Chemical fractionation of geogenic molybdenum and arsenic in a limestone aquifer and their impact on groundwater quality

Abstract:

Mobilization of naturally occurring molybdenum (Mo) and arsenic (As) in sedimentary rocks, mainly carbonates, contaminate groundwater in the Lithia area. The sedimentary rocks contains up to 825 mg kg^-1 Mo and 144 mg kg^-1 As. Mo and As groundwater concentrations reached up to 5000 µg L^-1 and 300 µg L^-1 and exceed the WHO guidelines of 70 and 10 µg L^-1, respectively. In order to asses their fractions and origins, a modified five-step (adsorbed/exchangeable, carbonates, hydrous iron oxides, crystalline iron oxides, sulfides and organic matter (OM)) sequential extraction procedure (SEP) was applied to 10 samples. Determination of the elements was carried out by ICP-MS. The SEP results were compared to values obtained by total digestion with aqua regia. The recovery ranged from 88 to 111 % for Mo and 75 to 116 % for As and RSD was better than 10%. In most samples up to 90 % of the Mo was present in the adsorbed/exchangeable fraction (step 1), characterizing a major Mo source. Pyrite, which is present in the aquifer matrix beneath Lithia, is generally considered a source for Mo. Electron microprobe analysis, did not confirm the presence of Mo in pyrite. Thus oxidation of OM is the main reason for the high Mo content in step 1 and groundwater. Another possible source for Mo in groundwater could be the mineral powellite (CaMoO₄). To investigate this possibility powellite saturation was calculated with PHREEQC under alkaline conditions. Powellite was super saturated once Mo concentrations exceeded 3000 μg L^-1. Thus powellite was not considered a source, but rather a sink for Mo released form OM.

In contrast to Mo, As was present in each extraction step in somewhat similar abundance: step 1 (17%), step 2 (11% ), step 3 (30 %), step 4 (23 %) and step 5 (18%). Hydrous and crystalline iron oxides which were dissolved in step 3 and 4 contained the highest As concentrations. Electron microprobe analysis of pyrite, which was dissolved in step 5, showed concentrations of up to 2000 mg kg^-1 As. Our results suggest the Mo found in groundwater is mainly related to OM rather than pyrite and powellite, whereas As could originate from OM, pyrite and iron oxides. A possible way to get more precise information about their distribution in the aquifer matrix beneath Lithia would be the application of the chromium reduction method.