Water repellency alters soil hydrology after periods of wildfire, potentially modifying the ecosystem recovery to such disturbance. Despite this potential importance, the extent and severity of water repellency within burned peatlands and its importance in regulating peatland recovery to wildfire disturbance remains poorly understood. We characterised the water repellency of peat in a burned (one year post-fire) and unburned peatland in the Western Boreal Plain utilising the water drop penetration time and ethanol droplet molarity tests. Burned Sphagnum moss and feather moss sites had a more severe degree of water repellency than unburned sites, with differences being more pronounced between burned and unburned feather moss sites. Burned feather moss exhibited the most extreme water repellency, followed by unburned feather moss, and burned Sphagnum. The severity of water repellency varied with depth through the near surface of the moss/peat profile. This was most evident within the burned feathermoss where more extreme water repellency was observed at the near-surface compared to the surface, with the most extreme water repellency found at 1 and 5 cm depths. Unburned Sphagnum was completely hydrophilic at all depths. We suggest that the extreme water repellency in near-surface feather moss peat acts as a barrier that impedes the supply of water to the surface that replaces that lost via evaporation. This leads to drying of the near-surface vadose zone within feather moss areas and a concomitantly large decrease in peatland evaporation within feather moss dominated peatlands. This negative feedback mechanism likely enhances the resilience of such peatland to wildfire disturbance, maintaining a high water table position, thereby limiting peat decomposition. In comparison, such a feedback is not observed strongly within Sphagnum, leaving Sphagnum dominated peatlands potentially vulnerable to low water table positions post disturbance. (c) 2014 Elsevier B.V. All rights reserved.

Most organic compounds, when dissolved in water, decrease the surface tension of the water and many change the wettability of porous media. This directly affects capillary pressures and can induce water flow and solute transport in the vadose zone. This study investigated a coupled unsaturated flow and transport phenomenon, solute‐dependent capillarity‐induced focused flow (SCIFF), that has significant implications for infiltration into contaminated unsaturated zones. A two‐dimensional laboratory flow cell and the two‐dimensional hysteretic saturated–unsaturated flow and transport numerical model HYDRUS‐2D, which was modified to include solute concentration‐dependent hydraulic functions, were used to demonstrate and quantify the effect of SCIFF on point‐source infiltration of clean water into contaminated sand. The difference between the SCIFF infiltration event and traditional water infiltration into uncontaminated sand is striking. Highly focused vertical flow with very little horizontal migration was characteristic of the SCIFF infiltration event from a constant‐flux point source. The SCIFF resembles a single finger as in classic flow instability fingering; however, the SCIFF system has the added complexities of aqueous solute concentration‐dependent hydraulic functions. The SCIFF mechanism may well be active at many sites where the vadose zone has significant contamination with organic compounds. Another significant finding was that the magnitude and trend in moisture content changes measured by time domain reflectometry (TDR) did not match point values from the simulations because the TDR sample area extended beyond the width of the SCIFF zone. A good match between TDR and simulated values was obtained by determining the average simulated moisture content within the same region sampled by the TDR.

It is known that surfactants can induce flow in unsaturated porous media due to the dependence of capillary pressure on surface tension. A commonly observed feature in systems with surfactant-induced flow is a transient wetting/drying/wetting sequence associated with the propagation of a surfactant solute front under monotonic flow conditions. Previous efforts to model surfactant-induced flow in relatively complex (e.g., two-dimensional systems) have not successfully incorporated hysteretic moisture retention properties. In this research, hysteretic, two-dimensional simulations of surfactant-induced flow were performed to assess the potential importance of considering hysteresis in such simulations. Hysteretic simulation results were compared to experimental data and to non-hysteretic simulations. The results suggest that the inclusion of hysteresis in numerical simulations can improve the match between simulated and experimental results in systems with surfactant-induced unsaturated flow. Furthermore, the inclusion of hysteresis in numerical simulations played a significant role in predicting the distribution of the contaminant and correct pressure head/moisture condition at the end of the experiment.

