An Assessment of Ground Water Levels and Flow
in the Vicinity of the Highway Pond

October 30, 2000
John Welhan
Idaho Geological Survey


 

Purpose

Relevant information is assembled and reviewed which bears on the physical context and possible impact of the Highway Pond gravel pit on local ground water and domestic wells. The purpose is to assist authorities in making informed decisions concerning environmental impacts on the water resource of gravel extraction, public access and recreational uses, and future reclamation plans for the gravel pit. It is not the intent of this report to identify and evaluate all possible contamination sources which may have an adverse impact on the water quality of nearby domestic wells, but rather to identify and evaluate those factors which could contribute to the Pond's possible impact on ground water quality.

 

Summary of Conclusions

Water Table - Pond Level Interpretation

Water table elevations were measured in five private wells immediately northwest of the Idaho Transportation Department's (ITD) Highway Pond gravel pit and related to the water level in the Pond. The survey was conducted April 18 and 21, 2000 prior to spring recharge. The results show that the Pond surface reflects the elevation of the local water table. The uniform decrease in water table elevation northwestward from the Pond indicates that ground water flows in this direction, essentially parallel to the low-permeability boundary of the aquifer defined by the edge of the Portneuf basalt. This conclusion is consistent with previous interpretations of water table elevation and also with the Pond being an area of the water table exposed by gravel mining. This, despite the recent installation and subsequent continuous operation of Pocatello Municipal Well 44 south of the gravel pit, which affects a large area of the aquifer south of the Highway Pond.

 

Ground Water Flow Direction

The specific direction of ground water flow away from the Pond was inferred from the water level data. A small area of exposed water table (as it currently is) does not greatly alter the ground water flow field and the aquifer is affected by infiltrating Pond water only in the area directly downgradient of the Pond; thus, under current low water table conditions the Pond does not affect the domestic well (Hildreth well 2) thought to be contaminated by fecal bacteria originating from the Pond. However, when the water table is higher, a much larger area of the water table is exposed in the gravel pit and ground water flow directions are substantially altered by the exposed water table.

 

Assessment of Historic Water level Data

Water level records from Pocatello Municipal Well 28 (Ross Park) show that the water table fluctuates on seasonal (due to pumping) and secular (due to variations in recharge) time frames. In the past, the water table has risen almost 25 feet above its present elevation, notably in the early 1970s and again in the mid-1980s. When the level in the Highway Pond rises by this much in future, the surface of the Pond will be less than 10 feet below the southern lip of the gravel pit, thereby submerging almost the entire pit area as it was in the 1971 topographic map. Under such conditions, the Pond would once again affect a much larger area of the aquifer down-gradient from the Pond, including Hildreth well 2.

 

Implications for Bacterial Contamination

Since the spring of 1996 and through at least mid-1998, the low-lying northern corner of the gravel pit directly upgradient of Hildreth well 2 contained exposed ground water. Bacterial analyses of Pond water collected since 1997 have shown coliform bacteria present in all samples and e.coli in six of seven samples. Although bacteria are known to move rapidly in flowing ground water in permeable aquifers, further information linking the bacteria in the Pond to bacteria detected in Hildreth well 2 is needed before a causal relationship can be demonstrated. However, the high permeability (740 ft/day) and coarse nature of the subsurface gravels suggests that bacteria may be able to migrate readily from the Pond.

 

Implications for Future Reclamation and Land Use

The response of the Highway Pond's level to such large water table fluctuations has implications for future reclamation of the gravel pit. Given past water table fluctuations, high water levels in the area of the gravel pit will recur and the pit will refill with water wherever low-lying areas intersect the level of the future water table. Whether a decision is ultimately made to retain an open water area for aquatic recreation or to fill in the deepest mined areas and landscape the entire area as parkland, the impact of water table rise on soil and vegetation stability and on the manner in which the reclaimed area can or will be used will have to be taken into account in future reclamation plans.

 

Introduction and Background

Geologic Background

All municipal drinking water originates from the aquifer whose gravel is mined at the Highway Pond gravel pit. The geological setting of the HP has been described in detail by Welhan and Meehan (1994) and Welhan et al. (1996). A layer of coarse, relatively well-sorted, highly permeable gravel overlies a deeper, poorly-sorted, silt- and clay-rich gravel whose permeability is much lower. The upper gravel unit hosts the aquifer from which all municipal and almost all private wells draw water in the southern LPRV. All wells measured in this study with the exception of Hildreth Well 4 are completed in the shallow, upper aquifer gravels; Hildreth Well 4 is completed in the deeper aquifer.

