Low Hydraulic Gradients Research Articles

Unusual development of sandur sedimentary succession, an example from the Pleistocene of S Poland

Unusual development of sandur sedimentary succession, an example from the Pleistocene of S PolandAn atypical lithological development of outwash deposits in the Carpathians Foreland (S Poland) shows lower and middle parts of the sedimentary succession that are characterized by sinuous palaeochannels. This channel facies consists of laterally accreted sands derived from side bars. The sedimentary environment was a proglacial system of anabranching channels, presumably of anastomosed type. The outwash channel pattern was most probably controlled by the raising base level of the fluvial system. Both proglacial and extraglacial waters were dammed by a sandur within a small upland valley. Aggradation and progradation of the glaciofluvial deposits resulted in progressive rising of the dammed lake level. The low hydraulic gradient of the outwash streams resulted in a sinuous planform as well as a low-energy style of deposition. Afterwards, the rising lake water was drained off through a low watershed and the entire valley became filled with outwash sediments. The bedrock morphology thus became buried and a typical unconfined sandur with a braided channel network developed during the last phase of the glaciomarginal sedimentation (upper part of the sedimentary succession under study).

Karstification in unconfined limestone aquifers by mixing of phreatic water with surface water from a local input: A model

Summary When water from the surface of a limestone plain seeps down through the fractured rock to the water table of an unconfined aquifer with low hydraulic gradient containing water saturated with respect to calcite, mixing of these waters causes renewed aggressivity. A model is presented, which describes the evolution of karstification by dissolutional widening of the fractures downgradient from the local input of surface water. The model couples flow in the fractures with dissolution rates. Dissolution rates are given by F = k (1 − c (x)/ceq)4, where c (x) is the calcium concentration at distance x from the entrance of the fracture, ceq is the equilibrium calcium concentration of the H2O–CaCO3–CO2 solution in the fracture, and k is a rate constant. The model describes two domains of waters saturated with respect to calcite at different partial pressure of CO2. At the borders of these domains the waters mix and create dissolutional widening of the fractures by mixing corrosion. A channel evolves along the border in the downgradient direction by about 100 m in 100 ky. Below this channel a zone of fractures with aperture widths up to 1 cm has originated. The change of the hydraulic conductivity in the mixing zone shifts the border of the domains, allowing the channel to grow in the downgradient direction. Below it the zone of widened fractures is invaded by saturated phreatic water and dissolution stops. This process continues at the downgradient part of the conduit. In summary, we find cave conduits evolving close to the water table, leaving significant cavernous structures below them. A variety of modelling scenarios with different choices of parameters show that this evolution is typical and changes only in details but not in its basic behaviour.

Comparing model sensitivities of different landscapes using the ecohydrological SWAT model

Abstract. Lowland areas are characterised by specific properties, such as flat topography, low hydraulic gradients, shallow groundwater, and high potential for water retention in peatland and lakes. The investigated mesoscale catchments Stör, Treene and Kielstau are located in Northern Germany within lowland areas. Covering areas from 50 to 517 km2, these rural catchments have sandy, loamy and peaty soils and are drained in high fraction by open ditches and tile drainage. Using the river basin model SWAT, sensitivity analyses were carried out through an automatic routine that is based on the Latin-Hypercube (LH) and a One-factor-At-a-Time (OAT) sampling. The objective of this study is to investigate how specific landscape features influence the model behaviour. There are two research questions: a) What are the most sensitive parameters in the studied lowland catchments? b) What differences occur between these landscape features in comparison to mountainous or low mountain range catchments? The results show that groundwater and soil parameters were found to be most sensitive in the studied lowland catchments and they turned out to be the most influential factors on simulated water discharge. The most sensitive parameter was the threshold water level in shallow aquifer for baseflow (GWQMN). In contrast, many studies of mountainous or low mountain range catchments show that the most sensitive parameters were the surface runoff parameters.