The natural remobilization of an initially static mixed dense non-aqueous phase liquid (DNAPL) pool due to dissolution was demonstrated by Roy et al. (2002, 2004) using a compositional mathematical model and laboratory experiments with open pools over a porous medium. The purposes of this study were to: a) demonstrate natural remobilization for a pool within porous media (as opposed to an open pool); and b) analyze the capillary effects associated with residual formation, a changing saturation profile, hysteresis, and aging, as these processes may reduce the potential for natural remobilization of pools in porous media. DNAPL pools comprised of tetrachloroethene and benzene were created within a zone of larger glass beads overlying smaller glass beads, in a water-saturated 2-D flow cell. In one case, remobilization occurred in the form of a DNAPL finger, after 56 days of flushing. In another case, no remobilization had occurred after 64 days of flushing, though the density increased by 430 kg m(-3) and remobilization was predicted by the compositional model. Comparison of observations with model predictions suggest that contact angle hysteresis, related to an observed change in wettability, was the most significant contributing factor causing overprediction of the potential for natural remobilization.

Mixtures of dense non-aqueous phase liquids (DNAPLs) trapped in the subsurface can act as long-term sources of contamination by dissolving into flowing groundwater. In general, the components of higher solubility are removed more quickly, thus altering the composition of the remaining DNAPL, and possibly leading to changes in its physical properties. Through the development of a simple compositional model, Roy et al. [J. Contam. Hydrol. 2002 (59) 163] showed that preferential dissolution of a mixed DNAPL could potentially result in changes in density and interfacial tension that could subsequently lead to remobilization of an initially static DNAPL pool. The laboratory experiments presented in this next paper provide a proof-of-concept for the previously presented theory, demonstrating and quantifying this process of remobilization. In addition, the experiments provide a data set for evaluation of the model presented by Roy et al. [J. Contam. Hydrol. 2002 (59) 163]. In the four experiments, a DNAPL pool comprised of tetrachloroethene and benzene was created as an open pool overlying glass beads within a water-saturated 2-D flow box. Experiments included rectangular and triangular pools. In each of the experiments, remobilization (as breakthrough) was observed more than 2 weeks after formation of the initial pool. During each experiment, the pool height declined as mass was lost by dissolution, while sampling indicated a decrease in the mole fraction of benzene, the more soluble component. Small protuberances formed along the bottom of the pool as its composition changed with time and the displacement pressure was achieved for various pore throats. Eventually one of the protuberances extended further, forming a finger (breakthrough). In general, the pool emptied as the finger proceeded further into the beads. It was also shown theoretically and experimentally that remobilization will occur sooner for pools with a triangular (pointing down), rather than rectangular, shape. The experimental results were simulated using the model developed by Roy et al. [J. Contam. Hydrol. 2002 (59) 163]. The model matched the observations well, suggesting that it accurately represents the primary mechanisms involved with natural remobilization under the conditions of the study.

There are numerous analytical solutions available for flow in unsaturated homogeneous porous media. In this paper, the stream tube model for one‐dimensional water movement is extended to two‐dimensional (2‐D) water movement from a line source as the stream plane model. As well, new solutions are derived to predict the mean and variance of pressure head of water movement under a surface line source in heterogeneous soil using the perturbation method with first‐order approximation (PM1) and with second‐order approximation (PM2). A variance expression was also developed based on the spectral relationship presented by Yeh et al. [1985a]. The new solutions were tested using the 2‐D stream plane model with parameters A = ln(α) and Y = ln(KS) and measurements from field experiments. Results show that the mean of steady state pressure head below the line source is not only a function of the mean parameter values but also a function of the variances of A and Y and the linear cross‐correlation coefficient (ρ) between A and Y. The PM2 model can predict the mean pressure head accurately in heterogeneous soils at any level of correlation between A and Y, except when both the soil variability and ρ are high. The pressure head variance estimation based on the PM1 model predicts the measured variance well only when both the soil variability and ρ are low. The field experimental results show that both the PM1 and the spectral models give reasonable predictions of the pressure head variance. Both the measured and predicted values of the variance of pressure head using the two models increase with the depth of soil. Both models show that the variance of pressure head decreases as the source strength increases, but on average, the pressure head variance was underestimated by both models.

