The distributions of , ,, titration alkalinity, Ca2+, and pH in interstitial waters collected from mine tailings and natural sediments in Rupert and Holberg inlets (Vancouver Island, British Columbia) are discussed. In natural sediments, dissolved ammonia, phosphate, and titration alkalinity increase with depth, and sulphate decreases, following established patterns for coastal sediments. In rapidly accumulating tailings, however, no dissolved phosphate enrichment is observed despite significant sulphate reduction. This phosphate depletion is accompanied by a calcium enrichment and a relatively high pH, which is probably due to the addition of lime to the tailings during milling. Solubility considerations suggest that the phosphate distributions in both tailings and natural deposits are controlled by equilibrium with respect to carbonate fluorapatite. Lime addition appears to increase the supersaturation of the pore water with respect to carbonate fluorapatite, resulting in phosphate consumption by the tailings. The tailings thus inhibit benthic phosphate regeneration, which is now known to be an important factor in the phosphorus budget of some coastal waters. It is demonstrated that such inhibition is not important in Rupert Inlet due to a short water residence time; however, it could be of significance in poorly mixed or stratified water bodies including lakes and some fjords.