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SECTION 2.0 ENVIRONMENTAL ASSESSMENT (pdf file--text only)

A thorough assessment of the physical environment of Portage County is required in order to understand the existing pattern of groundwater quality and the potential degradation of groundwater quality from a variety of activities and land uses, and to address the groundwater concerns critical to the residents of the County. The environmental assessment is the foundation for designing and implementing effective groundwater management strategies.

SECTION 2.1 AVAILABILITY OF DATA

The environmental assessment that follows is based on existing data. There is considerable general information available for Portage County's soils, surficial geology, bedrock geology, and groundwater hydrology and hydraulics. The environmental assessment is adequate for developing County management options. However, analysis of specific development proposals or possible problem areas will require additional site specific data acquisition. In addition to providing a basis for management of County groundwater resources, this document should also alert County planners, private developers, and landowners to potential problems, and help identify and direct further study needs. This assessment should be revised and updated as additional data is acquired.

SECTION 2.2 POLLUTION ATTENUATION IN THE PHYSICAL ENVIRONMENT

The pollution attenuation potential of a given site is the inherent ability of the site to remove, immobilize, or modify pollutants to reduce adverse impacts on groundwater quality. The primary environmental factors that must be considered in order to evaluate the pollution attenuation potential include:

The Soil Zone:

depth

texture (clay and silt content)

permeability

slope

organic matter content

The Unsaturated Zone:

thickness

texture

permeability

The Saturated Zone:

texture

permeability (hydraulic conductivity)

net recharge

direction of flow

Portage County is a complex mixture of these factors. Generally, the County can be divided into three areas on the basis of similar geologic and hydrologic conditions (Figure 2.1). The drift‑crystalline rock province occupies the northwest portion of the County, the sand plain province is located in the central and southwestern portions of the County, and the drift province occupies the eastern portion of the County. An evaluation of environmental factors will necessarily follow this general division.

A. Soils

There are 38 identified soil series in Portage County grouped into 11 soil associations. Four associations are generally related to the sand plain province, two to the drift province, three to the drift‑crystalline rock province, and two associations are related to alluvial or organic deposits (Figure 2.2).

Soils are generally considered to be the upper five feet (or less) of unconsolidated materials that support plant growth. The soil zone properties are the most important factors in determining the natural pollution attenuation capability in a given area. Pollutants can be removed or modified in the soil layer through physical processes such as filtration, chemical processes such as adsorption and precipitation, and biological processes such as plant uptake, denitrification, and decomposition. The soil layer, especially the top 18 inches, is generally the most chemically and biologically active zone between surficial pollutant sources and the aquifer, and is therefore considered the first line of defense. The roles of individual soil zone factors are summarized below.

1. Soil Texture ‑ The distribution of soil particle sizes influences the rate of water movement through the soil and the active surface area of the soil. Fine textured soils have greater surface area and lower permeability and, therefore, longer contact time and greater sorption area for pollution attenuation. High clay content can be especially advantageous for pollution attenuation because of very small pore size and tremendous surface area available for sorption of cations (positively charged molecules). Some pesticides are inactivated and degraded by sorption to clay colloids. Factors that can alter the effective texture, such as macropore phenomena, should also be considered. This would be most noticeable in otherwise fine textured soils, such as shrinking clay.

2. Soil Permeability ‑ Defined as the rate of water movement through the soil, this factor is extremely important to the attenuation potential. As noted in the discussion of soil texture, a slow rate of water movement increases the contact time between waterborne pollutants and the soil particles and, therefore, allows the natural contaminant-removal processes to function more effectively.

3. Soil Depth ‑ Influences the amount and time of contact between the pollutants and the soil particles. Deeper soils increase the contact and potential attenuation from inherent physical, chemical, and biological treatment processes.

4. Soil pH ‑ The acidity of the soil influences the solubility of pollutants and the rate of biological processes that may remove pollutants. In general, acid soils tend to increase pollutant solubility, reduce sorption onto soil particles, and reduce the effectiveness of biological treatment processes.

