Soil Infiltration Test in Hard Rock Areas—A Case Study

The study carried out in Ankasandra watershed covering on area of 375 km², falling in parts of Tiptur and C.N. Halli taluks of Tumkur district, Karnataka. Three major rock types identified in the area are gneisses, schists and granites belong to Archaean age. Groundwater is being extracted through borewells tapping fractures in depth ranges of 10–249 m below ground level. Over a period of time, due to increase in number of borewells and increasing groundwater extraction, the water level has gone down to more than 100 m bgl at certain location. The soil type in the area is fine red soils followed by clayey skeletal and loamy skeletal soils, and the rainfall is the main source of recharge. Twenty infiltration tests were conducted at various soil types using double-ring infiltrometer to understand the infiltration characteristics. The results of studies revealed that highest infiltration rates were observed at Madapurahattithanda, Kaval and Siddaramanagara sites characterised by sandy roils and lowest rates were noticed at Halkurike, Bommanahalli, Bhairanayakanahalli, Kuppur and Sasalu sites located on tank beds, pointing towards, need for desiltation of the tanks so that the tanks contribute to recharge also.