Research Article | Open Access
Hydrogel and Organic Amendments to Increase Water Retention in Anthroposols for Land Reclamation
Using waste materials from industrial activities to build anthroposols (soils built or altered by humans) can provide soil for reclamation and reduce amounts of materials stored in landfills. Mines and other large industrial disturbances requiring anthroposols usually have large amounts of nonorganic waste materials with low water holding capacity and large amounts of coarse fragments. Thus, water holding capacity is a key property to build into anthroposols as all aspects of revegetation are strongly influenced by soil water content. This research assessed the effectiveness of hydrogel and organic amendments to increase the water retention in common mine wastes used to build anthroposols for reclamation in three greenhouse experiments. Waste materials were crushed rock, lakebed sediment, and processed kimberlite, from a northern diamond mine in Canada. Amendments were hydrogel, sewage, salvaged soil, and peat. Pots were filled with the material and weighed and saturated, followed by periodic weighing until the weight was near constant. Water retention was consistently highest in processed kimberlite, with and without amendments. Water retention increased most with hydrogel in processed kimberlite and crushed rock. Hydrogel application method impacted the initial water retention, but over time, the effect was limited. Water retention in lakebed sediment showed little difference relative to no amendment addition and had lowest increases relative to other substrates. Type of waste material and amendment, application rate, and application method impacted water retention and can be adapted to build anthroposols in the field using waste materials suitable for reclamation.
Mining and other industrial activities produce large amounts of waste and disturb large areas that require land reclamation. Some waste materials must be capped due to their chemical properties; others can be used for reclamation. Soils can be built by combining waste materials and adding amendments to ameliorate the limiting properties required for vegetation establishment and growth (e.g., [1–3]). These anthroposols, human-made or human-altered soils , are important in land reclamation as they use materials that would otherwise need to be handled as waste and may be confined to landfills.
Water holding capacity of soil materials is an important consideration for land reclamation. Suitable soil water content is essential for seed germination, seedling establishment, and plant survival as desiccation is a major risk on disturbed sites, especially in arid environments like the arctic [5–7]. Water holding capacity of materials varies with properties such as structure, pore size, particle size, proportion of coarse material, and organic matter content [8, 9]. Particle size plays an important role in water holding capacity [8, 9]. Sand textured materials have large particles resulting in few larger pores relative to clay or silt textured materials, which have smaller particles and numerous smaller pores and result in greater water holding capacity. The proportion of coarse fragments over 2 mm significantly influences the water holding capacity as it creates large pores that are unable to hold water [8, 9]. Organic matter increases the water holding capacity as it alters particle aggregation and pore size distribution [8, 10].
Unamended mine wastes tend to have low water holding capacity, and therefore, amendments are essential to reduce water limitation in reclamation soils. Numerous amendments have been proposed to increase water holding capacity, primarily organic amendments. An alternative is hydrogel, an acrylic polymer that absorbs water and releases it over time which can be mixed with soil or other soil building materials to increase water retention and reduce water stress on vegetation [11–14]. While it is known that both hydrogel and organic amendments can increase water retention, studies comparing their effect on water retention of different reclamation soil building materials are limited.
The objectives of this research were to determine whether water retention of soil building materials (hereafter substrates) was altered by various amounts and application methods of hydrogel and select organic amendments and whether water retention was altered with rehydration of the materials. We hypothesized that substrates would retain different amounts of water regardless of the amount of amendment and that amendments would alter water retention regardless of the substrate depending on amendment and application rate and method.
2. Materials and Methods
2.1. Substrates and Amendments
Three primary waste materials from Diavik Diamond Mine, Northwest Territories, Canada (latitude 64°30′41″, longitude 110°17′23″), processed kimberlite, lakebed sediment, and crushed rock were used for the experiments. Treated sewage sludge (hereafter sewage) and salvaged soil (hereafter soil) were sourced from Diavik Diamond Mine. Peat (Premier Horticulture Inc.) and Soil Moist were purchased from a commercial supplier. Soil Moist is a synthetic acrylic cross-linked polyacrylamide with a potassium salt base (hereafter hydrogel) that absorbs water and releases it slowly as soil dries, potentially reducing watering needs up to 50% (JRM Chemical Inc.) [15, 16]. It has a slightly acidic to neutral pH and potassium salt base, persists 3 to 5 years in soil, and is advertised as nontoxic to plants [15, 16]. Chemical properties of materials varied considerably (Table 1).
Data are presented as means with standard errors in brackets (adapted from ).