Controls on groundwater flow in the Bengal Basin of India and Bangladesh: regional modeling analysis

Groundwater for domestic and irrigation purposes is produced primarily from shallow parts of the Bengal Basin aquifer system (India and Bangladesh), which contains high concentrations of dissolved arsenic (exceeding worldwide drinking water standards), though deeper groundwater is generally low in arsenic. An essential first step for determining sustainable management of the deep groundwater resource is identification of hydrogeologic controls on flow and quantification of basin-scale groundwater flow patterns. Results from groundwater modeling, in which the Bengal Basin aquifer system is represented as a single aquifer with higher horizontal than vertical hydraulic conductivity, indicate that this anisotropy is the primary hydrogeologic control on the natural flowpath lengths. Despite extremely low hydraulic gradients due to minimal topographic relief, anisotropy implies large-scale (tens to hundreds of kilometers) flow at depth. Other hydrogeologic factors, including lateral and vertical changes in hydraulic conductivity, have minor effects on overall flow patterns. However, because natural hydraulic gradients are low, the impact of pumping on groundwater flow is overwhelming; modeling indicates that pumping has substantially changed the shallow groundwater budget and flowpaths from predevelopment conditions.

Compartmented flow at the Bátaapáti site in Hungary

Integration of hydrogeological and geological data into a conceptual model is critical for site investigation programs, since this model is the basis for hydrogeological modeling and for engineering. In the Hungarian Radioactive Waste Disposal Investigation Program, several methods have been used to characterize the potential host rock (granite) at the Bataapati site for a repository for low and intermediate level radioactive waste. Hydrogeological data acquisition revealed some characteristic aspects of the site. One of the most important is the presence of extensive rock deformation zones (faults) with low hydraulic conductivity, which strongly reduce the direct hydraulic communication between adjacent blocks of rock (compartmentalization). This characteristic of the rock mass results in a mosaic-like distribution of rock compartments, each with an almost constant hydraulic head. Within the compartments, hydraulic tests have shown that transmissivity is strongly scale dependent: the larger the scale, the higher the measured transmissivity. The extensive highly transmissive zones cause very low hydraulic gradients within each block, thus the transport processes are strongly influenced by the low or average transmissivity zones and the rock matrix.

Modelling hydrological processes in mesoscale lowland river basins with SWAT—capabilities and challenges

Lowland areas are characterized by specific properties, such as flat topography, low hydraulic gradients, shallow groundwater, and high potential for water retention in peatland and lakes. These characteristics and their dominating hydrological processes have to be assessed and considered for the analysis and modelling of water balances in lowland catchments. The capabilities and challenges of modelling hydrological processes and water balances in mesoscale lowland river basins with the SWAT model are presented. The investigated catchments Stör, Treene and Kielstau are located in northern Germany within a lowland area. Covering areas from 50 to 517 km2, these rural meso-catchments have sandy, loamy and peaty soils. Drainage, in terms of tile drainage and open ditches, has changed the natural water balance. The set-up and modifications of the model as applied in these case studies can be transferred to other similar catchments in lowland areas. The dominating hydrological processes were found to be mainly controlled by groundwater dynamics and storage, drainage, wetlands and ponds. Some groundwater parameters were found to be highly sensitive and they turned out to be the most influential factors for improving simulated water discharge.

Spatial relationship of groundwater arsenic distribution with regional topography and water-table fluctuations in the shallow aquifers in Bangladesh

The present study has examined the relationship of groundwater arsenic (As) levels in alluvial aquifers with topographic elevation, slope, and groundwater level on a large basinal-scale using high-resolution (90 m × 90 m) Shuttle Radar Topography Mission (SRTM) digital elevation model and water-table data in Bangladesh. Results show that high As (>50 μg/l) tubewells are located in low-lying areas, where mean surface elevation is approximately 10 m. Similarly, high As concentrations are found within extremely low slopes (<0.7°) in the country. Groundwater elevation (weekly measured by Bangladesh Water Development Board) was mapped using water-table data from 950 shallow (depth <100 m) piezometers distributed over the entire country. The minimum, maximum and mean groundwater elevation maps for 2003 were generated using Universal Kriging interpolation method. High As tubewells are located mainly in the Ganges–Brahmaputra–Meghna delta, Sylhet Trough, and recent floodplains, where groundwater elevation in shallow aquifers is low with a mean value of 4.5 m above the Public Works Datum (PWD) level. Extremely low groundwater gradients (0.01–0.001 m/km) within the GBM delta complex hinder groundwater flow and cause slow flushing of aquifers. Low elevation and gentle slope favor accumulation of finer sediments, As-carrying iron-oxyhydroxide minerals, and abundant organic matter within floodplains and alluvial deposits. At low horizontal hydraulic gradients and under reducing conditions, As is released in groundwater by microbial activity, causing widespread contamination in the low-lying deltaic and floodplain areas, where As is being recycled with time due to complex biogeochemical processes.

