Involved in resistance to arsenic. Facilitates efflux of arsenite [As(III)]. Arsenate [As(V)] enters the cell through phosphate transport systems and is reduced to arsenite by the arsenate reductase ArsC. Internally generated arsenite flows out of the cell by downhill movement through AqpS (PubMed:16199569). Can also transport the highly toxic methylarsenite [MAs(III)] and the relatively non-toxic methylarsenate [MAs(V)]. May be a component of an methylarsenite resistance pathway in which methylarsenite enters cells via AqpS, is oxidized by ArsH to methylarsenate, which exits the cells via AqpS. This pathway may confer a selective advantage for R.melliloti to grow in the presence of environmental methylarsenicals (PubMed:33418223). Aquaporins contain two tandem repeats each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA). Disruption of the gene results in increased tolerance to arsenite but not arsenate. Does not affect sensitivity to antimonate [Sb(V)]. Belongs to the MIP/aquaporin (TC 1.A.8) family. NIP (TC 1.A.8.12) subfamily.