Minerals, metals and mining are the cornerstone of technological development and play an essential role in achieving the United Nations Sustainable Development Goals. Mining, however, is not a wastefree process, with mine wastes being a source of a host of environmental problems. One of these is acid rock drainage (ARD), which forms through a complex series of acid formation (mainly via sulfide oxidation), neutralisation (primarily by carbonates) and gangue mineral dissolution reactions in waste rock and tailings storage piles. The resulting drainage waters are often acidic, highly saline and may contain elevated levels of deleterious elements. Effective ARD mitigation requires accurate ARD characterisation and prediction strategies. To date, standard guidelines recommend a suite of geochemical static (characterisation) and kinetic (prediction) tests. Characterisation tests such as acid-base accounting (ABA) and net acid generation (NAG) tests provide a quick and relatively inexpensive estimate of the "worst case" scenario for acid formation and neutralisation, while kinetic tests (commonly humidity or column leach tests) aim to predict the longterm weathering potential of waste material. The UCT biokinetic test (not currently industry standard practice) was developed to address the effect of microorganisms on ARD formation and allow for the collection of relative kinetic data on neutralisation and acidification within a shortened time period. None of these tests, however, account for the additional layer of complexity introduced by mineral texture, which describes the interrelationship of mineral grains to one another, their shapes and sizes, with some frequently studied textural parameters including mineral liberation, association, grain size distribution and particle size. Mineralogical and textural analyses are infrequently practiced in the context of ARD assessment due to the difficulty in obtaining statistically sound quantitative textural data, high costs of measurement, and standard ARD assessment protocols recommending (rather than necessitating) these assessments. An ARD assessment approach that includes static, kinetic, mineralogical and textural assessments has nonetheless been suggested by several researchers. This project assessed the dominating textural parameters on the scales of kinetic (humidity cell) test (HCT) feed material (meso-scale) and characterisation (static and UCT batch biokinetic) test (SCT) feed material (micro-scale) using four waste rock samples (A, B, C and D) from a greenstone belt gold deposit as a case study. More specifically, the study aimed to assess the role of mineralogy and texture in the ARD assessment "toolbox" and to investigate the role of coarse material sampling for ARD assessment. Data sets collected included the PSD of the micro- and meso-scale material, sample chemistry data obtained from XRF spectrometry and LECO total sulfur, bulk mineralogy data from QXRD and QEMSCAN, as well as textural and mineralogical data from QEMSCAN for sized and unsized micro- and meso-scale material. ARD-specific data sets included results of geochemical characterisation tests such as ANC and single-addition NAG tests, the UCT batch biokinetic test with and without pH control for samples C and D, as well as prediction test data from water-fed and modified humidity cell tests. The geochemical static tests performed on samples A, B, C and D classified them as PAF, PAF, uncertain and NAF, respectively. Non-pH-controlled UCT batch biokinetic tests remained circumneutral for samples B, C and D over the duration of 90 days, while for sample A the pH became acidic over time. The pH-controlled tests demonstrated a steady depletion of neutralisation potential over the first 30 days. Humidity cell test results demonstrated no acidic leachate formation for waterfed tests over 40 weeks, while modified tests showed a decreasing pH over time as the neutralisation capacity was reduced. The mineralogy was important for the interpretation of test results on both the micro-and meso-scales and was assessed in terms of both discrete minerals and reactivity groupings (Fe-Sulfide, other sulfide, dissolving (carbonate), fast weathering, intermediate weathering, slow weathering, inert and other). For sample A the mineralogy was dominated by the inert (quartz), slow weathering (magnetite, plagioclasealbite) and intermediate weathering (Fe-amphibole) categories, with lesser contributions from the FeSulfide (pyrrhotite), dissolving (calcite) and fast weathering (epidote) groups. The main groups contributing to the sample B mineralogy were the slow weathering (plagioclase-albite, magnetite, Kfeldspar), inert (quartz) and intermediate weathering (Fe-mica, chlorite) groups, followed by Fe-Sulfide (pyrite), dissolving (calcite) and fast weathering (epidote) mineral groups. Sample C mineralogy comprised predominantly inert (quartz, titanite), Fe-Sulfide (pyrrhotite), dissolving (calcite) and intermediate weathering (Fe-mica, chlorite) minerals, with lesser contributions from slow weathering (K-feldspar) and fast weathering (epidote) minerals. Sample D comprised intermediate weathering (Feamphibole, chlorite, Fe-mica) and slow weathering minerals, with lesser contributions from slow weathering (magnetite), dissolving (calcite) and Fe-Sulfide (pyrrhotite) minerals. Textural parameters (liberation and association, grain size distribution and liberation spectrum) were evaluated for the FeSulfide and dissolving minerals. On the micro-scale, a large portion of the Fe-Sulfide and dissolving minerals in the samples was found in the liberated category (50%) of the