Unsaturated Column Experiments Research Articles

Role of nonspherical DLVO and capillary forces in the transport of 2D delaminated Ti3C2Tx MXene in saturated and unsaturated porous media

The transport and retention of two-dimensional (2D) nanomaterials, such as graphene oxide, in porous media have attracted lots of attention. However, previous studies often simplified these 2D colloids as equivalent spheres for numerical simulations, which ignored the influence of particle shape on colloid retention at multiple interfaces. In this study, a novel 2D nanomaterial delaminated Ti3C2Tx (d-Ti3C2Tx) was adopted to fill this knowledge gap. Comprehensive analyses of the 2D colloid retention mechanisms were conducted based on colloid characterization, saturated and unsaturated column experiments, reactive transport modeling, 2D-based DLVO and nonspherical capillary energy simulations. Results show that d-Ti3C2Tx mobility in both saturated and unsaturated conditions enhanced with the increase in pH and decrease in ionic strength. The DLVO interaction energy of d-Ti3C2Tx at the sand-water-interface (SWI) decreased with the orientation angle of the colloidal major axis to the sand surface from 0° to 90°. The primary mechanism under saturated flow conditions was the irreversible attachment in the deep secondary minimum at the SWI with the major axis of d-Ti3C2Tx parallel to the sand surface. The attachment in the primary minimum at 0° was impossible due to the extremely high energy barrier, and the attachment in the primary and secondary minimum at other orientation angles were negligible. d-Ti3C2Tx only experienced repulsive electrostatic force when approaching the air-water-interface (AWI) no matter the particle orientation. The detaching capillary potential energy was 3 orders of magnitude larger than the attractive DLVO interaction energy of the SWI in the secondary minimum at 0°, suggesting that the capillary force-induced irreversible attachment at the AWI was the primary mechanism under unsaturated flow conditions. This study shows that the DLVO and capillary potential energies were significantly dependent on the particle-interface orientation and colloidal shape. A simplification of 2D colloids as spheres is not recommended.

Impact of declining oxygen conditions on metal(loid) release from partially oxidized waste rock

The best available technology for preventing the formation of acid drainage water from the sulfidic waste rock at mine closure aims to limit the oxygen access to the waste. There is, however, a concern that contaminants associated with secondary minerals become remobilized due to changing environmental conditions. Metal(loid) mobility from partially oxidized sulfidic waste rock under declining and limited oxygen conditions was studied in unsaturated column experiments. The concentrations of sulfate and metal(loid)s peaked coincidently with declining oxygen conditions from 100 to < 5 sat-% and to a lesser extent following a further decrease in the oxygen level during the experiment. However, the peak concentrations only lasted for a short time and were lower or in the similar concentration range as in the leachate from a reference column leached under atmospheric conditions. Despite the acid pH (~ 3), the overall quality of the leachate formed under limited oxygen conditions clearly improved compared with atmospheric conditions. In particular, the release of As was two orders of magnitude lower, while cationic metals such as Fe, Cu, Mn, and Zn also decreased, although to a lesser extent. Decreased sulfide oxidation is considered the primary reason for the improved water quality under limited oxygen conditions. Another reason may be the immobility of Fe with the incorporation of metal(loid)s in Fe(III) minerals, in contrast to the expected mobilization of Fe. The peaking metal(loid) concentrations are probably due to remobilization from solid Fe(III)-sulfate phases, while the relatively high concentrations of Al, Mn, and Zn under limited oxygen conditions were due to release from the adsorbed/exchangeable fraction. Despite the peaking metal(loid) concentrations during declining oxygen conditions, it is clear that the primary remediation goal is to prevent further sulfide oxidation.

