The influence of urbanization of sinkhole development in central Pennsylvania

The karsted limestone valleys of central Pennsylvania contain two populations of sinkholes. Solution sinkholes occur in the Champlainian limestone units along the margins of the valleys. Solution sinkholes are permanent parts of the landscape and, although a nuisance to construction, do not present other problems. The second population is the suffosional or soil-piping sinkholes These occur on all carbonate rock units including the Beekmantown and Gatesburg dolomites that comprise the two principal carbonate aquifers in the valley. Suffosional sinkholes are the principal land-use hazard. Suffosional sinkholes are transient phonomena. They occur naturally but are exacerbated by runoff modifications that accompany urbanization Suffosional sinkholes are typically 1.5–2.5 m in diameter depending on soil thickness and soil type. The vertical transport of soil to form the void space and soil arch that are the precursors to sinkhole collapse is through solutionally widened fractures and cross-joints and less often through large vertical openings in the bedrock. The limited solution development on the dolomite bedrock combined with soil thickness, seldom greater than 2 m, limits the size of the sinkholes. All aspects of suffosional sinkhole development are shallow processes: transport, piping, void and arch formation, and subsequent collapse take place usually less than 10 m below the land surface Factors exacerbating sinkhole development include pavement, street, and roof runoff which accelerates soil transport Such seemingly minor activities as replacing high grass and brush with mowed grass is observed to accelerate sinkhole development. Dewatering of the aquifer is not a major factor in this region     

Reliability of dipole-dipole electrical resistivity tomography for defining depth to bedrock in covered karst terranes

 Sinkhole collapse is one of the main limitations on the development of karst areas, especially where bedrock is covered by unconsolidated material. Studies of sinkhole formation have shown that sinkholes are likely to develop in cutter (enlarged joint) zones as a result of subterranean erosion by flowing groundwater. Because of the irregular distribution of pinnacles and cutters on the bedrock surface, uncertainties arise when "hit-or-miss" borehole drilling is used to locate potential collapse sites. A high-resolution geophysical technique capable of depicting the details of the bedrock surface is essential for guiding the drilling program. Dipole-dipole electrical resistivity tomography (ERT) was used to map the bedrock surface at a site in southern Indiana where limestone is covered by about 9 m of clayey soils. Forty-nine transects were conducted over an area of approximately 42,037 m². The electrode spacing was 3 m. The length of the transects varied from 81 to 249 m. The tomographs were interpreted with the aid of soil borings. The repeatability of ERT was evaluated by comparing the rock surface elevations interpreted from pairs of transects where they crossed each other. The average difference was 2.4 m, with a maximum of 10 m. The discrepancy between interpreted bedrock-surface elevations for a transect intersection may be caused by variations in the subsurface geology normal to the transect. Averaging the elevation data interpreted from different transects improved the ERT results. A bedrock surface map was generated using only the averaged elevation data at the transect junctions. The accuracy of the map was further evaluated using data from four exploratory boreholes. The average difference between interpreted and actual bedrock surface-elevations was less than 0.4 m. The map shows two large troughs in the limestone surface: one coinciding with an existing sinkhole basin, while the other is in alignment with a small topographic valley. Because sinkholes were observed at the same elevation interval in similar valleys in the vicinity, the delineated trough may have implications for future land use at the site. Received: 4 January 1999 · Accepted: 8 March 1999     

The application of GPR and electrical resistivity tomography as useful tools in detection of sinkholes in the Cheria Basin (northeast of Algeria)

Sinkhole collapse is one of the main limitations in the development of karst areas, especially where bedrock is covered by unconsolidated material. Studies of sinkhole formation have shown that sinkholes are likely to develop in cutter (enlarged joint) zones as a result of subterranean erosion by flowing groundwater. Ground-penetrating radar (GPR) and electrical resistivity imaging or tomography (RESTOM) are well suited to mapping sinkholes because of the ability of these two techniques for detecting voids and discriminating subtle resistivity variations. Nine GPR profiles and two-dimensional electrical resistivity tomography have been applied, with relative success, to locate paleo-collapses and cavities, and to detect and characterize karst at two sinkhole sites near Cheria City where limestone is covered by about 10 m of clayey soils. The survey results suggest that GPR and RESTOM are ideal geophysical tools to aid in the detection and monitoring of sinkholes and other subsurface cavities.     