5. Soil Organic Matter ‑ The amount of soil organic matter influences the sorption potential of the soil and the level of biological activity. Organic matter can bind volatile organic chemicals, metals, nutrients, pesticides, and some pathogens. Organic matter also serves as an energy source for microorganisms essential in the breakdown of organic wastes and pesticides. Wet organic soils may also remove nitrogen through denitrification. Wet organic soils often occur in groundwater discharge areas where pollutants are more of a surface water quality problem.

6. Soil Slope ‑ Can influence the amount of water that will infiltrate into a soil. Flat slopes tend to increase the infiltration of water and associated contaminants into the soil and, therefore, the potential local recharge to (and pollution of) the aquifer. Steeper slopes lead to transport of runoff water and associated contaminants to another location downslope where it either recharges the groundwater or contaminates surface waters.

B. Surficial Geology

Below the soil zone and above the bedrock are unconsolidated deposits. These may be weathered residuum from bedrock, glacial materials, or alluvial deposits of a more recent origin. For the most part, the unsaturated zone between the near surface (A and B) soil horizons and the saturated zone consists of these materials, although crystalline bedrock occurs near the surface in the northwestern portion of the County, and sandstone mounds are found in several areas. The deep and widespread glacial deposits of the sand plain and drift provinces are also the major water supply aquifer for the area and, therefore, the saturated zone is also generally characterized by these materials. Even in the drift‑crystalline rock province, most wells are shallow wells tapping water in the thin glacial drift or water collected in shallow fractured and weathered crystalline rock zones. Portage County does not rely on deep sedimentary rock aquifers for groundwater as do many areas in southern Wisconsin and the rest of the United States.

Figure 2.1. Surficial Geology of Portage County

Figure 2.2 General Soil Map

The unsaturated zone below the A and B soil horizons generally has less pollution attenuation potential than the upper soil zone, and is considered a secondary line of defense. This zone is below the active rooting zone of most plants and there is less organic matter and biological activity. Chemical and physical processes such as sorption can still slow downward movement of some pollutants, and thick unsaturated zones can significantly delay the transport of some contaminants to the water table. Thickness, textures (including occurrence of sand lenses, and silt and clay beds), and permeability of materials in this unsaturated zone are parameters of considerable importance in groundwater protection.

In the saturated zone, biological processes are also less important than physical‑chemical sorption processes. Texture and permeability of aquifer materials are obviously important parameters. The rate of water movement is also related to the overall groundwater flow systems. The amount of head, or difference in water table elevations between recharge and discharge areas, is the driving force and can be quite variable over short distances. Local impacts, such as pumping high capacity wells, can create a significant cone of depression, which can greatly increase the slope of the water table and change the rate and direction of groundwater movement.

Obviously, the glacial deposits characterizing the unsaturated and saturated zones of much of the County are of primary importance to this environmental assessment. In much of the County, the pollution attenuation potential is dependent on soils formed in glacial materials, and on subsurface materials of glacial origin. It is important to understand glacial processes and the resulting characteristics of the materials transported and deposited by the glaciers.

The present Portage County landscape primarily reflects the last or Wisconsin stage of the Pleistocene or glacial epoch. The glacial ice transported large amounts of rock debris known as drift. The drift is called till, if deposited directly by the ice, and outwash, if placed by glacial meltwater.

The hilly drift province of eastern Portage County is a series of end moraines (Figure 2.1). These ridges and hills are composed of sandy till up to 350 feet thick and represent the accumulation of ice‑transported debris that piled up at the forward edges of the ice sheets.

The Arnott moraine marks the western advance of an early glacier. It is generally an unsorted mixture of materials ranging in size from clay to boulders, with a large proportion of quartz sand. The outer, second, and Elderon moraines represent later advances of the ice sheet during the Wisconsin stage. These moraines are also a heterogeneous, unsorted mixture of materials with a predominant sand size fraction.

As the ice melted and the end moraines were formed, large amounts of ice‑transported materials were removed by the melt waters. This glaciofluvial (outwash) material was deposited between the moraines and in a large area to the west. The deep sand and gravel deposits of the sand plain province were formed in this way. The sand and gravel is well sorted and contains only small amounts of silt and clay. Deeper gravel deposits are found adjacent to the end moraines. The sands are generally finer further from the moraine. The thickness of outwash deposits ranges from less than 30 feet northeast of Stevens Point to over 200 feet near the outer moraine, and averages about 100 feet.