2.2. Experimental Procedure
Three greenhouse experiments were conducted using complete randomized designs, all replicated 4 times. The hydrogel experiment and rewetting hydrogel experiment evaluated 3 substrates × 10 hydrogel treatments (3 application rates × 3 application methods and a control of each substrate with no hydrogel). Hydrogel application rates were manufacturer recommended (488 kg·ha−1), half recommended, and double recommended. Application methods representing potential field methods were dry substrate and dry hydrogel mixed then wetted (dry/dry), dry substrate and wet hydrogel mixed then wetted (dry/wet), and substrate and hydrogel wetted separately then mixed and wetted again (wet/wet). The amendment experiment evaluated 3 substrates × 11 treatments (5 amendments × 2 application rates and a control of each substrate with no amendment). Application rates were manufacturer recommended for hydrogel and 10% by volume organic amendments, and double hydrogel and 20% by volume organic amendments. A 10% by volume rate was considered potentially feasible for field application for reclamation. Five amendments were dry substrate and dry hydrogel mixed then wetted (dry/dry), dry substrate and wet hydrogel mixed then wetted (dry/wet), peat, sewage, and soil.
For all experiments, 7 cm tall and 8 cm diameter round pots were used. Two layers of landscape fabric were placed in the bottom to reduce loss of material through holes in the pot base. Consistent volumes of dry substrate of known weight were mixed with required amendments to achieve target volume ratios and then placed in the pots. For the dry/wet treatment, hydrogel was saturated by placing it in a beaker of water for 24 hours, draining excess water and then mixing with dry substrates. For the wet/wet treatment, hydrogel was saturated as mentioned above and the substrate was saturated by placing it in a tray of water for 24 hours and then mixing. Weight of filled pots prior to saturation was determined based on the known weight of each component (substrate, amendment, landscape fabric, and pot) to facilitate assessment of water retention.
After mixing, all pots were wetted by placing them in a tray of water for 24 hours to approximate saturation. Pots were weighed upon removal from the water, representing the 0 hour or saturation weight. Pots were weighed approximately twice a day for the first two days and then daily until constant or near constant weight. For hydrogel rewetting experiment, pots were rewetted using the same method after drying and weighing as above.
Substrate particle size was determined by sieving five replicates of each through sieve sizes 19.0, 12.5, 9.5, 6.3, 4.0, 2.0, 1.0, 0.5, 0.25, 0.212, 0.106, and 0.053 and less than 0.053 mm. All large rocks, greater than approximately 4 cm, were removed in this experiment due to the size of the pots which could not accommodate that particle size. Sieves were stacked in piles of three based on the order above and material placed on the largest sieve, with a pan at the bottom. The stack was moved smoothly and consistently by hand for 1 minute. The material left in each sieve was weighed and what passed through into the pan was placed on the top of the next stack of three sieves, repeated until all sieves were used.
2.3. Data Analyses
Water retention was determined by subtracting prewatering weight from the weight at each assessment and calculating the % water by weight. Three time periods were assessed for each experiment: 0 hours, approximating saturation; 48 hours, approximating field capacity; and near dry (77.2 hours for hydrogel experiment, 73.5 hours for rewetting hydrogel experiment, and 124.6 hours for amendment experiment). Data were assessed for normality and homogeneity of variance and then analyzed using ANOVA for continuous data. The final time period in the amendment experiment was log 10 transformed to improve homogeneity. Tukey HSD tests were completed for post hoc analysis for a priori comparisons. A significance level of 0.05 was used. Statistical analyses were performed using RStudio Version 3.4.0 .
3.1. Substrate Texture
Substrate particle size varied (Table 2). Coarse material over 2 mm dominated crushed rock (58.5%) and lakebed sediment (42.3%) (keep in mind large-sized rock pieces were removed prior to sieving); lakebed sediment and processed kimberlite contained a large proportion of fine material under 212 μm, 17.9% and 16.5%, respectively.
Data are presented as means with standard errors in brackets (n = 5). Amounts presented for each sieve size represent the percentage of material caught in that sieve.
3.2. Hydrogel Experiment
There was significant interaction between substrate and hydrogel treatment (application method and rate) (saturation ; field capacity ; near dry ).
Substrate influenced the water retention, with processed kimberlite holding more water than lakebed sediment and crushed rock at saturation (significant in 100% of comparisons), field capacity (90%), and near dry (85%). At field capacity and near dry, lakebed sediment and processed kimberlite did not differ when no amendment was added. With no amendment addition, lakebed sediment held more water than crushed rock, statistically more at saturation and field capacity. At double application rates, crushed rock held significantly more water than lakebed sediment although the significance of effect decreased over time.