Recognition of the anthropogenic contribution to the input of fluoride in urban recharge

Fluoride is a common inorganic contaminant of geogenic origin around the world. Anthropogenic sources of fluoride could also contribute to increased fluoride concentrations in groundwater, although they have not been well-documented. This paper presents soil batches and unsaturated column experiments on fluoride-enriched recharge water percolating from a urine-polluted soil in an area where fluorosis is endemic. Results from the polluted soil collected near a main street in Addis Ababa (Ethiopia) are compared with an unpolluted soil collected in a garden inside the university campus, located 50 m from the polluted one. Soils were sampled at the end of the dry season and after the first intense rainfall at the beginning of the wet season. The objectives of the study were to evaluate the anthropogenic fluoride enrichment in urban recharge due to unmanaged human excretions along the streets and model recharge water fluxes and solutes leaching to evaluate the reactivity of dissolved species. Batch experiments found extremely high fluoride (42.1 ± 5.2 mg/kg) and urea (89.5 ± 8.1 mg/kg) concentrations in polluted soil before the rainy season. The fluoride concentration in the polluted soil was more than one order of magnitude higher than that recorded in the uncontaminated soil. This study proved that in areas where the population is subject to a large fluoride daily intake, recharge waters extremely enriched in fluoride can be leached from urine-contaminated soils in urban environments. The proposed methodology could be applied for a first screening for the anthropogenic sources of fluoride in urban recharge.

Investigating natural attenuation of pharmaceuticals through unsaturated column tests

The growing consumption of pharmaceuticals together with their incomplete removal in wastewater treatment plants (WWTPs) implies the occurrence of these compounds in natural water resources. To investigate the natural attenuation of selected pharmaceuticals (caffeine, acetaminophen, sulfamethoxazole, naproxen and carbamazepine) during vadose zone infiltration, unsaturated column (L 26.67 cm, Ø 7.62 cm) experiments, filled with a sandy-loamy soil, were performed using two input concentrations (100 and 1000 μg L−1). The software Hydrus 1D was used to simulate experimental data. Caffeine and acetaminophen were never detected at the column outlet indicating a low environmental concern. On the other hand, attenuation of the detected pharmaceuticals could be reproduced by a combination of retardation and removal approaches. Carbamazepine is among the selected contaminants the most persistent. A dependence of removal rates on input concentrations was detected for sulfamethoxazole (μw from 2.78 d−1 to 1.16 d−1) and naproxen (μw from 1.16 d−1 to 0.63 d−1) attributed mainly to decreased metabolism of microorganisms when a higher input concentration is applied. Two transformation products (TPs) (N4-Acetylsulfamethoxazole and epoxycarbamazepine) derived from sulfamethoxazole and carbamazepine transformation, respectively, were detected during the experiment with the highest input concentration.

Experimental and Numerical Investigations of Silver Nanoparticle Transport under Variable Flow and Ionic Strength in Soil.

Unsaturated column experiments were conducted with an undisturbed loamy sand soil to investigate the influence of flow interruption (FI) and ionic strength (IS) on the transport and retention of surfactant-stabilized silver nanoparticles (AgNP) and the results were compared to those obtained under continuous flow conditions. AgNP concentrations for breakthrough curves (BTCs) and retention profiles (RPs) were analyzed by ICP-MS. Experimental results were simulated by the numerical code HP1 (Hydrus-PhreeqC) with the DLVO theory, extended colloid filtration theory and colloid release model. BTCs of AgNP showed a dramatic drop after FI compared to continuous flow conditions. Evaporation increased due to FI, resulting in increased electrical conductivity of the soil solution, which led to a totally reduced mobility of AgNP. A reduction of IS after FI enhanced AgNP mobility slightly. Here the strongly increased Al and Fe concentration in the effluent suggested that soil colloids facilitated the release of AgNP (cotransport). The numerical model reproduced the measured AgNP BTCs and indicated that attachment to the air-water interface (AWI) occurring during FI was the key process for AgNP retention.

Accumulation of pharmaceuticals in groundwater under arid climate conditions – Results from unsaturated column experiments

Intense reuse of treated wastewater in agriculture is practiced all over the world, especially in arid and water-scarce regions. In doing so, pharmaceutical residues in the water are irrigated to the soil and subsequently can percolate into the local aquifers. Since evaporation rates in these areas are typically high, persistent substances might enrich in the groundwater recharge of closed catchments like the Jordan Valley. Against this background, unsaturated column tests were conducted to investigate the potential for evaporative accumulation of the two pharmaceuticals bezafibrate and carbamazepine under simulated arid climate conditions. Parallel tests were conducted with inhibited microbiological activity where both substances showed an increase in the effluent concentrations proportional to the evaporation loss of the inflow solution. The mean accumulation factors of the pharmaceuticals correspond to the evaporated water loss. The experiments indicate the accumulation potential for pharmaceuticals with high persistence against biodegradation. For the first time, the overall potential for evaporative enrichment could be demonstrated for pharmaceuticals. Under the given experimental conditions, the two investigated pharmaceuticals did not enrich faster than chloride, which might result in soil salting prior to reaching harmful pharmaceutical concentrations in soil water. The findings are relevant to future assessments of environmental impacts of persistent trace substances, which need to take into account that concentrations in the aquatic cycle might increase further due to evaporative enrichment.