Stratigraphic architecture and infill history of a deglaciated bedrock valley based on georadar, seismic profiling and drilling

The sediment fill of a silled bedrock valley in Western Norway has been investigated with respect to stratigraphy and infill history using a combination of mapping, georadar, seismic profiling and drilling. A small outlet glacier occupies the head of the valley that displays a stepwise down-valley profile and terminates in a lake at 29 m above sea-level. The valley is surrounded by high, steep bedrock slopes and is characterized by a series of filled basins each limited by sills of bedrock or moraine accumulations. Till, glacial outwash and/or rockslide deposits fill in the lower half of the two larger basins. (Fan) delta deposits fringed by the deposits of alluvial fans and colluvial cones dominate the upper fill of most basins. (Fan) delta deposits interfinger downstream with lake sediments in the larger basins and fluvial deposits comprise the top fill. The overall infill pattern was controlled by deglaciation as well as basin size and shape. An overall decreasing sediment supply following deglaciation is shown in the fill of a larger basin down-valley, whereas a recently increasing sediment supply during glacier growth is reflected primarily in an upstream basin. Only the lowermost basin was exposed to a sea-level drop from 75 m above sea-level to the present lake level associated with incision and river migration. This observation is in contrast to the basins above marine influence where incision has been limited due to fixed downstream sills resulting in insignificant erosion except for some fan-head entrenchment. It follows that the fills of these small valley basins display progradational and aggradational trends of deposition and paraglacial reworking has been limited. Additionally, the study demonstrates that georadar profiling, combined with other methods, is very useful for comprehensive investigation of valley basins.     

Mobile sediment in an urbanizing karst aquifer: implications for contaminant transport

 Here we investigate geochemical characteristics of sediment in different compartments of a karst aquifer and demonstrate that mobile sediments in a karst aquifer can exhibit a wide range of properties affecting their contaminant transport potential. Sediment samples were collected from surface streams, sinkholes, caves, wells, and springs of a karst aquifer (the Barton Springs portion of the Edwards (Balcones Fault Zone) Aquifer, Central Texas) and their mineralogy, grain-size distribution, organic carbon content, and specific surface area analyzed. Statistical analysis of the sediments separated the sampling sites into three distinct groups: (1) streambeds, sinkholes, and small springs; (2) wells; and (3) caves. Sediments from the primary discharge spring were a mix of these three groups. High organic carbon content and high specific surface area gives some sediments an increased potential to transport contaminants; the volume of these sediments is likely to increase with continued urbanization of the watershed. Received: 13 April 1998 · Accepted: 6 October 1998     

Geophysical methods for mapping Quaternary sediment thickness: Application to the Saint-Lary basin (French Pyrenees)

This study aims at mapping the sediment infill thickness in the Saint-Lary basin (Aure valley, French Pyrenees). For this purpose, we combine passive seismic and gravity surveys. The resonance frequencies of the sediment body are retrieved from seismic ambient measurements, while the gravimetric survey shows negative residual anomaly of about −3 mGal in the basin. Both methods reveal unexpected but consistent bedrock shape. The southern Saint-Lary basin appears deeper than its northern part, with maximal infill thickness of about 300 m and 150 m, respectively. Valley cross sections show regular and smooth “U”-shape in the southern Saint-Lary basin, in contrast to an irregular and asymmetric pattern in the northern basin. This basin shape may be related to Quaternary fluvio-glacial carving processes especially controlled by a regional fault (the Soulan fault), variations in bedrock hardness, and preferential ice flow paths.     

