Browsing by Author "van Schalkwyk, Roeline"

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    Evaluation of old store-and-release covers on discard dumps and backfilled pits to improve and predict their performance for rehabilitated mines at Mpumalanga Highveld, South Africa
    (Stellenbosch : Stellenbosch University, 2021-12) van Schalkwyk, Roeline; Hoffman, Josias Eduard; Van Zyl, Johan Hendrik Combrink ; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: Store-and-release covers (SRCs) are an important mitigation method to protect the environment at rehabilitated mines in the Mpumalanga Highveld, South Africa. The long-term performance of SRCs can be influenced by soil cover-, soil hydraulic-, and vegetation properties. Currently, a Technical Guideline for Soil Covers Development is not in place in South Africa. In addition, data sets of well- and poorly constructed covers, and the availability of data on appropriate input parameters for predicting long-term performance of such covers are limited. This need includes data for saturated hydraulic conductivity (Ksat), soil water retention curves (SWRCs), and photosynthetic active leaf- area index (LAI). Moreover, the measurement of Ksat and SWRCs is time-consuming, labour intensive and costly. Consequently, a multidisciplinary study to investigate the impact of soil cover-, soil hydraulic- and vegetation properties on long-term performance of SRCs was initiated. Most importantly, pedotransfer functions (PTFs) to predict Ksat and SWRCs were developed from particle- size distribution, soil organic matter (SOM) and bulk density. Leaf area index values for good and poor vegetation covers were determined for rehabilitated mines in Mpumalanga Highveld. Soil cover properties viz. cover configurations, soil texture, Atterberg limits, bulk density and soil nutrient availability were determined. Saturated hydraulic conductivity were measured using two types of double-ring infiltrometer, a single-ring infiltrometer, and a constant-head permeameter. Soil water retention curves were established using the pressure plate apparatus. The SRCs data-set was split into training and testing sets to validate the SWRC model. After the SRCs data-set was split into moderately- and very dense SRCs data sets, and an additional site was used to validate the moderately dense Ksat model. The data-set of very dense SRCs was also split into training and testing sets to validate the very dense Ksat model. Monthly LAI from September 2018 to August 2019 was destructively measured using a LI-3100C Area Meter. The dual-layered SRCs were constructed with sandier growth medium (top layer) underlaid by a loamy to clayey water retention layer (sub-layer). Monolithic SRCs were constructed of sandy loam or sandy clay loam soil covers. After the SRCs were split into moderately- and very dense soil cover conditions, the moderately dense SRCs performed significantly better and had acceptable bulk densities, good vegetation covers with good root distribution in the growth medium, steep slope in the desaturation function of the growth medium and high water-holding capacity (WHC) in the water retention layers. The Ksat and WHC of the moderate SRCs over 20 years had values similar to that of the soils, but the values of sandier soil cover layers were lower than critical threshold values due to low resistance to compaction. The statistical analysis of best-fit moderately- and very dense Ksat. models yielded an adjusted R2 of 0.749 and 0.999, respectively from sand-, silt- and clay content, SOM and bulk density. The statistical analysis of the best-fit SWRC model of 14 matric potentials had an adjusted R2 = 0.827 from three fractions of sand-, two fractions of silt-, clay content, SOM, and bulk density. The photosynthetic active LAI for good and poor vegetation cover of rehabilitated mines at Mpumalanga Highveld was ~1.2 and 0.8 m2.m-2, respectively. Poorly constructed soil covers result in high bulk density, low to very low Ksat and WHC values and poor vegetation properties and should be avoided at any cost. The critical threshold values for bulk density, Ksat and WHC of soils can be used to evaluate long-term soil cover performance. The developed PTFs can be used to predict soil covers’ hydraulic properties having soil physical properties similar to the old SRCs. These results can be considered as a possible amendment to the Technical Guidelines on Soil Cover Development in South Africa.