Groundwater


From The Changing Illinois Environment: Critical Trends, 1994, Summary Report of the Critical Trends Assessment Project

Enough water to supply a single home can still be found by drilling a well most places in Illinois. Today, groundwater is used for industry, irrigation, and cooling water as well as drinking supplies. In all, more than one billion gallons of groundwater per day are displaced and/or consumed in Illinois for one human use or another.

SUPPLY

In most parts of Illinois, groundwater is not a separate water resource, but merely one of the forms taken by stored water as it moves from the atmosphere into the ground and back again. The water table--the zone of total saturation near the surface--does not supply shallow wells alone. It also acts as a reservoir that slowly feeds confined aquifers beneath it. Where local geology allows it, groundwater often even flows into surface streams, providing most of the flow during periods of slack rainfall.

While no underground rivers churn beneath Illinois, subterranean water can be made to move slowly from one region to another. Heavy withdrawals made by Chicago suburbs from that region's Ironton-Galesville sandstone aquifer, beginning about 1950, eventually caused water levels in some local wells to drop more than 1,000 feet, with lesser declines recorded as far away as Wisconsin. Court action by that state led Illinois authorities to agree to a long-range plan to reduce pumpage from that aquifer system.

An aquifer is a water-bearing stratum of permeable sand, gravel, or rock. Aquifers sufficient to supply municipal or industrial uses are somewhat patchily located around Illinois (Figure 10-1 ), ranging in depth and differing in quantity and quality. Aquifer units may occur in alluvium along major rivers (shallow), in buried glacial alluvium deposits and buried valleys (shallow and deep), and in shallow and deep bedrock. The nature of the formation largely determines whether and to what extent water withdrawals may be economic. Units of fine-grained, low-permeability bedrock are poor water producers, while coarse-grained and/or well fractured bedrock units are generally good producers. Transmissivity is a measure of an aquifer's permeability or hydrologic conductivity; transmissivity of Illinois aquifers ranges from as much as 300,000 gallons per day per foot to as little as a few thousand gallons.

DEMAND

Overall, Illinois is relying less on groundwater for its public water supply. In 1991 Illinois' larger public water systems pumped slightly more than 340 million gallons of water per day; in 1980 daily pumpage by facilities of this size was roughly 410 million gallons. Most of the reduction in groundwater use occurred in the Chicago area. (Figure 10-2)

Figure 10-2. Groundwater Use in the Chicago Area

Source: Water Resources, Illinois State Water Survey, 1994

Water may not be withdrawn from a given aquifer faster than its hypothetical average annual recharge rate without depleting it. This average annual recharge rate depends on rainfall, especially in the spring. An analysis of 21 shallow wells during the drought of 1988-89 found six new low-water level records had been set in five regions of the state, and near-records were recorded at nine other sites.

Groundwater that is withdrawn faster than it can be recharged is said to be "mined." To date, groundwater mining has occurred only in northeast Illinois, where a trend of steadily increased pumpage from some aquifers persisted for roughly a century. In the 1980s many Chicago suburbs switched to Lake Michigan water, as did DuPage County in 1992. This reversed the trend and reduced groundwater withdrawals to levels closer to natural recharge rates. Other northeast Illinois communities have or are planning to augment their municipal wells with river or lake water. If this most recent trend continues, water levels at Elmhurst may rise by as much as 650 feet by 2010, and the region's Cambrian-Ordovician aquifer system may be restored to its natural balance for the first time since the 1950s.

As insurance against drought, Illinois farmers have been expanding their irrigated land, from 40,000 acres in 1970 to about 240,000 in 1987. Some 65% of this acreage is located in five counties, all but one of which are in the northern half of the state. Most of this irrigation water (96%) is groundwater, usually added to high-value specialty crops that make the costs of pumping and equipment worthwhile. Increased withdrawals by farmers, while locally significant, are not thought to pose a long-term threat to regional resources.

POLLUTION

Groundwater quality seldom varies under natural conditions, so significant changes often indicate degradation due to human action. Chloride, nitrate, and sulfates occur in groundwater naturally. However, high sulfate concentrations may betray the presence of acid wastes from (for example) metal pickling operations. Concentrated chlorides may be a clue that road salt or oil field brine has entered an aquifer. High nitrate levels are usually associated with farm fertilizer applications, feedlot runoff, or septic tanks.

Shallow aquifers are more vulnerable to contamination than deep ones, and aquifers in industrialized areas are more vulnerable to contamination than ones in rural areas. (Figure 10-3 ) Unlined or improperly lined impoundments pose obvious risks of chemical pollution of groundwater. More than a quarter of the surface impoundments in Illinois are thought to be in places whose near-surface geology leaves subsurface soils prone to infiltration; surface soils also may be contaminated when impoundments accidentally overflow. However, various studies confirm that contamination from human activity tends to be local in both cause--a broken storm sewer pipe, a leaky retention pond--and effect.

Landfills. A recent survey concluded that approximately 25% of public water wells in Illinois are located within one mile of a known landfill or open dump site, as are approximately 10% of the known private drinking water wells. (Figure 10-4 ) Mere proximity is no certain measure of the risk of contamination, as the geology and the engineered systems of each site determine whether leachate flows toward or away from local wells.

Underground injection. Since 1965 several classes of injection wells have been used in Illinois to dispose of a variety of wastes, from factory residues to stormwater runoff, sewage, and heat pump effluent. For example: * Five deep "Class I" wells each year dispose of more than 300 million gallons of certain industrial wastes in Illinois, mainly dilute (70-95% water by volume) acids. The number of waste injection wells in Illinois has been on the decline since the 1980s as new regulations began to make land disposal of various liquid hazardous wastes illegal or more costly.

* In 1990 there were some 13,000 wells in Illinois used to inject mainly brine from oil and gas recovery into depleted oil fields lying below Underground Sources of Drinking Water (USDW). Few instances of saltwater contamination of aquifers from injection wells have been documented in Illinois (although historical contamination of groundwater associated with near-surface storage of such wastes has been documented).

* Another nearly 1,800 wells in Illinois dispose of more general wastes from sources ranging from auto shops to household toilets. (In Illinois, most such "wells" are septic tanks.) Illinois regulations permit injection of these wastes into underground formations both above and below USDW, as long as groundwater quality standards are maintained.

Tanks. Buried storage tanks containing petroleum or hazardous chemicals have been recognized as probably the most important (if not the most widespread) source of groundwater contamination in the U.S. For example, as of 1987, the number of underground storage tanks in Illinois was estimated at 60,000. Many of these tanks are leaking. A cleanup fund established in 1986 has spurred the ongoing removal of leaking tanks and clean up of dirty sites. Installation of new tanks are subject to stricter standards than heretofore.

Industrial pollution. There are nearly 1,400 listed or potential Superfund sites in Illinois, many of which are suspected by the IEPA of contributing to groundwater pollution. As of 1992, 364 wells statewide had been closed or restricted in use because of contamination by heavy metals and organic compounds like chloroform or vinyl chloride.

A sizeable database of 50,000 records of private and public drinking water wells give information on trends in groundwater supplies that might suggest pollution's cumulative effect on this resource. A recent analysis of these water quality records from 1900 to 1992 focused on six dissolved chemicals indicative of groundwater quality--iron, total dissolved solids, sulfate, nitrate, chloride, and "hardness." Data showed some variability, but overall there were no statewide trends for this time period, which suggests that Illinois groundwater quality is at least not declining. v


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