texturally significant size fractions (>1mm), which comprised predominantly locked Fe-Sulfide and dissolving minerals. Evidence of a bimodal distribution was, however, found for sample C via the liberation spectrum and grain size distribution (early liberation size of 8mm), which accounted for the larger degree of liberation observed in the >1mm size fractions, and a larger degree of liberation for the sample overall. The association of Fe-Sulfide and dissolving minerals for all samples was found to be primarily to inert, intermediate weathering and slow weathering minerals, with a larger degree of association of Fe-Sulfide to dissolving minerals observed in sample C. On the micro-scale the mineralogy helped inform the placement of the samples on the geochemical classification plot based on the Fe-Sulfide, dissolving and intermediate weathering mineral contents. For the non-pH-controlled UCT batch biokinetic test, the presence and abundance of calcite was thought to dictate the PAF/NAF nature of the test, as even relatively low amounts of calcite rendered the pH circumneutral for the duration of the test (sample B). For pH-controlled tests, however, the calcite was depleted over time, which led to a favourable acidic environment for the acidophilic bacteria used in the batch biokinetic test. For both the geochemical characterisation and the pH-controlled UCT batch biokinetic tests there was evidence to suggest the contribution of intermediate weathering (Feamphibole, Fe-mica, chlorite) and slow weathering (magnetite) minerals to the neutralisation potential in the sample. On the meso-scale the effects of mineralogy were most prominent for the modified humidity cell tests, which showed some pH fluctuations and a steady depletion of the primary neutralisation potential. The pH fluctuation after the depletion of the dissolving minerals was attributed to the dissolution of intermediate weathering minerals over the 40 weeks of the tests. These effects were not observed during the 40 weeks of the water-fed experiments. Given sufficient time for the latter test, however, it would be expected that upon the onset of acidification, similar effects of the mineralogy on the leachate quality would be observed as in the modified tests. Knowledge of the Fe-sulfide and dissolving mineral texture yielded several insights. on the micro-scale, the liberation and grain size distribution data provided an indication that a sample-customised grinding size should be established to ensure adequate "worst case" scenario determination via characterisation tests, as material with fine Fe-Sulfide or dissolving mineral grains may not be fully liberated at the recommended 75µm top size. On the meso-scale, the texture yielded insight into the circumneutral behaviour of the water-fed HCT, as most of the acid-forming minerals were contained in size fractions where the liberation was either limited or negligible, with predominant association to slow weathering, intermediate weathering and inert minerals. These findings highlighted the importance of considering mineralogy, texture and the PSD of the material for HCT result interpretation. When considering texture as a parameter for ARD assessment, the potential for sampling and mineralogical errors arose due to the coarse material size (specifically on the meso-scale) and the limitations on the number of particles that could be assessed. Quantitative mineralogy and texture data allowed for the quantitative assessment of the sampling and mineralogical errors, which were investigated through Pierre Gy's fundamental sampling error (FSE) equation, the binomial distribution approximation and the plotting of confidence intervals over the Fe-Sulfide liberation data. The results showed that although tools such as Gy's "safety line" provide a useful quick means of sampling error assessment, this approach may yield excessively large sample mass requirements for coarse material. Calculating the sampling error from the textural and mineralogical data provided a useful tool to estimate sample representativeness. Additionally, the estimation of sampling errors may help in the planning of an appropriate sampling approach, which may ultimately provide a means to relate data sets to one another across scales based on how representative samples are of one another, and therefore of the parent lot. The current study showed how mineralogy and texture are not simply "tools" in the ARD assessment "toolbox", but rather a key means for interpreting characterisation and prediction test data. Additionally, the quantitative assessment of mineralogy and texture provided the opportunity to assess the materialspecific sampling error, which, in turn, may allow for the correlation of data sets across various scales and for the planning of appropriate sampling strategies. Recommendations for future work include: the quantitative assessment of the ARDI for meso-scale material; the assessment of detailed characterisation and prediction test leachate chemistry; trace element assessment and deportment throughout UCT batch biokinetic and humidity cell testing; mineralogical and textural assessment on characterisation and prediction test residues during and after tests; an in-depth analysis of the minimal/optimal sample block/sub-sample mass required for minimal error; the assessment of samples using X-ray microcomputed tomography to assess and decrease the effects of stereological bias prevalent in 2D measurements; and the application of a similar texture and mineralogy assessment to additional waste types (such as coal wastes, or waste material containing non-Fe-bearing sulfides).

