Combined MODFLOW‐FRACTRAN Application to Assess Chlorinated Solvent Transport and Remediation in Fractured Sedimentary Rock

Detailed field investigations and numerical modeling were conducted to evaluate transport and fate of chlorinated solvent contamination in a fractured sedimentary bedrock aquifer (sandstone/siltstone/mudstone) at a Superfund site in central New Jersey. Field investigations provided information on the fractured rock system hydrogeology, including hydraulic gradients, bulk hydraulic conductivity, fracture network, and rock matrix, and on depth discrete contaminant distribution in fractures (via groundwater sampling) and matrix (via detailed subsampling of continuous cores). The numerical modeling endeavor involved application of both an equivalent porous media (EPM) model for flow and a discrete fracture network (DFN) model for transport. This combination of complementary models, informed by appropriate field data, allowed a quantitative representation of the conceptual site model (CSM) to assess relative importance of various processes, and to examine efficacy of remedial alternatives. Modeling progressed in two stages: first a large‐scale (20 km x 25 km domain) 3‐D EPM flow model (MODFLOW) was used to evaluate the bulk groundwater flow system and contaminant transport pathways under historic and current aquifer stress conditions and current stresses. Then, results of the flow model informed a 2‐D DFN transport model (FRACTRAN) to evaluate transport along a 1,000‐m flowpath from the source represented as a 2‐D vertical cross‐section. The combined model results were used to interpret and estimate the current and potential future extent of rock matrix and aqueous‐phase contaminant conditions and evaluate remedial strategies. Results of this study show strong effects of matrix diffusion and other processes on attenuating the plume such that future impacts on downgradient well fields under the hydraulic stresses modeled should be negligible. Results also showed futility of source remediation efforts in the fractured rock, and supported a technical impracticability (TI) waiver for the site. © 2013 Wiley Periodicals, Inc.     

Characterization and pilot‐scale studies for chemical oxidation remediation of fractured shale

The distribution of volatile organic compounds (VOCs) in fractured shale overlain by thin (< 10 feet) overburden at the Watervliet Arsenal near Albany, New York, was initially determined by sampling water from the fracture network using packer systems in boreholes and also using conventional monitoring wells. Furthermore, short‐term pumping and injection tests were conducted and the boreholes were logged using a variety of geophysical and hydrophysical tools. Tetrachloroethene is the dominant VOC in the groundwater, with lesser concentrations of trichloroethene and degradation products (cis‐1,2‐dichloroethene, trans‐1,2‐dichloroethene, and vinyl chloride). The vertical VOC distributions in the rock matrix were obtained from continuous‐cored holes from which small rock samples, collected at many depths between 18 and 150 feet below ground surface, were analyzed. The rock core VOC concentrations were determined by methanol extraction of crushed rock followed by direct methanol injection onto a gas chromatograph and subsequent estimation of rock porewater VOC concentrations. The rock core data support the concept that diffusion‐driven mass transfer has caused nearly all the VOC mass initially present in the fractures to now reside in the rock matrix, which has a porosity three or four orders of magnitude larger than the bulk fracture porosity. The results of the site characterization indicate that an effective site investigation strategy in fractured shale must include characterization of both the fracture and matrix contaminant distribution. These results also indicate that the most favorable remediation technologies for this fractured shale are those that will destroy VOCs in the rock matrix, particularly contaminants in the sorbed phase, and also destroy the VOC mass in the fractures including both dissolved and immiscible phases. The site characterization resulted in the selection of potassium permanganate for an in situ chemical oxidation pilot study. © 2004 Wiley Periodicals, Inc.     

