Accumulation of heavy metals in typic sulfaquents of Kuttanad ecosystm

Abstract

An investigation entitled “Accumulation of heavy metals in Typic Sulfaquents of Kuttanad ecosystem” was conducted in the Department of Soil Science and Agricultural Chemistry, College of Horticulture, Kerala Agricultural University during 2014-2016. Sample collection was done from six acid sulphate soil series of Kuttanad viz., Ambalapuzha, Kallara, Thakazhi, Purakkad, Thottapally and Thuravur series. Twenty soil, sediment and water samples were collected from each series. Dominant aquatic macrophytes were also collected from each series. Mainly three mangrove species (Avicennia officinalis, Bruguiera gymnorrhiza and Rhizophora mucronata) were present in the area, and were collected for analysis. Soil and sediment samples were characterized for texture, pH, EC, organic carbon, macronutrients, micronutrients (Fe, Mn, Zn and Cu) and heavy metals (Pb, Cr, Hg and Al). Water samples were analysed for its irrigation quality parameters such as colour, temperature, turbidity, suspended solids, pH, EC, biological oxygen demand (BOD) and chemical oxygen demand (COD). Also the plant and water samples were analysed for its macronutrients, micronutrients (Fe, Mn, Zn and Cu) and heavy metals (Pb, Cr, Hg and Al). Based on the analytical data, maps were prepared by using specialized software ArcGIS ArcMap version 10 of ESRI. The soil samples from acid sulphate soil series of Kuttanad were extremely acidic whereas the sediment and water samples were slightly acidic. Highest EC values were recorded for soil, sediment and water samples collected from Purakkad series. Significantly higher OC and available N content were observed in soil and sediment samples collected from Kallara series, while the water samples taken from this series showed high ammoniacal N content. The available P content was low in all the series. Soil and sediment samples collected from Thuravur series exhibited highest content of N, P, K, Ca and Mg. In all the series, Cd and Hg were found to be below detectable levels in soil, sediment and water samples. The heavy metal accumulation was more in sediments when compared to soil and water. Toxicity of Fe and Al was recorded in soil and sediment samples of all series. Concentration of Zn and Mn was high in all soil samples of acid sulphate soil series. Even though accumulation of Pb and Cr were reported in soil and sediment samples, their availability were more in sediment samples than in soil samples. Soil pH and OC had significant influence on availability of heavy metals in soil and sediments. Concentration of Fe, Al and Cr in water samples were above the maximum permissible limit (MPL) as suggested by WHO, hence they are not suitable for drinking water purpose. The concentration of Pb was below maximum permissible limit in water samples of all series except Purakkad and Thottapally series. According to FAO, for irrigation water quality, all heavy metals were within the MPL in the water samples collected from study area and were safe for irrigation. Along with Cd and Hg, Zn and Cu were also below detectable levels in all water samples. The heavy metal content in Eichhornia crassipes were in the order of Fe >Mn > Al> Zn> Cr> Cu >Pb. Among the dominant macrophytes, Eichhornia crassipes accumulated high content of Pb. Hence it can be effectively used as a phytoremediator. Salvinia molesta contained high amount of heavy metals, especially Cr and Al. The content of Cd and Hg were below detectable level in all weed species. The content of Zn, Cd and Hg were found to be below detectable levels in all the three mangrove species. But these contained high concentration of Fe, Pb, Cr and Al in roots, while Cu and Mn were accumulated in aerial parts such as shoot and leaves. Hence the three mangrove species viz., Avicennia officinalis, Bruguiera gymnorrhiza and Rhizophora mucronata restricted the mobility of heavy metals to aerial parts but showed greater mobility of essential elements like Cu and Mn. Since they are having extensive root system, they can stabilise heavy metals within the roots. These plants can be effectively used for phytoremediation, especially for phytostabilisation of metals in mangrove ecosystem.