 

Historical Summary

The Idaho Transportation Department's (ITD) gravel pit south of Pocatello since the gravel pit was first excavated to supply gravel for the construction of Interstate-15. Since then, one or more surface water bodies have existed at various times and to varying degrees in the gravel pit; this collection of water bodies has locally come to be known as the Highway Pond. The U.S. Geological Survey 1:24,000 topographic map of the Pocatello-South quadrangle depicts the Highway Pond as a large, contiguous surface water body. Since the map was photo-inspected in 1974 prior to publication and represents features which existed at the time of compilation in 1971, it appears that the surface area of the Pond was quite large during this period of time.

The degree to which the pit has been water-filled, however, has varied from completely dry in the last major drought period ending in 1993, to completely full as it was in the early 1970's and again in the mid-1980's. In 1996, the Pond was at its highest level in the past decade, with almost the entire area of the gravel pit submerged. As of May, 2000, the gravel pit was almost completely dry but for an approximately one-acre area that was intentionally deepened to promote a year-round stocked trout fishery. Photographic records of the degree of water-filling are included in Appendix I.

Despite its temporal water level fluctuations, the Highway Pond has become a popular location for anglers since the Idaho Department of Fish and Game (IDFG) began stocking the Pond with trout fingerlings ca. 1977, three to four years after a local land owner demonstrated its feasibility. In addition, the Pond provides a recreational area conveniently close to the city for canoeists and kayakers, plus space and trails for off-road and all-terrain vehicle enthusiasts in the mined and unmined areas surrounding the Pond.

Public access to the Pond and gravel pit has not been regulated in the past, other than during gravel extraction operations in the pit. Thus, vehicles of all sorts, including cars and trucks, have driven to and parked at the water's edge, leaving litter, used motor oil, auto batteries, scrap metal and plastic, and assorted garbage in and around the pit; dog and gull feces are found over the entire pit area, and portable toilets installed by IDFG have been vandalized and on occasion overturned into the Pond.

Since 1996, a local resident, Mr. Bud Hildreth, has contended that coliform / e.coli bacterial contamination of his private well originates from the Highway Pond. Of seven samples of Pond water collected since 9/97 from open water in the gravel pit (including the north corner), all have contained total coliform and six have had e. coli bacteria. A study commissioned by ITD (Rocky Mountain Environmental, 1997) did not rule out the Pond as a possible source of bacterial contamination in the well, and pointed out that other possible sources were present, including local seepage along an improprer or nonexistent surface seal at the well in question.

Because of the exposure of the aquifer gravels in the Highway Pond pit, Mr. Hildreth's concern about the possible impact of the Pond on the quality of water in the aquifer has been shared by the City of Pocatello. In particular, the City is concerned that uncontrolled public access to the Pond increases the risk of accidental or intentional releases of contaminants other than bacteria to the Pond and thence to the aquifer. Discussions between the City, ITD and IDFG over the possible risk posed by the Highway Pond commenced in late 1996 and resumed in 1999 with a working group formed to assess the situation, including the original parties, Bannock Paving (a private gravel mining concern), Mr. Hildreth, and various regulatory agencies, including District Health, Idaho Division of Environmental Quality, and Idaho Department of Lands (Minutes of Highway Pond Working Group, 1999). Access controls, stricter best management mining practices, and enhanced reclamation were negotiated, and an agreement was reached to eventually cease gravel mining activities and reclaim and cover the pits. The City of Pocatello has purchased land mined by Bannock Paving immediately to the south of the Highway Pond pit and is currently in the process of reclaiming it. ITD has indicated it plans to extract gravel for one more project in 2003 after which it will cease mining and fully reclaim the pit.

In 1999 Mr. Hildreth initiated legal action against ITD, claiming the Pond was responsible for bacterial contamination of his drinking water well and seeking compensation for a new domestic supply well he drilled in 1999 to replace the contaminated well. At the recommendation of the Idaho Geological Survey the new well was completed in a deeper aquifer to ensure that any potential contamination originating from the Pond in the vicinity of the well via the shallow aquifer would not influence his water supply. This well has tested clean since it was disinfected and purged after drilling. Since the pump from Hildreth Well 2 was moved to the new well, sampling from the original well ceased in August, 1999.