Borehole‐scale testing of matrix diffusion for contaminated‐rock aquifers

A new method was developed to assess the effect of matrix diffusion on contaminant transport and remediation of groundwater in fractured rock. This method utilizes monitoring wells constructed of open boreholes in the fractured rock to conduct backward diffusion experiments on chlorinated volatile organic compounds (CVOCs) in groundwater. The experiments are performed on relatively unfractured zones (called test zones) of the open boreholes over short intervals (approximately 1 meter) by physical isolation using straddle packers. The test zones were identified with a combination of borehole geophysical logging and chemical profiling of CVOCs with passive samplers in the open boreholes. To confirm the test zones are within inactive flow zones, they are subjected to a series of hydraulic tests. Afterward, the test zones are air sparged with argon to volatilize the CVOCs from aqueous to air phase. Backward diffusion is then measured by periodic passive‐sampling of water in the test zone to identify rebound. The passive (nonhydraulically stressed) sampling negates the need to extract water and potentially dewater the test zone. The authors also monitor active flowing zones of the borehole to assess trends in concentrations in other parts of the fractured rock by purge and passive sampling methods. The testing was performed at the former Pease Air Force Base (PAFB) in Portsmouth, New Hampshire. Bedrock at the former PAFB consists of fractured metasedimentary rocks where the authors investigated back diffusion of cis‐1,2‐dichloroethylene (cis‐1,2‐DCE), a CVOC. Postsparging concentrations of cis‐1,2‐DCE showed initial rebounding followed by declines, excluding an episodic spike in concentrations from a groundwater recharge event. The authors theorize that there are three processes that controlled concentration responses in the test zones postsparging. First, the limited back diffusion of CVOCs from a halo or thin zone of rock around the borehole contributes to the initial rebounding. Second, aerobic degradation of cis‐1,2‐DCE occurred causing declines in concentrations in the test zone. Third, microflow from microfractures contributed to the episodic spike in concentrations following the groundwater recharge event. In active flow zones, the latter two processes are not measurable due to equilibration from groundwater transport between the borehole and active flowing fractures.     

Multi-Scale Characterization of Fractured Rocks Used as a Means for the Realistic Simulation of Pollutant Migration Pathways in Contaminated Sites: A Case Study

An integrated methodology is developed to quantify the geostatistical and transport properties of fractured media at multiple scales. Such information is helpful in developing numerical models and estimating the up-scaled transport coefficients of fractured formations. An oil-contaminated fractured site, overlying granite rock and situated in northern Spain, is investigated, and a macroscopic geological model that quantifies the regional distribution of faults and fractures over the entire area is established. The methodology is based on the measurement of fractured outcrops in the field (scale ～1-100 m), the collection of representative fractured samples and measurement of the fracture aperture (scale ～0.01-1 mm), and the analysis of macroscopic characteristics (scale ～1-5 km) of fracture/faults. The multi-scale fracture properties are utilized to construct a discrete fracture/fault network model which provides input data to a macroscopic simulator of contaminant transport in fractured porous media. The transient NAPL migration pathways are predicted for one scenario of pollution. Such information is helpful in the risk assessment of fractured contaminated sites.     

Anaerobic Biodegradation of Polymer Composites Filled with Natural Fibers

Polymer composites with natural fibers prepared by melt blending were investigated. Synthetic and natural macromolecules were used, including poly(lactic acid), polyhydroxybutyrate-co-polyhydroxyvalerate and low density polyethylene. These polymers were filled with flax fibers. Mechanical properties of the composites, biogas production and mass loss under anaerobic digestion have been presented. It has been shown that the mechanical properties sustain after 28 days of biodegradation. Such materials can be found in applications as packaging, as well as in medicine as polymeric scaffolds, and drug delivery systems etc.     

Biodegradability of degradable plastics exposed to anaerobic digested sludge and simulated landfill conditions

The biodegradabilities of various plastics by anaerobic digested sludge were measured and compared with the biodegradabilities under simulated landfill conditions. Bacterial poly(3-hydroxy-butyrate-co-3-hydroxyvalerate) (PHB/HV; 92/8, w/w), a natural aliphatic polyester, degraded nearly to completion within 20 days of cultivation by anaerobic digested sludge, while synthetic aliphatic polyesters such as poly-lactic acid, poly(butylene succinate), and poly (butylene succinate-co-ethylene succinate) did not degrade at all in 100 days. Cellophane, which was used as a control material, exhibited a similar degradation behavior to PHB/HV. Under simulated landfill conditions, PHB/HV degraded quite well within 6 months. Synthetic aliphatic polyesters also showed significant weight losses through 1 year of cultivation. The acidic environment inside simulators generated by the degradation of biodegradable food wastes which comprised 34 % of municipal solid waste seems to cause the weight loss of synthetic aliphatic polyesters.     