Effects of Hydraulic Variables and Well Construction on Horizontal Borehole Flowmeter Measurements

AbstractDue to differences in hydraulic conductivity and effects of well construction geometry, ground water flow in a wellbore typically differs from the flow field in the surrounding porous media. These differences must be well understood to use borehole flowmeters for ground water velocity and direction measurement. In this work, a lab‐scale sand tank test and a small‐scale field test were conducted to explore the differences between average linear velocities in the borehole and in the formation and flow directions. In the sand tank test, six variables were studied, colloidal borescope meters were used to measure flows in wellbores, and correction factors (= linear velocity in center of borehole/average linear velocity in the surrounding formation away from the well) for flow velocity were determined. In these tests, correction factors ranged between 3 and 6 for all conditions studied, without any strong correlation between correction factor and most of the variables studied. The most obvious relationship was that the correction factor appeared to increase with decreasing ground water velocity; when the flow velocity decreased to near or below 2 × 10−3 m/s, the value of the correction factors increased to equal or exceed 10. In the field test, two different types of flowmeter—the colloidal borescope meters and heat‐pulse flowmeters—were used under natural gradient and forced hydraulic gradient conditions. Field test results showed no differences in correction factors when either type of borehole meters or hydraulic conditions were used. The correction factors remained between 3.9 and 6.0 consistent with other previously published results. The flow directions measured by borehole flowmeters in wellbores were consistent with the flow directions in the geological formation. The results of the sand tank test indicated the larger well screen diameter and the lower hydraulic gradient seem to have the more accurate flow directions measured by borehole flowmeters. Field and lab measurements suggested that the errors of flow direction were less than 15°, with a maximum of 21°.

Simulating the dissolution of a complex dense nonaqueous phase liquid source zone: 2. Experimental validation of an interfacial area–based mass transfer model

A multiphase flow–aqueous phase transport numerical model (DNAPL3D‐MT) is developed to simulate the dissolution of complex source zones containing both pooled and residual dense nonaqueous phase liquids (DNAPLs). The multiphase flow model (DNAPL3D) is coupled to the aqueous species transport code (MT3D) via a flexible mass transfer function, which can employ the local equilibrium assumption or the single–boundary layer expression for rate‐limited dissolution either incorporating a lumped (correlation function) coefficient or explicitly accounting for the interfacial area (IFA) between the fluids. For the latter, this work employs the thermodynamically based Explicit IFA Submodel (Grant and Gerhard, 2007), which provides IFA as a function of saturation and saturation history. A bench‐scale experiment is presented involving the complete, natural dissolution of a DNAPL source zone emplaced by a point source release into heterogeneous porous media. DNAPL3D‐MT simulations of the experiment, involving no calibration to results, are compared with the observed evolution of both (1) measured downgradient dissolved phase concentrations and (2) DNAPL source zone configuration. The model, employing a mass transfer expression equipped with the Explicit IFA Submodel, simulates the experiment more accurately than when equipped with either a local equilibrium assumption or a published empirical correlation expression. Sensitivity simulations indicate that this model validation is sensitive to a number of the key assumptions in the Submodel derivation except one: the relationship between interfacial area and residual DNAPL saturations. The employed assumption of a single mass transfer coefficient value is supported by an analysis of the evolution of Peclet numbers throughout the DNAPL source zone, which reveals that the low hydraulic gradient employed resulted in diffusion‐dominated mass transfer conditions throughout the experiment. This study suggests that simulations of global mass flux from complex DNAPL source zones are sensitive to the interrelationship of rate‐limited mass transfer and groundwater velocity (and thus aqueous phase relative permeability and DNAPL saturation) at the local scale.

