Adsorption plays an essential role in geochemical reactions at environmental interfaces because it not only directly controls contaminant fate and nutrient availability in soils but also is an important precursor step in many other processes such as the nucleation of precipitates, microbial- or ligand-promoted dissolution of minerals, and surface-initiated redox reactions. Adsorption reactions occur in the mineral-water interfacial region, a chemical and structural transition zone where surface sites in multiple coordination states displaying distinct pH-dependent charging behaviour interact with both adsorbates and water. While general ion adsorption mechanisms and the effects of surface charge on interfacial reactions have been extensively studied, little is known about the role of water in affecting ion adsorption processes. In this study, resonant and non-resonant surface X-ray scattering measurements have been used to investigate how interfacial water responds to the addition of adsorbed arsenate at corundum (001)-water interfaces over a range of pH conditions. The surface of corundum (001) is terminated by doubly coordinated functional groups that are neutrally-charged over a wide pH range. In the absence of arsenate, our non-resonant X-ray reflectivity (XR) data show that interfacial water displays weak ordering on this surface and little variation in its structure is observed over the pH range of 5 to 9 in 0.01 M NaCl solution. This suggests that surface charging from functional group protonation and deprotonation is inadequate to induce extensive structural changes in interfacial water. In the presence of arsenate, resonant anomalous X-ray reflectivity (RAXR) analyses have been used to determine the arsenic distribution (i.e., surface coverage and location) as a function of total arsenate concentrations. This is then combined with XR measurements to isolate the interfacial water structure at each experimental condition. Our results show that interfacial water undergoes substantial restructuring upon arsenate adsorption, indicating that a charged adsorbate greatly affects the arrangement and order of interfacial water on this surface. In addition, the overall interfacial water structure varies proportionally with arsenate surface coverage, with adsorbed water sites moving closer to the surface and the extended layering of interfacial water showing reduced positional disorder as arsenate surface coverage increases. This systematic variations in interfacial water properties with increasing arsenate surface coverage are consistent with two distinct water structures (one near sites of adsorbed arsenate and the other on the pristine surface) that vary in proportion with arsenate surface coverage. These observations demonstrate that the adsorption of arsenate directly alters the structure of water near corundum (001) surfaces, possibly through the modification of the charge state of surface sites or by providing new sites to which water may hydrogen bond. Such adsorbate-induced restructuring of interfacial water may represent an overlooked energetic contribution to adsorption reactions, which may thus reveal a potentially new process affecting contaminant and nutrient fate in soil systems.