Leaching behaviour of synthetic aggregates

In the framework of EU project "Utilising innovative kiln technology to recycle waste into synthetic aggregate" (BRST-CT98-5234), the leaching behaviour of synthetic aggregates has been studied to assess its environmental compatibility in the various stages of its use. Since the conditions are very different for the different uses, the assessment calls for a variety of different leaching conditions. The pH dependence test is used to cover important differences in pH environment to which the materials are exposed to as well as for an assessment of the buffering capacity of the material. Synthetic aggregate features a low buffer capacity, which makes it sensitive to externally imposed pH conditions. Utilisation and storage exposed to acidic conditions needs to be avoided. The results of the pH dependence test and column leaching test are mutually consistent. The CEN TC 154 method appears to provide systematically low values due to the arbitrary selection of test conditions. Synthetic aggregate studied to date will not adversely affect the concrete in its service life. The main issue for aggregate use is the recycling and the "end of life" condition, when the material becomes construction debris. Not metals, but oxyanions, such as Cr VI and Mo are most relevant under these conditions. A concise test has been applied to assess crucial aspects of leaching for different production mixes.     

Colloids in the mortar backfill of a cementitious repository for radioactive waste

Colloids are present in groundwater aquifers and water-permeable engineered barrier systems and may facilitate the migration of radionuclides. A highly permeable mortar is foreseen to be used as backfill for the engineered barrier of the Swiss repository for low- and intermediate-level waste. The backfill is considered to be a chemical environment with some potential for colloid generation and, due to its high porosity, for colloid mobility. Colloid concentration measurements were carried out using an in-situ liquid particle counting system. The in-house developed counting system with three commercially available sensors allowed the detection of single particles and colloids at low concentrations in the size range 50-5000 nm. The counting system was tested using suspensions prepared from certified size standards. The concentrations of colloids with size range 50-1000 nm were measured in cement pore water, which was collected from a column filled with a highly permeable backfill mortar. The chemical composition of the pore water corresponded to a Ca(OH)2-controlled cement system. Colloid concentrations in the backfill pore water were found to be typically lower than approximately 0.1 ppm. The specific (geometric) surface areas of the colloid populations were in the range 240 m2 g(-1) to 770 m2 g(-1). The low colloid inventories observed in this study can be explained by the high ionic strength and Ca concentrations of the cement pore water. These conditions are favourable for colloid-colloid and colloid-backfill interactions and unfavourable for colloid-enhanced nuclide transport.     

Lignin-based Carbon Fibers: Effect of Synthetic Polymer Blending on Fiber Properties

Carbon fibers have been produced from hardwood lignin/synthetic polymer blend fibers. Hardwood kraft lignin was thermally blended with two recyclable polymers, poly(ethylene terephthalate) (PET) and polypropylene (PP). Both systems were easily spun into fibers. A thermostabilization step was utilized prior to carbonization to prevent fusion of individual fibers. For the lignin-based carbon fibers, careful control of heating rate was required. However, PET–lignin blend fibers can be thermostabilized under higher heating rates than the corresponding homofibers. Carbon fiber yield decreased with increasing incorporation of synthetic plastic. However, carbon fiber yield obtained for a 25% plastic blend fiber was still higher than that generally reported for petroleum pitch. Blend composition also affected surface morphology of the carbon fibers. Immiscible lignin–PP fibers resulted in a hollow and/or porous carbon fiber; whereas carbon fiber produced from miscible lignin–PET fibers have a smooth surface. Synthetic polymer blending also affected the mechanical properties of the fibers, especially MOE; lignin-based carbon fiber properties improved upon blending with PET.     

