Spring water is naturally filtered by passing through different media, like gravel and sand. The filtration may or may not remove harmful contaminants, and also may or may not add dissolved solids to the water passing through. Some of these dissolved solids could be heavy metals, pesticides, or antibiotics. The source/origin of the water needs to be considered too - this spring fed by rain/groundwater from a densely populated area, or possibly waste water from farm fields, or city storm runoff? Are there any industries nearby contaminating the soil?