Deanna S. Durnford

Unsaturated hyporheic zone flow in stream/aquifer conjunctive systems

Abstract Saturated flow is typically assumed for seepage from a stream underlain by an alluvial aquifer. However, if the water table falls a sufficient distance below a semipervious streambed, the head losses in this less conductive layer will cause the region beneath the stream, or hyporheic zone, to become unsaturated. Hyporheic zone flow is defined loosely in this research as the flow that occurs underneath the streambed. Unsaturated flow transforms streams from constant head boundaries to constant flux boundaries, impacting the biogeochemistry in the hyporheic zone. The objective of this paper is to discuss the development and implications of unsaturated flow beneath the streambed. Conditions under which saturated or unsaturated flow occurs and the characteristics of each flow regime are discussed. Next, the effect of unsaturated flow is illustrated for the case of stream leakage induced by a well pumping from an aquifer that is hydraulically interacting with a partially penetrating stream. Prior analytical solutions for alluvial well depletions fail to model unsaturated flow between the streambed and water table. An approximating solution is proposed to estimate aquifer drawdown and stream depletion under saturated/unsaturated hyporheic zone flow conditions.

Rapid fluid content measurement method for fingered flow in an oil–water–sand system using synchrotron X-rays

Abstract The complexity of simultaneous flow of water and non-aqueous phase liquids is largely unappreciated because few techniques permit accurate quantitative measurement of water and oil contents in rapidly changing flow fields. High intensity X-rays were used at the Cornell High Energy Synchrotron Source (CHESS) to obtain rapid, accurate, and non-destructive quantitative measurements of the changing fluid contents in a porous medium during infiltration events. Concomitant temporal pressure measurements were obtained for each liquid phase using rapidly responding tensiometers. The system was used for measuring temporal volumetric fluid content changes during a water finger infiltration into sand saturated with a NAPL (Soltrol-220) in a two-dimensional chamber. The fluid content distribution of a finger in the oil–water system was found to be similar to air–water systems. The hysteretic constitutive relationship between pressure and the content was developed from the data. The relationship was used to explain why the finger did not widen behind the tip, and why, upon re-infiltration, water followed the previously established path. These findings are relevant for cleanup of oil-contaminated sites because it aids in the understanding of hydrologic control which is an essential component of cost-effective in-situ remediation.

Porewater pressure increases in soil and rock from underground chemical and nuclear explosions

Abstract A review and analysis of chemical and nuclear explosive-induced porewater pressure increases and induced rise in groundwater table elevations (groundwater mounding) is presented. Our analysis indicates that residual pore pressure increases and groundwater mounding can be induced by underground chemical and nuclear explosions to scaled distances of 879 m/(kt) 1 3 . This relationship is linear over seven orders of magnitude of explosive energy ranging from a 0.01 kg chemical explosion to a 100 kt nuclear explosion and is valid for a wide variety of saturated geological profiles. Underground chemical explosions, and probably underground nuclear explosions have the potential to induce liquefaction of water-saturated soils to scaled distances of about 260 m/(kt) 1 3 .

Stochastic aggregation model (SAM) for DNAPL–water displacement in porous media

Abstract Source zone characterization of DNAPL in the subsurface is a necessary part of managing environmental risks. Modeling DNAPL–water displacement processes at the pore-scale can lead to greater insight and understanding of lateral spreading, the extent of vertical migration, and the final fluid distributions of dense non-aqueous phase liquids. A two-dimensional stochastic aggregation model is developed to study DNAPL–water displacement in porous media. The model is a modified diffusion limited aggregation model, and uses essential properties governing front stability as model input: DNAPL–water viscosity differences, DNAPL–water density differences, intrinsic permeability of the porous media, flow rate, and the inclination angle of the porous media from the horizontal. Due to the simplicity of the algorithm, this modeling technique can produce realistic DNAPL–water configurations much faster than conventional continuum approaches and allows for a Monte Carlo approach to be utilized. The model is validated through comparison to laboratory experiments involving 1,2-Dichloroethane, carbon tetrachloride, tetrachloroethylene, and mobile pyrogard 53. These experiments were performed at varying flow rates and angles of inclination from the horizontal.