Detection of sinkholes using 2D electrical resistivity imaging in the Cheria Basin (north–east of Algeria)

Sinkhole collapse is one of the main limitations on the development of karst areas, especially where bedrock is covered by unconsolidated material. Studies of sinkhole formation have shown that sinkholes are likely to develop in cutter (enlarged joint) zones as a result of subterranean erosion by flowing groundwater. Electrical resistivity imaging or tomography (RESTOM) is well suited to mapping sinkholes because of the ability of the technique for detecting resistive features and discriminating subtle resistivity variations. Two-dimensional electrical resistivity tomography surveys were conducted at two sinkhole sites near Cheria city where limestone is covered by about 10 m of clayey soils. A Wenner transect was conducted between the two sinkholes. The electrode spacing was 2 m. The length of transect is about 80 m. The survey results suggest that RESTOM is an ideal geophysical tool to aid in the detection and monitoring of sinkholes and other subsurface cavities.     

Poroelastic analysis of cover-collapse sinkhole formation by piezometric surface drawdown

Where the water table is above the soil-rock contact in karst regions, cover-collapse sinkholes in the soil and soft sediment above the rock commonly occur as a result of drawdown of the piezometric surface in the karst aquifer. Transient stresses and pore pressures around soil voids at the soil-rock contact can cause hydraulic fracturing of the soil near the wall of the void. After the first such fracture, successive sloughing of soil propagates the soil void rapidly to the surface, resulting in a cover-collapse sinkhole. Sinkhole formation by this mechanism should be strongly a function of rate and magnitude of piezometric surface drawdown, permeability and tensile strength of the soil, and the size, depth, and geometry of the initial soil void. Large soil voids and those with walls that are partly planar or of low curvature are most susceptible to hydraulic fracture and the resulting progression to sinkhole formation.     

Sinkholes related to discontinuous pumping: susceptibility mapping based on geophysical studies. The case of Crestatx (Majorca, Spain)

The Crestatx aquifer is the main source of water supply to the Bay of Alcudia, one of the largest resorts on the island of Majorca (Spain). This water has been used since the 1970s using several pumping wells, which draw an annual volume of 1.5 hm³. The seasonal exploitation of this karstic aquifer causes a substantial cone of depression with great variations in the piezometric level (up to 120 m) and dynamic water levels of down to 87 m below sea level. At the end of the 1990s, several sinkholes and subsidence depressions started being detected in a highly karstified area. Twenty subsidence and sinkhole morphologies have been inventoried in an area measuring 70,000 m², with diameters up to 23 m and depth more than 15 m. The intense and continuous rainfall during recent years (2008–2010) has considerably accelerated the process, increasing the dimensions of existing sinkholes and the appearance of new morphologies. By means of electrical tomography techniques, a ground study was carried out. Numerous cavities have been identified in the upper 30 m. Using a map of the surface morphologies and the geophysical profiles, we can determine that the propagation and orientation of the sinkholes lie along three main directions: N30°E, N130°E and N60°E. The first is the most relevant, which is parallel to the main tectonic structures in the area. The interpretation of the electrical profiles has enabled us to identify the potentially most unstable areas, which is an effective tool to assess risk in the area, as there are roads and a housing development nearby. The high, but discontinuous, exploitation of the aquifer is considered the main trigger for these sinkholes and subsidence depressions, as it causes large variations of pressure and accelerates the dissolution process in the underlying rock.     

Architecture of fluvial-dominated valley-fill deposits in the Cretaceous Fall River Formation