Ground Water Source of Highway Pond Water

The water in the Pond is chemically the same as ground water in the valley aquifer beneath the gravel pits (Meehan and Welhan, 1994; Welhan et al., 1996). As shown in Table 1, the chemical composition of well water in Hildreth Well 2 reported by Rocky Mountain Environmental (1997) is also very similar to previous analyses from this and other wells around the Pond - and to the Pond itself. The exception to this pattern is sulfate, whose concentration in Hildreth Well 2 was almost three times higher in June, 1994. Meehan and Welhan (1994) proposed and tested a chemical reaction model in which sulfate originating from a stockpile of crushed aggregate situated across the road from the Hildreth well infiltrated to the water table directly upgradient of the well. The well's sulfate level now appears to be significantly lower after this putative sulfate source was removed in the intervening time.

Pond water levels have fluctuated with the rise and fall of the aquifer's water table. It has been observed that pond water level lags rising river stage in spring runoff events (B. Brown, written communication, 2000). This is a further indication that the Pond is a reflection of the local water table, which also lags the Portnuef River's spring runoff (Welhan et al, 1996). Photos in Appendix I document the changes at different times as water table levels have varied.

On the basis of the foregoing evidence, the Pond was proposed to be the expression of the water table where it intersects the land surface, either through intentional deepening of the gravel pit by mining or during times of high ground water levels. Ground water in this area of the aquifer generally moves from southeast to northwest at rates of 10-40 feet per day (Welhan and Meehan, 1994; CH2M-Hill, 1995; Welhan et al., 1996). When the water table is high and the water table is exposed in the gravel pit, ground water would be expected to flow through the Pond, entering along its southern edge and exiting (reentering the aquifer) along its downstream side.

Impacts of Water Table Exposure on Water Quality

If ground water quality were unaffected by this subaerial emergence, then water reentering the aquifer at the north side of the Pond would have no impact on aquifer water quality. However, whenever ground water discharges into a surface water body it undergoes a variety of natural chemical modifications aside from any changes induced by additions of foreign substances. For example, by its exposure to air, the relatively high dissolved carbon dioxide content of ground water will be reduced, thereby raising the pH and promoting mineral precipitation. If the dissolved oxygen content of ground water has been lowered by chemical oxidation in the aquifer prior to its emergence, the oxygen content will increase upon exposure to air, also initiating a potential chain of chemical readjustments. Organic photosynthesis and respiration reactions due to surface water biota will affect dissolved gas concentrations, organic matter content, metals uptake and mobility, nutrient levels and other chemical characteristics.

In addition to these and other natural chemical changes, accidental or intentional releases to the surface water body of fertilizers, pesticides, petroleum hydrocarbons, metals, sediment, and sewage or fecal waste will alter the chemistry of water reentering the aquifer. Regardless of the particular chemical changes, the impact of exposing the water table in a situation such as the Highway Pond is always to alter ground water quality in the surface exposure and in the aquifer downgradient of the Pond. The chemical impact of surface water infiltration into an aquifer is well known; if these changes are minimal or of a nature that allows natural chemical reactions between ground water and the aquifer sediments to reestablish a new chemical equilibrium, then infiltrating surface water will have no impact. However, if water quality is altered in a way that natural chemical equilibrium cannot be reestablished then aquifer water quality can be detrimentally affected.

Scope of This Evaluation

This report has two objectives: 1) to assemble relevant background information on the Highway Pond in relation to the local water table, and 2) to evaluate the nature of impacts on the aquifer due to the existence of a Pond. Background information and knowledge have been assembled from existing sources; new water level information was collected from five private wells north of the Pond, and the water level of the Pond itself was measured and evaluated in relation to the local water table. Because of the drawdown created by Pocatello Municipal Well 44 south of the Pond, the water level survey and this analysis were restricted primarily to the area of the Pond itself and wells immediately to the northwest.

 

Description of Methods

Surveying

An elevation and position survey was carried out April 18, 2000 by a registered land surveyor contracted by Mr. Bud Hildreth. Surveying of all wellhead measuring points and Pond water surface was performed with Trimble 4800 Global Positioning System (GPS) instrumentation, with a vertical accuracy of 1 cm. Results were summarized as a digital file of x, y, and z coordinates (D. Klatt, written comm., 2000) and provided to the IGS. A temporary benchmark was installed on the southern lip of the main ITD gravel pit (location BM in Figure 1) as a reference point for monitoring future Pond water level changes. All location data were imported into ArcView GIS software for plotting and analysis.

Water Level Measurements

Water levels in private wells were measured with a Solinst electrical water level tape graduated in 0.05-foot increments. Measurements were made 4/18, checked for reproducibility on 4/21, and again for short-term changes on 5/09; a spot measurement was also made in Hildreth Well 3 on 10/13. Neither Hildreth or Grady wells were affected by irrigation pumping at the time of the survey and none of the domestic wells were being pumped during either visit to the wells. Because of the very high permeability of this aquifer and the resultant rapid recovery rate of water levels in pumped wells (Welhan et al., 1996), all water level readings were considered to be static readings. Reading accuracy is +/- 0.025 ft; an estimate of measurement precision is provided by the degree of reproducibility attained in measurements at the same well taken three days apart (Table 2) and is less than 0.03 ft (RMS difference).