The application method had a limited effect in crushed rock relative to lakebed sediment and processed kimberlite (Figure 1). Dry/wet application tended to have the greatest water retention and wet/wet the lowest, particularly in processed kimberlite. In crushed rock and processed kimberlite, higher application rates generally resulted in greater water retention than lower rates or no amendment addition (Figure 1). Application rate had little effect on lakebed sediment, between rates or no amendment.
3.3. Rewetting Hydrogel Experiment
There was significant interaction between substrate and hydrogel treatment (application method and rate) (saturation ; field capacity ; near dry ). Water retention was generally less upon rewetting than initial saturation.
Substrate affected the water retention when materials were rewetted especially at saturation and field capacity. Processed kimberlite held more water than lakebed sediment and crushed rock at saturation (statistically 100% of comparisons) and field capacity (95%). At near dry, the effect declined (70%) although processed kimberlite continued to have the highest water retention, especially when hydrogel was applied dry/wet and at recommended rates. When applied at high rates at near dry, crushed rock tended to be statistically similar to processed kimberlite; at low rates, lakebed sediment was similar to processed kimberlite. At field capacity and near dry, lakebed sediment and processed kimberlite did not differ when no amendment was added, nor did crushed rock to the processed kimberlite at near dry. Crushed rock and lakebed sediment had few significant differences (30% of comparisons differed at saturation and 10% at field capacity and near dry). Generally, crushed rock tended to have greater water retention at higher application rates and lakebed sediment at lower or no application.
Upon rewetting, the application method had a limited effect except in processed kimberlite where wet/wet tended to have the lowest retention (Figure 2). Higher application rates resulted in greater water retention in crushed rock and processed kimberlite (Figure 2). Addition of hydrogel at any rate tended to increase water retention relative to no amendment though significance varied. The rate had a limited effect on lakebed sediment.
3.4. Amendment Experiment
There was significant interaction between substrate and treatment (amendment and rate) (saturation ; field capacity ; near dry ).
Substrate affected water retention with processed kimberlite having higher retention than crushed rock and lakebed sediment at saturation (statistically 100% of comparisons) and field capacity (95%, no amendment did not differ from lakebed sediment). As substrates approached near dry, differences became less significant between processed kimberlite and lakebed sediment (statistically 36% of comparisons), whereas processed kimberlite remained greater than crushed rock (82%). Processed kimberlite held more water than crushed rock with all amendments except dry/dry, whereas it only held more than lakebed sediment in dry/wet and sewage. With hydrogel, crushed rock tended to have greater water retention than lakebed sediment, although statistical significance was reduced over time with only wet/dry double significant across the three time periods. Lakebed sediment tended to have greater retention than crushed rock with organic amendments and no amendment, except sewage, although by near dry, only soil and peat recommended and no amendment was significantly greater.
Amendment selection altered water retention differently between substrates (Figure 3). Hydrogel generally resulted in greater water retention in crushed rock and processed kimberlite compared to organic amendments (except peat in processed kimberlite). In lakebed sediment, water retention showed little consistent effect of amendment, although peat tended to have the greatest water retention. Application rate was important for hydrogel treatments especially in crushed rock and processed kimberlite, with less variation in organic amendments (Figure 3). Generally, addition of amendments at any rate increased water retention in crushed rock and processed kimberlite relative to no amendment, whereas in lakebed sediment, addition of amendment rarely increased water retention compared to no amendment.
Development of methods to increase water retention in anthroposols built from mine waste materials is essential for successful reclamation. Some clear trends emerged from this research showing the importance of substrate, amendment, hydrogel application method, and amendment application rate in altering water retention for reclamation anthroposols.
Substrate variability in water retention despite amendment amount, application rate, and amendment type clearly shows its importance for anthroposol building. Using results of this research, good choices can be made to use substrate material alone or in mixes, with and without amendments.
Greater water retention in processed kimberlite relative to crushed rock and lakebed sediment is due to its composition of particles under 2 mm. At saturation, unamended processed kimberlite held approximately 25% water by weight indicating high water retention even without amendments. Large amounts of coarse fragments in mine waste are common due to extraction and blasting, parent material, and treatment of waste . Coarse fragments create large pores that cannot hold water, reducing water holding capacity; large rocks take up volume that would have been composed of mineral soil under 2 mm that primarily holds water [8, 9, 19], as seen in crushed rock and lakebed sediment. Lakebed sediment has more fine textured material than crushed rock, resulting in greater pore space and surface area , leading to higher water retention than crushed rock when unamended. However, in the field, the finer texture of lakebed sediment may negatively impact water retention, as a high proportion of fine particles can result in poor structure, creating a hard, smooth surface which can reduce infiltration [9, 21, 22].