Law of nonlinear flow in saturated clays and radial consolidation

It was derived that micro-scale amount level of average pore radius of clay changed from 0.01 to 0.1 micron by an equivalent concept of flow in porous media. There is good agreement between the derived results and test ones. Results of experiments show that flow in micro-scale pore of saturated clays follows law of nonlinear flow. Theoretical analyses demonstrate that an interaction of solid-liquid interfaces varies inversely with permeability or porous radius. The interaction is an important reason why nonlinear flow in saturated clays occurs. An exact mathematical model was presented for nonlinear flow in micro-scale pore of saturated clays. Dimension and physical meanings of parameters of it are definite. A new law of nonlinear flow in saturated clays was established. It can describe characteristics of flow curve of the whole process of the nonlinear flow from low hydraulic gradient to high one. Darcy law is a special case of the new law. A mathematical model was presented for consolidation of nonlinear flow in radius direction in saturated clays with constant rate based on the new law of nonlinear flow. Equations of average mass conservation and moving boundary, and formula of excess pore pressure distribution and average degree of consolidation for nonlinear flow in saturated clay were derived by using an idea of viscous boundary layer, a method of steady state in stead of transient state and a method of integral of an equation. Laws of excess pore pressure distribution and changes of average degree of consolidation with time were obtained. Results show that velocity of moving boundary decreases because of the nonlinear flow in saturated clay. The results can provide geology engineering and geotechnical engineering of saturated clay with new scientific bases. Calculations of average degree of consolidation of the Darcy flow are a special case of that of the nonlinear flow.

Fate and Transport of 2,4,6-Trinitrotoluene in Loams at a Former Explosives Factory

Natural attenuation processes affecting 2,4,6-trinitrotoluene (TNT) were determined within loams for two study areas at the former Explosives Factory Maribyrnong, Australia. TNT fate and transport was investigated through spectrophotometric/High Performance Liquid Chromatography (HPLC) analyses of soil and groundwater, adsorption and microcosm testwork. A five tonne crystalline TNT source zone delineated within near surface soils at the base of a TNT process waste lagoon was found to be supplying aqueous TNT loading (7 ppm) to subsurface soils and groundwater. The resultant plume was localized within the loam aquitard due to a combination of natural attenuation processes and hydrogeological constraints, including low hydraulic conductivity and upward hydraulic gradients. Freundlich described sorptive partitioning was the main TNT sink (KF = 29 mL/g), while transformation rates were moderate (1.01 × 10-4 h-1) under the aerobic conditions. Increasing 2-amino-4,6-dinitrotoluene predominance over 4-amino-2,6-dinitrotoluene was discovered with depth (in situ) and time (microcosms). Simplified dissolution rate calculations indicate that without mitigation of the TNT source, contaminant persistence within the vadose zone may approach 2000 years, while ATRANS20 simulations demonstrate that the TNT plume propagates very slowly along the flow path within the aquitard.

Aroca (domaine marin côtier, Pays basque, France) : un karst continental ennoyé par les transgressions maritimes quaternaires