The glacial presence is less noticeable in the drift‑crystalline rock province in the northwestern portion of the County. Although this area is mapped as part of the driftless or unglaciated area of Wisconsin, there are thin, heterogeneous till and outwash deposits of clay, silt, sand, and gravel from an earlier glacial period. The average thickness is only four feet. The topography is controlled primarily by the shallow granitic bedrock, and soil properties reflect the underlying bedrock residuum and the loamy, silty nature of the unconsolidated materials.

Areas identified as alluvium (Figure 2.2) are post‑glacial deposits of materials eroded from uplands and accumulated in lower areas such as marshes (organic‑rich clay, silt, sand, and peat) and stream valleys (well‑sorted silt, sand, and gravel). These alluvial deposits range from a few feet to over 60 feet in thickness.

C. Bedrock Geology

The basement bedrock underlying the County consists of impermeable crystalline rocks of Precambrian age, mainly granite. As noted previously, this granitic material is at or very near the surface in the northwestern portion of the County. To the east and south, glacial outwash and morainal deposits cover the crystalline bedrock. In the southern part of the County, the crystalline rocks are overlain with medium to coarse grained sandstone of late Cambrian age. The sandstone generally has uniform composition, but varies in the degree of cementation. The sandstone is estimated to be up to 200 feet thick along the southern edge of the County, thinning and eventually disappearing in the central part of the County, except for isolated remnant mounds such as just east of Plover and east and west of Stevens Point. Although the sandstone is a potential aquifer, almost all wells presently utilize the thick overlying glacial deposits. The sandstone aquifer may someday have greater importance and will need to be studied in greater detail. This report will deal primarily with the unconsolidated materials.

There are no areas of carbonate bedrock in Portage County.

The impermeability of the shallow crystalline bedrock in the northwestern portion of the County makes it impossible to obtain significant quantities of water from wells terminated in the bedrock. The Village of Junction City, and several thousand County residents obtain, their drinking water from bedrock wells.

D. Groundwater

The unconsolidated glacial deposits are the most important aquifer in the County. Wells in the sand and gravel aquifer of the sand plain province generally have a potential yield exceeding one thousand (1,000 gpm) gallons per minute, and wells in the sand and gravel aquifer of the drift province potentially yield over five hundred (500 gpm) gallons per minute. All municipal water supplies (except Junction City) are from wells terminating in sand and gravel aquifers. Wells terminating in bedrock can produce flows below one (1) gallon per minute.

An aquifer is a dynamic component of the hydrologic cycle. Water falling on the earth's surface can run off as surface water or can infiltrate into the soil. Water in excess of plant needs and the soil moisture holding capacity will continue to move downward to recharge the (groundwater) aquifer. Water reaching the saturated zone continues moving underground until reaching discharge areas such as springs or streams. During times when the soil is frozen, or during extended periods of minimal precipitation, stream base flow depends on discharges from groundwater. The amount of water recharging the aquifer is an important factor when considering waterborne pollutants. The average annual recharge to the groundwater in Portage County ranges from 2 inches in the drift‑crystalline rock province to 10 inches in the sand plain province. Many variables influence the amount of infiltration and potential recharge and pollution of an aquifer at a given site, including slope of the land surface, permeability of the soils and unsaturated materials, distribution of precipitation, and land use practices.

The depth to the water table is quite variable across the County, ranging from often less than one (1) foot in the northwest to one hundred fifty (150) feet plus in the moraines of the drift province (Figure 2.6).

Aside from the parameters that directly relate to pollution attenuation and recharge potential, the groundwater flow system also needs to be considered in order to evaluate the potential impact of pollutants that reach the aquifer without significant attenuation.

