Laboratory Sand Columns Research Articles

Influence of Supplemental Acetate on Bioremediation for Dissolved Polycyclic Aromatic Hydrocarbons

The influence of supplemental acetate on in situ bioremediation for the removal of dissolved polycyclic aromatic hydrocarbons (PAHs) in groundwater was evaluated in laboratory sand columns. Sand columns, inoculated with a soil enrichment culture, were fed with dissolved PAHs (9 mg/L naphthalene, 0.8 mg/L phenanthrene, 0.09 mg/L pyrene), nutrients, and hydrogen peroxide to sustain aerobic microbial growth. Pore water PAH concentration profiles were obtained during the study. Determinations of viable biomass, carbohydrate, and PAH sorption capacity were obtained at the conclusion of the experimental runs. Pore water profiles indicated that PAH biodegradation capability became more quickly established after 45 days in sand columns amended with acetate versus the unamended control. The endpoint pore water PAH concentration profiles were similar for both acetate-amended and unamended columns. Higher biomass in acetate-amended columns increased the overall sorption capacity of the sand medium for PAHs by 24–47%...

Diffusive partitioning tracer test for nonaqueous phase liquid (NAPL) detection in the vadose zone.

This paper proposes the theory and practical application of a new partitioning tracer test for nonaqueous phase liquid (NAPL) detection in the vadose zone, which is based on diffusion. A mixture of chlorofluorocarbons as gaseous tracers is injected into the vadose zone to form a point source at the injection point. While the tracers diffuse away, small volumes of gas are withdrawn from the injection point. The quantitative determination of the NAPL saturation is based on a comparison of the concentration decline of tracers with different air-NAPL partitioning coefficients. The test has been evaluated in laboratory sand columns contaminated with dodecane. NAPL in saturations of 0.8-4% of the total porosity have been quantified in a wide range of different water contents. Actual and measured NAPL saturations calculated as an average from four different tracer pairs agreed within +/-30%. The new method was successfully used for repeated NAPL quantification in a large-scale field lysimeter contaminated with artificial kerosene. This rapid and inexpensive test is potentially of value for site investigations especially in combination with soil gas measurements, because it requires similar equipment. Possible applications are source delineation and repeated NAPL quantification in situ during a remediation.

Laboratory sand column study of encapsulated buffer release for potential in situ pH control

Encapsulation technology is being investigated as a method for controlling pH in situ at contaminated groundwater sites where pH may limit remediation of organic contaminants. This study examined the effectiveness of using KH 2PO 4 buffer encapsulated in a pH-sensitive coating to neutralize pH in laboratory sand columns (1.5-l) under a simulated groundwater flow rate and characterized the pattern of capsule release in the flow-through system. Denitrification was used in the columns to increase the pH of the pore water. Each of three columns was equipped with three miniature mesh wells to allow contact of the buffer with column pore water, but capsules (15 g) were inserted into only one column (amended). The two other columns served as amendment (no buffer) and abiotic (no denitrification) controls. Oxidation–reduction potential, dissolved organic and inorganic carbon, NH 4 +, NO 3 −+NO 2 −, PO 4 3−, and pH were measured in the influent, two side ports, and effluent of the columns over time. Near complete conversion of 80 mg N/l of nitrate and 152 mg/l of ethanol per day resulted in a mean pH increase from 6.2 to 8.2 in the amendment control column. The amended column maintained the target pH of 7.0±0.2 for 4 weeks until the capsules began to be depleted, after which time the pH slowly started to increase. The capsules exhibited pulses of buffer release, and were effectively dissolved after 7.5 weeks of operation. Base-neutralizing capacity contributed by the encapsulated buffer over the entire study period, calculated as cation equivalents, was 120 mM compared to 8 mM without buffer. This study demonstrates the potential for this technology to mediate pH changes and provides the framework for future studies in the laboratory and in the field, in which pH is controlled in order to enhance organic contaminant remediation by pH-sensitive systems.