Acid rock drainage (ARD), caused by the oxidation of sulphide bearing minerals, continues to be an environmental impact of both closed and operational mines, leading to waters with low pH values and high metal concentrations (Park et al., 2019). A potential treatment method for ARD is controlled eutrophication, where algae growth is stimulated in the water and metals are accumulated within the algae cells or sorbed to the outside of the cell walls (Samal et al., 2020). The aim of this study is to determine if the addition of wood ash to acidic mine waters can increase the pH, stimulate algal growth, and lower metal concentrations in the water. Two types of wood ash, as well as KNO3 and KH2PO4 were utilized for this study, along with three water sample types: neutral (pH of 7.0) water from Åkerberg pit lake, acidic (pH of 5.7) water from Maurliden mine site (Maurliden East), and very acidic (pH of 2.3) water from Maurliden mine site (Maurliden West). Two experiments were performed, Experiment I (Exp. I) involved a one-time addition of wood ash and KNO3 to samples from each of the three sites, and Experiment II (Exp. II) involved a feeding style ash addition where ash was added every five days to samples from Åkerberg and Maurliden East. Exp. II also included one sample set where artificial nutrients only (KNO3 and KH2PO4) were added to Maurliden East samples. Samples were placed inside of a climate chamber to provide daily irradiation and pH, electrical conductivity (EC), and fluorescence signal measurements were taken daily. Additionally, absorbance samples and O2 saturation measurements were taken daily for Exp. II. At the end of each experiment, samples were sent to an accredited laboratory for elemental analysis and chl-a analysis.  Algal growth was observed in Exp. I in Åkerberg samples only, and in Exp. II in Åkerberg samples and in Maurliden East samples with artificial nutrient additions only. The algal growth rate was similar in both experiments; however, Exp. II yielded a higher concentration of chl-a than Exp. I. Precipitates were observed in all samples in both experiments, including in control samples. pH results cannot confirm the ability of the wood ash to increase the pH in the samples from Maurliden East or West, and metal concentration decreases in these samples are most likely due to precipitation reactions. Metal concentration decreases in Åkerberg samples could be due to precipitation reactions and/or algal uptake.

Road construction through sulfidic materials in Virginia has resulted in localized acid rock drainage (ARD) that threatens water quality, fill stability, integrity of building materials, and vegetation management. The objectives of this study were: i) to develop a state-wide sulfide hazard rating map based on characterization of the geologic formations associated with acid roadcuts, ii) to estimate depth to sulfidic sediments in the Coastal Plain based on landscape relationships, and iii) to evaluate potential acidity testing procedures on diverse materials. Geologic formations associated with acid roadcuts were characterized by potential peroxide acidity (PPA) and S content, and grouped into four categories. Listed in order of increasing severity, these formations included: the Tabb Formation (Coastal Plain), the Lynchburg Group of the Ashe Formation (Blue Ridge), the Chesapeake Group and Lower Tertiary deposits (Coastal Plain), the Millboro shale, Marcellus shale, Chatanooga shale and Needmore Formation (Valley and Ridge), and the Quantico Formation (Piedmont). Evaluation of landscape parameters near Richmond, Virginia, indicated that the likelihood of encountering sulfidic materials within a given depth at a specific location was related to elevation and mapped soil types. Elevation and soil map units were assigned to risk classes to indicate the likelihood of encountering sulfides within a depth of 9 m. Comparison of PPA and S content for 296 diverse samples indicated that S may serve as a screening tool to evaluate materials without carbonates. Comparison of PPA and conventional Acid-Base Accounting (ABA) for 14 diverse samples indicated that PPA and ABA were highly correlated, with PPA yielding 0.60 to 0.95X the amount of acidity as ABA. Potential acidity by Soxhlet extraction and PPA were equivalent for 3 of 4 diverse samples. Average acidity and metal contents of leachate from Soxhlet extractors were correlated with acidity and metals of road drainage. Sulfide hazard analysis should be an essential step in the pre-design phase of highway construction and other earth-disturbing activities. / Ph. D.