Groundwater moves from topographic highs (recharge areas) to topographic lows (discharge areas). Vertically within an aquifer there may be different flow systems to consider. Local groundwater flow systems represent shallow saturated flow moving between recharge areas and adjacent discharge areas in relatively short time periods, such as months. Depending on the degree of undulation in the water table and the depth of the aquifer relative to its length, intermediate and regional flow systems may also develop below the local flow systems. In a regional flow system, water is recharged to a deep aquifer at major divides and recharge areas, and is discharged many years later at major hydrologic discharge areas, such as rivers and lakes.

Figure 2.6 Depth to Groundwater in Portage County

The concept of groundwater layering may also be applicable to flow within a given recharge/discharge flow system in fairly uniform aquifer materials. A vague layering of water and water quality will often develop as the water flows downgradient within the flow system because water is recharged to the flow system at various distances from the discharge point (as in the cover figure). Unless mixing occurs, recharge distant from the discharge point generally will be layered below recharge occurring closer to the discharge point. This layering feature has important implications in understanding the relationship between well depth, water quality, and land use patterns.

Due to the nature of glacial processes, even within the major (drift and sand plain) provinces, the materials were not uniformly deposited. Simple layering, though an easy concept to visualize, does not accurately depict the flow of groundwater and its contaminants from recharge zones to discharge zones. Within each flow system it is likely that water infiltrating further from the discharge area could reach the discharge area sooner than groundwater recharged closer to the discharge due to more rapid movement through coarser materials.

In the northwestern portion of the County, only local flow systems are likely to develop in the relatively thin and shallow sand and gravel materials overlying the crystalline bedrock. Groundwater flow that may occur in the crystalline bedrock, especially along larger fractures, represents deeper regional flow systems.

Within the drift province, the pronounced variations in topography induce extensive local flow systems that probably extend to considerable depths. Given the thickness of the morainal deposits, deeper intermediate or regional flow systems are also likely. Within the sand plain province, the very long flow paths relative to the thickness of the aquifer and the more uniform, linear nature of the water table suggest little opportunity for true regional flow systems to develop. Research has also shown that even drainage ditches in the sand plain area can seasonally intercept all groundwater flowing downgradient in the sand and gravel aquifer (Faustini, 1985). Generally, the Wisconsin River and major tributaries and ditches are the main discharge areas. Vertical mixing at high capacity wells common throughout the sand plain also obscures vertical aquifer distinctions.

Based on information such as aquifer thickness and anticipated pumping rates, it is possible to create a computer model of the recharge area for a well or well field (Kraft and Mechenich, 1996). Particle tracking analysis (Figure 2.3) can determine the geographic origin of contaminants likely to reach a given well, and the time required for pollutant travel. Perhaps more importantly from a municipal water supply perspective, it is possible to determine locations which do not contribute recharge to a particular well or well field. Without a detailed analysis of the flow systems in a given area, the assumption should be made that water and waterborne pollutants discharging at any particular site, such as a well field, could enter the aquifer system at any point upgradient to the regional groundwater divide.

SECTION 2.3 EVALUATION OF GROUNDWATER VULNERABILITY TO POLLUTION

A. Evaluation System

There are numerous qualitative and quantitative evaluation systems that might be used, depending on the objectives and the available resources. This report will discuss separately the three basic components: soil attenuation factors, subsurface attenuation factors, and groundwater flow factors.

B. Soils And Their Ability To Protect Groundwater

Section 2.2A described the role of individual soil zone factors relative to pollution attenuation and protection of groundwater quality. In the following section, these factors are considered for the soils of Portage County. This assessment of the pollution attenuation potential for Portage County soils is supported by numerical ranking procedures. To calculate the numerical rankings, the critical factors considered for each soil are given a relative value depending on their importance and relationship to pollution attenuation potential. The values are then summed for each soil to obtain an overall ranking value. These relative values are for comparison only and have no absolute value.

Figure 2.3 Particle Tracking Contaminant Source Analysis

The ranking system used in the following discussion of the pollution attenuation potential of Portage County is based on a ranking system developed by the Wisconsin Geological and Natural History Survey (WGNHS).

The WGNHS ranking system heavily penalizes organic/wet soils because of a high water table and possible interruptions in the pollution attenuation processes. In specific situations, for certain contaminants, areas of organic/wet soils may provide favorable attenuation potential. To consider this possibility, the WGNHS system was modified slightly in this report to address soil organic matter. A slope factor was also added. Possible soil scores range from 10 to 63; the higher the value, the greater the pollution attenuation potential and the lower the risk of groundwater contamination.