Mixtures of dense nonaqueous phase liquids (DNAPLs) trapped in the subsurface can act as long-term sources of contamination by dissolving into flowing groundwater. If the components have different solubilities then dissolution will alter the composition of the remaining DNAPL. We theorized that a multicomponent DNAPL pool may become mobile due to the natural dissolution process. In this study, we focused on two scenarios: (1) a DNAPL losing light component(s), with the potential for downward migration; and (2) a DNAPL losing dense component(s), with the potential for upward migration following transformation into a less dense than water nonaqueous phase liquid (LNAPL). We considered three binary mixtures of common groundwater contaminants: benzene and tetrachloroethylene (PCE), PCE and dichloromethane (DCM), and DCM and toluene. A number of physical properties that control the retention and transport of DNAPL in porous media were measured for the mixtures, namely: density, interfacial tension, effective solubility, and viscosity. All properties except density exhibited nonlinear relationships with changing molar ratio of the DNAPL. To illustrate the potential for natural remobilization, we modelled the following two primary mechanisms: the reduction in pool height as mass is lost by dissolution, and the changes in fluid properties with changing molar ratio of the DNAPL. The first mechanism always reduces the capillary pressure in the pool, while the second mechanism may increase the capillary pressure or alter the direction of the driving force. The difference between the rate of change of each determines whether the potential for remobilization increases or decreases. Static conditions and horizontal layering were assumed along with a one-dimensional, compositional modelling approach. Our results indicated that for initial benzene/PCE ratios greater than 25:75, the change in density was sufficiently faster than the decline in pool height to promote DNAPL breakthrough into the adjacent porous medium. In contrast, there was no potential for natural remobilization of a PCE–DCM mixture, primarily because the densities of the components are not sufficiently different. Dissolution of a DCM–toluene mixture decreased the density, reducing the tendency for downward displacement. However, the ultimate transformation from a DNAPL to an LNAPL may induce upward displacement. These results suggest that at sites with DNAPL pools containing a mix of components of sufficiently different densities and relative solubilities, natural remobilization may be an active mechanism, with implications for site evaluation and remediation.

Surface‐active solutes (surfactants) can induce capillary pressure gradients and water fluxes within the vadose zone. Common numerical models do not account for the effects of surfactant‐induced capillary pressure gradients on unsaturated flow and transport. We modified the variably saturated flow and transport model HYDRUS‐2D [Simunek et al., 1999] to incorporate the effects of surfactant concentration‐dependent changes to surface tension and viscosity on moisture retention and hydraulic conductivity, respectively. The model was tested by simulating the two‐dimensional surfactant infiltration experiment of Henry and Smith [2002]. The model successfully captured the major processes associated with the advance of a surface‐active contaminant plume through the vadose zone, including drainage of the vadose zone and depression of the capillary fringe associated with the solute front. The simulations also provided insight into unusual features of surfactant‐induced flow such as the upward flow of surfactant solution that was associated with capillary fringe drainage.

The difficulty in determining the effective interfacial tension limits the prediction of the wavelength of fingering of immiscible fluids in porous media. A method to estimate the effective interfacial tension using fractal concepts was presented by Chang et al. [Water Resour. Res. 30 (1994) 125]. We modified the method in that the macroscopic interface length was used instead of the system width. Methods to determine the macroscopic and the microscopic interface length are given. Lab experiments of dense nonaqueous phase liquid (DNAPL) penetrating into water-saturated glass beads were carried out in a two-dimensional (2-D) transparent chamber. The displacement processes were recorded using a 35-mm camera or a video camera, which was directly connected to and controlled by a computer. Unlike the method of Chang et al. (1994), the modified method used here gives a constant value of the effective interfacial tension over time. The predicted wavelengths of fingering are relatively close to those observed except for the fine beads.

