The salinity of brackish water is intermediate between seawater and fresh waters. Brackish water contains too much salt to be drinkable, but not enough salt to be considered seawater. The ocean has an average salinity of 35 parts per thousand (ppt), whereas freshwater contains 0.065–0.30 ppt of salts, primarily chloride, sodium, sulfate, magnesium, calcium, and potassium ions. The salt content of brackish water ranges between approximately 0.50 and 17 ppt. Brackish water occurs where freshwater flows into the ocean, or where salts are dissolved from subsoils and percolate into freshwater basins. The gradient between salt and fresh water in estuaries and deltas varies from sharp distinction to gradual mixing, and different levels of vertical and horizontal mixing depend on the influence of tide, current, and rate of freshwater inflow.