The calculated scores for each soil series or soil mapping unit in Portage County are given in Table 2.1. The values range from 11 to 47. Values of 0 to 30 are considered to have the least potential to protect the groundwater, 31 to 40 marginal potential to protect the groundwater, 41 to 50 good potential, and 51+ best potential. No soil in Portage County scores over 50.

Soils in the group offering the least groundwater protection (0‑30) generally include the coarse textured, highly permeable glacial outwash and sandy drift soils covering much of the County. Soils offering marginal groundwater protection (31‑40) generally have finer texture and lower permeability and/or are well developed, deep soils. Some of the soils in the northwestern portion of the County and some organic soils fall into this category. Soils offering good groundwater protection (41‑50) generally are deep, fine textured soils with low permeability or high organic matter content. Only two soils in the northwestern portion of the County, and two very deep organic soils, fall into this category. Individual soil characteristics used in the ranking procedure (Tables 2.2‑2.5) and the spatial extent of each soil is available from the Portage County Soil Survey (USDA, 1978, revised 2001).

The 71 discrete soil mapping units that have been identified for Portage County make any discussion of soil attenuation ability difficult. The grouping of soil series into associations determined by the parent materials in which they developed, and generally corresponding to the three physiographic provinces of the County, provides a convenient basis for summarizing relevant soil attenuation factors (Figure 2.2).

Table 2.1. Soils Listed by Potential for Attenuation of Surface Applied Materials

The soil associations formed in the outwash sand and gravel, and in the sand plain and drift provinces, account for 52% of the County, and include the Richford‑Rosholt‑Billett, Plainfield‑Friendship, Leola‑Pearl, and Roscommon‑Meehan‑Markey associations (Table 2.2). These soils are nearly level to gently sloping and are generally comprised of loamy sand, sandy loam, and sand, sometimes with coarse modifiers. These flat, coarse textured soils allow rapid infiltration and generally have high permeability. Some of the soils have higher organic matter content and/or greater depth, which are advantageous from a pollution attenuation perspective. Depth to groundwater in these soils is variable, ranging from less than one to greater than ten feet. These soils are usually cropped and often irrigated. As a general rule, these soils have low pollution attenuation capability. Exceptions are areas of finer-textured Rosholt loam and Oesterle loams, and Markey and Seelyeville organic soils. Area weighted composite rankings for these associations are 25, 19, 25, and 23 respectively. Overall, the outwash sand and gravel soil associations have an area weighted score of 23.

A majority of the population centers within the County are situated on these soil types. These communities include most of Stevens Point, and all of Park Ridge, Whiting, Plover, Rosholt, Amherst, Amherst Junction, Almond, Bancroft, Arnott, Custer, Stockton, Ellis, and Kellner. The extensive groundwater basin for the Stevens Point, Whiting, and Plover municipal water supplies lies almost entirely beneath areas covered by these soils. The low pollution attenuation ability, the numerous contaminant sources, and the potential severity of pollution impacts, dictate high management priority for these soil areas.

The soil associations related to the sandy glacial drift (eastern moraine province) account for 22% of the County, and include the Wyocena‑Rosholt and Kranski‑Coloma‑Mecan associations (Table 2.3). These soils are gently sloping to very steep, with sandy loam to loamy sand textures. While steep slopes in finer textured soils would encourage more surface runoff, these soils are often coarse textured with high permeability, and therefore still have a high potential to pass pollutants to the aquifer. Good soil depth and topsoil organic matter content are somewhat helpful in preventing groundwater contamination. The flatter areas of these soils are generally cropped, while the steeper areas are pastured or in woodlands. Runoff from the steeper areas provides additional water to these flatter areas, resulting in a localized increase in the amount of water carried contaminants potentially infiltrating to the groundwater. Generally these soils have low pollution attenuation potential, except for areas of Rosholt and Oesterle loams with marginal potential. Area weighted numerical ranks for these two associations are 25 and 26 respectively. Drift soil associations overall score 26. Communities within areas covered by these soil types include Nelsonville, Polonia, and Peru.