Estimation of relative hydraulic conductivity of sandy soils based on a sheet flow model

The sophistication of numerical methods that are available to solve the governing equation of unsaturated flow in soils (Richards’ equation) far exceeds the ability to accurately estimate parametric functions of the equation. One of these parametric functions that defines the relationship between the capillary pressure and saturation (retention function) is usually obtained by fitting one of a number of available models to measured laboratory or field data. A second relationship, the hydraulic conductivity–saturation function can also be obtained by fitting to measured data. More often, the measured data of hydraulic conductivity is not available and a model of hydraulic conductivity–saturation function is obtained by using measured capillary pressure–saturation data. Such a hydraulic conductivity model that is widely used in soil physics applications was developed by Mualem (Mualem, Y., 1976 Water. Resour. Res. 12 (3), 513–522). An empirical parameter in this model was obtained by fitting to retention data from non-sandy, agricultural field soils. This model also conceptualizes unsaturated flow to occur in pipes that assume no air–water interfaces. In most aquifer contamination problems sandy soils are encountered and the unsaturated flow in these soils is dominated by air–water interfaces. In a laboratory sand column experiment we have demonstrated that the Mualem's model under-predicts the hydraulic conductivity of sandy soils. A new model of unsaturated hydraulic conductivity that is specifically applicable for sandy soils is developed. This model approximates the sand grains by uniform spheres, and uses a flow geometry that allows for the incorporation of an air–water interface at the pore-scale. The model was then applied to predict the capillary pressure head in a laboratory sand column under conditions of vertical infiltration. The proposed model matched the observations much better compared to Mualem's model.

Effects of solute concentration–dependent surface tension on unsaturated flow: Laboratory sand column experiments

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.

Aquifer washing by micellar solutions:: 3 Field test at the Thouin Sand Pit (L'Assomption, Québec, Canada)

A field test was performed to recover DNAPL (Dense Non Aqueous Phase Liquid) in a shallow aquifer at the Thouin Sand Pit near Montreal, to evaluate a new technique of aquifer restoration involving surfactant solutions. Laboratory tests have shown that washing solutions containing alcohols, surfactants, and solvents are very efficient in recovering DNAPL as a miscible phase. The Thouin field test was designed to: (1) study in situ recovery of DNAPL; (2) evaluate an injection–pumping strategy; (3) test the use of polymer solutions to control the mobility of a washing solution slug and to improve the vertical sweep efficiency throughout the sand unit. The test was performed in a shallow medium sand aquifer containing both contaminated saturated and unsaturated zones. The washing experiment was done on 17 m 3 of the saturated zone with an average DNAPL initial concentration of 55 000 mg kg −1 dry soil. Solutions were injected through a central well and pumped into four wells arranged in a five-point square pattern. In the zone swept by the washing solution, 86% of residual DNAPL was recovered using only 0.9 pore volume of a surfactant solution. These results confirm laboratory sand column experiments although the washing solution used in the field experiment had not yet been optimized to meet site-specific criteria. The use of a polymer solution before and after injection of the washing solution proved beneficial in insuring that the washing solution effectively swept all the sand layers in spite of soil heterogeneities. As the rinsing cycles were not completed because of weather problems (freezing conditions), small but significant amounts of the washing solution ingredients were still present in the aquifer at the end of the test and their fate is being monitored. For this field test, DNAPL recovery could have been better if an optimal washing solution had been injected, a larger volume of the selected solution had been circulated, or a different injection–pumping strategy had been chosen. ©1997 Elsevier Science B.V.

