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

Tuesday, 16 December 2014
Ali Mozaffari Khalf Badam and Thomas Pichler, University of Bremen, Bremen, Germany
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.