freeze-coring for the analysis of fine-grained low-cohesive soils

This paper presents the coupling of freeze-core sampling with X-ray CT scanning for the analysis of the soil structure of fine-grained, low-cohesive soils. We used a medical scanner to image the 3D soil structure of the frozen soil cores, providing X-ray CT data at a millimetric resolution over freeze-cores that are up to 62.5cm long and 25cm wide. The obtained data and the changes in gray level values could be successfully used to identify and characterize different soil units with distinctly different physical properties. Traditional measurements of soil bulk density, carbon and particle size analyses were conducted within each of the identified soil units. These observations were used to develop a 3D model of soil bulk density and organic matter distribution for five freeze-cores obtained at a restored floodplain in Switzerland. The millimetric X-ray CT scanning was applied to detect the impact of freeze-coring on the soil structural integrity. This allows identifying undisturbed zones, a critical precondition for any subsequent assessment of soil structure. The proposed coupling is thought to be applicable to a wide range of other low-cohesive soil types and has a large potential for applications in hydrogeology, biology or soil science.

Improved geohydraulic characterization of river bed sediments based on freeze-core sampling – Development and evaluation of a new measurement approach

Summary Prior to hydraulic engineering work, it is essential to know the geohydraulic properties of river bed sediments as precisely as possible in order to predict and minimize potential effects on adjacent aquifers. Many different methods are available, but all have limitations. Some destroy the natural sediment structure, others lack a small-scale resolution, and all fail to determine anisotropy. In this study, we present a new measuring approach to determine hydraulic conductivity ( K ) and anisotropy at a detailed scale. Frozen, undisturbed sediment samples taken with the freeze-core technique form the basis of our approach. Orientated core cutter samples are taken from the freeze-cores at different depths and examined by means of falling-head lab permeameter tests, in order to obtain detailed profiles of vertical and horizontal K values and anisotropy. The approach was tested with natural sediment samples from the bed of a channel near Potsdam, Germany, and with samples from an experimental container. For comparison and evaluation, hydraulic conductivities were also determined by means of in situ permeameter tests and empirically, on the basis of grain-size distribution (GSD). The results show a large discrepancy between the empirically determined hydraulic conductivities and the directly measured hydraulic conductivity with lab and in situ permeameter tests. We conclude that empirical methods based on GSD are not suitable for determining in situ conductivity of river bed sediments. Our new approach enables an improved geohydraulic characterization of river bed sediments including the determination of anisotropy.

Depositional dynamics in the El'gygytgyn Crater margin: implications for the 3.6 Ma old sediment archive

Abstract. The combination of permafrost history and dynamics, lake level changes and the tectonical framework is considered to play a crucial role for sediment delivery to El'gygytgyn Crater Lake, NE Russian Arctic. The purpose of this study is to propose a depositional framework based on analyses of the core strata from the lake margin and historical reconstructions from various studies at the site. A sedimentological program has been conducted using frozen core samples from the 141.5 m long El'gygytgyn 5011-3 permafrost well. The drill site is located in sedimentary permafrost west of the lake that partly fills the El'gygytgyn Crater. The total core sequence is interpreted as strata building up a progradational alluvial fan delta. Four macroscopically distinct sedimentary units are identified. Unit 1 (141.5–117.0 m) is comprised of ice-cemented, matrix-supported sandy gravel and intercalated sandy layers. Sandy layers represent sediments which rained out as particles in the deeper part of the water column under highly energetic conditions. Unit 2 (117.0–24.25 m) is dominated by ice-cemented, matrix-supported sandy gravel with individual gravel layers. Most of the Unit 2 diamicton is understood to result from alluvial wash and subsequent gravitational sliding of coarse-grained (sandy gravel) material on the basin slope. Unit 3 (24.25–8.5 m) has ice-cemented, matrix-supported sandy gravel that is interrupted by sand beds. These sandy beds are associated with flooding events and represent near-shore sandy shoals. Unit 4 (8.5–0.0 m) is ice-cemented, matrix-supported sandy gravel with varying ice content, mostly higher than below. It consists of slope material and creek fill deposits. The uppermost metre is the active layer (i.e. the top layer of soil with seasonal freeze and thaw) into which modern soil organic matter has been incorporated. The nature of the progradational sediment transport taking place from the western and northern crater margins may be related to the complementary occurrence of frequent turbiditic layers in the central lake basin, as is known from the lake sediment record. Slope processes such as gravitational sliding and sheet flooding occur especially during spring melt and promote mass wasting into the basin. Tectonics are inferred to have initiated the fan accumulation in the first place and possibly the off-centre displacement of the crater lake.