Aroca (littoral, Basque country, France) : a continental karst drowned by quaternary maritime transgressions The rocky formations in shallow areas of the Atlantic coast are hardly known. Studies are rare because of the difficulties of direct observation (diving in always agitated, troubled water, depth between -20 and -40 m). Our first step was to make a detailed topography of a submarine plateau named Aroca, 4 km off Socoa harbour (bay of Saint-Jean-de-Luz). This plateau was already known for having a large variety of forms within a small surface (150 x 100 m). We gave names to most remarquable formations and defined five main characteristic zones : -in the exokarstic domain 1/ a top surface with channels, 2/ a dismantled surface with pinnacles ; -in the endokarstic domain 3/ caves, galleries, arches ; -at the limits 4/ three inclined plans, west, north and east, 5/ a cliff to the south. A typology of forms shows a predominance of ablation reliefs : aplanation, over-deepened channels, covered rooms and galleries, arches, residual pinnacles. Deposit accumulations regroup chaotic breakdown blocks, pebble accumulations and sand covers. Statement of explanations requires recognition of the nature and age of the outcrops and succession of erosional agents during the Pleistocene. Rocks are dated from Ypresien (limestones) to Bartonian (marls). Continental erosion during sea regressions is responsible of caracteristic Iandforms and deposits ; for example wall banks, allochthonous pebbles... The currently active marine erosion during sea transgressions is due to storms, tide, dissolution, biochemical action (lithophages) and gravity. We propose a paleogeographic reconstitution. After an essentially calcareous sedimentation in Eocene and an essentially marly sedimentation in Oligocene, the sea recedes during Miocene. From then, the platform, henceforward above the water, is subject to meteoric erosion, in Pliocene, evolution of the massif is isovolumic (under a marly cover and with a low hydraulic gradient). During the lower and middle Pleistocene, the erosion of the marly cover goes on. During the upper Pleistocene, the wurmian (-18000 BP) marine regression allows entrenchment of the hydrographie system thanks to an increase of hydraulic gradients (classic functional karst). From 15000 years onwards, a general transgression of sea level happens by successive steps. During the Boreal, a break in transgression allows the formation of a paleo-shore at -20 to -30 m, inducing a peneplanation phase in the tidal or infratidal zone. From 7500 BP onwards, a a rapid transgression from -23 to -8, then a slower one from -8 to the present level stops karstification on the massif. At present, only marine abrasion is active and tends to obliterate the previously built landforms.

Applied tracer tests in fractured rock: Can we predict natural gradient solute transport more accurately than fracture and matrix parameters?

Applied tracer tests provide a means to estimate aquifer parameters in fractured rock. The traditional approach to analysing these tests has been using a single fracture model to find the parameter values that generate the best fit to the measured breakthrough curve. In many cases, the ultimate aim is to predict solute transport under the natural gradient. Usually, no confidence limits are placed on parameter values and the impact of parameter errors on predictions of solute transport is not discussed. The assumption inherent in this approach is that the parameters determined under forced conditions will enable prediction of solute transport under the natural gradient. This paper considers the parameter and prediction uncertainty that might arise from analysis of breakthrough curves obtained from forced gradient applied tracer tests. By adding noise to an exact solution for transport in a single fracture in a porous matrix we create multiple realisations of an initial breakthrough curve. A least squares fitting routine is used to obtain a fit to each realisation, yielding a range of parameter values rather than a single set of absolute values. The suite of parameters is then used to make predictions of solute transport under lower hydraulic gradients and the uncertainty of estimated parameters and subsequent predictions of solute transport is compared. The results of this study show that predictions of breakthrough curve characteristics (first inflection point time, peak arrival time and peak concentration) for groundwater flow speeds with orders of magnitude smaller than that at which a test is conducted can sometimes be determined even more accurately than the fracture and matrix parameters.

A dye-tracing test as an aid to studying karst development at an artesian limestone sub-aquifer: Zagros Zone, Iran

A dye-tracing test is employed to study the karst development and flow regime at an artesian limestone sub-aquifer, the Khersan3 Dam site, Zagros Zone, Iran. Tracer breakthrough curves showed an early dominant peak followed by a pronounced tailing effect. The peak concentration was a response to induced pressure during dye injection. The results suggest that the dye was pushed into the small pores and fissures around the injection point during dye injection. Hence, the dye moved out as a result of matrix and fissure diffusive processes and created a long pronounced tailing. The maximum flow velocity in the upper artesian sub-aquifer ranged from 0.97 to 2.9 m/h. However, the mean tracer velocity ranged from 0.19 to 0.51 m/h based on the mean residence tracer time; consequently, the flow regime in the artesian sub-aquifer was determined to be mainly diffusive. The results reveal (1) a low hydraulic gradient from upstream of the dam axis to downstream; consequently, there is no considerable flow; (2) poor karst development and diffuse flow at the tracing test area; (3) a discharge zone at a location downstream of the dam axis which is the main terminal of general flow direction at the dam site.

