Importance of Physical Setting and Land Use to Groundwater

Groundwater contamination can be naturally occurring, but it is typically a result of land uses associated with modern society. Nearly anything that can be spilled or spread on the ground has the potential to leach or seep through the ground and into groundwater. The physical setting of an area usually determines how easily groundwater becomes contaminated if inadequate waste management or improper land uses occur.

The "physical setting" of a basin includes soil type, characteristics of the subsurface unconsolidated material, depth to bedrock, depth to groundwater, topography, and hydrologic characteristics of the area. The potential for groundwater contamination is determined by land use practices applied to an area in conjunction with the basin's physical setting.

When considering an area for specific land uses, it is vital to consider the relationships between the land, the proposed or actual use, the physical setting, and the contamination potential, to help ensure that groundwater contamination does not occur.

Soils characteristics

Soil characteristics are among the most significant factors determining groundwater contamination potential. The type, structure thickness, permeability, chemistry and slope of the soil and the subsurface unconsolidated material (deposits overlying bedrock) determine soils' ability to attenuate contaminants.

Soils of different composition and geographic location possess different properties. Soils are comprised of all types of materials of different sizes, ranging from boulders to very fine-sized particles of clay. Soils are classified according to the percent of gravel, sand, silt, clay and organic material, in combination with other physical properties.

Permeability

Permeability is a very important soil characteristic. Permeability is the rate at which water and gases move or percolate through the pores and cracks of underlying soil and rock. Many factors contribute to permeability, but most often soil depth is the limiting factor. Most of the shallow soils have moderate to high permeability, allowing water and gases to enter groundwater before biological, chemical and/or physical degradation of potential pollutants can take place. Water can move easily in soils with coarser, larger soil particles containing higher percentages of gravel and sand because the spaces between the soil particles are large and interconnected. Soils with these characteristics are said to exhibit high permeability. Water moves rapidly through highly permeable soil and is not in contact with the soil or surficial material very long; this allows little time for biological or chemical degradation of contaminants or for the contaminants to be bonded to soil particles or "removed" by uptake from vegetation.

In contrast, soils that consist of finer particles are less permeable to infiltration of water. Clay, silt and loam are likely to impart some protection to groundwater (with many caveats, including the type of contaminant) because water moves more slowly through the smaller, less interconnected voids. This "rule of thumb" is not invariable, however. Cracks in dried clays may easily and quickly transport contaminants to deeper aquifers, just as karst or other materials are susceptible to contamination. The only invariable aspect of groundwater is that its characteristics and flows are highly site-specific and often complicated. The best protection is to prevent pollution.

Factors that affect the rate of water movement into soils and substrata include:

• Percentage of sand, silt and clay in the soil
• Soil structure
• Amount and type of organic matter
• Soil compaction
• Soil chemistry
• Soil temperature
• Presence of fractures, sinkholes
• Land use practices

Major Soils Of The Basin

The soils of the basin include three of the 10 major soils regions in Wisconsin described by F.D. Hole (1980). These soils developed on the loamy glacial deposits and relatively significant amounts of loess, a wind blown silt. The major soils are defined below from most prominent to least extensive. More detailed descriptions and use limitations can be found in county soil surveys.

Region B - Soils of the southeastern upland are represented by Dodge and Miami soils that have formed from loess and limy glacial tills of loamy texture. Moderately deep (Casco) to shallow loams and associated stony soils of the Kettle Moraine extend in an irregular belt from western Waukesha County to central Manitowoc County. These soils present problems in drainage and liquid waste disposal. Wet soils occupy numerous depressions in this region.

Soils of this grouping are found throughout the basin and are the most common type in the eastern portions.

Region Bp - The darker prairie soils of Region B include silt loams (Plano) and loams (Warsaw) that overlie glacial sand and gravel in the plains of Rock and Walworth Counties and on glacial till of the rolling upland to the north.

These soils are the dominant type in the river valley in Rock County and up the Yahara River through central Dane County.

