From The Changing Illinois Environment: Critical Trends, Volume 2: Water Resources , Technical Report of the Critical Trends Assessment Project
Summary and Significant Results
The Critical Trends Assessment Project (CTAP) Water Resources volume examines environmental issues related to hydrologic processes in Illinois. It concentrates on those issues deemed of major concern in regard to surface and groundwater resources. Each chapter of this report describes historic information and possible trends over time, allowing a critical review of the state of the particular resource. This review, a combination of many sources of information, can aid in our understanding and potential management of these resources. The ongoing compilation and collection of these data are essential to this understanding and must be recognized as a valuable resource to the people of Illinois and one which merits continuing support. The following sections summarize the eight major issues addressed in the report.
Chemical Surface Water Quality
Ambient water quality data collection began in the early 1970s, and there are continuous data since that time for a number of locations in Illinois. Data on water quality parameters for Illinois streams and lakes were analyzed to determine water quality trends for 15 chemical constituents. These data are available on STORET, a database maintained by the Illinois Environmental Protection Agency (IEPA).
Water quality trends indicate a general improvement in Illinois streams and rivers. In particular, there is a decreasing trend in the concentrations in streams of metals (arsenic, cadmium, chromium, lead, and mercury). Decreasing trends in cadmium, lead, and mercury were especially strong. Significant decreasing trends were also identified in chlorides and chemical oxygen demand (COD). Increases in dissolved oxygen--a favorable trend--were observed, but not at a statistically significant level. However, two constituents experienced significant increasing levels, causing a degrading impact: phosphorous and nitrate/nitrite nitrogen (NO2 + NO3). The likely source of these constituents is nonpoint pollution from agricultural areas. All the other parameters that were examined (phenolics, fecal coliform, pH, total dissolved solids, and ammonia nitrogen) show little or no trend. The temporal trends described above were observed throughout most of the major watersheds in the state. However, selected quality parameters in a few watersheds displayed trends that were contrary to the statewide norm. The DesPlaines, Kankakee, and Illinois River basins show increasing concentrations of cadmium, chromium, and chloride, respectively. The Ohio River basin shows a decreasing concentration in nitrates.
No trends were observed for any of the water quality parameters examined for lakes in Illinois. The analysis was encumbered by limited data with many missing values for some parameters.
Long-term temporal trends in groundwater quality over selected areas of Illinois were examined using data from private and municipal wells. The concentrations of six chemical constituents were examined: hardness, iron, sulfate, chloride, nitrate, and total dissolved solids. The analysis indicates that on a countywide scale, ground water has not been degraded with respect to the six chemicals examined. Data limitations precluded study of trace-level contaminants, but it is imperative that such an assessment be undertaken in the future.
Much of the contamination of Illinois ground water is generally localized. It is clear that the quality of some ground water, particularly in the metropolitan Chicago area, has been degraded by anthropogenic activity, resulting in increased chloride and total dissolved solids. This contamination can render a private or municipal groundwater supply unusable. Once contaminated, ground water is very difficult and expensive to clean, and the process may take many years to complete. Preventing groundwater contamination is thus in the best interests of the people of Illinois.
Since the mid to late 1980s, national concern has focused on groundwater contamination and its potential impacts for those individuals who rely on ground water for their drinking water. This concern has led to the initiation of several laws and agency policy shifts to help ward off this imposing threat or to help create a monetary base for research and remediation of existing contamination. Illinois is one of only a handful of states that has adopted legislation in an attempt to respond to this concern. In 1987, P.A. 85- 0863 was introduced as a comprehensive, prevention- based policy focusing on beneficial uses of ground water and preventing degradation. This act, known as the Illinois Groundwater Protection Act (IGPA), relies upon a state and local partnership and, although directed toward protection of ground water as a natural and public resource, it specifically targets drinking water wells in Illinois.
The IEPA's synoptic analysis of public water supply wells indicates that the quality of the state's ground water is generally good. The analysis of the information used for this report tends to support this view. The IEPA and the Illinois State Water Survey (ISWS) also agree that chemical levels are limiting use of the resource in some areas in Illinois. The IEPA reported that 4.6 percent of the tested public water wells had detectable levels of organic chemical contamination.
