The chemical quality of ground water is the most important ground-water problem in Segment 8. Highly mineralized water is present in shallow and deep aquifers in many parts of the four States. In most aquifers, the dissolved minerals in the water are from partial dissolution of aquifer minerals; in some, mineralized water has leaked into the aquifer from shallower or deeper aquifers.
Shallow aquifers can become contaminated where deeper aquifers that contain saline water discharge the water by upward leakage. In northeastern North Dakota, for example, aquifers in rocks of Cretaceous and Paleozoic age on the eastern flank of the Williston Basin contain highly mineralized water that is under artesian pressure. Where the bedrock aquifers are directly overlain by unconsolidated deposits, saline water can leak upward into permeable beds in the unconsolidated deposits. Some of the saline water discharges from the unconsolidated deposits to streams, and some flows into ponds and small lakes in depressions (fig. 87). Such ponds and lakes, which are common in Pembina, Walsh, and Grand Forks Counties, N. Dak. contain only salt-tolerant plants and aquatic life.
Seeps and ponds that contain saline water are numerous in Montana and present locally in Wyoming and North Dakota. Water in these seeps and ponds commonly contains dissolved-solids concentrations that range from 10,000 to 50,000 milligrams per liter. Most of the seeps form where unconsolidated deposits are underlain by almost impermeable bedrock (fig. 88). Recharge from precipitation enters closed depressions in the unconsolidated deposits and is unable to percolate downward into the bedrock. Evaporation is the only means by which the water can discharge; as a result, the water becomes increasingly mineralized with time. The saline water might collect as ponds in depressions on the land surface or can totally evaporate (fig. 89), thus leaving salt deposits where little or no vegetation will grow. The problem is intensified where the land is cultivated because plowing not only eliminates grasses and other natural vegetation that might transpire some of the precipitation, but loosens the soil and thus allows the precipitation to infiltrate rapidly into the unconsolidated deposits.
Locally, concentrations of fluoride greater than those recommended for drinking water by the U.S. Environmental Protection Agency are in water from some aquifers in Segment 8. Large fluoride concentrations have been reported in ground water in eastern North Dakota and South Dakota. Excessive concentrations of fluoride can cause mottling of tooth enamel in children.
Human activities can degrade the quality of ground water. For example, if uncased wells are drilled deep enough to penetrate an aquifer that contains saline water under artesian pressure, then the saline water can rise through the borehole and spread outward to contaminate shallower aquifers that contain freshwater (fig. 90). This type of contamination is possible only where the hydraulic head in the shallower aquifers is less than that in the aquifer that contains saline water. In areas where coal or metallic ores have been mined, precipitation that falls on, and percolates into, mine tailings or spoil piles where wastes have been stacked or ground water that moves laterally into the wastes can dissolve mineral material and carry contaminants into underlying and adjacent uncontaminated ground water (fig. 91). The mineralized ground water can move into wells or surface streams and ponds. Large concentrations of sulfate, iron and other metals, or radionuclides can result from the leaching of mine tailings. Other human activities that affect the quality of ground water include application of fertilizer and pesticides to cultivated land, disposal of human wastes in septic tanks, cesspools, or waste-treatment plants, disposal of liquid and solid wastes in landfills, storage of petroleum or other liquids in leaky underground tanks, and disposal of oil and gas production water or liquid industrial wastes by injection wells.
Declining water levels are a problem locally in unconfined and confined aquifers in all four States of Segment 8. In unconsolidated-deposit or consolidated-rock aquifers that contain water under unconfined conditions, large withdrawals from wells completed in the aquifers can reverse the prepumping direction of movement of ground water (fig. 92). Water that moves toward and discharges to streams under prepumping conditions can be intercepted by wells, particularly if large volumes of water are pumped from the wells. A cone of depression created by withdrawal from a single well, or several wells, can extend outward until it reaches a stream. Water from the stream can then move into the aquifer and toward the pumping well. If withdrawals are large enough, then streamflow can be decreased or completely diverted to the wells. Large withdrawals from pumping and flowing wells completed in the lower Cretaceous aquifers that are overlain by thick confining beds in eastern North and South Dakota have caused the hydraulic head in these aquifers to decline 200 feet or more over large areas.