Region J - Soils of the stream bottoms and major wetlands include both mineral soils (Pella, Newton, Arenzville) and organic soils (Houghton Muck). Numerous bodies of these soils occur around lakes, bogs and in countless depressions and drainageways. Land use ranges from wildlife habitat to crop production.

Larger areas of wetland soils can be found in southern Jefferson and eastern Waukesha counties just north of the Green Bay terminal Moraine. They are also extensive in central and eastern Dane County where flat lying areas are common between drumlins.

Depth to Bedrock and Type of Bedrock

Attenuation of Contaminants

Water polluted by various land use activities can be filtered before reaching the water table. Pollutants react differently to a variety of physical and chemical properties of soils, determining both the quantity and type of pollutants attenuated. The process of attenuation can involve increasing the uptake of nitrogen (a common groundwater contaminant) by crops, immobilizing harmful metals, and removing bacteria (from animal, including human wastes). Microbial, physical and chemical processes that take place in the ground make the soil an integral part of groundwater protection.

The pollution of groundwater can occur when the attenuation capacity of the soil system is exceeded. To assess a soil's attenuation potential, seven physical and chemical soil properties are important:

• Organic matter content of the surface layer
• The pH of the surface layer
• Soil drainage
• Permeability of the top five feet of soil (regardless of type)
• Depth to bedrock
• Depth to gravelly or cobbly substratum
• Cation exchange capacity of the soil

The organic matter content is important because it increases the ability of the soil to:

• Hold nutrients, making them available for plant uptake;
• Bond heavy metals, making them less soluble in water;
• Adsorb organic chemicals such as herbicides and pesticides.

Also, soil provides a valuable medium for micro-organisms, which play an important role in the breakdown of organic waste, volatile organic compounds (VOCs) and pesticides. Attenuation by microbial processes is often optimal when the pH of the soil is neutral or mildly alkaline.

The effectiveness of the soil's ability to attenuate contaminants depends on the rate at which water moves through the soils. Thus, the permeability of the top five feet of soil is an important attribute to attenuation.

Soil drainage class, which indicates the high water table or zone of saturation, is also an important factor to consider. A deep, medium-textured, well-aerated soil offers the best opportunity for water to move through the soil with maximum contact between potential pollutants and the mineral and organic fractions of the soil, enhancing soil attenuation.

Depth to bedrock and depth to gravelly or cobbly substratum have major roles in the attenuation of contaminants. Pollutants can move rapidly into groundwater supplies through fractures in bedrock or the highly porous, permeable gravel and cobble substratum with little or no attenuation occurring.

Depth to Water Table

Topography

Hydrologic Conditions

Potential Threats To Groundwater

Any land use has the potential to cause groundwater contamination. By locating, constructing and operating facilities and systems appropriately, negative affects can be minimized. Within the Lower Rock River Basin the land use is 74 percent agricultural, 14.7 percent urban, 5 percent forested, 3.2 percent wetlands, 3.1 percent open water, and 0.6 percent barren land.

Land use can have a dramatic affect on the groundwater resources and the resultant base flow to streams. Several studies in Dane County have highlighted these affects and provide some tools for further study. Investigations in Rock County have also developed some management options. The geologic and physiographic differences across the basin preclude the use of generic management selections.

Dane County has some groundwater depletion problems due to pumping in the Madison metropolitan area. The large cone of depression has deprived springs and streams of base flow. The cone of depression extends past the surface water divide and draws groundwater from the Sugar River basin. Decreases in recharge due to urbanization are also evident.

The problem has been further exacerbated by the diversion of wastewater from the lower Yahara River valley. The diversion was done to comply with surface water quality limits. The Yahara River total flow at McFarland has been reduced by about 35 percent. The stream's low flow (Q7,10)--the time when base flow is most important--has been estimated as reduced by 70 percent. Fortunately the lakes act as a large reservoir and low flow minimums have been maintained thus far. Further reductions in base flow due to pumping and reduced infiltration could prove problematic in the future.