The Fall River Formation is a 45 m thick layer of fluvial-dominated valley-fills and shore-zone strata deposited on the stable cratonic margin of the Cretaceous Western Interior Seaway. Fall River deposits in Red Canyon, in the south-west corner of South Dakota (USA), expose a cross-section of a 3.5 km wide valley-fill sandstone and laterally adjacent marine deposits. The marine deposits comprise three 10 m thick upward-shoaling sequences; each composed of multiple metres-thick upward-coarsening successions. The lower two of these sequences are laterally cut by the valley-fill sandstone, and are capped by metres-thick muddy palaeosols. The upper sequence spans the top of the valley-fill sandstone, and is overlain by the Skull Creek Shale. The 30 m thick valley sandstone is partitioned into four distinct fills by major erosion surfaces, and each of these fills contain many metres-thick channel-form bodies. Deposits in the lower parts of these fills are sheet-like, top-truncated channel bodies, whereas deposits in the upper parts of fills are upward-concave, laterally amalgamated channel bodies, more completely preserved heterolithic channel bodies, or wave-deposited sheets. Each valley-fill basal erosion surface records an episode of valley incision and relative sea-level fall, and the gradual progression from fluvial to more estuarine deposits upwards within each fill records relative sea-level rise. All fills are dominantly channel deposits and are capped by marine flooding surfaces. The dominance of channel deposits, the gradual change to more estuarine facies in the upper parts of fills, and the location of flooding surfaces at valley-fill tops all suggest that sediment supply initially kept pace with relative sea-level rise and valleys filled during late marine lowstand and transgression, not during subsequent highstands. Recently proposed facies models have focused on variations in the relative strength of tide, wave and river currents as controls on valley-fill deposits. However, relative rates of sediment supply and basin accommodation change, and the shift in this ratio along the depositional profile during multiple-scale cycles in relative sea-level, are equally important controls on the style of valley-fill deposits.     

Geophysical response of filled sinkholes,soil pipes and associated bedrock fractures in thinly mantled karst,east-central Illinois

Karst aquifers are often protected by a thin mantle of unconsolidated sediment. Soil pipes and sinkholes may breach this natural protective barrier and open pathways for contaminants to quickly reach bedrock aquifers. Geophysical surveys offer a quick and noninvasive way to identify these features; such surveys may also be sequenced to reveal increasing detail in critical areas. At a study site in east-central Illinois, electromagnetic (EM) surveys mapped high conductivity anomalies over filled sinkholes and soil pipes that penetrated the unconsolidated cover. Two-dimensional inverted resistivity sections, made over these anomalies, depict filled sinkholes and soil pipes as conductive zones above deeply weathered bedrock fractures. Borings verified the geophysical models and suggest high conductivities associated with the filled sinkholes are the result of enhanced moisture near active soil pipes. EM surveys also identified conductive zones in the overburden above a probable bedrock fracture linking sinkhole areas 0.5 km apart. Resistivity and EM methods, used in a phased and sequential manner, thus proved useful in mapping filled sinkholes and in delineating the vertical and lateral connections between soil pipes and hydraulically active bedrock fractures.     

Assessment of Grouting Efficiency for Treatment of Underground Cavities

Ground subsidence incidents occurred in a residential suburb in an arid desert terrain due to sinkholes. Several investigation programs were conducted to understand the cause of the incidents attributed to the dissolution of the limestone bedrock and the subsequent ravelling of the overburden soil cover. A pilot area within the affected locale of the residential suburb in the state of Kuwait was selected for implementing treatment measures. The main purpose of the treatment measures was to fill the uppermost layer of the limestone bedrock in order to eliminate the risk of recurrence of ground subsidence. Cavity filling and permeation cement grouts were injected from the ground surface for filling the underground cavities and the fractured rocks. Results of the implemented grouting measures for treating underground voids and the GIN concept are presented in the paper. Reduction in the porosity of the bedrock layer due to the applied treatment measures was verified and percentage of treatment fill volume for the entire pilot area was estimated. An assessment method for the efficiency of the treatment measures coupled with the properties of grout materials, and lessons learned from the implemented treatment are also presented herein.     

Earth fissures triggered by groundwater withdrawal and coupled by geological structures in Jiangsu Province,China