Pocatello Municipal Well 28 in Ross Park is 2.5 miles directly downgradient of the Highway Pond. Water level data from Well 28 were collected from two sources: manual water levels collected monthly by City personnel from 1971 to 1993, and from a Unidata Macro data logger and pressure transducer installed in the well and recording at hourly intervals since 1993. Manual measurement precision is unknown but is estimated to be better than 2 feet; data logger measurement precision is +/- 0.1 feet. All data for Well 28 have been reported relative to an assumed measurement point elevation of 4457 ft amsl; the City of Pocatello had the floor of Well 28’s pump house surveyed in May, 2000 and its actual elevation is 4460.32 ft amsl. Therefore, water levels reported here should be corrected by ca. +4 ft for absolute comparisons to other wells. However, for the purposes of this discussion it is the relative water level variation that is of greatest interest.

Although Well 28 is an active production well, its water level information is still a useful gauge of static water level trends during non-pumping periods (non-summer months) and for estimating year-to-year differences in water table elevations. Only non-pumping measurements from the manually- collected water level data (pre-1993) were considered here; the automatically-recorded data (post-1993) include both pumping and non-pumping water levels.

Pond Area

A Trimble GeoExplorer II was used to survey the area of the currently exposed water table, the major low-lying areas of the gravel pit that have been inundated in the recent past (1996 to 1999), and the areal extent of gravel back-fill placed on another low-lying area in the northernmost corner of the pit. All GPS data were collected and differentially-corrected with base-station data logged at Idaho State University (http://134.50.65.125/GPS/); mean horizontal positional precision of the corrected coordinates varies between 5 and 10 feet.

 

Results and Interpretation

Figure 1 shows salient features in the study area between the Union Pacific mainline and the Portneuf basalt, including monitoring wells for which historic water table information is available, the locations of private wells surveyed in this study, and Pocatello Municipal Well 44. The area of the ITD gravel pit is approximated by the areal extent of the Highway Pond in the 1971 topographic map (U.S. Geological Survey, 1971). The dark area within the pit is the currently exposed area of the water table; other irregular areas within the pit are low-lying areas that have been chronically submerged in the past six years, including the back-filled low area in the north corner of the pit.

Current Water Table Gradient

Table 2 summarizes the survey and water level data. Water level elevations depicted as bold text in Figure 2 are in feet above a datum of 4440 ft (relative to mean sea level). Note that the water level in Hildreth well 4 is not considered representative of the shallow aquifer in which all other measured wells are completed because this well is completed in and draws water from a deeper aquifer. The water levels have been contoured manually; contours are shown in Figure 2 as solid lines extending between the rail line and the edge of the Portneuf basalt. The interpreted flow net is discussed in a later section.

The water level in the Pond reflects the level of the local water table, albeit averaged over its length in the direction of the water table slope. It is well known that where a water table intersects the topographic surface so as to create a surface water body such as the Highway Pond, the elevation of the surface of the Pond will be slightly lower than the elevation of the water table at the upgradient edge of the Pond and slightly above the elevation of the water table at its downgradient edge. Thus, ground water flows from the aquifer into the Pond, and subsequently back into the aquifer. This is reflected in the localized warping of water table contours around the Pond (as described in a later section on Ground Water Flow Direction).

The water table data are consistent with ground water flow that is parallel to the edge of the Portneuf basalt. Hence, the water table elevation difference between Hildreth well 1 and well 2 (1.16 ft) provides a good approximation of the hydraulic gradient (water table slope) between these wells (a distance of 1100 ft). The gradient so determined is 0.00105 or 0.11%; between Hildreth 2 and Grady, it is 0.00115. As discussed in the following section, the magnitude of these gradients is entirely consistent with previous water table interpretations based on more wells over a wider area of the aquifer (CH2M-Hill, 1995; Welhan et al., 1996).

The average gradient between the Pond and Grady's well is 0.00094, decreasing to the southeast from a high of approximately 0.0013 near Grady's well to ca. 0.0007 between the Pond and Hildreth well 2. The decrease appears to be systematic and may be due to several factors: the complex three-dimensional hydraulic interaction between ground water and the surface water body through which it flows (Townley and Trefry, 2000), aquifer inhomogeneity (that is, permeability around the gravel pit differs from that beneath Grady's property), or the effect of Well 44's essentially continuous pumping since it was put into production in August, 1999. Of these possible effects, the latter is probably of greatest significance.

