Rocky Mountain Section - 64th Annual Meeting (9–11 May 2012)

Paper No. 7
Presentation Time: 10:45 AM


GARCIA, Patricia K.1, SANCHEZ, Andrea1, STUART, Kevin1, WILLIAMS, Jason1, SAPKOTA, Ajit2, KHATRI, Sabina2, EMERMAN, Steven H.1 and ADHIKARI, Basanta Raj3, (1)Department of Earth Science, Utah Valley University, 800 West University Parkway, Orem, UT 84058, (2)Department of Geology, Tri-Chandra Multiple Campus, Tribhuvan University, Kathmandu, Nepal, (3)Department of Civil Engineering, Institute of Engineering, Pulchowk Campus, Tribhuvan University, Kathmandu, Nepal,

Elevated levels of As in groundwater in the floodplain of the Ganges River in Bangladesh, West Bengal (India) and southern Nepal have been well-documented over the past 15 years. Recent studies have shown that elevated As in groundwater occurs even in Kathmandu Valley in Nepal, a tectonic valley well upstream of the floodplain of the Ganges River. Moreover, studies in Kathmandu Valley showed surface water As to be statistically indistinguishable from groundwater As, which led to the fluvial recharge model in which elevated groundwater As results from losing streams with elevated As, which is a consequence of rapid erosion caused by a combination of monsoon climate, tectonic uplift and deforestation. The objective of this study was to further test the fluvial recharge model and other existing models in Pokhara Valley, another tectonic valley in the Nepal Himalaya far upstream from the floodplain of the Ganges River. In November 2010 water samples were collected from 20 hand-dug wells (depths 2 – 18 m), 12 machine-drilled wells (depths 34 – 220 m), six springs, 15 streams, two lakes and two caves. Samples were analyzed for As, sulfate, and the transition elements Fe, Cu, Ni, Co, Mn, Cr, and Zn, with the Hach DR-2700 Spectrophotometer. Arsenic concentrations in all but one of the 57 samples (a stream) exceeded the WHO As Standard (As = 0.01 mg/L) with the highest concentration being As = 7.9 mg/L in a hand-dug well. All statistical analysis was carried out based on the lognormal distributions of the geochemical data. Whether machine-drilled wells and hand-dug wells were considered together or separately, groundwater As was uncorrelated with either sulfate or any combination of the transition elements, which is inconsistent with both the reductive-dissolution model and the sulfide-oxidation model. The As concentration of shallow hand-dug wells (geometric mean As = 0.133 mg/L) was statistically indistinguishable (P = 0.12) from that of deep machine-drilled wells (geometric mean As = 0.047 mg/L). The As concentration of all surface water (geometric mean As = 0.067 mg/L) was statistically indistinguishable (P = 0.44) from that of all groundwater (geometric mean As = 0.085 mg/L), which is consistent with the fluvial recharge model. Current work involves searching for spatial clustering of As concentrations.