Earth fissures in Jiangsu Province, China have caused serious damages to properties, farmlands, and infrastructures and adversely affected the local or regional economic development. Under the geological and environmental background in Jiangsu Province, this paper presents the earth fissures caused by excessive groundwater withdrawal and coupled by distinctive geological structures such as Ancient Yellow River Fault in Xuzhou karst area, and Ancient Yangtze River Course and bedrock hills in Suzhou, Wuxi, and Changzhou area. Although all the earth fissures are triggered by groundwater exploitation, the characteristics are strongly affected by the specific geological and hydrogeological settings. In particular, in the water-thirsty Xuzhou city, the cone of depression caused by groundwater extraction enlarged nearly 20 times and the piezometric head of groundwater declined 17 m over a decade. As groundwater is extracted from the shallowly buried karst strata in the Ancient Yellow River Fault zone, the development of earth fissures is highly associated with the development of karstic cavities and sinkholes and their distribution is controlled by the Ancient Yellow River Fault with all the 17 sinkholes on the fault. On the other hand, in the rapidly developing Southern Jiangsu Province, groundwater is mainly pumped from the second confined aquifer in the Quaternary, which is distributed neither homogeneously nor isotropically. The second confined aquifer comprises more than 50 m thick sand over the Ancient Yangtze River Course, but this layer may completely miss on the riverbank and bedrock hills. With a typical drawdown rate of 4–6 m per annum, the piezometric head of groundwater in the second confined aquifer has declined 76 m at Maocunyuan since 1970s and 40 m at Changjing since mid-1980s, and a large land subsidence, e.g., 1,100 mm at Maocunyuan, is triggered. Coupled with the dramatic change of the bedrock topography that was revealed through traditional geological drilling and modern seismic reflection methods, the geological-structure-controlled differential settlement and earth fissures are phenomenal in this area.     

Application of high-resolution seismic techniques in the evaluation of earthquake site response, Ottawa Valley, Canada

High-resolution seismic surveys, including P- and S-wave studies, have been conducted in an area of the Ottawa River valley located 80 km east of Ottawa (Canada). Based on dating of paleolandslides, the existence of paleoearthquake activity has been postulated in this area. The target zone for the seismic survey is characterized by surface disturbance and sediment deformation. P-wave seismic imaging was used to map the overburden–bedrock interface as well as to indicate reflecting boundaries within the overburden. The area of surface disturbance was found to overlie a buried bedrock basin, 8 km in diameter, infilled with a maximum thickness of 180 m of unconsolidated Quaternary sediments. Preliminary results of core logging show the presence of sand overlain by deformed fine sediments within the disturbed area. Shear-refraction studies reveal differences in the velocity–depth profiles between the disturbed area and the surrounding undisturbed areas. The shear-wave reflection method was used to produce a fundamental resonant period map for the area. Surface sediment disturbance was probably due to a combination of ground-motion amplification due to the basin (thick soft sediments) and the presence of water-saturated sand at depth.     

Hydrogeological studies on the Nubian sandstone aquifer in El-Bahariya Oasis, Western Desert, Egypt

The current study aimed to evaluate hydrogeologically the Nubian sandstone aquifer in El-Bahariya Oasis. It represents the main water-bearing horizon in the study area and consists of continental elastic sediments, mainly sandstone alternating with shale and clays. The general flow lines are directed from SW to NE direction, as detected from the constructed potentiometric head contour map. The piezometric surface reaches 149 m in El-Heiz area at the southern part, while it reaches 90 m at the northern, reflecting higher pressure head of the aquifer in the southern part. The map also illustrates that the southern part is considered as the most promising location for development. The structural elements play an important role in the deposition and distribution of the sedimentary succession of the Nubian sandstone sediments. Consequently, this sedimentary pattern affects the occurrences and movements of the groundwater within the aquifer system. Along the structurally high areas, in the study area, the piezometric head increases, while the reverse is recorded along the structurally low areas. The step-drawdown tests data were carried out by calculating the aquifer loss coefficient (B) and the well loss constant (C). The B values are smaller compared with C values, indicating that the aquifer under pressure has a behavior of leaky aquifer; therefore, it shows hydraulic connection with surrounding formation. The values of well efficiency range from 78.50% to 87.76%. Analysis of 12 pumping test data (constant discharge tests) was carried out in order to calculate the Nubian aquifer hydraulic parameters (transmissivity, hydraulic conductivity, and storage coefficient). The transmissivity values decrease from 3,045 m²/day in the southern part (El-Heiz area) to 236 m²/day in the northeastern part (El-Harra area). Accordingly, the aquifer classified as a high to moderate potentiality. Transmissivity contour map observes gradual increase of transmissivity values from the southern to northeastern direction. This may be due to the increase of shale or clay content in the concerned aquifer in that direction. The storage coefficient values range between 1.04 × 10^?4 and 5.22 × 10^?3, as obtained from the results of pumping test analysis, which ensure that the Nubian sandstone aquifer is classified as semi-confined to confined aquifer type. The S values show a decrease from southwest to northeast direction as detected from S-map. The hydraulic conductivity values vary from to 0.46 m/day in the northern part to 10.88 m/day in the southern part with an average of 5.67 m/day. According to the classification based on K values, the aquifer is mainly composed of coarse sand.     