With hydrogel, water retention in crushed rock can be greater than in lakebed sediment. The hydrogel can fit into the large pores for maximum expansion and more held water. The high proportion of sand and large pores in processed kimberlite also allows hydrogel expansion. Expansion is essential for long-term success of field applications as hydrogel needs to be rewetted through natural precipitation. Success under field conditions may be reduced in crushed rock as hydrogel crystals may slip into gaps between rocks that are not accessible to plant roots . Fine particles in lakebed sediment may have restricted expansion of hydrogel, reducing water retention and effectiveness in this substrate. Abedi-Koupai et al.  compared the effects of hydrogel application on water retention in sandy loam, loam, and clay, finding highest water content increases in sandy loam and least in clay, likely due to reduced expansion of the hydrogel. Saline conditions can reduce water uptake of hydrogel , and lakebed sediment had the highest electrical conductivity of the substrates.
The small increases found in lakebed sediment with amendment addition may be too small to be practical in the field, especially relative to greater increases in water retention in crushed rock and processed kimberlite with amendment addition. A more promising method in anthroposol construction may be to mix lakebed sediment with crushed rock or processed kimberlite to gain the best from each of the substrates.
4.2. Hydrogel Application Method
While substrates differ intrinsically in water holding capacity, methods to increase water retention are essential as use of waste materials is influenced by many factors beyond water holding capacity, including nutrient content, presence of metals and salts, availability, and regulatory requirements. Hydrogel application research had focused on dry application [11–13] whereas this research tested three strategies. With initial wetting, dry/wet application was most successful since wetting hydrogel in advance allowed the crystals to expand to their maximum size without being impeded by the substrate.
Wet/wet application was less successful likely from loss of material due to challenges with mixing and hydrogel migration to the surface of the pots, especially in lakebed sediment, reducing crystal expansion with water when there is a lack of contact with the substrate. The rough and irregularly shaped pieces in crushed rock may have impeded hydrogel migration. Wet/wet application is difficult to scale up to an industrial level, limiting its effectiveness for large disturbances; dry/wet may have similar challenges. The reduced success of dry/dry application relative to dry/wet may be due to the hydrogel expanding within the substrate, potentially limiting full expansion.
Rewetting hydrogel demonstrated there would likely be a limited effect of the application method in the field. Although dry/wet still tended to be the most successful method, differences were smaller than with initial wetting. As pots were allowed to dry before rewetting, all of the treatments started the same. Small differences, especially with wet/wet having lower water retention, may be due to how the hydrogel originally settled in the pots, limiting its effectiveness. Overall, the limited effect of application method of hydrogel means method can be determined by what is industrially feasible, likely dry/dry application.
4.3. Amendment Selection
Effects on water retention of adding organic amendments relative to hydrogel is important for reclamation, especially knowing that the effectiveness varied with substrates. The greater increases in water retention with hydrogel, especially dry/wet, in crushed rock and processed kimberlite than lakebed sediment is likely due to the coarser textured substrates potentially allowing hydrogel to expand more, holding more water . Hydrogel can hold 40 to 500 times its weight in water, depending on type, size, and chemical makeup [11–13], with the primary goal of increasing available water and acting as a reservoir.
Organic amendments are commonly added to soil or substrates to address physical, chemical, and biological limitations, rather than the single problem of low water holding capacity. Organic matter increases soil water and nutrient content, water and nutrient holding capacities, and cation exchange capacity; improves soil texture and pH; and provides energy sources for microorganisms [1, 23–25]. Organic amendments, like peat, take up physical space, whereas hydrogel has to expand, which may result in greater water retention in lakebed sediment, although peat showed limited statistical differences from hydrogel, indicating that all amendments had poor success in lakebed sediment. Peat has increased water holding capacity in many experiments [24–27] and resulted in the greatest increase relative to other organic amendments in this research. The limited increase in water retention with sewage may have resulted from its form. The sewage was collected after being dewatered, which involved pressing between two belts to remove excess water. The sewage remained very wet and therefore may have had a reduced ability to take up large amounts of additional water. The limited increase in water retention with addition of soil may be due to its low organic carbon content, relative to peat and sewage and the high proportion of sand (coarse fragments 28.9%; sand 74.3%, silt 20.7%, and clay 4.9%).
