School of Applied Sciences and Engineering, Monash University, Australia

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1 Geotechnical and Hydrogeological Evaluation of Artificial Soils to Remediate Acid Mine Drainage and Improve Mine Rehabilitation An Australian Case Study Michael Taylor 1, Mohan Yellishetty 2,*, and Barbara C. Panther 1 1 School of Applied Sciences and Engineering, Monash University, Australia 2 Division of Mining & Resources Engineering, Department of Civil Engineering, Monash University, Wellington Road, Clayton 3800, VIC Abstract. This project investigates the feasibility of combining overburden (OB) produced from an open cut brown coal mine, rejected wood chip waste from a Kraft Paper Mill (KMR), and industrially derived compost. The outcome is an artificial soil that mitigates Acid Mine Drainage (AMD) and enhances soil heath and suitability for rehabilitation. The three separate industries are local to each other, facilitating economical transportation of waste streams. The study identified a suitable artificial soil mixing ratio that would ultimately neutralise AMD and amplify nutrient content (8 parts OB, 1 part KMR and 0.6 parts compost), based on net acid producing potential derived for each component. The ph of the mixtures increased compared with the raw materials eg. from ph 3.24 to ph 6.51, which was well within ideal conditions for plant growth and inhibition of acidophilic bacteria that catalyse AMD reactions. The artificial soil also demonstrated increased water retaining characteristics (field capacity) and enhanced vegetation growth, with an extreme example illustrated by one acid OB sample (ph 1.75, originally unable to support vegetation) effectively supporting grass growth after mixing. Synthetic Precipitation Leaching Procedure (SPLP) results showed no regulatory levels being breached in regards to metals leaching out of the artificial soils. However, in some samples, the artificial soil leachate exhibited higher concentrations of metals than the original samples. Electrical Conductivity values increased on average from 0.8 ds/m to 1.66 ds/m). These soils, once proven safe and effective for use, could be laid over the waste dumps in brown coal mines for reclamations purposes. Keywords: AMD, Overburden, Kraft Mill Rejects, Mine Rehabilitation. * Corresponding author. C. Drebenstedt and R. Singhal (eds.), Mine Planning and Equipment Selection, 855 DOI: / _83, Springer International Publishing Switzerland 2014

2 856 M. Taylor, M. Yellishetty, and B.C. Panther 1 Introduction In regions such as the Central Gippsland region of Victoria, where a number of industries are located within close proximity, there is potential for industrial symbiosis to be practiced, where individual waste streams can be combined and engineered into a useful product [1], thus decreasing waste quantities generated. This study investigated the feasibility of creating an artificial soil by combining waste streams from three different industries: a brown coal mine, a Kraft paper mill and a green waste and bio-waste composting facility. Mining of brown coal produces large volumes of overburden (interseam materials and inferior coal) which is often backfilled into the mine. This process has the potential to expose sulphide minerals such as pyrite to oxygen and water causing oxidation and the production of sulphuric acid, commonly known as Acid Mine Drainage (AMD) [8]. This acid generation can lower the ph of soil and mobilise heavy metals [3]. Furthermore, final rehabilitation of such mines once backfilling is completed, poses a challenge due to the often poor health of the soil for sustaining vegetative growth (i.e. low nutrients and low ph). Kraft Mill rejects (KMR) are the rejected wood chips that fail to sufficiently breakdown when reacted with white liquor (NaS 2 and NaOH) during the Kraft digesting process. This waste stream is alkaline, contains traces of black liquor (spent white liquor), and is traditionally disposed of to landfill. This project aimed to create an artificial soil that exhibited sufficient buffering capacity (from the KMR component) to remediate acidity from the overburden, and to elevate levels of nutrients through addition of compost. Such a soil could be very useful to mine rehabilitation efforts. Water retaining characteristics (field capacity) was also increased, further attributing to healthier soil. If the artificial soil proved applicable, numerous beneficial aspects would occur such as: - Providing a use for KMR that would otherwise be disposed of to landfill. - Mitigate AMD and heavy metal mobilisation which could be experienced in open cut mine OB dumps - Significantly improve soil health and suitability for vegetation growth, benefitting rehabilitation efforts, mitigating erosion issues, eliminating ongoing treatment for AMD, and providing a long-term solution to mine rehabilitation. - Provide large scale use of compost, inducing further recycling of the original raw materials (one part biosolids three parts green waste) A variety of mixes were developed, assessed for geotechnical and geochemical characteristics, including leaching characteristics. Grass was grown in all the mixes in order to determine their suitability for future rehabilitation use.