Sensitivity analysis for the EPIK method of vulnerability assessment in a small karstic aquifer, southern Belgium

Applying the EPIK parametric method, a vulnerability assessment has been made for a small karstic groundwater system in southern Belgium. The aquifer is a karstified limestone of Devonian age. A map of intrinsic vulnerability of the aquifer and of the local water-supply system shows three vulnerability areas. A parameter-balance study and a sensitivity analysis were performed to evaluate the influence of single parameters on aquifer-vulnerability assessment using the EPIK method. This approach provides a methodology for the evaluation of vulnerability mapping and for more reliable interpretation of vulnerability indices for karst groundwater resources. Received, March 1999/Revised, December 1999, February 2000/Accepted, February 2000     

Multi-tracer investigation of groundwater residence time in a karstic aquifer: Bitter Lakes National Wildlife Refuge, New Mexico, USA

Several natural and anthropogenic tracers have been used to evaluate groundwater residence time within a karstic limestone aquifer in southeastern New Mexico, USA. Natural groundwater discharge occurs in the lower Pecos Valley from a region of karst springs, wetlands and sinkhole lakes at Bitter Lakes National Wildlife Refuge, on the northeast margin of the Roswell Artesian Basin. The springs and sinkholes are formed in gypsum bedrock that serves as a leaky confining unit for an artesian aquifer in the underlying San Andres limestone. Because wetlands on the Refuge provide habitat for threatened and endangered species, there is concern about the potential for contamination by anthropogenic activity in the aquifer recharge area. Estimates of the time required for groundwater to travel through the artesian aquifer vary widely because of uncertainties regarding karst conduit flow. A better understanding of groundwater residence time is required to make informed decisions about management of water resources and wildlife habitat at Bitter Lakes. Results indicate that the artesian aquifer contains a significant component of water recharged within the last 10–50 years, combined with pre-modern groundwater originating from deeper underlying aquifers, some of which may be indirectly sourced from the high Sacramento Mountains to the west.     

The role of geomorphic processes in the transport and fate of mercury in the Carson River basin, west-central Nevada

 The historic processing of precious metal ores mined from the Comstock Lode of west-central Nevada resulted in the release of substantial, but unquantified amounts of mercury-contaminated mill tailings to the Carson River basin. Geomorphic and stratigraphic studies indicate that the introduction of these waste materials led to a period of valley-floor aggradation that was accompanied by lateral channel instability. The combined result of these geomorphic responses was the storage of large volumes of mercury-enriched sediment within a complexly structured alluvial sequence located along the Carson River valley. Much of the contaminated sediment is associated with filled paleochannels produced by the cutoff and abandonment of meander loops, and their subsequent infilling with contaminated particles. Geochemically, these deposits are characterized by variations in mercury levels that exceed three orders of magnitude. Continued lateral instability, coupled with an episode of channel-bed incision, followed the decline of Comstock mining, and has reexposed contaminated debris within the banks of the river. Erosion of bank sediments reintroduces mercury-enriched particles to the modern channel bed. It is suggested on the basis of geochemical and sedimentological data that during the bank erosion process, much of the mercury associated with fine (<63 μ) valley-fill deposits are carried downstream without being incorporated to any appreciable extent within the channel-bed sediments. In contrast, mercury associated with larger and denser particles, particularly mercury-gold-silver amalgam grains, are accumulated in the channel-bed sediments as the river traverses polluted reaches of the Carson River valley. Concentration patterns developed along the modern channel indicate that the valley fill is the primary source of mercury to the river today. Thus, these data imply that efforts to reduce the influx of mercury to the aquatic environment should examine methods for reducing bank erosion rates. Received: 13 December 1996 · Accepted: 15 April 1997     