Many organic compounds depress the surface tension of water relative to their aqueous concentration. These surface-active organic solutes have been shown to cause water flow in unsaturated porous media. Flow in these systems typically occurs from contaminated (high concentration) regions toward cleaner (lower concentration) regions. That flow is characterized by significant drainage and rewetting associated with the advance of the solute front. In the literature, modeling of unsaturated flow and transport in systems with solute concentration-dependent surface tensions has been limited to simulations without hysteresis in the hydraulic functions. However, experimental evidence is also presented which shows that hysteresis is a factor. We modified the hysteretic unsaturated flow and transport numerical model HYDRUS 5.0 to include concentration-dependent effects of a mobile organic solute. The moisture content-pressure head and unsaturated hydraulic conductivity functions were scaled for concentration-dependent surface tension and viscosity, respectively. The modified model successfully simulated data from surfactant-induced flow in 1-D horizontal column experiments. The effects of hysteresis were shown to be important to accurately simulate flow in these systems. For example, at final steady state, hysteretic simulations predicted uniform concentration and pressure profiles, but moisture contents that varied with distance. The final simulated moisture contents ranged from 0.12 to 0.25 along the column. In contrast, a non-hysteretic simulation would predict uniform distributions of concentration, pressure and moisture content. Dispersivity also had an effect on flow simulations. Lower dispersivity values caused sharper surfactant concentration gradients, which resulted in larger capillary pressure gradients and higher fluxes near the solute front. The modeling approach used here is expected to be applicable to many organic compounds of environmental interest that depress surface tension and thereby are capable of inducing unsaturated flow in porous media.

Abstract Thirty-five azooxanthellate (non-photosynthetic) corals belonging to 18 species were collected at sites ranging from the Norwegian Sea to the Antarctic and of depths ranging from 10 to 5220 m. All specimens showed distinct, well-defined linear correlations between carbonate oxygen and carbon isotopic composition, with slopes ranging from 0.23 to 0.67 (mean 0.45 ± 0.9) and linear correlation r2 values that averaged 0.89. These pronounced isotopic disequilibria have, to date, rendered azooxanthellate corals unsuitable for use in paleothermometry. Most, but not all, of the heaviest skeletal δ18O values reached or approached equilibrium. If the isotopically-heavy ends of the δ18O vs δ13C regression lines reliably approximated isotopic equilibrium with seawater, these values could be used to estimate the temperature of the water in which the coral grew. The δ13C values of the heavy ends of each line, however, were always depleted compared to carbon isotopic equilibrium with ambient bicarbonate by varying amounts. Despite the disequilibria, a reliable method for obtaining paleotemperature data was obtained. It was found that, if a δ18O vs δ13C regression line from an individual coral could be generated, the δ18Oarag value corresponding to δ13Carag = δ13Cwater and corrected for δ18Owater was a linear function of temperature: δ18O = −0.25 T(°C) + 4.97.

It has been demonstrated that local surface tension depression due to the presence of an insoluble surfactant, myristyl alcohol, can result in capillary pressure gradients that cause significant flow in unsaturated porous media. We hypothesized that the effects of a soluble surfactant, butanol, would differ from those of myristyl alcohol despite the fact that both give a similar reduction in the surface tension of water. We investigated this hypothesis through a series of horizontal column experiments and unsaturated flow modeling. We found that both surfactants induced water flow from regions of high surfactant concentration to regions of low concentration but that the shapes of the resultant moisture content profiles were fundamentally different. Because surface tension depression from myristyl alcohol was confined to the original source zone, we were able to simulate the system using a standard unsaturated flow model by assigning separate sets of hydraulic functions to the initially clean and source zones. Modeling showed that the redistribution of water due to the initial surfactant-induced capillary pressure gradient created moisture content induced capillary pressure gradients which acted to balance the system over time. The moisture content profile within the butanol system indicated a propagation of the zone of reduced surface tension. Because the surface tension of water is a function of solute concentration, surfactant-induced capillary pressure gradients are expected to remain and influence the flow system as long as concentration gradients exist.