3 Geotechnical and Hydrogeological Evaluation of Artificial Soils Methods 2.1 Sampling A 60 L composite sample of compost was taken on the 2/08/2012 and a 20 L composite sample of KMR was collected on the 20/08/2012. The homogeneity of both materials was consistent. On the 30/7/12, samples of the open cut mine OB were taken from two different depths, at the surface approximately 0.5 m deep. Three different areas of the OB dump were sampled: an old OB dump no longer in use (samples labelled S1), a number of areas where AMD was evident in the dump, characterised by no vegetation and grey sandy appearance (S2), and an active OB dump (S3). In total, 8 small point samples and 5 large composite samples were taken from within the dump. This large number of samples was warranted due to the extreme heterogeneity of the material in the dump. 2.2 Determination of Mixing Ratio All raw samples were tested using the Sobek Method with peroxide siderite (FeCO 3 ) correction to establish Acid Neutralisation Capacity (ANC) [5], followed by Net Acid Generation (NAG) analysis [10]. The Net Acid Producing Potential (NAPP) of each sample was calculated based on the ANC and NAG results. From the NAPP results obtained for OB and KMR, an estimated mixing ratio that could ultimately produce a neutral mixture was derived. Compost was simply treated as an additive. 2.3 Geotechnical and Physical Characteristic Tests Moisture content of all samples were determined by oven drying each sample at 105 C. Selected OB samples from each area sampled were mechanically sieved to 75 μm to determine particle distribution as outlined in AS Atterberg Limit analysis were also conducted as per AS (liquid limit), AS (plastic limit) and AS (plastic index). The mixtures could not be tested for PSD and Atterberg Limit due to large particle size of the KMR within the mixtures. Containers of identical dimensions were filled with raw materials and mixtures (oven dried). Each container was then saturated with known amounts of water and drained in a controlled manner until equilibrium was reached between gravity and soil water retaining forces (i.e. capillary). Differences between water retaining characteristics of each sample could then be compared. Wettability was conducted as described by Appendix C of AS [2]. The filter ring specified in the standard was too small for the artificial soil due to the consistency and textural influence induced by the KMR waste (large wood chips). Therefore a larger filtration ring was used to improve compatibility with

4 858 M. Taylor, M. Yellishetty, and B.C. Panther the artificial soils. Appropriate calculations were conducted to account for variations from the standard method. 2.4 Grass Growing Analysis Grass was grown in the mixtures to assess the suitability for vegetative growth over a six week period. Water application was controlled and conditions were kept uniform across all soil plots. The soil profile was approximately 20 cm deep with a surface area of 573 cm 2 giving an approximate soil volume of cm 3. Grass was chosen because it is fast growing and simulated the initial stage of the rehabilitation sequence, where grass is grown first to provide the initial stabilisation, and ultimately create the first layer of organic content. 2.5 Geochemical Analysis The ph was determined in duplicate for each raw material and mixture (EPA Method 9045D Soils and Waste ph, 2004). 20 g of oven dry soil was mixed with 20 ml of deionised water, thoroughly mixed and left to completely settle before measuring the ph. The electrical conductivity (EC) of each raw material and mixtures was determined in duplicate, as per the method stipulated in Department of Sustainable Natural Resources (2012), where a 1:5 soil to water suspension ratio was created to measure EC. Method 1312 Synthetic Precipitation Leaching Procedure was applied to all samples and artificial soil mixtures. Inductively coupled plasma (ICP) analysis was then undertaken on each sample to determine concentrations of a range of metals. 3 Results and Discussion 3.1 Suitable Mixing Ratio All OB sampled were found to be acid producing to varying extents, while KMR and Compost were both acid neutralising (fig. 1). A large variability in NAPP was demonstrated across the OB samples. Locations with high NAPP have the potential to develop very acidic soils. An ideal dry mass ratio was found to be one part KMR: eight parts OB. Compost was treated as an additive based on literature for similar compost (one part biosolid, three parts green waste) and similar applications [6], [7], [9], [13]. The resultant ratio of the mixture was 8 parts OB, 1 part KMR and 0.6 parts compost.

