Fluvial Arsenic in Utah Valley, Salt Lake Valley and the Wasatch Range: Analogy with the Himalayan Range and the Ganges River Floodplain

Abstract

Elevated arsenic in groundwater in the floodplain of the Ganges River has been well-documented over the last fifteen years. Measurements of arsenic in the Himalayan Range and Ganges floodplain found that dissolved arsenic was elevated in the Himalayan Range, but fell to undetectable in the Ganges floodplain. The sudden change in dissolved arsenic across the Himalayan-Ganges boundary was accounted for by the residence time in the vicinity of a sediment particle necessary for the large multivalent arsenate ion to adsorb onto sediment, so that arsenate can adsorb onto sediment only when the stream velocity drops. The result that dissolved arsenic falls to undetectable as a river passes from a steep mountain range onto a flat valley floor is so startling and has such major implications for understanding the arsenic cycle and its implications for global public health that the result must be tested in analogous geological environments, such as the Wasatch Range and the corresponding flat floors of Utah and Salt Lake Valleys. Twenty samples from Provo River were collected and analyzed for As and associated transition elements. Upstream of the mouth of Provo canyon, As is low (0.011-0.095 mg/L), and shows low correlation with most transition elements, except Cu (R² = 0.38). At the mouth of the canyon, As values increase significantly (0.367 – 0.436 mg/L) and are also moderately correlated with Cu (R² = 0.48); As continues to be elevated until nearing Utah Lake, where As values drop again (0.048-0.052 mg/L). The sharp increase in As may be due to historic mine tailings piling up at the mouth of the canyon and the formation of chalcopyrite co-precipitating Cu and As. The subsequent drop near Utah Lake may be where stream velocity actually decreases.