Well 44’s zone of influence is distorted by its proximity to the aquifer boundary, but nevertheless the well’s drawdown creates an artificial ground water divide and flow reversal between the Pond and the well. Southeast of this divide, the slope of the water table and direction of ground water flow is toward Well 44; at the divide, the hydraulic gradient is zero; and northwest of it, the gradient gradually steepens to the northwest. The capture zone shown in Figure 2 under-represents the actual extent of the well’s impact on aquifer water levels; in particular, water levels northwest of the well would be reduced as the aquifer seeks a new quasi-equilibrium. Because Well 44 has been pumping continuously since coming on line in August, 1999 (F. Ostler, pers. comm., 2000), its hydraulic impact on the aquifer is assumed to have reached a quasi-steady state for the purposes of this analysis.

Water levels in Hildreth well 2 and monitoring wells PA-9 and PA-10 measured on May 9 (Table 2) corroborate the expected gradient reversal. The apparent hydraulic gradient between Hildreth well 2 and PA-9 is 0.0005 (sloping to the northwest), whereas between PA-9 and PA-10, it is reversed (sloping southeast) and much steeper (0.0018); the gradient at the production well exceeds 0.020 (based on well TH-5’s water level relative to PA-10).

Comparison with Past Water Table Variations

Figure 3 summarizes water level data measured in a number of wells in the study area on May 12, 1994 (CH2M-Hill, 1995) prior to the installation of Pocatello Municipal Well 44. Water levels are shown relative to the 4440 ft datum. Contours of the water table indicate a general ground water flow direction that is parallel to the valley axis and the aquifer's boundaries. This is consistent with historic water level records dating back to 1981 (Welhan et al., 1996) and underscores the uniform nature of the water table in this portion of the valley in the absence of pumping disturbances. From the spacing of these water table contours, the average hydraulic gradient in this area of the aquifer is 0.00090. This is almost identical to the average hydraulic gradient of 0.00094 determined from the April, 2000 water level survey discussed above.

The water level record at Well 28 (measured when the well was not pumping) is shown in Figure 4, together with total annual precipitation recorded at the National Research and Conservation Service's SnoTel station on Wildhorse Divide in the Bannock Range, the aquifer’s principal recharge area (Welhan et al., 1996). Well 28's response over three decades shows a consistent pattern of (a) pumping-induced drawdowns of the order of 1-2 feet at pump rates of 800-1200 gallons per minute, (b) a general summertime pumping period decline of the order of 5-10 feet, followed by (c) a post-pumping period of recovery and a variable amout of spring recharge-induced water level increase, and (d) a suggestion of a secular correlation between total precipitation (maximum available recharge) and aquifer water level. It is apparent that static water levels in the Ross Park area have varied significantly in the past. They have been almost 25 feet higher than current levels, notably during the early-1970s and mid-1980s.

A comparison of water level variations at Well 28 and in wells near the Highway Pond is illuminating. As shown in Appendix I, the degree of inundation in the gravel pit appears to correlate with long-term changes of water table elevation measured at Well 28. Measurements at Well 28 appear to provide a reasonable representation of changes in water table elevaton in the vicinity of the Pond. Water levels measured 18 days apart (between April 21 and May 9, 2000) in Hildreth Well 2 and Well 28 showed very similar changes (declines of 0.61 and 0.72 feet, respectively), within the precision of the measurements. Between May 12, 1994 and April 21, 2000 (2171 days), water level in Hildreth 2 decreased by 6.40 feet; water levels in the Hildreth 1 and 2 and Grady wells (compare Figures 2 and 3) decreased an average of 6.38 feet (range: 5.90 to 6.83 ft). In the same period, water level at Well 28 decreased 5.17 feet. Between April 21 and October 13, 2000, Hildreth Well 3's water level declined a further 4.32 feet (Hildreth 2 went dry) compared to xxx feet at Well 28. The greater rate of decline in the Pond area may be a reflection of the proximity to Well 44, which has been pumping almost continuously since August, 1999.

Based on the above information, Well 28's water level record appears to provide a reasonably good approximation of relative water table fluctuations in the vicinity of the Highway Pond.