We initially studied the treatment of acid rock drainage using a sulfate-reducing bioreactor with zero-valent iron as the electron donor. The results demonstrate that this electron donor can serve as the sole exogenous slow-release electron donor to drive sulfate reduction over 400 operational days at short HRTs (1-3 days). The synthetic acid rock drainage contained high heavy metal concentrations (up to 50 mg/L of copper) and pH values ranging from 3.0 to 7.0. Treatment of this acid rock drainage efficiently removed Cu, Cd and Pb (>99.7%) and increased pH to circumneutral values (7.3-7.7). Elemental analysis indicated that formation of insoluble metal sulfides was responsible for the effective metal removal in the zero valent iron columns. In the second study, three inoculated columns containing anaerobic granular sludge were fed a synthetic medium containing H₂SO₄ and Cu²⁺ during the experimental period of 4 months. Algae biomass promoted 80% of sulfate removal (12.7 mg SO₄²⁻ d-1), enabling near complete Cu removal (>99.5 %), and alkalinity generation, raising the effluent pH to 6.5. In the algae amended columns Cu²⁺ was precipitated with biogenic H2S produced by sulfate reduction. Whole cell algae and lipid extracted algae biomasses were both shown to be effective e-donors in driving sulfate reduction of ARD, thus enabling the precipitation and removal of Cu²⁺. The precipitate retained in the columns was composed mostly of insoluble copper sulfide formed from the biogenic sulfide, as shown by sequential extraction and X-ray diffraction. In the third study, several pretreatments, i.e., thermal, chemical, sonication and combinations thereof, that enhance anaerobic biodegradability of Chlorella protothecoides biomass were evaluated. The results demonstrate that anaerobic digestion of pretreated Chlorella protothecoides biomass generates energy-rich methane and recovers nitrogen nutrients. Sonication of algal biomass under optimized conditions provided a significant increase in the methane yield (327 mL STP CH₄ g⁻¹ VS) compared to untreated algae (146 mL STP CH₄ g⁻¹ VS), as demonstrated in anaerobic digestion experiments incubated for 41 days. In contrast, thermal pretreatment provided only a moderate increase of the methane yield and alkaline treatment led to a decrease of the methane yield compared to the untreated algal biomass. Additionally, sonication treatment provided a 4.1-fold increase in the release of ammonia nitrogen during anaerobic digestion of the algal biomass. In the fourth study, the nutrient recovery and biogas generation from the anaerobic digestion of waste biomass from algal biofuel production was investigated. Anaerobic digestion of whole cell and lipid extracted Chlorella sorokiniana-1412 released 48.1 and 61.5% of the total algal nitrogen as NH₄⁺-N, and 87.7 and 93.6% of the total algal P as soluble P, respectively. The biochemical methane potential, quantified through the methane yield of whole cell algae and lipid extracted algae, was 0.298 and 0.253 L methane/g algal volatile solids, respectively. The conversion of lipid extracted algae and whole cell algae biomasses to methane was very similar (38 and 41% on a COD basis, respectively), indicating that the energy yield was not significantly lowered by extraction of the lipid fraction (which accounted for 9% of algal dry weight). Sonication improved the access of hydrolytic enzymes to algal biopolymers, compensating in part for the energy lost due to lipid extraction. The above results demonstrate that algal waste from the biodiesel industry has the potential to be recycled through anaerobic digestion into valuable nutrients and energy. These studies indicate that zero valent iron and algae biomass are promising reactive materials for the treatment of acid rock drainage in sulfate-reducing permeable reactive barrier systems. Additionally, to promote algae cultivation for the biodiesel industry, the anaerobic digestion of algae residues can generate nutrients and energy, making algae cultivation more fiscally attractive.