Effect of water content on strontium retardation factor and distribution coefficient in Chinese loess

Geological burial and landfill are often employed for disposal of nuclear wastes. Typically, radionuclides from nuclear facilities transport through the unsaturated zone before reaching the groundwater aquifer. However, transport studies are often conducted under saturated and steady-state flow conditions. This research aimed to examine the effects of unsaturated flow conditions and soil water content (θ) on Sr sorption and retardation in Chinese loess through 1D column transport experiments. Reagent SrCl2 was used as a surrogate for the radioactive isotope (90Sr) in the experiment because of their analogous adsorption and transportation characteristics. The spatial distribution of Sr along the column length was determined by segmenting the soil bed and analysing the Sr content in each soil segment following each column breakthrough test. The single-region (SR) and two-region (TR) models were employed to interpret the transport data of Sr as well as a tracer (Br−), which resulted in the dispersion coefficient (D) and retardation factor (Rd) under a given set of unsaturated flow conditions. For the tracer, the SR and TR models offered nearly the same goodness of fitting to the breakthrough curves (R2 ≈ 0.97 for both models). For the highly sorptive Sr, however, the TR model provided better fitting (R2, 0.80–0.96) to the Sr retention profiles than the SR model (R2, 0.20–0.89). The Sr retention curves exhibited physical non-equilibrium characteristics, particularly at lower water content of the soil. For the unsaturated soil, D and the pore water velocity (v) displayed a weak linear correlation, which is attributed to the altering dispersivity as the water content varies. A much improved linear correlation was observed between D and v/θ. The retardation factor of Sr increased from 69.1 to 174.2 as θ decreased from 0.46 to 0.26 (cm3 cm−3), while the distribution coefficient (Kd) based on Rd remained nearly unchanged at various θ levels. These results illustrated that water content must be taken into account in determining radionuclide Rd values in Chinese loess, while Kd values can be derived from the unsaturated column experiments and can be considered constant at various levels of θ.

Quantification of Colloid Retention and Release by Straining and Energy Minima in Variably Saturated Porous Media

The prediction of colloid transport in unsaturated porous media in the presence of large energy barrier is hampered by scant information of the proportional retention by straining and attractive interactions at surface energy minima. This study aims to fill this gap by performing saturated and unsaturated column experiments in which colloid pulses were added at various ionic strengths (ISs) from 0.1 to 50 mM. Subsequent flushing with deionized water released colloids held at the secondary minimum. Next, destruction of the column freed colloids held by straining. Colloids not recovered at the end of the experiment were quantified as retained at the primary minimum. Results showed that net colloid retention increased with IS and was independent of saturation degree under identical IS and Darcian velocity. Attachment rates were greater in unsaturated columns, despite an over 3-fold increase in pore water velocity relative to saturated columns, because additional retention at the readily available air-associated interfaces (e.g., the air-water-solid [AWS] interfaces) is highly efficient. Complementary visual data showed heavy retention at the AWS interfaces. Retention by secondary minima ranged between 8% and 46% as IS increased, and was greater for saturated conditions. Straining accounted for an average of 57% of the retained colloids with insignificant differences among the treatments. Finally, retention by primary minima ranged between 14% and 35% with increasing IS, and was greater for unsaturated conditions due to capillary pinning.