Impacts of the Pond on Local Ground Water Flow

To infer ground water flow directions from the water level measurements on April 21, a flow net was created to approximate the two-dimensional areal nature of ground water flow and the effect of the exposed water table. A flow net is a map showing contours of equal water table elevation and resultant ground water flow directions. Because of the three-dimensional complexity of flow that arises around surface water bodies communicating with ground water (Townley and Trefry, 2000), the effect of aquifer inhomogeneity, and Well 44's known impact on ground water elevations south of the Pond, an analytical model of the flow net was not computed. An approximate flow net was created manually using standard methods (Freeze and Cherry, 1979). The flow net interpretation was constrained by measured water levels, the exposed area of the Pond, and the assumptions of a homogeneous, isotropic porous medium, a hydrologic steady-state, and laminar (Darcian) flow. The adjacent aquifer boundary along the basalt was assumed to be impermeable.

The flow net shown in Figure 2 expresses the relationship between water table elevation and inferred ground water flow direction arising from that water table configuration. Solid lines extending southwestward from the basalt are contours of equal water table elevation; dashed lines represent ground water flow moving into and emanating from the Pond. Note that the flow net shown up-gradient of the Pond is unconstrained because of the lack of measurements and the influence of Well 44.

The flow net analysis provides a visual approximation of the areas of the aquifer affected by infiltration of water from the Highway Pond. Currently, the area of impact is limited to the area directly downgradient of the exposed Pond. The area of impact would not include Hildreth well 2 unless locally induced water table gradients distorted the flow lines shown in Figure 2. Hildreth well 3 is an irrigation well some 30 feet from Hildreth 2; during the growing season it pumps at more than 300 gallons per minute (B. Hildreth, pers. Comm., 2000). Aquifer permeability has not been determined at this well, but if it is similar to that at Well 44 (740 ft/day, unpubl. data and analysis), Hildreth 3 could not operate at any substantial pumping rate over the growing season without capturing water leaving the Pond area. The dotted area shown in Figure 2 converging on Hildreth 3 represents a six-month capture zone for a continuous 30 gallons per minute pumping rate, approximately the maximum continuous pumping rate at which its capture zone would not intercept Pond-derived water. Since Hildreth 3 pumps substantially more than this during the irrigation season (ca. 300 gpm), its actual capture zone would encompass a much larger area. The implication is that Hildreth 3’s pumping impact could draw Pond water toward Hildreth well 2 even under current low-water table conditions.

In the past, when the water table was considerably higher, a much larger area of the gravel pit was flooded. Although we do not have measurements of the Pond area or water level data to construct a flow net under such conditions, an approximate scenario can be evaluated. Based on photographs and personal visits to the Highway Pond following the rapid rise in Pond level in the spring of 1996, an essentially contiguous area spanning the length of the main pit was submerged through most of 1996 and 1997. The extent of this area is approximated in Figure 5.

Under such moderate- to high-water table conditions, a larger area of the water table becomes exposed and the pattern of ground water flow around the Pond is altered over a much larger area. Figure 5 depicts an approximate representation of the ground water flow net under such conditions; unlike Figure 2’s flow net, it is constrained solely by the area of the exposed water table and the same assumptions used to constrain the flow net in Figure 2. Note how the water table contours are warped immediately upgradient and downgradient of the Pond, thereby spreading water which seeps from the Pond over a much larger area of the aquifer (including areas west of the rail line). Although water table data during the high-water period of 1996-97 are unavailable to substantiate this interpretation, Figure 5 provides a reasonable approximation of the nature and magnitude of the hydraulic effect that would be expected. The conclusion that a larger area of pond surface would expose a larger area of the aquifer downgradient of the Pond to water originating from the Pond is not surprising (e.g. Townley and Trefley, 2000).

The above flow net analysis suggests that, regardless of the size of the Pond and the area of water table exposed by gravel mining, it is reasonably likely that Hildreth Well 2 has been exposed to water seeping from the Highway Pond under both low and high water table conditions. Whether this exposure has been continuous or intermittent during the life of the gravel pit cannot be determined with current information.

Bacterial Contamination

Based on photographs of the pit (Appendix I) and my walk-throughs of the pit area in the past decade, plus information supplied by Mr. Hildreth, it appears that the northern corner of the gravel pit has chronically had water exposed in it. The northern corner contained water in the spring of 1993 and retained it through at least 1994; it was entirely flooded in 1996-97, and held a dimishing pool of water before going dry sometime in 1998. Photographs show the pit area was dry in 1992 and, based on the relative water level variations in Figure 5, it is likely the pit was dry at least back to 1990.

Thus, the water table in the northern corner of the gravel pit has been exposed almost continuously for about four years (spring, 1993 - summer, 1998) directly upgradient of Hildreth well 2. During this interval, Hildreth 2 was within 500 feet of exposed water, continuously, and within about 200 feet when the water table was high in 1996-97. From the USGS topographic map, the northern corner of the pit is known to have been full in the early 1970s and from Well 28’s record, it is probable that the water table was exposed in this area of the pit in the mid-1980s, also.