An acid rock drainage (ARD) stream emanating from the Zn-Pb XY-deposit in the Yukon Territory was examined in order to evaluate the physico-chemical and geochemical processes governing the distribution of dissolved elements from the creek. The creek showed very high concentrations of metals (300 mg/L Fe, 500 mg/L Zn, 15 000 µg/L Ni, 1300 µg/L Cu and 4500 µg/L Cd), low water temperatures (1 – 12°C) and was acidic to moderately acidic (pH 3.1 – 5.0). It was found that this stream experienced a strong seasonal evolution, with increased sulphate and metal concentrations and decreased pH over the course of the summer. The mineral precipitates that formed under low pH conditions were a mixture of schwertmannite, goethite, jarosite and barite, while those that formed under moderately acidic conditions were a mixture of jurbanite, hydrobasaluminite, gibbsite and an X-ray amorphous Al-sulphate phase. Most of the mineral precipitates were of inorganic origin, although microbes may have played a role in mineral formation and trace metal sequestration in some of the precipitates. All of the mineral precipitates contained anomalous concentrations of trace elements (up to 1.5 % wt Zn) and showed a seasonal evolution in their mineralogy, both of which were determined to be a function of the pH and prevailing geochemical conditions. The geochemistry of the ARD creek draining the XY-deposit was compared to another ARD creek in the area that was likely draining shales. The two creeks were compared in order to determine if ARD geochemical characteristics can be used as a tool for the mineral exploration industry.

An acid rock drainage (ARD) stream emanating from the Zn-Pb XY-deposit in the Yukon Territory was examined in order to evaluate the physico-chemical and geochemical processes governing the distribution of dissolved elements from the creek. The creek showed very high concentrations of metals (300 mg/L Fe, 500 mg/L Zn, 15 000 µg/L Ni, 1300 µg/L Cu and 4500 µg/L Cd), low water temperatures (1 – 12°C) and was acidic to moderately acidic (pH 3.1 – 5.0). It was found that this stream experienced a strong seasonal evolution, with increased sulphate and metal concentrations and decreased pH over the course of the summer. The mineral precipitates that formed under low pH conditions were a mixture of schwertmannite, goethite, jarosite and barite, while those that formed under moderately acidic conditions were a mixture of jurbanite, hydrobasaluminite, gibbsite and an X-ray amorphous Al-sulphate phase. Most of the mineral precipitates were of inorganic origin, although microbes may have played a role in mineral formation and trace metal sequestration in some of the precipitates. All of the mineral precipitates contained anomalous concentrations of trace elements (up to 1.5 % wt Zn) and showed a seasonal evolution in their mineralogy, both of which were determined to be a function of the pH and prevailing geochemical conditions. The geochemistry of the ARD creek draining the XY-deposit was compared to another ARD creek in the area that was likely draining shales. The two creeks were compared in order to determine if ARD geochemical characteristics can be used as a tool for the mineral exploration industry.

Water contaminated due to mining activities is often acidic and can contain high concentrations of dissolved metals. Cement kiln dust (CKD) is a fine-grained, alkaline material that is generated as a by-product of cement production. Its high lime (CaO) content makes it attractive as a substitute for quicklime in the generation of slurries for the treatment of mine water. The first part of this study analyzed six CKD samples for several physical and chemical properties to determine their variability and to compare them to the characterization of a commercial quicklime sample. Neutralization and precipitation experiments using acidic mine water containing high concentrations of zinc and iron determined that all slaked CKD slurries performed comparably to the quicklime slurry in terms of precipitation of soluble metals. The results of this research show that CKD can be effectively used to neutralize mine water and precipitate and remove dissolved metals.

No description available.