Crosslinked Polyacrylamide Composite Hydrogels Impregnated with Fly Ash: Synthesis,Characterization and Their Application as Fractures Sealant for High Water Producing Zones in Oil and Gas Wells

Polymeric hydrogels are designed to serve many purposes in various fields of human endeavor. Herein, crosslinked polyacrylamide (PAM) composite hydrogels impregnated with coal fly ash (CFA) were synthesized, characterized and tested as fracture sealing agents in high water producing permeable zones in petroleum industry. The concept of utilizing CFA as an inorganic additive in the matrix of hydrogel emanates from the fact that CFA constitutes majorly alumina and silica. Both chemical oxides can induce the CFA to promote the inherent properties of hydrogel. Polyethyleneimine (PEI) was chosen as the crosslinking agent. Neat PAM/PEI hydrogel and PAM/PEI–CFA hydrogels at various CFA loadings (0.5, 1 and 2 wt%) were synthesized via a transamidation reaction pathway. The developed hydrogels were characterized by hybrid rheometer, FTIR, SEM and XRD instruments. Rheological results reveal that the PAM/PEI–CFA composite hydrogels embedded with various CFA quantities were more elastic than the neat PAM/PEI hydrogel, indicating the dispersion and reinforcing effect of CFA. The functional groups of these hydrogels were confirmed by the FTIR while SEM analysis show that the surface micrographs of neat PAM/PEI hydrogel and PAM/PEI–CFA1 hydrogel were porous in several regions. In contrast, the micrographs of PAM/PEI–CFA2 and PAM/PEI–CFA3 hydrogels demonstrated dense and “net-like” structural patterns. Further, XRD analysis revealed that CFA impregnation has a significant effect on the bulk structural properties of the fabricated hydrogels. The swelling rates of these hydrogels were determined by the gravimetric method and their diffusion parameters evaluated using Fickian diffusion and Schott-order kinetic models. Efficacy of the PAM/PEI–CFA composite hydrogel as fractures sealant in oil and gas wells was conducted at a typical reservoir temperature, 90 °C, and the outcome demonstrated considerable sealing potency.     

Remediating TCE‐contaminated groundwater in low‐permeability media using hydraulic fracturing to emplace zero‐valent iron/organic carbon amendment

A field pilot test in which hydraulic fracturing was used to emplace granular remediation amendment (a mixture of zero‐valent iron [ZVI] and organic carbon) into fine‐grained sandstone to remediate dissolved trichloroethene (TCE)‐contaminated groundwater was performed at a former intercontinental ballistic missile site in Colorado. Hydraulic fracturing was used to enhance the permeability of the aquifer with concurrent emplacement of amendment that facilitates TCE degradation. Geophysical monitoring and inverse modeling show that the network of amendment‐filled fractures extends throughout the aquifer volume targeted in the pilot test zone. Two years of subsequent groundwater monitoring demonstrate that amendment addition resulted in development of geochemical conditions favorable to both abiotic and biological TCE degradation, that TCE concentrations were substantially reduced (i.e., greater than 90 percent reduction in TCE mass), and that the primary degradation processes are likely abiotic. The pilot‐test data aided in re‐evaluating the conceptual site model and in designing the full‐scale remedy to address a larger portion of the TCE‐contaminated groundwater plume. © 2012 Wiley Periodicals, Inc.     

