Land-applied arsenical pesticides have contributed to elevated soil arsenic (As) levels in agricultural fields, which are being rapidly converted to residential properties in fast-expanding cities. In-situ remediation using chemical amendments is a promising approach to cleaning up of such As-enriched soils compared to ex-situ remediation methods, which are expensive and damaging to the ecosystem. One such emerging in-situ chemical remediation technique is application of drinking water treatment residuals (WTRs). The WTRs are byproducts of drinking water treatment plants; in many cases where Fe- and Al-salts are used in the water purification process, these byproducts are composed of amorphous Fe/Al oxides with high As-retention capacity. Hence, once land-applied, these WTRs are expected to convert the natively soluble As to stable As-bearing Fe/Al phases, which are typically not bioaccessible. A 3-mo incubation study was performed to evaluate the effects of Fe- and Al-WTRs on geochemical speciation of As in two chemically variant soils of Texas. The Euphaula soil (sandy, low pH, low Fe/Al) and the Orla soil (saline, high pH, high Fe/Al and Ca/Mg) were spiked with sodium arsenite at 2 rates (45, 450 mg/Kg), which were then amended with 2 rates of Al- and Fe-WTR (2.5%, 10%). Because phosphorus (P) is a competitor with As for surface sites, 2 rates of P (1:1 and 1:5 of As:P ratio where initial As concentration is 45 mg/Kg) as TSP fertilizer were added to the WTR-amended soils. A sequential extraction scheme was employed to identify the following “operationally-defined” soil-As phases: soluble As, exchangeable As, Fe/Al-bound As, Ca/Mg-bound As, organically and S-bound As, and residual As. Results showed that prior to the application of the WTRs, native soil chemistry dictated the geochemical forms of As. However, upon WTR application, the majority of the applied As was bound to Fe/Al phases, thereby rendering it potentially non-bioaccessible.