Capillary pressure overshoot for unstable wetting fronts is explained by Hoffman's velocity‐dependent contact‐angle relationship

Pore velocity-dependent dynamic contact angles provide a mechanism for explaining the formation of fingers/columns in porous media. To study those dynamic contact angles when gravity is present, rectangular capillary tubes were used to facilitate observation of the complete interface without geometric distortion. Results show that the Hoffman (1975) relationship between dynamic contact angle and water velocity applies to gravity-affected flow fields, and that it (when adjusted for nonzero static contact angles) can be used to model dynamic capillary pressures for unstable wettings fronts in porous media by assuming that (1) pressure at the wetting front is discontinuous, (2) the flow field behind the fingertip is highly heterogeneous, and (3) the front line advances one or a few pores at the time. We demonstrate the utility of the Hoffman relationship for porous media with a published infiltration experiment by calculating the capillary pressure successfully at the unstable wetting front as a function of the flux of water in the finger and the grain size diameter.

Homemade bone charcoal adsorbent for defluoridation of groundwater in Thailand

High levels of fluoride in groundwater are a significant environmental and health problem in Thailand, as in many parts of the world. Small household defluoridators have several advantages over centralized treatment systems. In Thailand, however, use of bone char for water treatment has met resistance because of objectionable taste and odours of the water produced and the social resistance to handling fresh bone. This paper presents a method that uses bone charcoal as an absorbent for removing fluoride from groundwater. The commercially provided boiled bone is burned in a simple homemade furnace that can be constructed, operated and maintained easily by small rural householders. The method to produce the Thai bone char eliminates the odour and objectionable taste and also does not require the user to handle fresh bone, thus eliminating the social resistance. To evaluate the efficacy of the absorbent, batch experiments compare Thai and Indian bone char. Sorption isotherms are fit to the Freundlich and Langmuir equations and the kinetics are modelled using the pseudo first-order Lagergren equation. Results show that the sorption characteristics of Thai bone char compare favourably with the Indian bone char, with approximately 80% of the fluoride removed in both cases.

Moisture retention of a swelling soil under capillary and overburden pressures

Abstract The moisture content in a rigid soil varies with capillary pressure. On the other hand the moisture retention properties of a swelling soil are functions of capillary pressure and overburden pressure. An unloaded moisture retention curve for a swelling soil can be determined following the same laboratory procedures used for the rigid soils. Determination of the moisture contents due to combined effects of capillary pressure and overburden pressure, however, require a sophisticated experimental setup. The present study presents a method to obtain the moisture content constitutive surface for a swelling soil as function of capillary and overburden pressures by using more easily measured unloaded moisture retention curve and consolidation curve. The method considers that the moisture ratio contours at the constitutive surface can be represented by an ellipse equation. The proposed method was compared with laboratory results and found to be acceptable, particularly in view of the relative ease of the method.

RAPID DENSITY PROFILING OF CONSOLIDATING CLAY USING SYNCHROTRON RADIATION

The high-energy, highly collimated X-ray beam at the Cornell High Energy Synchrotron Source (CHESS) was used to provide rapid and high-resolution temporal and spatial density distributions in a consolidating saturated clay specimen. Porewater pressure at the undrained base of the clay was obtained by a porewater pressure transducer. Test results show good agreement between theoretical and experimental density distributions and indicate that synchrotron radiation is a promising and unique method to study consolidation.

Defluoridation with Locally Produced Thai Bone Char

The fluoride sorption ability of a locally available bone char is quantified. Both a synthetic solution and natural groundwater samples from several sites are studied and compared to Indian bone char, which is widely accepted and used successfully in India and elsewhere. The Freundlich and Langmuir sorption isotherms were used to quantify sorption properties. Results show that the Thai bone char is as effective as the Indian bone char for removing fluoride from contaminated water, despite the more rigid physical and social constraints found in rural Thailand. Sorption studies with fluoride-contaminated natural groundwater samples also show that chlorides, nitrates, and sulfates had little effect on the removal of fluoride by the homemade bone char.

Unsaturated Hyporheic Zone Flow in Analytical Models for Stream/Aquifer Interaction

Pumping from groundwater resources that are hydraulically interactive with adjacent streams induces a water flux from the stream to the aquifer. Analytical models of stream/aquifer systems assume saturated flow within the region between the streambed and aquifer, or hyporheic zone. However, pumping next to a stream may cause the region between the bottom of the streambed and the aquifer to become unsaturated. When this perching occurs, the flux from the stream to the underlying aquifer approaches a constant limit. Unsaturated, hyporheic zone flow transforms streams from constant head boundaries to specific flux boundaries. This research presents an analytical solution for drawdown and stream depletion that accounts for unsaturated hyporheic zone flow. The saturated and unsaturated hyporheic zone flows are linearly superimposed in the analytical model to account for transient stream disconnection along the length of the stream. The effect of unsaturated hyporheic zone flow on the total flux from surface water to groundwater will be investigated using the proposed analytical solution.