The evolution of groundwater in the Tyrrell catchment, south-central Murray Basin, Victoria, Australia

The Tyrell catchment lies on the western margin of the Riverine Province in the south-central Murray Basin, one of Australia’s most important groundwater resources. Groundwater from the shallow, unconfined Pliocene Sands aquifer and the underlying Renmark Group aquifer is saline (total dissolved solids up to 150,000 mg/L) and is Na-Cl-Mg type. There is no systematic change in salinity along hydraulic gradients implying that the aquifers are hydraulically connected and mixing during vertical flow is important. Stable isotopes (18O+2H) and Cl/Br ratios indicate that groundwater is entirely of meteoric origin and salts in this system have largely been derived by evapotranspiration of rainfall with only minor halite dissolution, rock weathering (mainly feldspar dissolution), and ion exchange between Na and Mg on clays. Similarity in chemistry of all groundwater in the catchment implies relative consistency in processes over time, independent of any climatic variation. Groundwater in both the Pliocene Sands and Renmark Group aquifers yield ages of up to 25 ka. The Tyrrell Catchment is arid to semi-arid and has low topography. This has resulted in relatively low recharge rates and hydraulic gradients that have resulted in long groundwater residence times.

Steady- and transient-state inversion in hydrogeology by successive flux estimation

A calibration method to solve the groundwater inverse problem under steady- and transient-state conditions is presented. The method compares kriged and numerical head field gradients to modify hydraulic conductivity without the use of non-linear optimization techniques. The process is repeated iteratively until a close match with piezometric data is reached. The approach includes a damping factor to avoid divergence and oscillation of the solution in areas of low hydraulic gradient and a weighting factor to account for temporal head variation in transient simulations. The efficiency of the method in terms of computing time and calibration results is demonstrated with a synthetic field. It is shown that the proposed method provides parameter fields that reproduce both hydraulic conductivity and piezometric data in few forward model solutions. Stochastic numerical experiments are conducted to evaluate the sensitivity of the method to the damping function and to the head field estimation errors.

Hydrological modelling of a re-wetted peatland on the basis of a limited dataset for water management

The valley mire ‘Mittleres Trebeltal’ in Mecklenburg-West Pomerania (northeast Germany) has recently been re-wetted in the course of an EU-funded conservation project. On the basis of geological cross-sections, a hydrological model applying the Boussinesq-equation has been developed to investigate the hydrological dynamics of the study area and to evaluate various management strategies. Modelled water levels corresponded well to measured ones. The optimised values of two of the main model parameters—the hydraulic conductivity and the proportions of vegetation types—agreed with measurement values, and, respectively, the real vegetation cover, while the third one, the specific yield, should pragmatically be used as a calibration parameter. Due to a proportion of around 30% of tall fen vegetation, the modelled evapotranspiration clearly exceeded grass evapotranspiration resulting in low groundwater levels in some parts of the fen during summer. Generally, the water levels are controlled by topographic features (i.e. former peat cuttings) and weather conditions during the summer months. When analyzing different management scenarios, even an optimal water supply (ditch water levels near the ground surface) could not raise the groundwater table to ecologically sound levels during summer. However, during winter, these ditch water levels enhanced flooding. The groundwater fluxes towards the river, and, respectively, towards the fen were low. The minor importance of these fluxes for the water balance was caused by very low hydraulic gradients. Therefore, it can be concluded that the risk of nutrient export from the fen into the river is very limited and compound transformations take place within the mire system itself. Hydrological analysis as a prerequisite for the assessment of nutrient and pollutant transport and transformation processes can be carried out—as a first approach—with simple models on the basis of a limited dataset.

Surfactant-enhanced electrokinetic remediation of polycyclic aromatic hydrocarbons in heterogeneous subsurface environments

