Over the years MTL Consulting limited has successfully undertaken numerous soil survey for many projects as it is one of the principle requirement by the relevant legislative frameworks in the country and with some other project internationally set standards and guidelines. The general objective of conducting soil surveys within the project sites has been to characterize soils of the proposed project sites for the purpose of ascertaining their physical and chemical characteristics so as to evaluate their general physical standards on any observed changes prior to existing environmental management plan and the suitability of such soil characteristics for rehabilitation/restoration actions. Ideally, a good soil that will favor plant growth ought to have the selected chemical properties presented here under. Studies on soil physical and chemical properties have been proposed in many studies and also incorporated in the soil quality standards as indicators of nutrient supplying capacity of agricultural, forest soils and rangeland that will henceforth support stable ecosystems hosting relatively diversities of flora and fauna species.
As for the heavy metals, mining are regarded as a significant source of heavy metals contamination of the environment owing to activities such as mineral exploitation, ore transportation, smelting and refining, disposal of the tailings and waste waters around mines. Toxic chemicals are released into the environment following weathering of the heaped waste materials. Such harmful metals include Lead (Pb), Mercury (Hg), Copper (Cu), Nickel (Ni), Uranium (U) and Cadmium (Cd) are among heavy metals reported in many mine polluted soils in Tanzania and are amongst extensively studied heavy metals in the soil elsewhere. The selected heavy metals are prescribed in the soil quality standards. See the National environmental management (soil quality standards) regulations (2007) of Tanzania which are also in concurrence with East Africa Regulations and standards. The following description below shows different methodological approaches that are normally conducted in site and other in the laboratory analyses work;
Soil Sampling and Sample Preparation
Soil samples within the project footprints are collected within grid transects which are laid in the interest of capturing a broad sample of representative soil types. A maximum of 45 soil samples representative of the sampled sites are normally collected systematically for chemical analyses which is conducted in a certified laboratories namely SGS laboratories based in Mwanza. Soil sampling focuses on additional sites earmarked for mining and location of infrastructures.
Soils is cored at specific intervals (350 m X 350 m grid) on site using a 150 mm soil auger up to 1.2 m deep or to refusal depth where the underlying parent rock is reached prior to the set depth. At least one soil profile is excavated per site in natural exposures and in some areas in cuttings or gullies and is thoroughly described for each major soil types. For each test pit excavated, soil horizons is described based on their respective soil colour, texture, wetness, occurrence of concretions or rocks, underlying material, wetness and presence of roots. Types of soil within the project areas are classified based on USDA soil taxonomy which is widely used by soil scientists around the world to describe different systems. All types of soil orders are achieved by reference to the properties of soil series.
Determination of Soil physical Characteristics
Soil textural classes are established according to the modified method by Thien (1979). This method entails estimating percentages of sand, silt, and clay particles in a soil by rubbing the moist soil between the fingers and the thumb and estimating the amount of the various separates present. The estimation of various separates are normally done by a skilled and experienced field soil scientist.
Soil colour is determined in the field by comparing a piece of soil to the standard color chips in a Mussel color charts.
This is established in the field by observing type (shape) of the structural peds present and their relative size (e. g fine, medium or coarse) and degree of development or distinctness of the peds such as strong, moderate or weak.
Prior the analyses the collected soil samples for laboratory analyses are pre-treated based on the following procedures:
- Drying the soil samples by placing them in a shallow tray in a well ventilated, dust and wind free area.
- Breaking the clay clods, crushing the soil lumps in order to remove gravel, roots and large organic residues and
- Sieving the soil samples through a 2 mm sieve, and ground them in a mortar in order to pass through a 60 mesh screen ready for chemical analysis
Soil pH and electrical conductivity (EC) is measured in H2O suspension (1:2) and detected directly using a probe. Total Nitrogen determination employs semi-kjeldahl digestion method in accordance to Allen (1999) while soil organic matter is evaluated using Walkley-Black potassium method (Nelson and Sommers, 1999). The available phosphate is established using Bray P-1 method developed by Emteryed (1989). Amounts of exchangeable bases (K, Ca, and Mg) and cation exchange capacity (CEC) is measured after successive extraction using 1 mol L-1 ammonium acetate (pH 7.0) and 100g L-1 NaCl. The amount of NH+4 replaced by sodium the then determined by the steam distillation method. An exchangeable base concentration is resolved by atomic absorption spectrometry.
Analysis of the selected heavy metals (Hg, Cd, U, Pb, Cu, Ni) is conducted by ICP-OES after extraction with DTPA. Testing of heavy metal levels is conducted based on the permissible levels by the National environmental management (soil quality standards) regulations (2007) of Tanzania