Evaluation of the ground-water resources of the lower Susquehanna River basin, Pennsylvania and Maryland

Abstract

Sensitivity analysis 32 Calibration of model under transient conditions 34 General procedure 34 Modeled flow components 37 Groundwater conditions 37 Surface-water conditions 37 Contents III Calibration of model under transient conditions Continued Specific yield and storage coefficient 38 Initial estimates 38 Adjustments 38 Recharge 38 Boundary conditions 41 Results 41 Sensitivity analysis 45 Digital-model evaluation of groundwater resources 45 General procedure 45 Qualitative ranking of hydrogeologic units 46 Standardized potential yield 46 Procedure 46 Results 47 Analysis by hydrogeologic unit 47 Analysis in a selected area 52 Reliability of model results 54 Suggested uses of model 58 Summary 59 References cited 61 Appendixes: A: Model program listing 64 B: Instructions for use of model program 108 C: Water-level measurement data from observation-well network 118 Metric conversion factors 128 PLATES [Plates in pocket] Contents V 42 19. Statistical comparison, by general lithology, of observed and model-generated changes in water-table altitude in calibrated transient model 45 20. Ranking of hydrogeologic units, based on effects of withdrawing 1 inch of ground water per year from modeled area 46 21. Standardized potential yield for each hydrogeologic unit, based on hypothetical withdrawal scheme 47 22. Time, since start of hypothetical withdrawal scheme, for each hydrogeologic unit to reach steady-state conditions 49 23. Percentage reduction of average annual base flow during 20-year simulation of hypothetical withdrawal scheme 49 24. Median decline of water table during 20-year simulation of hypothetical withdrawal scheme 50 25. Groundwater budget for each hydrogeologic unit after 91 days of hypothetical withdrawal scheme 51 26. Ultimate groundwater budget for each hydrogeologic unit resulting from hypothetical withdrawal scheme 52 VI Contents

Evaluation of the ground-water resources of parts of Lancaster and Berks Counties, Pennsylvania

“…4) is virtually identical to the conceptual model used in the lower-basin evaluation (Gerhart and Lazorchick, 1984). For the same reasons discussed in that report, it was assumed that porous-media methods of analysis could be used in this investigation.…”

“…Ground water below this layer is not stagnant, but data on water-bearing zones in wells indicate a marked reduction in secondary permeability. The results of the lower-basin evaluation (Gerhart and Lazorchick, 1984), in which this lower part of the system was modeled as an active layer, indicate that less than 8 percent of the total ground-water flow occurs in this deeper layer. Therefore, in this investigation, ground-water flow below the base of active ground-water circulation was considered negligible and modeling it was not warranted.…”

“…A three-dimensional ground-water flow model documented in Trescott (1975) and revised by Trescott and Larson (1976) was modified for use in the evaluation of the ground-water resources of the entire lower basin (Gerhart and Lazorchick, 1984). The modified model, with only minor additions, was used in this investigation to simulate two-dimensional ground-water flow in a single-layer aquifer; the model was calibrated under steady-state and transient conditions.…”

“…The ground-water resources of the Gettysburg and Hammer Creek Formations, including parts of Lancaster and Berks Counties, were described by Wood (1981). Gerhart and Lazorchick (1984) presented a model used for a regional ground-water resource evaluation of the lower Susquehanna River basin, which included the area in this study. the eastern part of the area ( fig.…”

“…The first part is an evaluation of the ground-water resources of the lower basin through the use of a regional ground-water flow model (Gerhart and Lazorchick, 1984). The second part involves a more detailed model of the most developed area in the lower basin an area in Lancaster and Berks Counties that is underlain largely by carbonate rocks.…”