Geotechnical properties of municipal solid waste at different phases of biodegradation

This paper presents the results of laboratory investigation conducted to determine the variation of geotechnical properties of synthetic municipal solid waste (MSW) at different phases of degradation. Synthetic MSW samples were prepared based on the composition of MSW generated in the United States and were degraded in bioreactors with leachate recirculation. Degradation of the synthetic MSW was quantified based on the gas composition and organic content, and the samples exhumed from the bioreactor cells at different phases of degradation were tested for the geotechnical properties. Hydraulic conductivity, compressibility and shear strength of initial and degraded synthetic MSW were all determined at constant initial moisture content of 50% on wet weight basis. Hydraulic conductivity of synthetic MSW was reduced by two orders of magnitude due to degradation. Compression ratio was reduced from 0.34 for initial fresh waste to 0.15 for the mostly degraded waste. Direct shear tests showed that the fresh and degraded synthetic MSW exhibited continuous strength gain with increase in horizontal deformation, with the cohesion increased from 1 kPa for fresh MSW to 16–40 kPa for degraded MSW and the friction angle decreased from 35° for fresh MSW to 28° for degraded MSW. During the triaxial tests under CU condition, the total strength parameters, cohesion and friction angle, were found to vary from 21 to 57 kPa and 1° to 9°, respectively, while the effective strength parameters, cohesion and friction angle varied from 18 to 56 kPa and from 1° to 11°, respectively. Similar to direct shear test results, as the waste degrades an increase in cohesion and slight decrease in friction angle was observed. Decreased friction angle and increased cohesion with increased degradation is believed to be due to the highly cohesive nature of the synthetic MSW. Variation of synthetic MSW properties from this study also suggests that significant changes in geotechnical properties of MSW can occur due to enhanced degradation induced by leachate recirculation.     

The horizontal reactive media treatment well (HRX Well®) for passive in situ remediation: Design,implementation, and sustainability considerations

A new in situ remediation concept termed a Horizontal Reactive Media Treatment Well (HRX Well^®) is presented that utilizes a horizontal well filled with reactive media to passively treat contaminated groundwater in situ. The approach involves the use of a large‐diameter directionally drilled horizontal well filled with solid reactive media installed parallel to the direction of groundwater flow. The engineered contrast in hydraulic conductivity between the high in‐well reactive media and the ambient aquifer hydraulic conductivity results in the passive capture, treatment, and discharge back to the aquifer of proportionally large volumes of groundwater. Capture and treatment widths of up to tens of feet can be achieved for many aquifer settings, and reductions in downgradient concentrations and contaminant mass flux are nearly immediate. Many different types of solid‐phase reactive treatment media are already available (zero valent iron, granular activated carbon, biodegradable particulate organic matter, slow‐release oxidants, ion exchange resins, zeolite, apatite, etc.). Therefore, this concept could be used to address a wide range of contaminants. Laboratory and pilot‐scale test results and numerical flow and transport model simulations are presented that validate the concept. The HRX Well can access contaminants not accessible by conventional vertical drilling and requires no aboveground treatment or footprint and requires limited ongoing maintenance. A focused feasibility evaluation and alternatives analysis highlights the potential cost and sustainability advantages of the HRX Well compared to groundwater extraction and treatment systems or funnel and gate permeable reactive barrier technologies for long‐term plume treatment. This paper also presents considerations for design and implementation for a planned upcoming field installation.     

Effects of hydrodynamic chromatography on colloid-facilitated migration of radionuclides in the fractured rock

The performance assessment of high level radioactive waste disposal has emphasized the role of colloids in the migration of radionuclides in the geosphere. Previous literature [Nagasaki S, Tanaka S, Suzuki A. Fast transport of colloidal particles through quartz-packed columns. J. Nucl. Sci. Technol. 1975;30(11):1136] indicates that owing to hydrodynamic chromatography the colloid velocity may not be equal to that of groundwater. Using hydrodynamic chromatography, this work investigates the effects of the size of colloidal particles on the radionuclide migration facilitated by colloids in a single fractured porous rock. Also, a methodology is proposed to develop a predictive model to assess transport within the fracture rock as well as various other phenomenological coefficients, particularly the size of colloidal particles. In addition, a fully developed concentration profile for non-reactive colloids in the fracture is developed to elucidate hydrodynamic chromatography of colloids in geological media. The external forces acting on colloidal particles hypothesized in the model proposed herein include inertial force, van der Waals attractive force, double layer force as well as gravitational force. The dispersion coefficient of colloids and the distribution coefficient for radionuclides with colloids are also considered as they pertain to the size of the colloid. In addition, the size distributions of colloids are utilized to investigate the effects of polydispersed colloids.     