Polycyclic aromatic hydrocarbon (PAH) contamination exists at numerous sites and poses a substantial risk to public health and the environment. Electrokinetic remediation technology has the potential to treat PAH-contaminated soils, but the effect of soil heterogeneities such as layers, lenses, and mixtures of different soils on the electrokinetic process has not been adequately studied. This study evaluates surfactant-enhanced electrokinetic remediation under heterogeneous soil conditions. A series of bench-scale experiments was conducted using two soils (sand and kaolin) spiked with a representative PAH compound (phenanthrene) in a two-dimensional electrokinetic test apparatus under various layered, lens, or mixed soil configurations. In addition, the homogeneous sand and kaolin soils were each tested alone for comparison purposes. All the experiments employed the same nonionic surfactant (5% Igepal CA-720) flushing solution and a low (0.05) hydraulic gradient. The results showed that the surfactant flushing under the low hydraulic gradient alone was sufficient for complete removal of the contaminant from the homogeneous sand profile, whereas the electroosmotic flow generated by the application of a DC 2.0 V/cm electric potential in a periodic mode considerably enhanced the removal efficiency for the homogeneous and heterogeneous soil profiles containing kaolin. The voltage gradient varied spatially and temporally through the soil profiles and affected the electroosmotic flow and contaminant removal. Key words: clays, electrokinetic remediation, electroosmosis, flushing, heterogeneity, polycyclic aromatic hydrocarbons (PAHs), phenanthrene, soils, sorption, solubilization, surfactants.

Seismic geomorphology of buried channel systems on the New Jersey outer shelf: assessing past environmental conditions

Quantitative geomorphologic analysis of shallowly buried, dendritic channel systems on the New Jersey shelf provides estimates of paleo-hydrologic parameters needed to link channel morphology to the former hydrodynamic setting. These channels, observed in 1–4 kHz deep-towed chirp seismic data, formed presumably as fluvial systems when the shelf was exposed during the Last Glacial Maximum (LGM). The presumed fluvial origin of these channels is supported by their incision into underlying Pleistocene strata, a chaotic seismic fill unit at their bases which may be indicative of non-marine gravel lag, and measured stream junction angles that are consistent with a riverine origin. The channels would also have been subjected to estuarine/tidal environments during ensuing sea-level rise. We employ empirically derived hydraulic equations for modern rivers and estuaries to estimate paleo-discharges, velocities and maximum shear stresses, using the preserved and interpolated paleo-channel geometries as a guide. Generally, trunk/main channels have box-like, symmetric cross-sections, with width/depth ratios of >100, whereas smaller, tributary channels have more v-shaped, asymmetric cross-sections with width/depth ratios of ∼40–70. The high width/depth ratios, along with low sinuosities (∼1.1) and slopes (<0.02°), are consistent with braided streams as specified by a modern river classification system. However, the channels show no evidence of braiding. We hypothesize instead that these channel systems are immature, having had insufficient time to develop high sinuosities that would otherwise be expected before they were drowned by the Holocene transgression. Mean paleo-flow estimates for these systems assuming a tidal environment (1.0–1.5 m/s) are consistent with modern tidal creeks comparable to the sizes of channels observed here (<2 km wide and <25 m deep). Estimated tidal shear stresses would be sufficient to initiate sediment transport of grains 2–8 mm in diameter (coarse sand and fine gravel) as bedload and finer grained material in suspension. However, paleo-flow estimates assuming a fluvial environment (1.1–2.0 m/s) are generally too high for a non-tidal creek, given the presumed low hydraulic gradients in this coastal plain setting. Retrodicted fluvial discharge and boundary shear stresses would have been sufficient to transport particles up to ∼15 mm in diameter (gravel) as bedload; these grain sizes are too coarse to be transported by sluggish coastal plain rivers. We conclude that either flows were quite high when this system was first incised fluvially, perhaps due to meltwater pulses following the LGM, or that tidal influences have modified the original fluvial geometry.

Equilibrium nonaqueous phase liquid pool geometry in coarse soils with discrete textural interfaces

This paper presents a model for the geometry of nonaqueous phase liquid (NAPL) pools and mounds in homogeneous soils and soils with discrete textural interfaces. It is shown that the concepts of capillary pressure–saturation curve hysteresis and entry pressures are integral to the complete conceptualization of pool and mound geometry. Unless hysteresis is included in the analysis, light NAPL (LNAPL) in homogeneous soils cannot exist in pools at all, and dense NAPL (DNAPL) will not mound on horizontal textural interfaces unless lateral confining boundaries are present. The proposed model also implies that remobilization of DNAPL pools will occur at lower hydraulic gradients than those predicted with previous models. Comparing predicted and experimental DNAPL and LNAPL pool thicknesses and the location of an LNAPL lens with respect to the top of the capillary fringe validate the model.