Arsenic Speciation as a tool for groundwater quality assessment and sustainable water management: Evidence from the Almopia Basin, Northern Greece

Arsenic (As) is referred as a potentially toxic element (PTE) and is commonly present in rocks, soils, and water. Bioaccumulation of As poses significant risks to human health. The World Health Organization (WHO) and the European Union have established a maximum guideline value of 10 μg·L⁻¹ for As in drinking water. Elevated As concentrations have been reported in thermal spring- and ground- waters globally. The distribution of arsenic species, specifically trivalent arsenite [As(III)] and pentavalent arsenate [As(V)], in natural waters is primarily influenced by geochemical conditions such as redox and pH [1]. Under oxidizing conditions, such as those found in surface waters, As(V) predominates and is mainly present as oxyanions (H₂AsO₄⁻, HAsO₄²⁻). In mildly reducing environments, As(III) is the dominant species and remains mostly as neutral arsenious acid (H₃AsO₃) at typical natural water pH levels (<9) [1].

This study examines As speciation in the Almopia Basin, Central Macedonia, Northern Greece, an area known for the Pozar thermal baths. The main geological formations include Quaternary and Neogene sediments (alluvium, marls, conglomerates, sandstones), Cretaceous and Triassic carbonate formations (limestones, dolomites), schists, ultramafic and mafic rocks (serpentinites, peridotites, diabases), Pliocene and Upper Jurassic volcanic rocks (andesites, dacites, trachytes), and pyroclastic rocks (volcanic tuffs and clasts).

Twenty-six surface water and groundwater samples were collected from irrigation and drinking wells (16), natural springs (4), and surface water bodies (6) during the wet period in 2023. All samples were analysed for physical parameters (i.e., pH, EC, Temp, D.O., Eh), major ions (i.e., Ca²⁺, Mg²⁺, NO₃^- etc) and 72 trace elements (i.e., As, B, Cr, Fe, Mn etc) in accordance with all established analytical protocols. For As speciation, an additional sample from each site was filtered in situ using disposable cartridges and analysed for As [2]. Spatial distribution maps were generated using ArcGIS Pro, and descriptive statistics were calculated.

The results show that total concentration of (As_tot) in the whole samples range from 8.4 μg·L⁻¹ to 382 μg·L⁻¹, whereas As(III) concentrations, analysed in the filtered samples as As_tot range from 0.5 μg·L⁻¹ to 25.2 μg·L⁻¹. Trivalent As in the samples accounts for approximately 1.6% to 17.5% of the As_tot. The prevalent presence of As(V) suggests mildly oxidizing hydrogeochemical conditions in the Almopia Basin. Quantifying As(III) and As(V) is essential because As(III) is significantly harder to remove from water, while As(V) responds readily to standard treatments. This research highlights the importance of As speciation analysis in groundwater, as it informs the selection of suitable remediation strategies and determines the necessity for pre-oxidation to convert As(III) to As(V).

[1] Cullen, W. R., & Reimer, K. J. (1989). Arsenic speciation in the environment. Chemical Reviews, 89, 713–764.

[2] Meng X, Wang W. Speciation of arsenic by disposable cartridges. Proceedings of the third International Conference on Arsenic Exposure and Health Effects, San Diego, CA, July 12–15; 1998.