A first estimate of ground water ages for the deep aquifer of the Kathmandu Basin, Nepal, using the radioisotope chlorine-36

The Kathmandu Basin in Nepal contains up to 550 m of Pliocene-Quaternary fl uvio-lacustrine sediments which have formed a dual aquifer system. The unco nfined sand and gravel aquifer is separated by a clay aquitard, up to 200 m thick, from the deeper, confined aquifer, comprised of Pliocene sand and g ravel beds, intercalated with clay, peat, and lignite. The confined aquifer currently provides an important water supply to the central urban area but there are increasing concerns about its sustainability due to overexploita tion. A limited number of determinations of the radioisotope Cl-36 have been made on bore waters in the basin, allowing us to postulate on the age of ground water in the deeper, confined aquifer. Ground water evolution scenarios ba sed on radioisotope decay, gradual dissolution of formational salts as the ground waters move downgradient, and flow velocity estimations produce comp arable ground water ages for the deep waters, ranging from 200,000 to 400,0 00 years. From these ages, we deduce a mean ground water flow velocity of o nly 45 mm/year from recharge in the northeast to the main extraction region 15 km to the southwest. We thus estimate current recharge at about 5 to 15 mm/year, contributing 40,000 to 1.2 million m(3)/year to the ground water system. Current ground water extraction is estimated to be 20 times this am ount. The low specific discharge confirms that the resource is being mined, and, based on current projections, reserves will be used up within 100 yea rs.