Complex sentence: While many of the chemicals and substances that are regulated may be naturally occurring (calcium, sodium, iron, manganese, etc.) the concentration is often the key in determining what is a natural component of water, and what is a contaminant.
Split into simpler sentences: While many of the chemicals and substances that are regulated may be naturally occurring (calcium, sodium, iron, manganese, etc.). The concentration is often the key in determining what is a natural component of water, and what is a contaminant.