LNAPL Removal from Unsaturated Porous Media Using Surfactant Infiltration

A series of unsaturated column experiments was conducted to evaluate light nonaqueous phase liquid (LNAPL) removal during surfactant solution infiltration. Surfactant‐LNAPL phase behavior tests were conducted to optimize the remedial solutions. Packed sand and site sediment columns were first processed to establish representative LNAPL smear zones under unsaturated conditions. Infiltration of low‐concentration surfactant was then applied in a stepwise flush mode, with 0.33 column pore volume (PV) of solution in each flush. The influence of infiltrated surfactant solution volume and pH on LNAPL removal was assessed. A LNAPL bank was observed at the front of the first surfactant infiltration in each column, indicating that a very low surfactant concentration is needed to reduce the LNAPL‐water interfacial tension sufficiently to mobilize trapped LNAPL under unsaturated conditions. More LNAPL was recovered as additional steps of surfactant infiltration were applied. Up to 99% LNAPL was removed after six infiltration steps, with approximately 2.0 PV of total surfactant solution application, suggesting surfactant infiltration may be an effective method for vadose zone LNAPL remediation. The influence of pH tested in this study (3.99∼10.85) was insignificant because the buffering capacity of the sediment kept the pH in the column higher than the zero point charge (pHzp) of the sediment and therefore surfactant sorption was negligible.

Thermo-hydro-mechanical behaviour of sodium silicate-cemented paste tailings in column experiments

The coupled evolution of the main performance properties (thermal, hydraulic, mechanical) of underground cemented paste tailings that contain sodium silicate as chemical admixture are studied by means of unsaturated column experiments. Two columns have been built to simulate the thermo-hydro-mechanical (THM) behaviour of the aforementioned cemented paste tailings (called gelfill, GF) under drained and undrained conditions. The GF columns are cured at room temperature for 28days. The suction and temperature developments during the hydration process are monitored. When the curing phase has been completed, the cylindrical samples are taken out from the columns to evaluate the thermal, hydraulic and mechanical properties of GF. Uniaxial compressive strength (UCS), saturated hydraulic conductivity, and thermal conductivity tests are conducted. The results show that the mechanical, hydraulic and thermal properties of GF columns are coupled. The UCS values of GF are strongly coupled to heat development, saturation and suction development within the GF. In addition, the UCS outcomes, saturated hydraulic conductivity, and thermal conductivity tests for samples obtained from the GF columns show higher values than small samples. This study has demonstrated that the coupled THM effect on GFs is an important factor for consideration in the designing of cost-effective, safe and durable GF structures. The results presented in this paper will contribute to a better understanding of the THM behaviour of underground GF structures.

Multiphase flow and reactive transport model in vadose tailings

Weathering processes affecting pyritic wastes may generate huge amounts of acid waters with high concentrations of potentially toxic contaminants (acid mine drainage). Acid mine drainage is mainly produced in the vadose zone. In the present study, a coupled non-isothermal multiphase flow and reactive transport model of the vadose zone of sulfide mine tailings was developed. The geochemical model included kinetically controlled reactions for Fe(II)-oxidation and for the dissolution of sulfide and aluminosilicate phases and the Pitzer ion-interaction model to describe the behavior of the pore-water solutions. Model results were compared with experimental observations in unsaturated column experiments under strongly evaporative conditions similar to arid or semiarid climates. Evolution trends for temperature, water saturation, evaporation rates, pore-water hydrochemistry and mineral phases observed during the drying experiment were adequately reproduced. The coupled model reproduced the increase of solute concentrations in the column top and the precipitation of a crust of secondary mineral phases. This crust became a barrier for water vapour diffusion to the atmosphere and modified the thermohydraulic behavior of the tailings. Enhanced downward migration of water vapour and its condensation in this colder end of the column were correctly taken into account by the model, which reproduced the dilution observed in the lower part of the column during the experiments.

Advective Desorption of Uranium(VI) from Contaminated Hanford Vadose Zone Sediments under Saturated and Unsaturated Conditions

Sedimentary, hydrologic, and geochemical variations in the Hanford subsurface environment, as well as compositional differences in contaminating waste streams, have created vast differences in the migration and mobility of U within the subsurface environment. A series of hydraulically saturated and unsaturated column experiments were performed to (i) assess the effect of water content on the advective desorption and migration of U from contaminated sediments and (ii) evaluate the U concentration that can develop in pore water and/or groundwater as a result of desorption/dissolution reactions. Flow rate and moisture content were varied to evaluate the influence of contact time, pore water velocity, and macropore desaturation on aqueous U concentrations. Sediments were collected from the T‐TX‐TY tank farm complex and the 300 Area Process Ponds located on the Hanford Site, southeastern Washington State. The sediments vary in depth, mineralogy, and in contamination events. Experiments were conducted under mildly alkaline/calcareous conditions representative of conditions commonly encountered at repository sites across the arid western United States and, in particular, the Hanford site. Results illustrate the release of U from these sediments is kinetically controlled, and low water contents encountered within the Hanford vadose zone result in the formation of mobile–immobile water regimes, which isolate a fraction of the reactive sites within the sediments, effectively reducing the concentration of U released into migrating pore waters.

