Aluminum in drinking water has two very different origins. Some is natural, since aluminum is among the most abundant elements in the earth's crust and appears in groundwater particularly where the water is acidic. Most of it in a municipal supply is there because it was added on purpose, as part of the treatment that makes surface water clear.
Why treatment plants add it
Surface water carries fine particles too small to settle out on their own. Aluminum sulfate, known as alum, is dosed as a coagulant: it neutralizes the charge keeping those particles apart so they clump into flocs large enough to settle and be filtered. It is one of the most effective and widely used treatment chemicals there is, and the vast majority is removed with the floc. A small residual carries through, and that residual is what appears in the finished water.
How to find out what yours carries
Read the report rather than guessing. The EPA requires every community water system to publish an annual Consumer Confidence Report and maintains a finder for them, and aluminum appears there where it is monitored. Aluminum carries a secondary standard, which is aesthetic rather than health based, concerned with color and with deposits in the distribution system. A utility running well keeps the residual low, and a rising figure usually reflects a plant working hard on difficult raw water rather than a hazard.
The health question, stated fairly
The link between aluminum in drinking water and neurological disease has been studied for decades and remains unresolved rather than established, and drinking water is a small contributor to total aluminum intake next to food, cookware and medicines such as antacids. That is not a reason to dismiss the question, but it is a reason to keep it in proportion. If it concerns you, treat it as one item on a full water analysis rather than the item, and decide with the whole picture.
Removing it if you choose to
Reverse osmosis removes aluminum effectively and is the practical domestic answer at a drinking tap. Distillation does too. Carbon does not remove dissolved aluminum, though a fine sediment filter catches the particulate fraction where water is cloudy with floc carryover. On a private well with acidic water, aluminum can be mobilized from the surrounding geology, and correcting the pH addresses the cause rather than filtering the symptom. Our drinking water filter guide covers the point of use options.
Questions people ask about aluminum in water
Why is aluminum in my tap water?
Usually because alum was dosed as a coagulant during treatment to clump fine particles for settling. A small residual carries through.
Is it regulated?
It carries a secondary, aesthetic standard concerned with color and deposits rather than a health based limit.
How do I find out how much my supply has?
The annual Consumer Confidence Report. Every community water system publishes one and the EPA maintains a finder for them.
What removes it?
Reverse osmosis or distillation. Carbon does not remove dissolved aluminum, though sediment filtration catches particulate floc carryover.