Variations of Ni, Cr and Mn Concentration in Soils Formed Along a Toposequence of Ultrabasic Rocks in Western Mashhad

Parent materials as one of the main soil formation factors have a great impact on the concentration of heavy metals in the soil. Heavy metals are released to the soil during weathering and pedogenic processes. Ultrabasic rocks are known as the potential natural source of heavy metals, especially Ni, Cr and Mn in the soil. Average concentrations of Ni and Cr in the soils are 84 and 34 mg kg-1, respectively; while, in soil derived from ultrabasic parent material, the concentration of these elements may reach up to 100000 mg kg-1. Binaloud zone in northeastern composed of different geological materials. There is a narrow band of ophiolitic rocks in this zone that located along Mashhad city. The geochemical behavior of ultrabsic rocks and the associated soil have been frequently studied mostly in humid regions. But, there are a few research works done in arid environments. The objective of this study was to investigate the physical and chemical properties and concentrations of Ni, Cr and Mn in soils formed along a toposequence of ultrabasic rocks in western Mashhad. The study area is located in the hilly land landscape of Binaloud zone in the Western part of Mashhad. Mean annual precipitation and temperature is 260 mm and 13.7 oC, respectively. Soil temperature and moisture regimes are thermic and aridic boarder on mesic, respectively. Studied soils developed on hornblendite rocks that are ultrabasic rocks with SiO2 less than 45% and contain ferromagnesian minerals. A toposequence was selected and, three soil profiles on shoulder, backslope and footslope geomorphic positions were described acoording to key to soil taxonmy 2014 and the soil horizons were sampled. Air-dried samples were passed through 2 mm sieve and were used for laboratory analysis. Pseudo-total concentrations of Ni, Cr and Mn were extracted by aqua regia digestion procedure. Free iron oxides (Fed) and amorphous iron oxides (Feo) were extracted by citrate-bicarbonate-dithionite (CBD) and oxalic acid methods, respectively and were measured by atomic absorption spectroscopy. The soil was extracted by ammonium acetar 1N and concentration of Ca and Mg were measured by EDTA titrimetric method. Calcium carbonate equivalent, gypsum, pH, Sand, silt and clay fractions and soil organic materials were measured using custom laboratory methods. Solum thickness of the studied soils is less than 45 cm. Calcification and gypsification are the two main soil formation processes leading to formation of calcic (Bk) and gypsic (By) horizons. Calcium carbonate equivalent and gypsum contents in the studied soils varied from 5.1 to 30 and 5.9 to 40.1 %, respectively. Regarding the type of parent material, presence of large amounts of gypsum and carbonates can be attributed to aeolian addition to the soil system. The presence of discontinuous and thin loess deposits in the study area confirms the dustfall deposition. High amount of these minerals cause Ca/Mg ratio is up to 33.3. Concentration of Fed and Feo were less than 6.8 and 0.2 g kg-1 reflecting weak wethering state of the soils. Morphological characteristics are the indications of weak soil development and weathering. Concentrations of Ni, Cr and Mn varied from 52.6 to 312.5, 35.2 to 135.3 and 375.3 to 628.9 mg kg-1 that are low values in comparison to soils in humid regions due to weak soil weathering and eolian addition of materials containing gypsum and carbonates. The Ni and Cr contents increase from shoulder to foot slope. Direct and concordant variations of Ni with Cr and Mn with Fed indicate the similar mineralogy and trend of weathering of these elements. Regarding the high concentration of Ni and Cr in the studied soils, the bioaccessibility of these elements should be investigated. Results of this study indicated the weak development of soil formed on ultabasic rocks in the western Mashhad that was expected regarding the arid climate of the study area. Because of the low weathering status of the soil, the concentration of Ni, Cr and Mn were less than that of similar soils in humid areas. Also aeolian addition of carbonates and gypsum to the soil system dilutes the concentration of these elements. To evaluate risk assessment of Ni, Cr and Mn in the studied soils, successive extraction and pot experiments are suggested.