The recent addition of the Verona to the Madison Metropolitan Sewerage District has increased the effects of urbanization on the Sugar River Basin base flow. A plan to pipe the Verona discharge back to the Sugar River basin is being implemented. The treated wastewater will be discharged to Badger Mill Creek. This will revitalize a stream that has lost most of its base flow due to the extensive development of the area. The quality and delivery of this water may not be as good for the stream as the lost base flow but should help sustain the aquatic community.

The Dane County Regional Planning Commission, Madison Metropolitan Sewerage District and the City of Middleton recently funded a study by the U.S. Geological Survey and Wisconsin Geographic and Natural History Survey. The Dane County Regional Groundwater Flow Model was developed as a planning tool for assessing various management alternatives. The model will help determine how new well placement may affect groundwater and surface water and what affects land use changes will have on groundwater conditions. The model will also be used to evaluate alternative zones of contribution as the basis for delineating Wellhead Protection Areas (WHPA) for municipal wells.

Preliminary results of the modeling effort show that most of the groundwater in the county originates within the county boundaries. This highlights the need for water conservation and proper land use planning to maintain groundwater supplies and base flow to the counties streams.

Rock County's geology is quite different from Dane County's. The larger cities utilize the alluvial valleys and enjoy abundant water supplies. Back in the 1960s Janesville began buying up land along the valley floor for recreation and water management. The glacial drainageways have very high infiltration rates and are good areas for recharge. Municipal wells are located in these areas and stormwater from the housing on higher terraces is allowed to infiltrate in these areas. This type of planning and land use is excellent for the groundwater resource, as long as the quality of runoff can be maintained. Research has shown the importance of recharge and base flow maintenance. Proper land use planning should include protecting recharge areas and the quality of the recharging waters. This is one of the concepts behind the development of wellhead protection areas. Another aspect of this is proper well siting. By spacing wells to minimize drawdown and promoting recharge, base flows can be maintained.

The rapid infiltration near Janesville has lead to some groundwater problems that are also evident in parts of the Kettle Moraine where recharge rates are high. Because the infiltration is so high, contaminants reach the water table very easily.

Most groundwater contamination is related to poorly-sited land uses, such as agricultural manure storage and handling facilities located near sinkholes or shallow depth to groundwater. Once groundwater has been contaminated, depending on the pollutant, successful remediation can take years and may never occur. Remedial techniques are often extremely expensive. In the meantime, health-threatening contaminants can spread from one well to the next without well users' awareness of the problem. This difficult situation can be prevented through safe storage and handling techniques for pesticides, for example, or through reducing the use of the problematic chemicals in the first place.

To succeed in pollution prevention, it is necessary to identify all potential sources of groundwater contamination in a specific geographic area. Many of the potential source locations within the basin are listed in Bureau of Solid and Hazardous Waste Management publications.

Within the basin there are:

• 194 leaking underground storage tank (LUST) sites,
• 4 sites that are listed on the Hazard Ranking list category 1 that pose a "substantial danger" to the public or environment,
• 2 sites that are listed on the Hazard Ranking list category 2 that do not pose a "substantial danger" to the public or environment,
• 87 licensed waste disposal facilities,
• 8 active municipal and industrial landfills,
• 13 sites being investigated under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) or "Superfund" program,
• 8 sites on the National Priorities List,
• 88 historic waste disposal sites.

Within the seven counties that the basin crosses 3,087 reported spills have occurred. It cannot be determined how many of these are within specific watersheds due to poor locational recording in the database. Other potential sources include: improperly abandoned or constructed wells; agricultural practices; waste disposal; materials storage and handling; and other causes (Zaporozec). See also PUBL-SW-108-93, PUBL-SW-501-94, PUBL- SW-504-95

For more information on groundwater, visit the WDNR - Drinking Water and Groundwater Program