Our aim was to systematically investigate the influence of anions (HPO₄ ²⁻), cations (Ca²⁺, Mg²⁺) and neutral H₄SiO₄ on Fe flocculation and As(III) removal in the complex natural water matrix. For this purpose, three different anaerobic groundwaters were selected and manipulated by dosing of Ca²⁺, Mg²⁺, HPO₄ ²⁻, or by their removal by cation – and anion exchange. The change in Fe floc volume and of dissolved Fe and As were followed in aerated jar experiments. Fe floc growth was improved by addition of Ca²⁺ or Mg²⁺, and hindered by their removal. This hindered floc growth was more severe for groundwaters with higher P:Fe ratios, where Fe flocs carry a larger net negative surface charge, and rely stronger on Ca²⁺ or Mg²⁺ for charge neutralisation. When expressing the charge balance of the different groundwaters as the molar ratio (Ca²⁺ + Mg²⁺)/P, a linear relationship was found with the cumulative Fe floc volume, with a plateau at molar ratios >500. At environmentally relevant concentrations, H₄SiO₄ was found more likely to compete with As(III) for adsorption capacity than HPO₄ ²⁻. As(III) removal was strongly related to Fe removal - independent of Ca²⁺ or Mg²⁺ presence - indicating that As(III) is primarily adsorbed at an early stage in the flocculation process.