While organic amendments hold water, their greatest influence on water holding capacity of the substrates will result from their effect on structure and aggregation . As microorganisms decompose the organic matter, products help form aggregates, which increase water holding capacity and improve infiltration and percolation [9, 25]. Therefore, organic amendments are likely to have a greater effect in the long term than hydrogel. While hydrogel showed a large increase in water retention in the short term of the experiments, its long term effectiveness is less. Hydrogel decreased in effectiveness over time in other studies due to degradation from environmental exposure [12, 14]. Rowe et al.  found an 85% reduction in water held after 42 months. In this research, all treatments held slightly less water between the initial wetting of the hydrogel and subsequent rewetting. However, increased water retention over the first few critical growing seasons may result in greater plant growth, which over time will increase organic matter content as plants die and decompose, improving soil structure.
4.4. Application Rate
The more important effect of application rate than application method for hydrogel, especially in crushed rock and processed kimberlite, is important for anthroposol building in reclamation. In other research, increasing the amount of hydrogel also increased the amount of water held [11, 12] as seen with crushed rock and processed kimberlite. The relationship was less clear in lakebed sediment as water retention was high even when no amendment was applied, although generally higher application rates slightly increased water retention. The reason for the low water retention of dry/wet hydrogel applied at a double rate in lakebed sediment compared to other treatments in lakebed sediment for the amendment experiment is unknown; however, it demonstrates the uncertainty related to using hydrogel in lakebed sediment where the patterns are not clear.
Determination of the appropriate application rate is influenced by several factors, including cost of material, amount available, depth of mixing, and substrate. Many reclamation sites are remote, creating challenges for economically transporting large amounts of material to sites. Use of onsite materials, such as sewage, topsoil, or manure (depending on the site), reduces transportation costs but supplies may be limited for reclamation. Hydrogel represents an option for remote sites as it is easy to transport and small in volume and weight, though it may have a higher cost depending on the source of the organic materials.
4.5. Outstanding Questions and Recommendations
This research begins to address the challenge of increasing water retention in mine waste materials used for soil building. Additional research to focus on outstanding questions will further the knowledge required for successful reclamation. As these experiments were completed in the greenhouse, they had limited exposure to environmental conditions. Hydrogel is estimated to last 3 to 5 years in soil (JRM Chemical Inc., 2012); however, mine waste materials may have chemical and physical properties that result in faster hydrogel breakdown. Harsh environmental conditions, including short summers, cold winters, freeze-thaw cycles, and extended light periods, may negatively affect both hydrogel (reduced ability to hold water as it breaks down) and organic amendments (slow decomposition limiting the effect on soil structure). In the field, substrates may not be saturated often due to short rainfall events which could influence effectiveness of amendments. Rowe et al.  found that their hydrogel went from anhydrous to full capacity in 30 minutes when exposed to saturation by water. However, amendments, especially hydrogel, may not expand to their full capacity in partially saturated conditions. Increased water retention also only represents one factor required to build successful anthroposols for revegetation. Research is needed to compare the plant response to both hydrogel and organic amendments in these substrates.
The addition of hydrogel and organic amendments has potential for building anthroposols for reclamation simply by its effects on water retention. Water retention was increased with use of amendments and varied with substrate materials. Processed kimberlite generally held the most water with and without amendments. A double application rate for both hydrogel and organic amendments resulted in greatest increases in water retention. Applying wet hydrogel to dry substrates resulted in the greatest initial increase in the amount of water held, but over time, there were only small differences. Dry hydrogel would be easiest to apply at an industrial scale. Differences among substrates showed that hydrogel resulted in the greatest increases in water retention in crushed rock and processed kimberlite, whereas lakebed sediment showed little increase with amendment addition, especially hydrogel, and was only slightly greater with peat.
The data used to support the findings of this study are available from the corresponding author upon request.
Funding sources had no influence on the study design, collection, analysis and interpretation of data, writing the report, or the decision to publish.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding this study.
This work was supported by the grants to Dr. M Anne Naeth from the Diavik Diamond Mine, the Helmholtz-Alberta Initiative, and the Land Reclamation International Graduate School, through the NSERC CREATE program and through an NSERC Alexander Graham Bell Canada Graduate scholarship to Valerie Miller. We gratefully acknowledge the assistance of Dr. David Chanasyk, Stacy Campbell Court, Sarah Wilkinson, and the Environment Department of the Diavik Diamond Mine. In-kind support was provided by the Diavik Diamond Mine.
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Copyright © 2019 Valerie S. Miller and M. Anne Naeth. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.