Groundwater is the water that exists below the land surface and fills the spaces between sediment grains and fractures in rocks. A geologic formation saturated with groundwater is considered to be an aquifer if it is sufficiently permeable as to allow the groundwater to be economically extracted. It is replenished naturally through the infiltration of rainfall and artificially through the irrigation of crops. Soluble chemicals in rainwater (like NO_x in acid rain) or at the land surface (like pesticides) can be transported downward with percolating water to reach groundwater. Underground petroleum storage tanks (USTs) or buried pipelines also pose threats if they should leak. Over 400,000 leaking USTs have been identified in the United States as of 2001. Dissolved chemicals are transported with the flowing groundwater. Once groundwater is contaminated, remediation can be expensive and time-consuming; billions of dollars are spent annually in the United States on the remediation...