Cost-effectiveness analysis of the seamistTM borehole liner system

SEAMIST^TM is an innovative technology that facilitates measurements of contaminants in both vertical and horizontal boreholes. The essence of SEAMIST^TM is an airtight membrane liner pneumatically emplaced inside the borehole and maintained with positive pressure. Sampling ports, absorbent collectors, and various in-situ measuring devices can be fabricated into the liner. Small instruments and cameras can be guided through the borehole to obtain real-time data. This article reports on the cost and performance effectiveness of this new technology. In this report, SEAMIST^TM is evaluated as a tool for obtaining data on volatile organic compounds, semivolatile organic compounds, other water-borne contaminants, and radionuclides. SEAMIST^TM is also compared to traditional borehole casing, to conventional soil vapor probes, and to conventional pore-fluid collecting lysimeters. The most cost- and performance-effective applications of SEAMIST^TM are shown to be those applications for which multiple characterization requirements can be combined into one SEAMIST^TM membrane system.     

Treatment of creosote-contaminated groundwater in vegetated sorption/bio-barriers in cold climates

Permeable barriers are structures installed in situ to treat contaminated groundwater. Pollutants are removed as contaminated groundwater flows through a barrier material. A compost/sand barrier and a plant covered permeable barrier with soil/sand and peat/sand were tested in pilot-scale to treat creosote-contaminated groundwater by sorption and biological removal in situ. Outlet concentrations of the barriers were consistently low during the 29 months of operation. Although sorption sites were filled up with polycyclic aromatic hydrocarbons, they seemed to be regenerated because of biodegradation under aerobic conditions. The vegetated section was least efficient, probably because of lack of oxygen, hence it could not be determined if the plants had a positive effect. As long as biodegradation is efficient the barrier is expected to function for several more years.     

The Development of a GIS-based Inventory of Standing Waters in Great Britain together with a Risk-based Prioritisation Protocol

An inventory of standing waters (freshwater lakes and lochs) wasderived from Ordnance Survey digital map data at a scale of 1:50 000 and represents the most comprehensive survey of its kind for Great Britain. The inventory includes 43 738 water bodies in England, Scotland, Wales and the Isle of Man and contains basic physical data such as location, surface area, perimeter and altitude. Catchment areas were computed for water bodies with a surface area larger than 1 ha from a digital terrain model (DTM) using customised routines in a geographical information system (GIS). The resulting polygons were then used to derive catchment-related information from a variety of national datasets including population density, livestock density, land cover, solid and drift geology, meteorological data, freshwater sensitivity status, acid deposition and conservation status. Using data derived from the inventory a risk-based prioritisation protocol was developed to identify standing waters at risk of harm from acidification and eutrophication. This information is required by the Environment Agency, Scottish Environmental Protection Agency and the U.K. statutory conservation bodies to co-ordinate actions and monitor change under international, European and national legislation.     

Photoacoustic infrared spectroscopy for conducting gas tracer tests and measuring water saturations in landfills

Gas tracer tests can be used to determine gas flow patterns within landfills, quantify volatile contaminant residence time, and measure water within refuse. While gas chromatography (GC) has been traditionally used to analyze gas tracers in refuse, photoacoustic spectroscopy (PAS) might allow real-time measurements with reduced personnel costs and greater mobility and ease of use. Laboratory and field experiments were conducted to evaluate the efficacy of PAS for conducting gas tracer tests in landfills. Two tracer gases, difluoromethane (DFM) and sulfur hexafluoride (SF₆), were measured with a commercial PAS instrument. Relative measurement errors were invariant with tracer concentration but influenced by background gas: errors were 1-3% in landfill gas but 4-5% in air. Two partitioning gas tracer tests were conducted in an aerobic landfill, and limits of detection (LODs) were 3-4 times larger for DFM with PAS versus GC due to temporal changes in background signals. While higher LODs can be compensated by injecting larger tracer mass, changes in background signals increased the uncertainty in measured water saturations by up to 25% over comparable GC methods. PAS has distinct advantages over GC with respect to personnel costs and ease of use, although for field applications GC analyses of select samples are recommended to quantify instrument interferences.     