The soil associations related to the drift‑crystalline rock province and the Arnott moraine account for 16% of the County and include the Kert‑Norgo variant, Meadland‑Rozellville‑Dolph, and Point‑Dancy‑Mosinee associations (Table 2.4). These soils are nearly level to sloping. A significant trend towards loam textures and moderate permeability indicates better soil pollution attenuation potential than for the soils formed in sandy glacial deposits. Drainage is variable and land use is a mix of crops, pasture, and woodland. A majority of these soils have marginal pollution attenuation potential. The fine textured, slowly permeable Altdorf and Dolph series have good attenuation potential. Area weighted numerical ranks for these three associations are 30, 39, and 29 respectively. Drift‑crystalline rock soils score 34 overall. Communities located in areas covered by these soil types include Junction City and a portion of northwestern Stevens Point.

There are also two associations related to alluvial or organic deposits that account for 10% of the County, and include the Alluvial land, wet‑Dunnville, and Markey‑Seelyeville‑Cathro associations (Table 2.5). These soils are nearly level to gently sloping, and are often subject to flooding or ponding. The predominant soil texture is sapric muck or flood plain alluvial deposits, and permeability is generally high. These areas are often left in native vegetation, although some pasturing and specialty cropping occurs. Pollution attenuation potential ranges from poor to good. Deep organic soils, especially the Lupton and Seelyeville series, are rated favorably for pollution attenuation. Area weighted numerical ranks for these two associations are 18 and 38 respectively, with an overall score of 34. A portion of the Stevens Point, Whiting, and Plover urban area is located on these soils along the Wisconsin River corridor.

The preceding discussion indicates that a majority of the County's soils have little pollution attenuation potential. An evaluation of the pollution attenuation potential for Portage County soils using the WGNHS system was completed in 1987 by Good and Madison. Their analysis also clearly illustrates the low pollution attenuation potential for soils over much of the County, especially in the sand plain and other sandy outwash areas. Some moraine soils have marginal potential, and only a relatively small area in the northwest has good potential. No Portage County soils rank in the best attenuation classification.

The Stevens Point-Whiting-Plover urban area is in an especially vulnerable location. Obviously, soil conditions can vary considerably across a given area, and site specific evaluations of soil pollution attenuation potential are necessary for evaluating specific developments or land uses. It is also important to realize that numerical systems generalize the complex physical, chemical, and biological pollution attenuation processes, and that characteristics of individual pollutants (such as solubility or leachability) should also be considered.

The most important characteristics to be considered in the evaluation of the pollution attenuation potential of subsurface materials, are the unsaturated zone thickness, and texture and permeability of the materials.

In the sand plain province, very little subsurface pollution attenuation potential exists. The well sorted, thick, homogeneous, coarse textured sands and gravels that constitute the saturated and unsaturated zones of this province have high permeability, suggested by the production of water in numerous high capacity wells. A simple index of the ability of these subsurface materials to transmit water is the specific capacity of the irrigation wells.

The specific capacity is defined as the ratio of the water yield to water level drawdown in a well, and is a function of the aquifer materials and the particular well construction. It is, therefore, valuable as a general indicator of aquifer permeability. The average specific capacity for 52 irrigation wells in the sand plain province is 60 gallons per minute per foot of drawdown (Holt, 1965). For comparison, typical specific capacities for wells in sandstone in the central Wisconsin area range from 8 to 33 gpm/foot (Devaul and Green, 1971).

The hydraulic conductivity or coefficient of permeability is a direct measure of the potential water transmitting ability of the material. As reported by Holt (1965), the average hydraulic conductivity at four wells in the sand plain, as determined from pumping tests, is approximately 1,900 gallons per day (gpd) per square foot (.09 cm/sec). In comparison, the hydraulic conductivity for silt may be less than 1 gpd/square feet (.00005 cm/sec).

