LABORATORY SIMULATION OF THE ACCELERATED EROSION MODEL FOR ARSENIC CONTAMINATION OF GROUNDWATER: EFFECT OF FLOW VELOCITY ON SORPTION RATE

For the past decade Prof. Steven Emerman and his students have been developing the model that As contamination of groundwater in south Asia is due to the accelerated erosion that results from monsoon climate, tectonic uplift, deforestation, and overgrazing. The consequence of the accelerated erosion is elevated As in rapid overland flow and mountain streams, resulting in elevated arsenic in the losing valley rivers that recharge groundwater. The elevated As in overland flow and mountain streams results from the inability of the large, multivalent arsensate oxyanion to adsorb onto sediment in rapid flow conditions for two reasons. First, sediment tends to have many fewer positively-charged sorption sites than negatively-charged sorption sites. Second, the sorption sites must be sufficiently widely-spaced to accommodate the large arsenate oxyanion. As a consequence of the first two reasons, the arsenate oxyanion cannot attach to multiple positively-charged sorption sites unless it has sufficient residence time in the vicinity of the sorption sites for all necessary sites to become simultaneously vacant. On the other hand, a competing factor is that a rapidly-moving arsenate oxyanion will encounter a greater number of possible sorption sites. Thus far, the predictions of the accelerated erosion model have been consistent with field studies in Kathmandu, Mustang and Pokhara Valleys of Nepal Himalaya. The objective of this study is to carry out the first laboratory simulation of the effect of flow velocity on the rate of arsenate sorption. The objective is being addressed by circulating water spiked with an initial As concentration past sediment in a home-made flume and periodically measuring the As concentration of the water as the As sorbs onto the sediment. Experimental treatments thus far have included the initial As concentration (As = 0.050 mg/L), sediment type (fritted clay, marble), grain size (125-250, 250-500, 500-1000 μm), and flow rate (2.5, 4.5, 6.5 gal/min). Preliminary results have shown a large initial sorption followed by a slow sorption of the remaining As at the higher flow rate, in contrast to a small initial sorption followed by a rapid subsequent sorption at the lower flow rate, which is consistent with the theoretical predictions. Further results and interpretation will be reported at the meeting.