4. A scientist is investigating the source of a petroleum contaminant in a high-yield drinking-water well. The well is deep and draws from a confined aquifer. Test wells drilled immediately to the north and to the east of the well are found to be contaminated. Test wells drilled to the south and west are not. The scientist concludes that the contaminant must be coming from the north and east. Which of the following factors would most reduce the validity of the scientist's conclusion?