Erosion and Sedimentation
The estimated annual gross soil erosion from croplands in Illinois is 158 million tons, or nearly 90 percent of the total gross erosion for the state (180 million tons). It is estimated that 2 to 9 inches of Illinois topsoil has eroded since its initial cultivation by early settlers. The most severe erosion has occurred in southern Illinois, a hilly and highly erosive area that was settled and farmed earlier than the rest of the state. Erosion has also been significant in the western Illinois, where 6 to 7 inches of topsoil has eroded. Central and northeastern Illinois have fared better with 2 to 4 inches of erosion.
Three agencies, the ISWS, the U.S. Geological Survey (USGS), and the IEPA, have collected instream sediment data over the last 20 years. Analysis of the measured sediment concentrations for selected streams in northern, central, and southern Illinois shows that since 1980, sediment concentrations have been decreasing in the Rock River in northern Illinois, have been essentially constant in the Sangamon River in central Illinois, have been decreasing slightly in the Kaskaskia River in south central Illinois, and have been increasing slightly in the Cache River in southern Illinois. The spatial distribution of instream sediment shows that the highest amounts of sediment are from regions along the Illinois River and in west-central Illinois, along with some areas in southern Illinois. The northeastern and central sections of the state show the least instream sediment.
The major impact of soil erosion is the eventual accumulation of the eroded soils in lakes and reservoirs. The gradual loss of capacity in water supply lakes due to sedimentation has been a serious problem and has been investigated intensively for a long time. More recently, greater concern has focused on the impact of sedimentation on environmental quality and on the impact of continuing sedimentation on the Illinois River valley. The Illinois River drains nearly half of the state and many major streams drain into it. The main sources of sediment to the Illinois River valley are watershed erosion, stream bank erosion, and bluff erosion. The sediment yield calculations show that, on the average, 13.8 million tons of sediment are delivered to the Illinois River annually. The average annual outflow of sediment from the Illinois River at Valley City is 5.6 million tons. Thus, on the average, 8.2 million tons of sediment are delivered from tributary streams and deposited in the Illinois River valley. The temporal trend analysis indicates that sediment concentration and load have been decreasing in the Illinois River at Valley City since 1980. But the primary cause, at least for the sediment load, is the decrease in average streamflow over the same period.
Major areas impacted by sediment deposition in the Illinois River valley are backwater lakes. Sediment rate calculations show that the backwater lakes had lost from 20 to 100 percent of their capacities by the year 1990. The average capacity loss is 72 percent. The conditions in Peoria Lake over the years have clearly illustrated the impact of sedimentation in the Illinois River valley. As of 1985, Peoria Lake had lost 68 percent of its 1903 capacity due to sedimentation. The average depth of the lake had been reduced from 8 feet to 2.6 feet.
Most of the chemicals found in lake sediments are transported from source areas and upland watersheds, which include urban and agricultural areas, through stream networks. Profiles of lead and zinc concentrations in lake sediments from Lake Peoria indicate that the highest concentration of lead was in the late 1960s, while that for zinc was highest in the early 1950s. The concentrations of the two heavy metals in the sediment have been decreasing since the peak periods of deposition in the lake. The top sediment layer shows much lower concentrations of lead and zinc than the lower layers, which reflect earlier periods. Similar patterns are also observed for many other chemicals in Illinois River sediments.
Ground water is a finite resource that is not uniformly distributed throughout the state. In some areas of Illinois, demand for this resource has caused it to become well developed. Demand can exceed supply, especially in urbanized areas of Illinois, and potential problems can arise from competition for the resource. The major concern is that annual use, in the long term, should not exceed the average annual recharge available to a specific aquifer system. When annual use does exceed average annual recharge over a prolonged period, ground- water mining occurs.