Partitioning tracer tests as a remediation metric: Case study at naval amphibious base little creek,Virginia Beach,Virginia

The partitioning tracer test (PTT) is a characterization tool that can be used to quantify the porespace saturation (S_N) and spatial distribution of dense nonaqueous phase liquids (DNAPLs) in the subsurface. Because the method essentially eliminates data interpolation errors by directly measuring a relatively large subsurface volume, it offers significant promise as a remediation metric for DNAPL‐zone remediation efforts. This article presents, in detail, the design and results of field PTTs conducted before and after a DNAPL‐zone treatment at the Naval Amphibious Base Little Creek, Virginia Beach, Virginia. The results from different tracers yield a relatively large range in S_N estimates, indicating notable uncertainty and presenting significant challenges for meaningful interpretation. Several potential interpretation methods are presented, resulting in an estimated DNAPL removal range of 15 to 109 L. While this range is large, it is consistent with the DNAPL removal (～30 L) determined from analysis of effluent concentration measurements collected during the remediation efforts. At this site, the initial and final S_N values are low, and the relatively inconsistent performance of the various tracers indicates that these levels are near the lower practical quantification limit for these PTTs; however, the effective lower quantification limit for these tests is unknown. Generally, an understanding of lower quantification limits is particularly important for interpretation of post‐remediation PTTs because S_N values are likely to be low (due to remediation efforts) and the S_N estimated from the PTT may be used to predict long‐term dissolved plume behavior and assess associated environmental risk. Partitioning tracer test quantification limits are test‐specific, as they are dependent on a variety of factors including analytical uncertainty, tracer breakthrough characteristics, and tracer data integration techniques. The results of this case study indicate that methods for estimating lower quantification limits for field PTTs require further development. © 2004 Wiley Periodicals, Inc.     

Long-term Performance of a Permeable Reactive Barrier in Acid Sulphate Soil Terrain

Deep drainage technique utilised for flood mitigation in low-land coastal areas of Australia during the late 1960s has resulted in the generation of sulphuric acid in soil by the oxidation of pyritic materials. Further degradation of the subsurface environment with widespread contamination of the underlying soil and groundwater presents a major and challenging environmental issue in acid sulphate soil (ASS) terrains. Although several ASS remediation techniques recently implemented in the floodplain of Southeast Australia including operation of gates, tidal buffering and lime injections could significantly control the pyrite oxidation, they could not improve the long-term water quality. More recently, permeable reactive barriers (PRBs) filled with waste concrete aggregates have received considerable attention as an innovative, cost-effective technology for passive in situ clean up of groundwater contamination. However, long-term efficiency of these PRBs for treating acidic groundwater has not been established. This study analyses and evaluates the performance of a field PRB for treating the acidic water over 2.5 years. The pilot-scale alkaline PRB consisting of recycled concrete was installed in October 2006 at a farm of southeast New South Wales for treating ASS-impacted groundwater. Monitoring data of groundwater quality over a 30 month period were assessed to evaluate the long-term performance of the PRB. Higher pH value (~pH 7) of the groundwater immediately downstream of the PRB and higher rates of iron (Fe) and aluminium (Al) removal efficiency (>95%) over this study period indicates that recycled concrete could successfully treat acidic groundwater. However, the overall pH neutralising capacity of the materials within the barrier declined with time from an initial pH 10.2 to pH 7.3. The decline in the performance with time was possibly due to the armouring of the reactive material surface by the mineral precipitates in the form of iron and aluminium hydroxides and oxyhydroxides as indicated by geochemical modelling.