Coupled thermal, hydraulic and geochemical evolution of pyritic tailings in unsaturated column experiments

The evolution of pore-water and the composition of solid phases in the vadose zone of pyritic tailings was studied by means of unsaturated column experiments. Several columns of water-saturated mine tailings were dried during 125 days under controlled laboratory conditions. The columns were dismantled at four successive drying stages and the evolution of pore-water, mineralogy, water content and temperature was characterized. Sulfide and aluminosilicate minerals present in the waste dissolved, releasing sulfate and other solutes (mainly Fe, Zn, Cu, Al, Mg and Ca) to the pore-water. Evaporation caused a crust of efflorescent, water-soluble sulfates to develop over the complete top surface of the columns and into the pores of the underlying waste material. This crust, which has also been identified in the field, changed the hydraulic properties of the tailings and produced a decrease in the evaporation rate of the columns. Moreover, these water-soluble precipitates (mainly rozenite, szomolnokite, halotrichite, hexahydrite, mirabilite and gypsum) acted as temporary sinks for Cd, Pb, Co and Ni, which could be released to the surface run-off or the groundwaters during rainfall events under field conditions. Pore-water evolution was determined not only by geochemical processes (dissolution of sulfides and aluminosilicates, precipitation of secondary phases) but also by thermal and hydraulic processes. Progressive dilution was observed in the lower part of the columns. Dilution was caused by the thermally driven vapor flux from the top of the column to its colder bottom and subsequent condensation therein. This process, which may also occur in tailings under sub-arid climate, played a key role on the evolution of pore-water with increasing drying.

Rate controls on the chemical weathering of natural polymineralic material. I. Dissolution behaviour of polymineralic assemblages determined using batch and unsaturated column experiments

Chemical weathering rates for a mudstone obtained from a mining environment were investigated using a combination of batch reactors and hydrologically unsaturated column experiments. Results of tracer tests were combined with relationships between solute concentrations, mass fluxes, flow rates and residence times, and used to calculate element release rates and infer rate-controlling mechanisms for the two different experimental environments. Elements that eluted from column experiments exhibited either: (1) concentrations independent of flow rate and column length coupled with mass flux increasing with flow rate, or (2) an inverse relationship between concentration and flow rate coupled with mass flux increasing with both column length and flow rate. The former is attributed to equilibrium-controlled release of particular elements (Si, Al), while the latter is ascribed to transport-controlled release of others (Mg, Mn, Ca, Na, K, S). Tracer tests using NaBr solutions revealed that some elements were also affected by ion exchange (Mg, Mn, Ca, Na), but these effects were temporary and did not mask underlying dissolution rate-controlling mechanisms. Analysis of characteristic diffusive lengths was used to distinguish between transport rates limited by the transfer of solutes between immobile and mobile water within the columns, and rates limited by slow diffusion across partially reacted mineral layers. The analysis suggests that transfer between immobile and mobile water limited the element release rates, with diffusion hindered by low diffusion coefficients within the unsaturated medium, and by low interfacial areas between mobile and immobile fluids. Results of the batch experiments showed different characteristics. Element concentrations either rose to a plateau or increased linearly with time. Rate-controlling mechanisms associated with these characteristics were equilibrium (Fe) and surface kinetic reactions (Si, Mg, Mn, Ca, K, S), respectively. Surface area-normalized element release rates for Mg, Mn, Ca and S are consistently a factor of 4 higher than those from the column experiments. This is a significant difference and cannot be attributed to differences in mineral preparation or to factors influencing individual minerals. It must, therefore, reflect the difference in the rate-controlling mechanism, that is, transport vs. surface-kinetic control. These results suggest that some proportion of the commonly recorded discrepancy between laboratory and field weathering rates is due to hydrological differences between the two environments, and that hydrological characterization of weathering environments is, therefore, as important as physical–geochemical characterization of reacting solids. An important practical implication of the work is that substantial reservoirs of solutes are held in immobile water in the unsaturated environment, and that these could be released by soil disturbance or flooding.