The thickness of the unsaturated zone in the sand plain province varies from less than one (1) foot in some areas of the southwestern portion of the province, to over thirty (30) feet in other areas. With the combination of coarse, highly permeable materials and relatively shallow depth to groundwater, little contact time and pollution attenuation can be expected. For management purposes, the sand plain province area, characterized by highly vulnerable soils, should be considered to have minimal overall pollution attenuation capability and should be managed accordingly.

In the drift province, the subsurface materials were more irregularly deposited than in the outwash sand plain province. Aquifer characteristics can change considerably over short distances, and with depth, in any given location. As previously noted, the glacial till comprising the moraines of the drift province is an unsorted mixture of clay, silt, sand, gravel, and boulders, with the sand size fraction predominating. Although the permeability in this area is lower overall than in the well sorted, fairly uniform outwash to the west, the predominance of coarse materials reduces the potential pollution attenuation.

High capacity wells in the drift province generally utilize the outwash areas associated with the moraines. These areas are hills, terraces, and deltas of washed till deposits and stratified silt, sand, and gravel. These materials have higher permeability than the till moraines, but lower permeability than the sand plain outwash. The average specific capacity for 11 wells in outwash areas of the drift province is 24 gallons per minute per foot of drawdown, compared to the average 60 gpm/foot for the sand plain (Holt, 1965).

The thickness of the unsaturated zone in the drift province is more variable than in the sand plain, and is generally in the range of 20 to 200 feet. Because of the high degree of variability, it is difficult to generalize the pollution attenuation potential. In general, the greater unsaturated zone thickness and the slightly lower permeability of the subsurface materials indicate a poor to moderate attenuation potential. Combined with variable soil pollution attenuation ability, the province overall rates a poor to marginal pollution attenuation capability. Due to the heterogeneous nature of the area and the predominance of coarse textured materials, it would be advisable to be conservative without site-specific subsurface data.

The drift‑crystalline rock province is considerably different from the rest of the County, in that the basement granitic bedrock is close to the surface, and the unconsolidated aquifers above it are very limited. The depth to bedrock is generally less than 20 feet, and the depth to groundwater is generally less than 10 feet. Seasonally, depths to groundwater can decrease to less than one foot.

Given the very thin or nonexistent unsaturated zone, there exists little or no second line defense against pollutants regardless of the nature of the subsurface materials. Although some of the soils ranked moderate to good in pollution attenuation, this area of the County should be considered vulnerable overall, given the shallow depth to groundwater and bedrock.

The WGNHS utilized a numerical ranking system in Rock County to evaluate subsurface materials. Map overlays, with type and depth of subsurface materials, and depth to groundwater ranges were utilized to identify subsurface areas that are most, moderately, and least vulnerable to pollution. The most vulnerable areas occur where shallow bedrock or thick sand and gravel is combined with shallow depth to groundwater. The least vulnerable areas occur where medium to thick till is combined with a depth to groundwater of more than 50 feet. Using this system, the sand plain and drift‑crystalline rock provinces are ranked as the most vulnerable to pollution. The drift province receives the intermediate rank of moderate vulnerability to pollution because of greater depth to water.

The direction and rate of groundwater flow are extremely important when considering the potential impact of pollutant sources. If the attenuation factors are not adequate to prevent a pollutant from reaching the aquifer or to dilute its adverse impacts, analysis of groundwater flow is necessary to predict where such impacts will occur, and to devise clean‑up strategies, if necessary. Analysis of the groundwater flow is also important to develop pollution prevention strategies to protect critical areas, such as community well fields.

Using the water table elevation map (Figure 2.5), generalized groundwater flow directions can be estimated. There are three groundwater basins in the County. The sand plain and the drift‑crystalline rock provinces west of the groundwater divide are within the Wisconsin River basin. The drift province east of the groundwater divide has two major drainage basins, the Tomorrow-Waupaca River and the Little Wolf River watersheds.

