Sulfide (Rotten Egg Odor)
Sulfide can be formed naturally as a by-product of the decomposition of organic material possibly aided by the presence of sulfur reducing bacteria. It can also be produced by chemical reactions of soil and bedrock minerals containing sulfur. At the concentrations typically found in drinking water, it is not hazardous to human health. It is also important to note that the odor threshold for sulfide is considerably lower that the point at which our laboratory test detects it. Therefore, you may smell the sulfide before it reaches a detectable level for lab analysis.
For further information, plese refer to the NH DES Fact Sheet
WD-DWGB-3-16 Hydrogen Sulfide in Drinking Water