Regional reconstruction of subglacial hydrology and glaciodynamic behaviour along the southern margin of the Cordilleran Ice Sheet in British Columbia,Canada and northern Washington State,USA

Subglacial landsystems in and around Okanagan Valley, British Columbia, Canada are investigated in order to evaluate landscape development, subglacial hydrology and Cordilleran Ice Sheet dynamics along its southern margin. Major landscape elements include drumlin swarms and tunnel valleys. Drumlins are composed of bedrock, diamicton and glaciofluvial sediments; their form truncates the substrate. Tunnel valleys of various scales (km to 100s km length), incised into bedrock and sediment, exhibit convex longitudinal profiles, and truncate drumlin swarms. Okanagan Valley is the largest tunnel valley in the area and is eroded >300 m below sea level. Over 600 m of Late Wisconsin-age sediments, consisting of a fining-up sequence of cobble gravel, sand and silt fill Okanagan Valley. Landform–substrate relationships, landform associations, and sedimentary sequences are incompatible with prevailing explanations of landsystem development centred mainly on deforming beds. They are best explained by meltwater erosion and deposition during ice sheet underbursts.During the Late-Wisconsin glaciation, Okanagan Valley functioned as part of a subglacial lake spanning multiple connected valleys (few 100s km) of southern British Columbia. Subglacial lake development started either as glaciers advanced over a pre-existing sub-aerial lake (catch lake) or by incremental production and storage of basal meltwater. High geothermal heat flux, geothermal springs and/or subglacial volcanic eruptions contributed to ice melt, and may have triggered, along with priming from supraglacial lakes, subglacial lake drainage. During the underburst(s), sheetflows eroded drumlins in corridors and channelized flows eroded tunnel valleys. Progressive flow channelization focused flows toward major bedrock valleys. In Okanagan Valley, most of the pre-glacial and early-glacial sediment fill was removed. A fining-up sequence of boulder gravel and sand was deposited during waning stages of the underburst(s) and bedrock drumlins in Okanagan Valley were enhanced or wholly formed by this underburst(s).Subglacial lake development and drainage had an impact on ice sheet geometry and ice volumes. The prevailing conceptual model for growth and decay of the CIS suggests significantly thicker ice in valleys compared to plateaus. Subglacial lake development created a reversal of this ice sheet geometry where grounded ice on plateaus thickened while floating valley ice remained thinner (due to melting and enhanced sliding, with significant transfer of ice toward the ice sheet margin). Subglacial lake drainage may have hastened deglaciation by melting ice, lowering ice-surface elevations, and causing lid fracture. This paper highlights the importance of ice sheet hydrology: its control on ice flow dynamics, distribution and volume in continental ice masses.     

Use of sinkhole and specific capacity distributions to assess vertical gradients in a karst aquifer

The carbonate-rock aquifer in the Great Valley, West Virginia, USA, was evaluated using a database of 687 sinkholes and 350 specific capacity tests to assess structural, lithologic, and topographic influences on the groundwater flow system. The enhanced permeability of the aquifer is characterized in part by the many sinkholes, springs, and solutionally enlarged fractures throughout the valley. Yet, vertical components of subsurface flow in this highly heterogeneous aquifer are currently not well understood. To address this problem, this study examines the apparent relation between geologic features of the aquifer and two spatial indices of enhanced permeability attributed to aquifer karstification: (1) the distribution of sinkholes and (2) the occurrence of wells with relatively high specific capacity. Statistical results indicate that sinkholes (funnel and collapse) occur primarily along cleavage and bedding planes parallel to subparallel to strike where lateral or downward vertical gradients are highest. Conversely, high specific capacity values are common along prominent joints perpendicular or oblique to strike. The similarity of the latter distribution to that of springs suggests these fractures are areas of upward-convergent flow. These differences between sinkhole and high specific capacity distributions suggest vertical flow components are primarily controlled by the orientation of geologic structure and associated subsurface fracturing.