Within these basins, general groundwater flow is from the basin divides, which generally coincide with the topographic highs (except near Almond), toward the major stream in the basin. There are complex local flow systems superimposed on the general flow. For example, a less permeable, buried sandstone ridge just east of Plover and Whiting blocks some of the westward groundwater flow toward the Wisconsin River and increases discharge to the Little Plover River. High capacity wells can create local flow systems that can temporarily reverse the natural direction of flow, drawing surface water out of streams and into the aquifer. This has been noted in the Plover River adjacent to the Stevens Point wellfield, and in the Little Plover River east of I-39. Man‑made drainage ditches can intercept groundwater flow at considerable depth, as documented in the Buena Vista basin (Bahr, 1990). As a further complication, groundwater flow systems can also change seasonally, especially near groundwater divides. Detailed site studies are needed to define this level of flow detail.

The County groundwater flow map can be used in conjunction with the soil attenuation mapping, and identified pollutant sources, to delineate special priority areas (such as wellhead protection zones) as needed, and to serve as a preliminary screening tool for more detailed studies.

While all wells, private and municipal, should be considered for protection, the municipal well fields have historically been a higher priority because of the concentrated number of users dependent on the water .

The rate of groundwater movement within an aquifer is related to the slope of the water table, the media properties (permeability), and the fluid properties (viscosity). The water velocity can be approximated from the formula: V=KI/n where V is velocity in feet per day, K is the hydraulic conductivity in feet per day, I is the hydraulic gradient in feet per foot, and n is the effective pore volume as a fraction of total media volume. The hydraulic conductivity factor includes consideration of media and fluid properties, and is most often determined through aquifer pump tests. The hydraulic gradient is the slope of the water table and can be calculated from Figure 2.5. The hydraulic gradient reflects the specific combination of changes in permeability of the media, variations in the aquifer thickness, and variations in recharge and withdrawals from the system. The n value is needed in order to correct the velocity for the true cross‑sectional area that actually transmits the water.

For the sand plain province, Holt (1965) noted an average hydraulic conductivity value of 1,900 gpd/sq.ft. or 252 feet/day, as determined from pump tests of four wells. The n value is approximately 22%. The water table slope for the sand plain ranges from 3 to 25 feet/mile and averages 5 feet/mile. The calculated velocity, therefore, ranges from 0.7 to 5.4 feet/day and averages 1.1 feet/day. However, recent research has shown that differential flows can exceed average flows by several fold, (Kung, 1995) in some cases reaching 100 feet per day.

Velocity of groundwater flow is much more variable in the drift province. The sandy drift of this province has a lower overall hydraulic conductivity than the sand plain outwash, and can be quite variable over short distances. Although the hydraulic gradient is steeper, ranging from 5 to 40 feet/mile, the average groundwater velocity in the sandy drift is probably less than 1 foot/day. Velocities in well sorted outwash areas, and through stream deposits, within the drift province, are probably more similar to those of the sand plain, depending on the specific hydraulic gradient.

The drift‑crystalline rock province is also more difficult to generalize than the sand plain. The thin unconsolidated drift and outwash deposits over impermeable bedrock have variable hydraulic conductivity and porosity. The lower permeability of these materials in general would indicate groundwater flow rates considerably less than 1 foot/day. Within significant fractures, however, groundwater can travel much further and faster than would otherwise be expected.

Withdrawals from high capacity wells (municipal, irrigation, industrial) in the sand plain province can considerably increase local flow rates. Within the steeper region of the well's cone of depression, flow rates can increase to as high as 1,000 feet/day, with a strong vertical component. This can greatly increase the downward migration of fertilizers, pesticides, gasoline, and other materials discharged to the ground surface. Another result can be vertical mixing of contaminated water throughout portions of the aquifer, eliminating the possibility of obtaining better quality water deeper in the aquifer.

The preceding sections have discussed the various environmental factors that must be considered for the proper management of groundwater resources. Review of any potentially polluting activity, in terms of soil and subsurface attenuation and groundwater flow, is a logical point of beginning for groundwater management planning.

In Portage County, the areas of low pollution attenuation potential for soil and subsurface materials include most of the sand plain province. This highly vulnerable zone includes the groundwater basins for the water supplies for a majority of the County population.

Much of the rest of the County has only slightly better attenuation potential. Obviously, composite generalizations of pollution vulnerability and potential pollutants facilitate overall management plan development. Yet great care must be exercised in the interpretation of these generalizations because of the complex nature of the environmental processes and the highly variable nature of the pollutants.