Environmental geology, Allegheny County and vicinity, Pennsylvania: Description of a program and its results

Abstract

Past land-use practices, including mining, in Allegheny County, Pa., have resulted in three principal environmental problems, exclusive of air and water contamination. They are flooding, landsliding, and subsidence over underground mines. In 1973, information was most complete relative to flooding and least complete relative to landsliding. Accordingly, in July 1973, the U.S. Geological Survey (USGS) and The Appalachian Regional Commission (ARC) entered into an agreement by which the USGS undertook studies chiefly aimed at increasing knowledge of landsliding and mine subsidence relative to land use, but having other ramifications as well, as part of a larger ARC 'Land-use and physical-resource analysis' (LUPRA) program. The chief geographic focus was Allegheny County, but adjacent areas were included in some investigations. Resulting products, exclusive of this report, are: 1. Forty-three provisional maps of landslide, distribution and susceptibility and of land modified by man in Allegheny County, 1:24,000 scale, 7½ -minute quadrangle format, released to open files. 2. Four USGS Miscellaneous Field Studies (MF) maps of Allegheny County showing (a) bedrock, MF685A; (b) susceptibility to landsliding, MF-685B ; (c) coal-mining features, MF-685C; and (d) zones that can be affected by flooding, landsliding and undermining, MF-685D; all at the scale of 1:50,000. 3. Two MF maps showing coal-mining activity and related information and sites of recorded mine-subsidence events, and one MF map classifying land surface by relative potentiality of mine subsidence, in Allegheny, Washington, and Westmoreland Counties, Pa., at a scale of 1:125,000--MF-693A through MF-693C. 4. A companion report to the Allegheny County map of susceptibility to landsliding--USGS Circular 728. 5. Five MF maps, largely in chart form, describing interaction of the shallow ground-water regime with mining-related problems, landsliding, heavy storm precipitation, and other features and processes, largely in Allegheny County--MF-641A through MF-641E. Map products are directly applicable to general classification of land for susceptibility to landsliding and mine subsidence and, to a lesser extent, flooding and engineering characteristics. The hydrogeologic charts enable greater understanding of environmental effects of ground water. All products are guides to expected conditions, but none are substitutes for detailed investigations of specific sites by competent technical personnel on the ground. Specific results and findings are: 1. Knowledge of .susceptibility to landsliding in Allegheny County now is adequate for application to countywide land-use planning. 2. About 110 mi 2 (285. km 2) , or 15 percent, of the county has some significant degree of susceptibility to landsliding. 3. Although a general classification of land in Allegheny, Washington, and Westmoreland Counties relative to mine-subsidence incidents was prepared, data are wholly inadequate for even moderately precise prediction of subsidence events over previously mined-out areas; the accumulation of adequate data might not repay the effort in terms of damage prevention. 4. Commonwealth-of-Pennsylvania regulations, have been very successful in limiting mine-subsidence damage over areas mined after 1966. 5. Undermining and consequent subsidence may have affected the ground-water regime more widely than heretofore believed. Except for the earth-disturbance inventory that resulted in the maps of susceptibility to landsliding and man-modified land, methods used in the studies largely were conventional. The inventory and ensuing analysis combined aerial photographic interpretation with field work and incorporation of existing data. The method worked very well for the purposes of defining distribution of landslides and areas having different susceptibilities to landsliding. However, if susceptibility to landsliding alone had been the goal, this could have been determined adequately by using photographs at smaller scales and appreciably less expenditure of effort per unit of ground area.