A design of experiments approach to arsenic retention: Interactions between sediment properties and water chemistry

The mobility of arsenic (As) in groundwater is governed by dynamic interactions at the sediment-water interface, where both mineral reactivity and aqueous chemistry play key roles. In the Como aquifer (Northern Italy), As distribution is vertically heterogeneous: shallow horizons remain below the WHO limit of 10 μg/L, whereas deeper zones reach concentrations up to 250 μg/L. Although this variability is well documented, the underlying mechanisms remain only partially understood. To address this gap, we performed batch experiments on shallow and deep aquifer sediments with contrasting composition, systematically varying pH, electrical conductivity, and dissolved organic carbon in a full-factorial design of experiment. Results revealed nonlinear adsorption responses and sediment-specific behaviours. Shallow sediments exhibited stronger and more stable As retention, associated with higher contents of Fe-, Al-, and Mn-(hydr)oxides and greater cation exchange capacity. In contrast, deepaquifer sediments showed weaker and more variable adsorption, especially under acidic, DOC-enriched, and high-conductivity conditions. Response surface models confirmed that sediment composition governs As retention capacity, while water chemistry modulates its efficiency. This integrative approach links laboratory adsorption patterns with field-scale As distribution, providing a mechanistic explanation for the observed vertical heterogeneity in the Como aquifer and supporting predictive assessment of groundwater vulnerability.