GEOCHEMISTRY OF ARSENIC IN THE GROUND WATER OF FERNLEY, NEVADA

Fernley is a small town located about 35 miles east of Reno in the Truckee river basin. In 1994, Broadbent & Associates ('Project Gold') sampled 100 wells and found high arsenic in the ground water; one of the wells was above 1000 parts per billion (ppb). Our project (still on-going), started with the objectives of: identifying the sources of the high arsenic, characterizing the chemical processes that mainly control the release of As, observing the change in concentration of As with time, and describing the fate and transport of As in the groundwater. In August 2000 we sampled 70 wells in Fernley, out of which 43 were previously sampled by the 'Project Gold', and analyzed them for As and major ions. The analysis result of these samples shows that arsenic concentration in the in most of the wells is 100- 300 ppb; in some wells the concentration is higher than 600 ppb. The principal aquifers in the area are alluvial deposits of Holocene age, lake sediments of Tertiary age, and Tertiary volcanic rocks, and the town is surrounded by mountain ranges, which contain exposed rocks of mostly Tertiary age, and predominantly of volcanic origin (Lico, 1992). The potential sources of As in the area might therefore be these arsenic rich volcanic rocks and lake deposits. From the groundwater chemistry result we were able to see a generally low phosphate and higher pH values in the groundwater of Fernley, which may indicate that sorption process is playing a major role in releasing As from the aquifer surfaces. The 43 wells, which were also sampled in a previous study in 1994, showed similar concentration of arsenic in the year 2000 as in 1994. This may be due to a steady state transport of As in the groundwater. In this project the geochemical model PHREEQC and a compartment model will be utilized to test these potential answers.

Key words: Arsenic, ground water, geochemistry, transport.