Given Hildreth Well 2's location and proximity down-gradient of exposed water in the pit, it is possible that water quality variations originating in the Pond have impacted the well. Previous studies of ground water quality around the Pond (Meehan and Welhan, 1994) found elevated sulfate in Hildreth well 2 (60-127 mg/l) at a time when a stockpile of crushed slag on the asphalt-mixing tarmac adjacent to the north corner of the pit provided a source of readily leachable sulfate (Meehan and Welhan, 1994). After the stockpile was removed, sulfate levels in Hildreth Well 2 returned to normal levels of 40-50 mg/l (Rocky Mountain Environmental, 1997).

Table 3 lists Pond water samples collected by Mr. Hildreth and analyzed at District 6 Health Department. Coliform bacteria have been detected in the Pond since 1997 when the first samples were collected, and six of seven samples contained e.coli bacteria. E. coli are known to migrate rapidly even through structured soils where preferential flow paths such as root channels, cracks, and macropores exist (McCurry et al., 1998).

The coarse, permeable gravels of the aquifer similarly may offer little filtration capacity to retard bacterial migration. The size of pore throats hosted in gravels of the Highway Pond are relatively large compared to fine-grained soils. A sediment's effective grain size (defined as the size at which 10% by weight of a soil is finer) provides an estimate of the characteristic pore diameter that directly controls soil permeability (Freeze and Cherry, 1979). Based on sieve analyses of Highway Pond gravels (B. Brown, written comm., 1996), the effective grain size of gravels sampled in the ITD gravel pit is of the order of 0.15 - 0.25 mm. In comparison, the effective grain size for silt loam soil is two orders of magnitude smaller (Pudney, 1994). If relatively large pore diameters in the Highway Pond gravels promote bacterial mobility in a manner similar to macropore flow in structured soils, then rapid bacterial migration in these gravels is possible. However, other possible sources for the coliform contamination in Hildreth well 2 have not been ruled out; it is also possible that more than one source may be responsible.

Tracer experiments in permeable aquifers demonstrate that bacteria can migrate rapidly when injected into flowing ground water. For example, in a sandy aquifer on Cape Cod where ground water flow is more than ten times slower than in the Highway Pond aquifer, bacteria moved 30 feet in three weeks (Harvey and Garabedian, 1991). Where bacteria are transported in the aquifer together with dissolved or suspended organic matter to sustain their growth, bacterial migration in excess of 3000 feet from the source is possible (Harvey et al.,1989). Since water in the Highway Pond is visibly rich in organic matter (derived from a variety of sources, e.g., algal mats, fish, fish waste, gull and dog feces, food waste and garbage, etc.), conditions appear to be conducive for allowing bacteria originating in the Pond to survive and migrate through the aquifer over considerable distances from the Pond.

The northernmost corner of the gravel pit was back-filled with rejected gravel in mid-1999 to prevent exposure of the water table immediately upgradient of Hildreth well 2 and as a precaution against the possible impact of this exposure on ground water quality. Based on the results of the flow net analysis, this measure should afford a level of protection for Hildreth well 2 in all but periods of very high ground water by minimizing the well's exposure to Pond water. However, if in the future the Highway Pond rises as much as it has in the past (25 feet), the surface of the Pond would rise to within 5-10 feet of the southern lip of the gravel pit, thereby submerging almost the entire pit area as it was in the 1971 topographic maps. Under such circumstances, Pond water exposed in the northern corner of the pit would once again infiltrate directly toward Hildreth well 2.

Implications for Future Pit Reclamation

The direct coupling between water level in the Highway Pond and aquifer water table variations has implications for future planning of alternative land uses in the gravel pit. As shown above, the water level record at Municipal Well 28 can be used as an approximate indicator of water level variations in the Highway Pond. Given the magnitude of past water table fluctuations (a ca. 20 foot variation between high and low water levels) and the recurrence frequency of high water level conditions (e.g., the water table at Well 28 has been 15 feet or more above current low water levels during 5 out of 27 years of record), we can expect that such high water levels are not unusual and that the pit will refill with water wherever low-lying areas intersect the level of the future water table. For example, at 15 feet above current levels the Pond would be essentially full (approximating the area of open water in the 1971 topographic maps). As it did in 1996, such large increases in water level can occur very quickly (within a few weeks) during large spring runoff events.