In some environmental situations, environmental effects caused by elevated metals resulting from past mining and smelting activities can be observed in nearby receiving water bodies several decades after mine and smelter closure. There is a growing need for managing the hazardous solid wastes such as mining wastes as well as for assessing water quality and for sustainable management of sediment quality. The work presented in this thesis examined the mobilization of metals from two metal sources: mining wastes from a mine site in Vietnam and sediments from a contaminated lake in Sweden in order to test the hypothesis that mobilization of metals will be increased, when the environmental conditions change by e.g. exposure of mining wastes to oxidative weathering, change of redox conditions at the water-sediment interface and resuspension of sediments. The results from this work under field and laboratory conditions have verified the hypothesis. The exposure of sulphidic mining wastes in oxidative weathering conditions may cause long-term production of ARD and the resultant long-term mobility of metals. The oxidation/resuspension of sediments is an important factor for the release of trace metals Zn, Cu and Cd into the solution and substantial amounts of particles and, hence, associated metals into overlying water. The concomitant changes in pH during oxidation/resuspension of sediment play a significant role in the metal release both to redox sensitive elements Fe and Mn and trace elements Zn, Cu and Cd. The concomitant change in DOC during oxidation/resuspension can also contribute to the increased mobility of study metals. The field study was coupled to intermittent operation of a hydropower plant. The mobility of the metals was higher under operation compared to non-operation and, thus, the potential impacts on dispersal of metal pollution to downstream aquatic environments. The sudden increase in water flow upon the hydropower plant upon shifts from inactive to active state could cause immediate release of particles and thus particulate metals in the overlying water. However, the magnitude and its integrated effects in fluxes of metals over the season call for further research. There is a need to further investigate the impacts of hydropower generation in a longer period of time and at a higher frequency of observations at the very start of the hydropower operation. The results from this multidisciplinary approach would give a basis for an optimal operation of the hydropower plant to minimize the metal pollution associated with the water flow. / <p>The series number is changed from 410 to 430 and the ISBN is changed from 978-91-85895-56-4 to 978-91-7393-926-3.</p>

An experimental waste-rock pile (50m x 60m x 15m, 0.053 wt. % S) was constructed at the Diavik Diamond Mine, in the Northwest Territories, Canada to evaluate the generation of acid-rock drainage and the seasonal and annual release of various metals in drainage leachate. A dense internal instrumentation network enabled well resolved observations of temperature, air content, water content, fluid flow, microbiology, mineralogy and geochemistry within the waste-rock pile. Water samples were collected from soil water solution samplers (SWSSs) to measure core pore-water characteristics, from 4 m2 to 16 m2 scale basal collection lysimeters (BCLs) to measure core leachate characteristics, and from basal drains (3000 m2 basal area) to measure aggregated leachate characteristics. Monitoring of pore-water geochemistry within the core of the test pile indicated an evolving weathering front characterized by changes in predominant acid-consuming mineral-dissolution reactions. Initially, acid neutralization occurred through dissolution of carbonate minerals. A subsequent decline in pH was limited by acid neutralization through dissolution of Al- and Fe-bearing minerals. This lower pH environment was accompanied by increasing concentrations of SO4, Al, Fe, Ni, Co, Cu, Zn, Cd, Ca, Mg, K, Na and Si. Annual drainage cycles in the core of the test pile were characterized by distinct, high concentration ‘spring flushes’ followed by a steady decline of all dissolved constituents with minimums prior to freeze-up. Core trends were typical of freshets observed in polar environments and primarily explained by a combination of fluid residence time and the build-up of oxidation products over the winter. The opposite trend was observed in the aggregated pile drainage, whereby early-season low-concentration leachate was derived from snowmelt and batter flow and late-season high-concentration leachate was dominated by contributions from the core of the pile. Thermal data demonstrating the annual freeze-thaw cycle was used to delineate core and batter subsystems within the pile. Mean annual solute concentrations and geochemical speciation modelling results revealed two different environments within these subsystems. Concentrations in the core of the test pile were 2.5 to 8.5 times greater than concentrations in the batter. Dilution through snowmelt contributions and shorter flow pathways were expected to control solute concentrations in the batter subsystem. Aggregated basal leachate loading exhibited a linear annual increase for all ARD reaction products, with the maximum annual release observed in 2012. Between 2007 and 2012 core flow accounted for 13 % of the total drainage volume; whereas 35 to 51 % of major and trace metal loads were attributed to this zone. Parameter correlation analysis and core contribution estimates were used to identify common source minerals for elements and infer mechanisms controlling the mobility of dissolved metals (sorption, co-precipitation and precipitation-dissolution). By 2012, the release of pyrrhotite oxidation products from the < 5 mm reactive fraction of waste rock reached as high as 5.1 %, 9.0 %, 7.2 % and < 0.1 % for S, Ni, Co and Fe respectively. The results of this study indicate that a comprehensive understanding of thermal cycling is imperative when estimating seasonal and annual releases of weathering products from waste rock. Observations of active zone dynamics and the temporal and spatial evolution of waste rock drainage loading recognized in this study will assist in the advancement of reactive transport models describing ARD in cold climates.