Retention and Transport of Amphiphilic Colloids under Unsaturated Flow Conditions: Effect of Particle Size and Surface Property

The purpose of this study was to examine the mechanisms responsible for deposition and transport of amphiphilic colloids with a wide range of particle sizes (20-420 nm) through porous media. A series of saturated and unsaturated column experiments were conducted using amphiphilic latex microspheres and a hydrophilic silica colloid. We found that the amphiphilic latex particles were retained to a greater extentthan the hydrophilic silica colloid in unsaturated media. This was attributed to colloidal attachment atthe air-water interface due mainly to hydrophobic interactions. We also found that dependence of colloid retention on particle size was nonlinear. There existed a fraction of colloids with greater mobility than other fractions, which we referred to as the most mobile colloids. As particle size increased from 20 to 420 nm, colloid deposition rate first decreased to reach a minimum value at --100 nm then increased, indicating that different retention mechanisms were involved. We showed that conducting saturated transport experiments and analysis using filtration theory may be an effective approach for determining the most mobile colloid size(s) in porous media, perhaps even for unsaturated flow conditions. This study highlights the importance of including size effect and surface properties when predicting concentrations and fluxes of amphiphilic colloids or colloid-bound amphiphilic and hydrophobic contaminants in the subsurface environment.

Physicochemical factors controlling the release of dissolved organic carbon from columns of forest subsoils

SummaryRetention of dissolved organic carbon in soil depends on the chemical and physical environment. We studied the release of organic carbon from three carbonate‐free forest subsoil materials (Bs1, Bs2, Bg) in unsaturated column experiments as influenced by (i) variations of the flow regime and (ii) varied chemical properties of the irrigation solution. We investigated the effect of flow initiation, constant irrigation, interruptions to flow, and variation in the effective pore water velocity on the release of organic C. The influence of ionic strength and cation valence in the irrigation solution was studied by stepped pulses of NaCl and CaCl2. The release of C from all materials was characterized by an initial large output and a decline to constant concentrations under long‐term irrigation. Interrupting the flow increased its release when flow was resumed. The release from the Bs1 material was not related to the duration of the interruption. The Bs2 material, in contrast, released organic carbon in a way that was successfully described by a kinetic first‐order model. Increased pore water velocity decreased the concentrations of C in the effluent from it. The pH of the irrigation solution had negligible effects on the mobilization of C. Increased ionic strength reduced the release, whereas rinsing with distilled water increased the concentrations of C in the effluent. The response of dissolved C to pulses of weak solutions, however, was sensitive to the type of cation in the previous step with strong solutions. The results suggest that the release of organic matter in the soils depends on its colloidal properties.

Radionuclide Uptake and Transport on Microbes in Potential Repository Drifts at Yucca Mountain, Nevada

ABSTRACTRadionuclide adsorption onto microbes, microbial retention in the engineered barrier system (EBS), and their potential release from the EBS as microbial colloids have been investigated. The microbial source term for these calculations was derived using MING V 1.0 software code [1]. Multiple model calculations from MING representing variations in possible microbial communities in the EBS were abstracted into two equations representing one meter segments of potential repository drift containing either commercial spent nuclear fuel (CSNF) or defense high level waste (HLW) packages. These two equations (Equations 1 and 2) represent the average cumulative microbial biomass generated in the EBS at any given time. A distribution for uranium uptake onto microbes (162.88 ± 133.05 mg U/gm dry cell) was applied to the microbial source term. The distribution was derived from the data set in Suzuki and Banfield [2] representing 45 different species of bacteria and fungus, covering uranium uptake at optimum pH values of 1 to 7. The mass of uranium sorbed onto the biomass was either sequestered in the EBS or transported as a microbial colloid based on a regression of data from Jewett et al. [3] representing microbial sorption onto air-water interfaces in unsaturated column experiments. Over one million years, it is estimated that EBS microbes may adsorb from 77 to 2302 kg of uranium [2302 kg U &gt; 100% of the uranium available in a one meter segment of a CSNF waste package] per meter of waste package depending on the saturation of the invert and type of waste package. Over the same time, microbial colloids may transport from 8 to 1250 kg of adsorbed uranium per meter of waste package from the EBS.

