Chemistry - What chemicals are in tap water that gives it a slightly basic pH level
Solution 1:
The tap water is likely "hard," i.e., contains some dissolved mineral salts, most likely (primarily) calcium and magnesium carbonates and bicarbonates. The anions of these salts are slightly basic, consuming $\ce{H^+}$ ions and thereby raising the pH. Carbonates (with the obvious exception of alkali metal salts) are only sparingly soluble in water, while bicarbonates are typically very soluble. A couple of other subtle factors may affect the pH:
Lower atmospheric pressure encourages the evaporation of dissolved carbon dioxide from water. Since dissolved carbon dioxide exists in equilibrium with carbonic acid, it would tend to lower water pH. If you live in an area with comparatively low atmospheric pressure, your tap water might have a lower concentration of $\ce{CO_2}$, which would favor higher pH.
At higher temperatures, bicarbonate decomposes to water, carbonate, and carbon dioxide. Carbonates are relatively insoluble, and the consumption of bicarbonate should drive forward the dissociation of any dissolved carbonic acid, so a higher temperature should have the effect of lowering pH.
Solution 2:
Water treatment plants typically buffer the pH of the water supply to be slightly alakaline, with a pH of ~7.5-8 normally (the standard is that it must be between 6.5 and 9.5) with various chemicals including Sodium Hydroxide (lye) or Sodium Carbonate (soda ash).
Take a look at this photo from Wikipedia which shows all of the chemicals added at this particular water purification plant.
From left to right: sodium hypochlorite for disinfection, zinc orthophosphate as a corrosion inhibitor, sodium hydroxide for pH adjustment, and fluoride for tooth decay prevention.)
For treatment systems which add fluoride to tap water, a buffer is further necessary - the most commonly used additive for fluoride is fluorosilicic acid.
The reason tap water is kept slighly alkaline is because acidic water will leach chemicals from the pipes which it passes through. This can be especially problematic for homes with older plumbing (i.e. lead/copper/zinc). It also increases the corrosion and wear on the pipes/fixtures it travels through.