Freeze Core Sampling to Validate Time‐Lapse Resistivity Monitoring of the Hyporheic Zone

A freeze core sampler was used to characterize hyporheic zone storage during a stream tracer test. The pore water from the frozen core showed tracer lingered in the hyporheic zone after the tracer had returned to background concentration in collocated well samples. These results confirmed evidence of lingering subsurface tracer seen in time-lapse electrical resistivity tomographs. The pore water exhibited brine exclusion (ion concentrations in ice lower than source water) in a sediment matrix, despite the fast freezing time. Although freeze core sampling provided qualitative evidence of lingering tracer, it proved difficult to quantify tracer concentration because the amount of brine exclusion during freezing could not be accurately determined. Nonetheless, the additional evidence for lingering tracer supports using time-lapse resistivity to detect regions of low fluid mobility within the hyporheic zone that can act as chemically reactive zones of importance in stream health.

Effects of freezing on soil temperature, freezing front propagation and moisture redistribution in peat: laboratory investigations

Abstract. There are not many studies that report water movement in freezing peat. Soil column studies under controlled laboratory settings can help isolate and understand the effects of different factors controlling freezing of the active layer in organic covered permafrost terrain. In this study, four peat Mesocosms were subjected to temperature gradients by bringing the Mesocosm tops in contact with sub-zero air temperature while maintaining a continuously frozen layer at the bottom (proxy permafrost). Soil water movement towards the freezing front (from warmer to colder regions) was inferred from soil freezing curves, liquid water content time series and from the total water content of frozen core samples collected at the end of freezing cycle. A substantial amount of water, enough to raise the upper surface of frozen saturated soil within 15 cm of the soil surface at the end of freezing period appeared to have moved upwards during freezing. Diffusion under moisture gradients and effects of temperature on soil matric potential, at least in the initial period, appear to drive such movement as seen from analysis of freezing curves. Freezing front (separation front between soil zones containing and free of ice) propagation is controlled by latent heat for a long time during freezing. A simple conceptual model describing freezing of an organic active layer initially resembling a variable moisture landscape is proposed based upon the results of this study. The results of this study will help in understanding, and ultimately forecasting, the hydrologic response of wetland-dominated terrain underlain by discontinuous permafrost.

Organic matter and fine grains as possible determinants of spatial and seasonal variability in bed sediment fauna: A case study from a Hercynian gravel stream

The spatial and seasonal distributions of organic matter and fine grains were tested as possible determinants of fauna distribution in bed sediment of a Hercynian gravel stream. Invertebrate densities and the amounts of fine grains and organic carbon were assessed in freeze-core samples taken along 70 cm depth profiles at three different positions in the stream channel. Sampling was conducted on five occasions of low discharge over two years. The variability in invertebrate community composition was analysed using Detrended Correspondence Analysis with posterior projection of explanatory variables; Variation Partitioning was used to estimate the independent and shared effects of the explanatory variables. We found that the best predictors of the invertebrate community were spatial variables (depth, position in the channel) and then variables influenced by seasonal patterns (surface water temperature and discharge). The influence of organic matter and fine grain content was significant only after eliminating spatial autocorrelation. High amounts of organic matter, randomly accumulated in the sediment, improved the model by explaining high fauna densities. The fine grain content was not a limiting factor to fauna at our study site. It is possible that the large amount of mica flakes in the sediment has caused the arrangement of grains with a pore space sufficient for fauna even when fine grain content was high.

Bio-Archive Core Storage and Subsampling Procedure for Subseafloor Molecular Biological Research

No abstract available. doi:10.2204/iodp.sd.8.05.2009

Biases of submerged bulk and freeze‐core samples

AbstractFreeze‐coring and bulk sampling are routine methods used to sample subsurface spawning gravel under shallow water. Both methods have limitations. Freeze‐coring is not believed to representatively sample coarse grain sizes and the sample volumes are relatively small. Conversely, when bulk sampling, even within an enclosure, some fine sediment is suspended and washed away from the sample. This paper assesses the biases in sampling performance between the two methods and determines whether the loss of fines that occurs when bulk sampling could be predicted and thus corrected for.At six riffles the spawning substrate was sampled under approximately 50 cm of water with a bulk sample and three adjacent freeze‐cores. For each riffle, data from the two samples were combined using the method of Fripp and Diplas (1993) and the resultant composite sample was compared with the original freeze‐core and bulk samples to assess the relative precision and biases of the two techniques.On average, the D50 of the bulk samples was 4 mm larger and a one‐third loss of the <2 mm fraction occurred compared with the composite samples. In contrast, freeze‐core samples contain on average 32% more sediment >16 mm compared with composite samples. Based on six samples, taken from six riffles, the amount of sediment finer than 0·5 mm lost using our bulk sampling technique with an enclosure appears to be predictable and correctable. Copyright © 2005 John Wiley & Sons, Ltd.