The Role of Colloidal Kaolinite in the Transport of Cesium through Laboratory Sand Columns

Recent laboratory and field studies indicate that colloids are capable of adsorbing and enhancing the transport of a variety of contaminants in groundwater. We examined the influence of kaolinite particles on Cs movement through sand columns. The times for transport of the main part of the 137Cs pulse decreased by over a factor of 2 as the concentration of kaolinite in the column influent increased from 0 to 200 mg/L. Very early breakthrough of a relatively small amount of 137Cs was observed and attributed to the existence of sorption sites on the kaolinite that were kinetically controlled. This early breakthrough was essentially unretarded, whereas the initial breakthrough in the absence of kaolinite showed a retardation of about 15 pore volumes. Comparison of experimental observations with calculations from a transport model demonstrates that kaolinite deposition onto the sand was described best by an irreversible, first‐order kinetics process and that retention of 137Cs by these clay particles was described by linear equations for two‐site solute adsorption. Uptake of 137Cs by the sand was represented by the Langmuir version of the two‐site equations. This work indicates the need to account for facilitated transport of contaminants by inorganic colloids and provides a basis for quantifying the phenomenon.

Use of injected helium as a hydrological tracer

Abstract Helium has several characteristics which make it attractive for use as a tracer in hydrological studies. These include its inert nature, relatively high solubility in water (~ 1 %), low molecular diffusion in water, ready availability in commercial quantities, nontoxic nature, and low background atmospheric concentration. The use of helium as a tracer of water movement has become possible through the development of an instrument which takes advantage of the fact that at room temperature helium diffuses through a quartz glass membrane at a rate of three to four orders of magnitude greater than any other gas. This paper describes: (i) a set of experiments undertaken to compare breakthrough of helium with common salt (NaCl) tracer through laboratory sand columns; (ii) a set of groundwater tracing experiments conducted in a basaltic aquifer in Central O'ahur, Hawaii and (iii) two laboratory experiments undertaken to evaluate the applicability of helium instrumentation for the tracing of submerged plumes in open water conditions. The test results demonstrate that helium behaves as a conservative tracer during saturated flow through porous media. During unsaturated flow, exchange of helium with air entrained in the porous media reduces its usefulness. During submerged flow of a labelled plume, helium behaves like fluorescein in a relatively tranquil laboratory tank environment for hours but is gradually lost through the air-water interface, thus limiting the usefulness of helium in shallow plume tracing studies to short duration experiments.

Ground‐Water Tracing with Injected Helium

AbstractHelium has several characteristics that make it attractive for use as a tracer in hydrological studies. Two types of experiments were conducted to investigate applicability of helium as a tracer of ground‐water movement. The experiments included studies using laboratory sand and soil columns and field ground‐water tracing in a basaltic aquifer. A water helium analyzer comprised of a thin quartz glass membrane and diode ion pump (making use of the preferential permeation of helium through the quartz glass into an evacuated space) was developed and used for the experiments.Results of our studies demonstrated that breakthrough curves of specific conductance and helium were similar under saturated conditions. In the unsaturated sand/soil columns, breakthrough curves of helium were retarded relative to specific conductance reducing the usefulness of helium as a tracer.

Colloid transport through fractured and unfractured laboratory sand columns

We examined the effects of artificial fractures on colloidal transport in laboratory sand columns. Fractures were simulated by inserting small (0.2- or 1-mm-diam) tubes of controlled length and density into a sand column with dimensions 5-cm-diam by 65-cm-length. We injected 1-μm-diam fluorescent microspheres, bacteria, and colloidal organic matter into columns with tubes and columns without tubes. Breakthrough curves were analyzed using an equilibrium transport model to obtain values for percent retention, retardation, and dispersivity. Fractures significantly increased the amount of microsphere transport; 80–99% of the spheres were retained in experiments with tubes inserted compared to 99–100% without tubes. Bacteria used in this particular experiment had a lower percent retention (increased transport), 73% with tubes and 93% retention without tubes. Colloidal organic matter had 34% retention with tubes and 55% retention without tubes. Microspheres travelled faster than a conservative salt tracer (retardation factor < 1), probably caused by exclusion of travel at slower velocities in the matrix. This effect was enhanced when tubes were present. In some experiments, the tubes created multiple flow paths manifest in irregular rather than smooth breakthrough curves. There was no measurable difference in dispersion for the “fractured” versus the “unfractured” system The retention coefficient was determined by sectioning the column at the end of the experiment and counting the number of microspheres retained in sections along the column. The exponential retention rate for spheres was on the order of 0.1 cm −1, with no measurable variation between sand columns with and without tubes. This retention coefficient is needed for models to predict colloid transport. In addition to providing parameter values for models, these observations have implications for monitoring colloid transport in natural systems. It is necessary to consider the possibility of travel faster than a conservative tracer, irregular breakthrough curves, and irreversible retention when sampling for colloids.