Skeletal banding has been found in the deep-water scleractinian coral Desmophyllum cristagalli, an important animal in studies of climate change. This banding pattern sheds light on skeletogenesis and suggests methods by which the record of climate change contained within the coral skeletons may be interpreted. A central wall built of trabeculae forms the interior of the septa and rings the theca. Lamellae form a sheath over the trabecular frame, showing continuity from thecal edge to septum. Skeletal bands are added by the tissue layer, which overlaps and seals the internal coral and upper portion of the outer theca. Truncated inner bands on the outer theca indicate a pattern of skeletal deposition and dissolution dependent on the presence or absence of the live tissue layer. A long-term record will be difficult to collect from D. cristagalli since lamellae are less than 10 μm thick and band position is unpredictable. Density banding in shallow-water coral skeletons has long been recognized as a valuable paleo-oceanographic tool, and deep-water corals are now being used to reconstruct deep-ocean environments. Pattern of skeletal growth must be carefully considered if deep-water corals are to be used as proxy climate recorders.

Laboratory and numerical modeling studies were conducted to investigate the effects of solute concentration–dependent surface tension on unsaturated flow and transport of a dissolved organic compound. The laboratory experiments were conducted in a 2 m long unsaturated/saturated sand column with a water table at the bottom and butanol as the solute. Pressure head was measured with tensiometers connected to pressure transducers, and water content was measured using time domain reflectometry probes. The results show that changes in surface tension caused by changes in concentration of a dissolved organic compound can have significant effects on unsaturated flow. A highly localized large flow perturbation was shown to be associated with the solute front. Relatively good agreement was found between the numerical model and experimental results. The results suggest that in many cases where dissolved organic solutes are present in the vadose zone their effects on surface tension should be included in conceptual and numerical models.

The data from transient-flow air permeameters often exhibit curvature in the theoretically linear plots of the natual log of pressure vs, time, This was originally attributed to errors in water manometer data caused by the inertia of the water in the manometer. The resulting recommendation was to ignore the early time data when calculating the air permeability of a soil by these methods, We have demonstrated and quantified that the exhibited curvature is a direct consequence of small changes in temperature in the source air tank as a result of the cooling of the expanding air during the permeability determination. This was accomplished by interfacing the air permeameter with a computer to collect pressure and temperature data at a relatively high frequency, The data acquisition procedure facilitated the calculation of the change in pressure and temperature with respect to time, which allowed the direct solution of the differential form of the equations describing the mass flux of air from the tank through the core. The data is compared with the theoretical relationship expected for the core based on air permeability determined using a steady-state method.

We present the first δ18O and δ13C data from mollusc aragonite from Lake Erie for the 4.6–0.2 ka (4600 BP to 200 BP 14C yrs) time interval and describe single and composite species isotope trends. Composite species δ18O data show an almost 2.0%o increase from 3.3 to 3.0 ka followed by a nearly 2.5%o decrease at 2.8 ka. Oxygen isotope values then fluctuate by<l‰ until 0.2 ka. This trend in oxygen values is also evident in single species analyses of Sphaerium striatinurn. The most dramatic changes in isotope values, which occur from 3.3–2.8 ka, may reflect a pattern of water level changes in Lake Erie which occurred during the Nipissing flood and its lower water aftermath. Carbon isotope data show progressively more 13C enriched values from 4.6 ka (averaging–6.5‰ PDB) to the present (–0.57‰). This trend may reflect the dilution of isotopically light CO2 from the oxidation of organic matter due to rising lake levels. The short-term increase in δ18O values is coupled with a corresponding decrease in δ13C values. A similar pattern for Lake Erie prior to 10.5 ka was also associated with a lowering of water levels in the lake.