Where I work in southern California, "aggressive" water is a major problem. This catch-all term refers to high alkalinity or high mineral or gas content in the water that causes the premature corrosion of copper and galvanized piping (see Figure 1).

	Figure 1. Water with low pH levels or high CO2 content, often referred to as aggressive water, can prematurely wear out copper piping (top). Thermal-galvanic corrosion, caused by a variety of factors, usually damages hot water pipes first (bottom).

There are two main types of corrosion related to aggressive water: pitting, which affects an entire system, and thermal-galvanic corrosion, which first affects hot water pipes. Pitting accounts for about 75% of all the repipes my company does, with the other 25% stemming from thermal-galvanic corrosion. Depending on the severity of the problem, new copper supply lines can fail within one to ten years. In some cases, entire developments plagued by aggressive water have had their piping systems fail. The upshot of all this is that my business focuses almost entirely on residential repiping. If pitting is present in a system, then it must be entirely repiped. Thermal-galvanic corrosion, on the other hand, may only require new pipes on the hot water supply lines. I almost always insist on using plastic piping because the plastic isn't affected by aggressive water and won't doom residents to additional future whole-house repiping. In most areas of California, state code doesn't allow anything but copper for residential potable water distribution - even though most water travels through PVC right up to home service entrances. Fortunately, though, some municipalities are beginning to allow for CPVC or PEX piping in new construction, based on local conditions. And most jurisdictions will allow a home to be repiped with CPVC or PEX instead of copper if the presence of aggressive water can be proven.