As problems of waste disposal become more critical, and environmental constraints more demanding, serious consideration is being given to isolated zones in groundwater basins as burial grounds for hazardous wastes. Proponents have suggested various geologic environments--thick impermeable clay or shale layers, heavily pumped subsidence basins, deep saline water bodies, etc.--as being safe disposal sites. However, several hydrogeologic constraints must be understood fully before a prospective site can be considered isolated. Unconsolidated deposits, even at great depth, are highly sensitive to changes in applied stress. Interstitial fluids may be squeezed from even relatively impermeable beds by modest stress changes. Compaction of the deposits below 2,500 ft has been induced in the San Joaquin Valley by groundwater pumping. As much as 30 ft of vertical shortening in 3,000 ft of water-bearing deposits has been measured. At Alpaugh, more than 4 ft of surface subsidence, principally due to the compaction of fine-grained beds at depth, has resulted from 250 ft of artesian head decline. Interstitial arsenic, presumably locked in clay interbeds for thousands of years of interaquifer circulation, now is being squeezed into the aquifers. Hydraulic stresses in a developing groundwater basin may affect beds considerably below the deepest pumping wells. Theoretically, seepage stresses, and thereby groundwater movement, ultimately would affect all beds down to basement or some underlying truly impermeable layer. Slow drainage continues from each interbed as long as excess pore pressures exist in that interbed. Injection of wastes into clay or shale interbeds would cause a sharp increase in pore pressures in these interbeds. Subsidence areas are usually poor candidates for waste-disposal sites. Generally, they have demonstrated their close communication with the groundwater system, their direct response to pressure changes, and their high compressibility. Heavily depleted artesian aquifers are not empty reservoirs ready for refilling. Rather, the groundwater systems are full and adjusting to the various stresses imposed on the basin. Compaction recorders show that pore pressures throughout the vertical section respond rapidly to pumping stresses. Horizontal and vertical ground movement, caused by horizontal and vertical groundwater flow, can be a serious threat to surface or buried structures. Subsidence affects probably 7,000 sq mi in California alone in amounts to 28 ft in a few places, and has caused the failure of thousands of deep and shallow water and oil wells. Horizontal shear stresses and surface tension cracks are a continued threat in a developing groundwater basin. Saline-water bodies in a basin are not necessarily permanently out of touch with the circulation system of the basin. Hydrologic stresses may force some of this saline water into the circulating groundwater system. Evidence suggests that upward seepage stresses may extend below the freshwater-saline water interface; also, in some areas, saline water may be moving upward into the freshwater zones. Few regions of a groundwater basin are isolated from the circulating flow system. Although injecting liquid wastes deep underground is sometimes expedient, the effects are complex and never ending. Long-lived radioactive wastes require isolation from the hydrosphere for periods of time ranging from 1,000 to several hundreds of thousands of years, but much waste material, to be safe, must be contained forever. And who can anticipate future natural and manmade stresses on the system? End_of_Article - Last_Page 1600------------
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