In Illinois the most significant problem, in terms of groundwater mining, occurs in the Chicago region. This problem started during the 1950s when demand exceeded what the natural systems could effectively recharge. Groundwater levels in one of the major aquifer systems of this region have declined by almost 1,000 feet since pumping began. The economic result has been increased pumping costs due to increased lift requirements. The mining of the natural system also puts it at risk of compaction, which could permanently damage the aquifer. A lawsuit brought by Wisconsin ultimately required the state of Illinois to decrease its pumpage from the major aquifer supplying the Chicago region.
Steps are being taken to relieve the stress on the aquifer system. The substitution of Lake Michigan water for ground water is having positive impacts, but this strategy will only buy a few years. It is projected that by the year 2005, groundwater mining of the principal aquifer supplying the Chicago region will resume.
No other area in Illinois has had a more historically adverse impact on groundwater levels than has the Chicago region. There are other areas where heavy groundwater use has affected the groundwater flow system. These include two industrial centers, Peoria and the American Bottoms region near East St. Louis, and one agricultural area in eastern Kankakee and northern Iroquois Counties. None of these areas has a groundwater mining problem similar to the Chicago situation. But groundwater mining has the potential to be a critical concern in each of these areas.
Drought Impacts on Water Resources
Droughts occur in Illinois on average once every eight to ten years. The identification of these droughts generally requires some socioeconomic impact resulting from a lack of water. The most widely recognized impacts of water shortages are those associated with public water supply and agriculture, but aquatic habitat, river navigation, and recreation can also suffer due to water shortages. This chapter concentrates on drought impacts from the perspective of streamflow, ground water, and public water supply. From this perspective, the most severe droughts occurred in 1930-1931, 1952-1955, and 1962-1964. The recent minor droughts were those of 1976-1977, 1980-1981, and 1988- 1989. Though the most severe droughts have not occurred in the last 30 years, there is no evidence that their frequency of occurrence has been altered.
The impacts of each of these droughts on public water supplies have always brought about a renewed awareness of inadequacies in the existing water supply systems. Thus, the greatest amount of activity to upgrade water supply systems occurs following droughts. Many systems throughout the state are much better equipped to handle shortages associated with drought conditions than in the past. Yet it is estimated that, of the surface water systems susceptible to drought impacts, almost 40 percent could be severely impacted during a 50-year drought. While this is an improvement over the percentage of systems that had shortages during the severe historical droughts, it is not a particularly significant improvement. Most groundwater systems are buffered from the impacts by drought and few of these would be impacted by the same type of drought.
Reducing water use during drought is one way to reduce the impact of shortages on water supply systems. During the 1988-1989 drought, more than 20 percent of the public water supply systems imposed or requested water conservation measures. Lawn watering was the most often restricted water use, followed by car washing and domestic water use. Water conservation practices were reportedly successful for 40 percent of the public water supplies that used them. But despite the water conservation measures, most systems indicate a large increase in total water use during drought con-ditions, particularly during the early stages before the drought is actually recognized. Understanding the con-ditions that lead to drought may allow us to develop better strategies to help eliminate or reduce its impact. Yet at this point it is apparent the occurrences of droughts still catch us off-guard, and that mitigative measures are not begun until the drought impacts become obvious.
Water Supply and Use
Water use in the state has increased a modest 27 percent since 1965. Most of that increase is in power generation. Water use for public water supplies has risen only about 7 percent during that time, less than the concurrent percentage increase in population. The number of public groundwater supply facilities within Illinois has also risen significantly, yet the total amount supplied by ground water remains near 25 percent.
A dependable, adequate source of water is essential to sustain the existing and potential population demands and industrial uses in Illinois. Modifications and practical management of the use of both surface and ground water have helped make this vital resource reliable in Illinois. As individual facilities experience in-creases in water use, innovative alternative approaches to developing adequate water supplies must arise. In particular, this is likely to involve conjunctive use of surface and ground waters. Major metropolitan centers such as the Chicago area, Peoria, Decatur, and Bloomington-Normal have already developed both surface and ground water to meet their needs for devel-opment and to sustain growth. The construction of impounding reservoirs has become and will remain economically and environmentally expensive, making it a less common approach.