Laboratory and numerical modelling studies of iron release from a spoil heap in County Durham

Abstract At present there is no suitable method to predict either the longevity of contaminant sources within spoil heaps, or the evolution of their strength over the contaminating life time of the sites. Existing acid-base accounting techniques provide little information relevant to the prediction of field contaminant concentrations and time scales. For robust prediction, the relative rates of contaminant generation and attenuation must be evaluated then extrapolated to the physical scale and environmental conditions of real field sites. Laboratory unsaturated column experiments on colliery spoil from a well-documented site in County Durham have been set up to assess its contamination potential, and the results compared with results from a mathematical model for contaminant release and transport. The random walk method, a form of particle tracking, is used to transport iron and sulphate ‘particles’, released by oxidative weathering of pyrite minerals. The model also includes the oxidation of ferrous iron to ferric iron in an attempt to account for contaminant ‘sinks’, for example where ferric iron spontaneously precipitates as ferric oxyhydroxide and is effectively removed from the transport process. In general, the modelled results compare favourably with the laboratory results and any discrepancies can be accounted for.

The Remediation of Tetrachloroethylene by Surfactant Flushing in Both the Saturated and the Unsaturated Zone

Column and box experiments were performed to evaluate the efficiency of surfactant flushing for remediation of non-aqueous phase liquid (NAPL) in water saturated and unsaturated sands under controlled conditions. Tetrachloroethylene (PCE) was selected as the NAPL and a 1 % solution of sorbitan monooleate as the surfactant to be studied. Four column experiments using Ottawa sand and sand from the aquifer at Canadian Forces Base Borden were performed by flushing a 1 % sorbitan monooleate solution through the columns under saturated and unsaturated conditions, and their results were compared to the results of column experiments that flushed water only. Two-dimensional box experiments with Ottawa sand were duplicated in the unsaturated condition. Both in the saturated and the unsaturated column experiments, less than 0.5 % of the original mass of PCE remained in the column after flushing 12 pore volumes of a 1 % sorbitan monooleate solution. Mass removed from the saturated column as dissolved phase accounted for more than 98 % of PCE removed and more than 91 % of PCE was removed from the unsaturated column. Approximately 9 % of the mass of PCE was estimated to have been lost by volatilization during the unsaturated column experiments. In the unsaturated box experiment, 85 % of the initial PCE was removed from the effluent within 5 pore volumes of surfactant flushing and 0.5 % of the initial mass of PCE remained in the box.

Flow and Reactivity Effects on Dissolved Organic Matter Transport in Soil Columns

AbstractDissolved organic matter (DOM) plays a prominent role in the transport of contaminants in porous media. As DOM has to be considered as a reactive component, flow regime and sorbent reactivity should affect overall DOM transport in an important way. We focused on DOM transport in unsaturated column experiments using quartz sand (QS) and goethite‐coated quartz sand (GS). Rate constrictions to DOM sorption were investigated by varying the volumetric flow rate, while extent and reversibility of sorption were studied in consecutive adsorption and desorption steps. In the QS, DOM retention was low and unaffected by changes in flow rate. Desorption‐step breakthrough curves (BTCs) and mass balances show full reversibility of the sorption process. However, DOM retention in GS was significant and sensitive to flow variation, indicative of nonequilibrium sorption. At lower flow rates, DOM breakthrough exhibited a change in curvature (shoulder) due to the superimposition of two BTCs representing reactive and nonreactive DOM fractions. Transport was successfully modeled assuming these two fractions governed overall DOM mobility. At higher flow rates, the BTC shoulder vanished due to reduced contact time between the DOM and the solid phase (rate‐limited sorption). Sorption of DOM on GS is accompanied by a marked rise in effluent pH, indicative of a ligand‐exchange mechanism. Recovery of DOM during desorption was incomplete due to either partially irreversible sorption or strongly rate‐limited desorption. Increased DOM mobility in the consecutive adsorption step resulted from partial blocking of sorption sites by the initial pulse of DOM.