Denitrification in laboratory sand columns: Carbon regime, gas accumulation and hydraulic properties

Microbiological denitrification in a sandy matrix was studied by means of laboratory sand columns operated at continuous and pulse feed regimes. Gas production resulting from the biological activity played a major role in modifying the hydraulic properties of the column, leading to decreases in hydraulic conductivity and porosity, higher water velocities through the column, higher dispersion and anomalies in the head difference to flow rates ratios. All of these effects were more pronounced when formate, the carbon source used, was supplied continuously: microbial activity and gas production were concentrated at the top of the column, leading to almost complete clogging. When the formate was supplied in pulses, activity and gas production were dispersed, leading to relative uniformity in the physical parameters measured and a homogeneous appearance of the column. The results suggest that in a future in situ aquifer denitrification plant, pulse application of the carbon source is prefereable to a continuous supply regime.

Interactions among nitrogen-transforming bacteria and nitrogen-fixingPisum sativum L. in laboratory sand columns

An open laboratory sand column system was developed to study interactions between N2-fixing pea,Pisum sativum L. and the nitrifying organisms,Nitrosomonas europaea andNitrobacter winogradskyi or the denitrifier,Pseudomonas fluorescens in the presence of NH4-ion or NO3-ion, respectively. In columns watered continuously with 1 mM (NH4)2SO4, nitrification began immediately after inoculation and NO3-ion production was not inhibited by the plants. Above ambient levels of nitrous oxide were detected in all columns containing nitrifiers. In the presence of plants, nitrifiers were evenly distributed throughout the column; in their absence, they were found predominantly within the top 8 cm of the column. When15N-enriched (NH4)2SO4 was added to columns containing nitrifiers and plants,15N-enriched NO3-ion was detected in the xylem sap. Nitrogenase activity (as indicated by acetylene reduction rates and atom %15N of shoot N) was inhibited more in the presence of nitrifiers and NH4-ion than with NH4-ion alone. In columns continuously watered with 2 mM KNO3 and containing a denitrifier, low levels of denitrification were detected in columns maintained at 21% O2 or at 5% O2. Nitrate uptake was inhibited from days 26 to 31 in peas grown at 5% O2. Denitrification may have been limited by available carbon in the root exudates or by competition with roots for NO3-ion and rates were insufficient to counteract the inhibitory effects of NO3-ion on symbiotic nitrogen fixation.

Nonaqueous phase liquid transport and cleanup: 2. Experimental studies

An analysis of the movement of nonaqueous liquids such as organic solvents and gasoline in part 1 (Hunt et al., this issue) showed that separate phase liquids are not completely displaced by groundwater flow under typical pumping conditions and that removal of the compounds by dissolution is mass transfer limited. Steam displacement was proposed as a more efficient cleanup strategy for separate phase organic liquids present in porous media. This paper presents the results of a series of laboratory sand column experiments designed to confirm the theoretical analyses presented in part 1 (Hunt et al., this issue). Experiments with trichloroethylene, a benzene‐toluene mixture, and a commercial gasoline, showed that water flow at rates as high as 15 m d−1 could not displace the separate phase liquids when present in a sand matrix in quantities corresponding to a column average saturation of 2.5%. Steam injection, on the other hand, displaced the contaminants as a separate phase just ahead of the steam front, producing a concentrated, small‐volume waste stream. Analysis of the laboratory data consisting of pressure gradients, temperature profiles, and water flow velocities shows that the laboratory results are consistent with the theoretical predictions. Moreover, computations of energy requirements show that steam displacement of separate phase contaminants is economically attractive.