Proper management of water resources is also necessary to ensure a reliable, high quality supply for the population. Water conservation practices will be- come increasingly important to reduce total demand and avoid exceeding available supplies. Both our ground- water resources and surface reservoir storage must be preserved to maintain reliable sources for future generations.
Streamflow Conditions, Flooding, and Low Flows
Most streams throughout Illinois have experienced an increase in flow conditions during the last 25 years, in particular for normal flows and low flows. These increases are particularly significant throughout much of the northern third of Illinois. The greatest cause for these increases appears to be climate variability; much of this area has experienced precipitation increases lessthan 10 percent. For many streams low flows are also highly impacted by increases in the amount of waste water returned to streams.
Trend analysis indicates that much of the northeastern quarter of Illinois and a part of northwestern Illinois have experienced increased average flow and low flows. This widespread trend in streamflow is attributed to changes in climate, particularly in total precipitation. Streams in central Illinois have also seen above average flows in the last 25 years, but not to a scale such that they produce a significant trend. Examination of average flow conditions to determine cyclical or gradual trends indicates that the increased flows have occurred over the last 25 years and that their departures from the long-term normal flows appear to be related to concurrent increases in the average precipitation. For most locations, high flows and flooding have not been noticeably affected by climate variability. One region where an increase in high flows is identified is the Kankakee River basin. Urbanization is the one land use change where an impact on streamflow conditions is easily detected. Many smaller urban streams in northeastern Illinois have considerable increases in flood volumes and peak flows, regardless of whether stormwater detention facilities are present. Increases in low flows are com-mon, but not universal among urban streams. Many streams show sizable increases in low flows as the result of wastewater effluents.
The existence of reservoirs normally creates a large change in the amount of streamflow downstream. Although high flows and flooding are reduced in reservoirs that are designed for flood control, they may not be greatly impacted by other reservoirs. The impact on low flows is most greatly affected by the reservoir operation and use of its outlet facilities. Most large reservoirs provide for a minimum release of water, which often increases the amount of downstream flow compared to natural drought conditions. Reservoirs that do not provide minimum releases and use their storage primarily for water supply are most apt to cause a decrease in flow downstream. But most of these reservoirs occur on small watersheds that ordinarily have no flow during dry conditions.
Low flows in Illinois streams are most greatly impacted by return flows of wastewater coming from municipalities and industry. For many large streams in the state the amount of return flows can often comprise over 30 percent of the total flow in the stream during low flow. Several small streams in northeastern Illinois virtually lack low flows because wastewater returns comprise over 90 percent of the flow during dry conditions.
Instream Flow Uses, Needs, and Protection
A fundamental issue in all future water management programs and decisions is the protection of instream flow uses. Instream flows are valuable in maintaining: 1) aquatic habitat during low-flow or critical periods, 2) water-based recreation and associated streamwater quality and quantity, 3) a stream's assimilative capacity to receive effluents from wastewater plants, 4) stream integrity in terms of biodiversity and strength of biotic communities, and 5) sufficient water quantity for downstream municipal and industrial water supplies during emergency and severe drought conditions.
The protection of instream flows has been under serious consideration by the state natural resource agencies for the last two decades. The absence of suitable aquatic habitat assessment models (in terms of hydraulic and habitat simulations) and the lack of financial support for an integrated statewide study of desirable protected flow levels have seriously hindered instream flow regulation and the development of an associated infrastructure. The adoption of a protected flow standard involves consideration of conflicting goals and needs. Both tangible and intangible benefits are associated with a protected flow level, and these benefits vary with the level of protection. There is also an associated cost for adopting and maintaining a protected flow level in a stream. A cost-benefit approach will provide a framework for analyzing the economics of objectively selecting and adopting a particular protected flow level to meet various needs
Low-flow releases from reservoirs may have to be mandated to protect downstream interests. Preference or suitability curves for Illinois fish species and their life stages need to be developed or improved with respect to flow parameters and channel substrates. Field studies for streams in various physiographic areas are also necessary to determine various parameters that affect habitat evaluations.
Continue to Background