UTILIZATION OF PALEOFLOOD DEPOSITS TO DETERMINE TRACE METAL BACKGROUND CONCENTRATIONS IN ALLUVIAL DEPOSITS, RIO LOA BASIN, CHILE

Determination of background concentrations in alluvial deposits is a critical component of the assessment and remediation of contaminated rivers. Previous investigations have demonstrated that toxic trace metal concentrations in riverine deposits vary as a function of the depositing discharge, among other factors. The question arises, then, as to how to determine background concentrations for alluvial sediments deposited by rare, high magnitude events that are often responsible for the bulk of sediment-borne trace metal dispersal in semi-arid to hyperarid environments. This investigation documents background concentrations in both the silt/clay and bulk (<2 mm) fractions of alluvial sediments in the hyperarid Rio Loa Basin of northern Chile by documenting the sedimentology and geochemistry of paleoflood deposits, and analyzing the magnitude and frequency of the floods responsible for their deposition.

Both water and sediment within the Rio Loa are highly contaminated by toxic trace metals derived predominately from the El Tatio Geyser Basin and 3 large Cu mines. The bulk of the metal-enriched sediments are transported during extreme events, such as the 2001 flood (largest event on record; ~310 cms) which resulted in the deposition of >1 m of sediment over 100s of km². The analysis of paleoflood deposits, dating back to ~390±30 YBP, exhibited flood magnitudes estimated using inverse modeling to range between 240 and 515 cms; recurrence intervals ranged from about 70 to >500 years. Geochemical data demonstrate that background trace metal (e.g., Cd, Cu, Ni, Zn) and metalloid (e.g., As, Sb) concentrations of flood deposits varied by as much as 4 orders of magnitude between events for the silt and clay fraction. Variations may reflect differences in sediment source. Concentrations within the bulk fraction, dominated by quartz, were less variable. Estimated background concentrations were subsequently used to assess the degree of contamination of post-mining deposits.