5 Geotechnical and Hydrogeological Evaluation of Artificial Soils 859 Fig. 1 Net Acid Producing Potential of all raw samples 3.2 Geotechnical and Physical Characteristic Tests Particle size distribution (PSD) and Atterberg Limit were conducted for soil samples but could not be applied to mixtures or compost and KMR due to the large particle sizes. The following two tables summarise results from the tests and classify the soils accordingly (tab. 1 and tab. 2). These tests provided better understandings of the soil type within each mixture to aid in evaluating observed differences. Table 1 Particle Size Distribution results Soil Sample Gravel Sand S S S Fines Cu Cc ASTM D 2487 ASTM D 2487 (clay/silt) Group Symbol Description SM (>15% N/A N/A Silty sand with gravel Gravel) SW (>15% Well graded sand with silt Gravel) and gravel N/A N/A N/A N/A

6 860 M. Taylor, M. Yellishetty, and B.C. Panther Table 2 Atterberg Limit results Soil Sample Liquid Limit Plastic Limit Plastic Index Method PI- LL Chart soil type S1 31 S2 22 S AS AS AS N/A N/A CL (Clay Low Compressibility) The paper mill rejects had a moisture content of 57.9% and compost 31.1% %. Considerable drying would be required prior to transportation of these wastes to maximise efficiency. On average OB moisture contents were between 9.5% and 16.9%. Higher levels of clay in the fresh OB samples impacted water content, with S3 samples, high in clay (tab.1 and tab. 2), producing higher moisture content. Water retention, or field capacity, of soil is a very important factor when assessing suitability for vegetative growth, since it dictates plant available water within the soil. Field capacity was not quantitatively determined, but the influence mixtures induced on water retaining was demonstrated. The mixtures noticeably retained more water than the raw samples (fig. 2). Fig. 2 Water retention comparison between mixtures and soils The recommended value for wettability of landscaping and garden soil is >5 mm/min [2], with higher values desired. The wettability of the mixtures was increased when compared to raw samples (tab. 3). When water was added to S3 Mixture, infiltration was rapid accompanied by crackling sound of water reacting

7 Geotechnical and Hydrogeological Evaluation of Artificial Soils 861 with the dry mixture. The higher clay content was unique to this sample and may have influenced such results. S3 Mixture also produced strange results in the grass growing analysis (see 3.3). Table 3 Wettability results for mixtures and raw samples Soil Sample Trial #1 (min) Trial #2 (min) wetting rate (mm/min) S S2 Mixture S S3 Mixture Grass Growing Analysis Figure 3 shows the grass growth over time for the mixtures and in compost. The compost clearly showed the best growth (visual observation and grass length measurements), followed by S2 Mixture, S1 Mixture, and S3 Mixture. The mixtures were all capable of supporting grass growth, even the most acidic soil (see fig. 4). However, in one samples (S3 Mixture), the growth was minimal, scattered, and located in areas of least resistance (i.e. soil cracks and up against container wall). Fig. 3 Soil plot grass length recorded over time

8 862 M. Taylor, M. Yellishetty, and B.C. Panther Fig. 4 Before and after photos of acid soil illustrating successful remediation A number of observations during the grass growth phase demonstrated the complexity of developing such an artificial soil. In one of the samples (S3 Mixture), germination was delayed by a number of days. Clay content seemed to have an impact on permeability and thus drainage of the S3 Mixture was small compared to other plots. Leachate from the Kraft mill rejects had a very high electrical conductivity and appeared to create a sodic environment in the S3 Mixture. This particular mixture became increasingly resistant to water infiltration and developed a solid crust on the surface of the plot. Sodic soils with clay content >20%, such as S3 Mixture (tab. 1 and tab. 2), exhibit poor structural integrity due to the destruction of aggregation resulting from adsorption of the hydrated Na+ ions on clay particle surfaces [12]. When excessive sodium is present, the sodium ions (Na+) replace divalent cations (i.e. Ca2+, Mg2+ etc.) on the clay particle. This

9 Geotechnical and Hydrogeological Evaluation of Artificial Soils 863 decrease in cation valencee results in increased interparticle repulsion, causing clay particles to disperse in the presence of water. Dispersed clay particles can fill pores between soil particles and aggregates, forming a hard crust when dried. This restricts seedling emergence, water and air movement and root penetration; all of which were observed in the S3 Mixture. Regardless of this, grass growth was observed, albeit inconsistent over the surface. 3.4 Determine Geochemical Characteristics and Suitability Compost is relatively neutral (ph 6.5), KMR waste is alkaline with ph 10.1 and all OB samples were acidic. Figure 5 illustrates the influence mixtures had on ph, with all mixtures increasing ph from unsuitable levels to within ideal ranges between ph 5 and ph 8 [11]. These ph levels are outside of ideal conditions for acidophilic bacteria (i.e. Thiobacillus Ferroxidans, optimal ph around 2) [4], which are known to catalyse AMD reactions. Fig. 5 ph of Mixtures compared to original soils Electrical conductivity values for compost (4.6 ds/m) and KMR (11.8 ds/m) were very high in comparison to the OB samples. This is as expected as these wastes are known to contain more inorganic components than the overburden. In particular, KMR had an elevated EC, which may be due to sodium ions from the Kraft process liquors. The overburden samples ranged between <1.2 ds/m and 2.5 ds/m, with a number slightly higher than recommended [2]. In each case, mixing the OB with KMR and compost resulted in an increase in electrical conductivity commensurate with the amount of ions present in the added KMR (ratio of 8: 1)