Do sandy beaches accumulate nitrogen?

Do sandy beaches accumulate nitrogen?

Denitrification in Laboratory Sand Columns

ABSTRACT A high percentage of nitrate was re-moved using methanol as a substrate. This was done at temperatures as low as 13 C and at pore velocities as high as 16.4 cm per day.

Solutions for Miscible Displacement of Soil Water with Time‐Dependent Velocity and Dispersion Coefficients

AbstractMiscible displacement processes with time‐varying velocity and dispersion coefficients are examined. Simplified solutions utilizing arbitrary initial conditions are presented and used to simulate step and slug inputs of solutes into soil during infiltration of water. The solutions are used in the analysis of experimental data, both for field infiltration with a slug of solute and for psychrometric measurements of salt fronts in a laboratory sand column. Experimental data obtained for solute movement was more accurately described using time‐increasing dispersion relationships than for constant values.

Oxygen Relationships in Intermittent Sand Filtration

A model for processes in intermittent sand filtration is described and an expression for oxygen transfer is formulated. The model assumes that aerobic bacterial activity within the sand or soil matrix is limited by oxygen deficiency while the surface is ponded. Atmospheric oxygen reenters into the soil after infiltration ends. Aerobic activity is resumed, but oxygen penetration is limited and some depths may be always anaerobic. This condition leads to large variations in the concentration of certain contaminants in the percolate. Analyses of soil moisture in field studies and of effluent from laboratory sand columns substantiated the model. The oxygen content at sufficiently long times after addition of wastes can be described by a quasi steady-state diffusion equation including a term for an oxygen sink. Measurements of oxygen content during laboratory and field studies show that the oxygen profile changes only slightly up to two days after the quasi-steady state is attained. Results can be applied in operating existing facilities and in interpreting data from pilot-plant studies.

The Conduction of Heat Incident to the Flow of Vaporizing Fluids in Porous Media

Published in Petroleum Transactions, AIME, Volume 204, 1955, pages 282–284. Introduction and Purpose Problems relating to thermal methods of oil recovery have been given increasing attention during the past year. The nature of the physical and chemical processes underlying thermal recovery are not yet well understood. The need for pertinent basic information is recognized generally. One of the processes involved is the transfer of heat by conduction in oil reservoir rocks containing moving reservoir fluids. This technical note deals with heat conduction in a laboratory sand column filled with a moving hydrocarbon and therefore should be contributory to the presently inadequate fund of knowledge on the over-all subject. In an analysis of the flow of vaporizing propane through a horizontal column of sand, one of the present authors demonstrated correspondence between the measured values of pressure along the length of the column and those predicted by a theory which postulated that the enthalpy of the propane was invariable along the length of the column. This postulate was implied as a deduction from the principle of conservation of energy but the details of the deduction were not given. The specific purpose of this technical note is the presentation of these details both as an adjunct to the earlier analysis and to supply some additional details on the processes involved in this type of flow. Results disclose that rates of heat transfer by conduction, in the direction of fluid motion, generally would be small for systems of the type studied. Fundamental Energy Equation The principle of conservation of energy may be applied to a section of the porous material on the basis that heat transfer, like fluid flow, is possible in the x direction only. Transfer of heat in the y and z directions is prevented by suitable insulation or by taking the medium as infinite in directions perpendicular to the x axis. For steady state heat conduction and conditions of steady fluid flow the principle of conservation of energy specifies that any reduction in the enthalpy rate increases the flow of heat in the same proportion.