Six areas of the Lower Susquehanna River Basin in southcentral Pennsylvania were studied as part of the U.S. Geological Survey's National Water-Quality Assessment. Five of the six sampling areas with population densities ranging from 53 to 410 per square mile (mi2) had predominantly rural land use; three underlain by limestone, one underlain by crystalline bedrock, and one underlain by interbedded sandstone and shale. One of the six sampling areas with a population density of 1,300 per square mile had predominantly urban land use and was underlain by limestone. The urban setting was in and around Shippensburg, Carlisle, Harrisburg, Hershey, and Lebanon. To determine the effect of land use on the occurrence and concentrations of MTBE in otherwise similar hydrogeologic settings, the urban area and adjacent rural area, both underlain by limestone, were compared. Samples were collected in 1993-95 from 118 wells to depths ranging from 30 to 226 feet. Of the 118 wells sampled, 99 were domestic wells used as drinking water supplies, 9 were domestic wells not used as drinking water supplies, 9 were monitoring wells (4 drilled for this study), and 1 was a public supply well. Samples were analyzed by gas chromatography/mass spectrometry. The analytical method employed for MTBE had a method detection limit of 0.06 ug/L (micrograms per liter). Blanks and spiked samples comprised a 26-sample quality-assurance program. Resampling was used to confirm the presence of MTBE in selected well waters.
In the urban area, concentrations of MTBE, detected in water from 10 of the 20 wells, ranged from 0.11 to 51 ug/L. For 20 wells in each of the adjacent urban and rural areas, both underlain by limestone, MTBE was detected 10 times in the urban area and once (at a concentration of 0.11 ug/L) in the rural area. Occurrence of MTBE in well waters in the urban area corresponds with detections of MTBE at springs and spring-fed streams that are discharge points for ground water from the limestone aquifer in the urban area.
In rural areas, concentrations of MTBE, detected in water from 6 of the 98 wells, ranged from 0.11 to 1.3 ug/L. In areas of lowest population densities (53 and 110 per mi2) there were no detections of MTBE in water from 38 wells completed in limestone bedrock and sandstone and shale. In water from 30 wells representing a crystalline bedrock area and a limestone area with population densities of 210 and 290 per mi2, respectively, there was one detection of MTBE. In an area with a population density of 410 per mi2 in limestone terrane there were 4 detections of MTBE in water from 30 wells sampled. The rural area with the highest population density had the greatest number of MTBE detections with concentrations ranging from 0.3 to 1.3 ug/L.