10 864 M. Taylor, M. Yellishetty, and B.C. Panther (fig. 6). Higher EC can impact vegetation growth by inducing salinity, particularly as sodium is the primary metal present in the leachate of KMR. SPLP tests and ICP results showed no regulatory levels being breached in regards to metals leaching out of the artificial soils. Fig. 6 EC of Mixtures compared to original soils 4 Conclusion A range of mixtures of overburden, KMR and compost were created (8 parts OB; 1part KMR; 0.6 part Compost), which beneficially altered soil characteristics such as ph, plant growth, wettability and water retaining characteristics. In one case (S2 Mixture), highly acidic soils that originally could not support vegetation were successfully remediated and demonstrated ability to grow healthy grass. SPLP tests followed by appropriate ICP analysis, demonstrated no regulatory levels for heavy metals were breached. Although all samples supported grass growth, a number of issues were identified requiring further consideration: - Leachate from mixtures showed an increase in electrical conductivity, which may indicate potential leaching issues when used in the environment. - One mixture appeared to induce a sodic environment upon grass growth, severely impacting on soil structure and growth of vegetation as demonstrated by S3 Mixture. This illustrates the great variability in overburden composition throughout an OB dump and needs to be considered when creating artificial soils on a large scale.

11 Geotechnical and Hydrogeological Evaluation of Artificial Soils 865 Further studies are required to monitor the long-term processes that occur in such mixtures, but the potential for useful artificial soils created from waste streams such as these have been demonstrated in this project. References [1] Agarwal, A., Strachan, P.: Is Industrial Symbiosis only a Concept for Developed Countries? The Journal for Waste & Resource Management Professionals, 42 (2008) [2] Australian Standard, Soils for landscaping and garden use. Standards Australia International, Sydney (2003) [3] Evangelou, V.P., Zhang, Y.L.: A Review - Pyrite Oxidation Mechanisms and Acid- Mine Drainage Prevention. Critical Reviews in Environmental Science and Technology 25(2), (1995) [4] Herman, D.C., Maier, R.M.: Consequences of Biogeochemical Cycles Gone Wild. Academic Press. Inc., s.1. (2009) [5] Leavitt, B., Skousen, J., Ziemkiewicz, P.: Effects of siderite on the neutralisation potential in the acid-base account. In: Proceedings of the 17th Annual West Virginia Surface Mine Drainage Task Force Symposium, Morgantown, West Virginia (1995) [6] Kelly, G.: Recycled Organics in Mine Site Rehabilitation. Department of Environment and Conservation NSW, Parramatta (2006) [7] Kelly, G.: Application of recycled organics in mine site rehabilitation. Department of Environment and Climate Change NSW, Sydney (2008) [8] Lottermoser, B.G.: Mine Waste, 3rd edn. Springer, Heidelberg (2010) [9] Mercuri, A.M., Duggin, J.A., Grant, C.D.: The use of saline mine water and municipal wastes to establish plantations on rehabilitated open-cut coal mines. Forest Ecology and Management (204), (2005) [10] Miller, S., Robertson, A., Donahue, T.: Advances in acid drainage prediction using the Net Acid Generation test. In: Proceedings of the 4th ICARD, International Conference on Acid Rock Drainage, Vancouver, pp (1997) [11] NSW Agriculture, Understanding Soil ph. Yanco Agricultural Institute, NSW (2000) [12] Pittaway, P.: How to interpret if your soil is sodic, using soil physical and chemical test results. Chrysalis Landscape Consultants, Laidley (2002) [13] Sanchez, E.S., Richard, T.L.: Using Organic Nutrient Sources. The Pennsylvania State University, Pennsylvania (2009)

Engineering Review of the Use of Leached Brown Coal Ash in Soil Covers

Engineering Review of the Use of Leached Brown Coal Ash in Soil Covers Gavin M Mudd a*, Srijib Chakrabarti b, Jayantha Kodikara b a Institute for Sustainable Water Resources/Department of Civil Engineering,