Isotopic analysis of groundwater flow systems in a wet alluvial fan, southern Nepal.

Results are reported of an isotopic and geochemical study of groundwater in wet alluvial fan deposits in the Terai Plain of southern Nepal. Local, intermediate, and regional scale groundwater flow systems are suggested by the areal and vertical distributions of the isotopes and dissolved solids along principal flow paths within the alluvial fan deposits. Most groundwater is recharged by precipitation during the annual monsoon and by the Tinau River where it leaves the Himalayan uplift. The delta ¹⁸O and delta ²H of the groundwater range from -10.3 to -6.4per mille and from -79 to -51per mille, respectively (relative to SMOW), and plot on a line parallel to the global average meteoric water line. The delta ¹⁸O and delta ²H contents generally increase from the recharge area to the discharge area, and coincide with an increase in dissolved solids from approx 100 to 400 mg/l. All groundwater is of the calcium-magnesium bicarbonate type. The tritium content of groundwater reflects variations in atmospheric bomb tritium during the past three decades. Tritium increases downgradient from approx 20 TU in the recharge area to approx 80 TU in the discharge area, and then decreases below detection limits (10 TU) near the southern margin of the study area. An average linear groundwater velocity of 2-3 units per day, estimated from the tritium data, compares favourably with velocities determined from aquifer tests. (Authors' abstract)-A.W.H.