Preservation of inorganic arsenic species in groundwater

The objective of this research was to develop a robust preservation method for stabilizing inorganic As(III/V) species in synthetic and actual groundwaters. Ethylenediaminetetraacetic acid (EDTA), H2SO4, H3PO4, and EDTA-acetic acid (HAc) were evaluated in synthetic groundwater containing 3 mg/L Fe(III) in the pH range 6.5-8.4 and Eh range -100 to +200 mV at room temperature. In the absence of strong UV light, only EDTA-HAc was found to be an effective preservative under all the experimental conditions tested. A total of 89 samples (including 16 samples in triplicate) from 55 drinking water wells in Minnesota, California, and North Dakota were preserved with a combination of EDTA-HAc and speciated to evaluate its effectiveness for preserving inorganic arsenic species in actual groundwater samples. The preserved and field-speciated samples were repeatedly speciated and analyzed in the laboratory for up to 85 days after collection. Field-speciated As(III) concentrations were compared with the As(III) concentrations in the preserved samples. The results were highly correlated (slope 0.9773, R-2 = 0.9986), which indicates that during sample transportation and storage the distribution of arsenic species did not change for samples preserved with EDTA-HAc.