Detecting patterns in hyporheic community structure: Does sampling method alter the story?

Hyporheic invertebrates were sampled in six small headwater streams draining pine, pasture, and native forest catchments. Sites were sampled in autumn and spring using three different hyporheic sampling methods (colonisation pots, pump sampling, and freeze‐coring). Total invertebrate abundance and taxon richness differed significantly among sites on both sampling occasions for pump samples, whereas freeze‐coring did not discriminate among sites, and pot samples showed significant differences in autumn, but not in spring. Differences among sites with respect to community composition were not consistent between sampling methods. Community ordination indicated that samples from colonisation pots tended to group together, whereas pump and freeze core samples were widely separated. Differences across land uses were also apparent (native forest sites formed a distinct cluster separated from pine and pasture sites). Results suggest that colonisation pots may provide a pathway for the entry of epigean organisms into the hyporheic zone even when hydraulic connectivity between surface and hyporheic zones is low. Invertebrate abundance was very low in pump samples, and this method also captured the fewest taxa of the three methods. The size bias known to exist with pump sampling techniques may restrict the usefulness of the method when comparing streams of differing hydraulic conductivity.

Freeze core sampling of the hyporheos: implications of use of electropositioning and different settling periods

Invertebrate densities and taxon richness in freeze cores were compared between three different settling periods (zero, two and seven days) as well as between samples taken with two variations of electropositioning and samples taken without electropositioning with a settling period. Species composition differed significantly among the three settling periods, but few significant differences were found in density or richness. Eriopterini were more abundant in hyporheic sediments (10-50 cm below sediment surface) in samples settled for two days than in those settled for seven days. Benthic densities (0-10 cm) of nematodes were higher in samples taken without a settling period than with either of the two settling periods. Electropositioned samples did not differ from non-electropositioned samples, except that benthic densities of leptophlebiid mayflies were significantly higher in electropositioned samples taken with a settling period than those without a settling period. The results of this study indicate that the length of settling period is of minor consequence when sampling hyporheic invertebrates (10-50 cm) and also provide little evidence of a benefit from electropositioning before freeze coring.

In Situ Ground Freezing to Obtain Undisturbed Samples of Loose Sand

In situ ground freezing with liquid nitrogen can be an excellent technique for obtaining high quality undisturbed samples from a wide range of loose sand deposits, with their in situ conditions preserved. This paper describes the techniques used during the CANLEX project to carry out in situ ground freezing and undisturbed sampling of the loose sand deposits encountered at six test sites. Specific details, with respect to the effects of difficult subsoil or ground-water conditions encountered at the sites, are provided. Modifications made to the ground freezing and sampling system to accommodate difficult ground conditions are described. The actual advance of the freezing front and the consumption of liquid nitrogen are compared with theoretical predictions. The dry coring technique utilized to recover undisturbed frozen sand samples is described. The void ratios measured for the frozen core samples are compared with void ratio measurements obtained using geophysical logging in boreholes located within a 5-m radius of the freeze pipe. The costs associated with in situ ground freezing utilizing liquid nitrogen and coring of the frozen sand at six test sites are presented.

A Field Comparison of the Substrate Composition of California Golden Trout Redds Sampled with Two Devices

Stream substrates are frequently sampled with several different devices, and it is often assumed that the resulting samples have comparable compositions. We compared substrate samples taken with an excavated corer (15-cm-diameter plastic pipe) with those taken with a freeze corer from redds of California golden trout Oncorhynchus mykiss aguabonita in a stream dominated by small substrates (geometric mean diameter of redd substrates, approximately 6 mm). The proportions of individual particle size-fractions and the percent of fine sediment in the upper portion of excavated cores were similar to those in freeze cores. Geometric mean particle diameters of excavated-core and freeze-core samples were influenced by differences in the proportional weight of particles larger than 12.5 mm; as a result, geometric mean diameters were most similar when these larger size-fractions were omitted. Samplers did not differ in their probability of encountering egg pockets. A power analysis of the substrate size-fraction comparisons showed that the power of our analyses to detect differences between samplers was usually high (≥0.8). Therefore, the general lack of significant differences in the substrate composition from samples taken by the excavated corer and freeze corer is probably the result of few true differences and not of shortcomings in our sampling design. For small streams dominated by fine substrates, we conclude that the inexpensive excavatedcore sampler we used is a viable alternative to the expensive and cumbersome freeze-core sampler, even when sampling in redds, and that is has several advantages over other excavated-core samplers or a shovel when sampling outside of redds.

