ASSESSMENT AND MAPPING OF GROUNDWATER QUALITY IN THE THIRIRIKA SUB CATCHMENT KIAMBU COUNTY, KENYA BY EBENEZER ASHUN (BSC. ENVIRONMENTAL SCIENCE) REG. No: I56F/23073/2011 A Thesis Submitted in Partial Fulfilment of the Requirements for the Award of the Degree of Master of Science in Integrated Watershed Management in the School of Pure and Applied Sciences of Kenyatta University. JANUARY, 2014