Local variability of gold in active stream sediments

The distribution of gold in a short 120 m reach of an upland stream in Scotland has been investigated using an established freeze-core sampling method that avoids problems of elutriation. Thirty cores were taken from six sites chosen to represent the variety of geomorphological settings. Bed sediments varied between sites(24.3> D 50>9.2 mm). Gold distribution is described in terms of concentration, mass and number of grains. Data are erratic in coarse fractions (>500 μm) because of the low number of individual gold grains and the high frequency of barren samples. The < 63 μm fraction gave the most consistent results, especially for gold loads (g · kg −1) reflecting the high number of individual grains. The distribution of gold is discussed in relation to the geomorphological controls and sediment transport processes that lead to enrichment and dilution of gold deposits. The sampling method is shown to provide a practical approach for obtaining representative and quantitative data on fine-grained gold distributions.

Biases Associated with Four Stream Substrate Samplers

We compared samples collected from 10 substrates of various compositions with a single-probe freeze-core sampler, a triple-probe freeze-core sampler, a McNeil sampler, and a shovel. The accuracy with which these devices sampled particles larger than 50 mm in diameter varied; they were oversampled by the freeze-core devices, sampled in proportion to their availability by a shovel, and sampled inconsistently by the McNeil sampler. The geometric mean particle size and variance of single-probe freeze-core samples consistently exceeded those of samples collected with the other devices. Most sample means also exceeded the test substrate means. By excluding the proportions of particles larger than 50 mm in diameter in our analyses, we found that proportions of several particles sizes in samples collected by different methods differed significantly from the actual proportions in test substrates. There were few differences between the single- and triple-probe freeze-core samples or between McNeil and shovel samples. All four samplers were biased, but the McNeil sampler most frequently produced samples that approximated the true substrate composition.

Field Comparison of Three Devices Used to Sample Substrate in Small Streams

Abstract We conducted a field study to compare the composition of substrate samples collected from small streams with three types of samplers: an excavated-core sampler, a single-probe freezecore sampler, and a shovel. Large particles (>50 mm in diameter) occurred most frequently in excavated-core samples (76%) and least frequently in freeze-core samples (52%), but they had the greatest influence on overall sample composition when they occurred in freeze-core samples. Excavated-core and shoveled samples did not differ significantly in composition, but freeze-core samples differed significantly from both excavated-core and shoveled samples for some particle sizes. When freeze-core samples were divided into halves, the lower portion contained significantly more fine particles than the upper portion. We concluded that the freeze-core sampler does not produce samples similar to excavated-core samples and is too expensive and cumbersome for routine management applications. Conversely, the shovel produces subst...

Substrate Alteration by Spawning Brook Trout in a Southeastern Wyoming Stream

To measure the changes in substrate composition produced by spawning brook trout Salvelinus fontinalis, we collected 187 freeze-core samples, both before and after spawning, from egg pockets, inside redds but excluding egg pockets, and outside redds. In the upper strata of reddassociated samples, we found that the proportions of particles 0.85–1.70, 0.425–0.85, 0.212–0.425, and less than 0.212 mm in diameter were significantly lower in egg pockets than outside redds, and the proportions of particles 0.425–0.85, 0.212–0.425, and less than 0.212 mm in diameter were also lower in egg pockets than in other areas inside redds. Samples collected outside redds were similar to samples collected before spawning. We observed no significant correlations between the proportion of fine sediment in samples from outside redds (and presumably the proportion present before spawning) and the proportion of fine sediment in egg pockets. In vertically stratified samples, the lower strata contained a greater proportion of the smaller substrate particles than the upper strata. Spawning brook trout removed fine sediment from the substrate during redd construction, and this removal was most apparent in the egg pocket. Thus, egg pockets, rather than entire redds, should be the focus of studies of survival to emergence.