The impact of high water levels on any reclamation performed in the pit (e.g., landscaping, soil cover, vegetation) should be considered in any engineering design for a reclamation plan. Whether a decision is ultimately made to retain an open water area for aquatic recreation or to fill in the deepest mined areas and landscape the entire area, the impact of a rapid rise in the water table on the stability of landscaping and vegetation and on the manner in which the reclaimed area will be used should be an important factor in any future reclamation design.

Acknowledgements

The elevation survey was sponsored by Mr. Bud Hildreth. Mr. Hildreth and Mr. Maurice Grady kindly granted permission to access their wells for water level measurements; Mr. Hildreth also made available the bacterial analysis results on his wells and the Pond. Mr. Fred Ostler, City of Pocatello Water Superintendent, provided access to municipal wells for water level measurements and made city records available for analysis. I thank Jerome Hansen, Idaho Fish and Game, and Ed Bala and his staff at the Idaho Transportation Department for reviewing the draft report and providing many valuable comments and suggestions. A ground water database created for Pocatello, Chubbuck, Bannock County, and Fort Hall was used to create supporting GIS coverages used in this analysis. Personnel in Idaho State University's Geology GIS computer laboratory and the GIS Training and Research Center provided access to GIS software and assistance with use of the database, GPS equipment, and GPS base station data.

 

References

CH2M-Hill, 1995, Hydrogeology and assessment of TCE contamination in the southern portion of the Pocatello aquifer; Phase I Aquifer Management Plan Draft Report, 100 p.

Freeze, R.A. And Cherry, J.A., 1979, Groundwater, Prentice Hall, New Jersey, 604 p.

Harvey, R.W. and Garabedian, S. P., 1991, Environmental Science and Technology, 25, p. 178-185.

Harvey, R.W., George, L.H., Smith, R.L. and LeBlanc, D.R., 1989, Transport of microspheres and indigenous bacteria through a sandy aquifer: results of natural- and forced-gradient tracer experiments; Environmental Science and Technology, 23, p. 51-56.

McCurry, S.W., Coyne, M.S. and Perfect, E., 1998, Fecal coliform transport through intact soil blocks amended with poultry manure; Journal of Environmental Quality, 27, p. 86-92.

Meehan, C. and Welhan, J., 1994, Impact of leachable sulfate on the water quality of ground water in the Pocatello aquifer; Proceedings, 30th Symposium, Engineering Geology and Geotechnical Engineering, p. 19-35.

Pudney, W., 1994, Physical properties of sediments affecting saturated vertical water flow at the Idaho National Engineering Laboratory; M.S. thesis, Idaho State University, 92 p.

Rocky Mountain Environmental Associates, 1997, Coliform bacteria in a domestic well owned by Bud Hildreth; report and recommendations in memorandum of Dec. 1, 1997.

Townley, L.R. and Trefley, M.G., 2000, Surface water-ground water interaction near shallow circular lakes: flow geometry in three dimensions; Water Resources Research, 36, p.935-949.

U.S. Geological Survey, 1971, Pocatello South topographic quadrangle, 1:24,000 scale.

Welhan, J. and Meehan, C., 1994, Hydrogeology of the Pocatello aquifer: implications for wellhead protection strategies; Proc., 30th Sympsium, Engineering Geology and Geotechnical Engineering, p. 1-18.

Welhan, J., Meehan, C. and Reid, T., 1996, The lower portneuf River valley aquifer: a geologic / hydrologic model and its implications for wellhead protection strategies; Final Report, EPA Wellhead Protection Demonstrations Program and City of Pocatello Aquifer Characterization Project, 48 p. plus appendices.

 

Appendix II - Summary of Available Lithologic Information

Available Lithologic Logs:

PA-series Monitoring Wells

Purpose: characterize and monitor extent of Trichloroethylene contamination

Source: CH2M-Hill (1995)

Thickness of Silt Unit: 12 ft (PA-9), 11 ft (PA-10)

Depth to base of Upper Gravel: 98 ft (PA-9), >69 ft (PA-10)

Pocatello Test Drilling for Well 44 Siting

Purpose: locate sufficient saturated thickness for production well

Source: unpublished IGS descriptive lithologic logs

Thickness of Silt Unit: 5-8 ft (TH-3, 4) to 10-12 ft (TH-1, 2, 5, 6)

Depth to base of Upper Gravel: 29-32 ft (TH-1, 2, 3) to 64-74 ft (TH-4, 5, 6) bls

Hildreth Well 4

Purpose: replacement drinking water supply

Source: unpublished IGS descriptive lithologic log

Thickness of Silt Unit: 7 ft

Depth to base of Upper Gravel: 70-75 ft bls

 

Other Lithologic Data:

ITD test borings

Purpose: for gravel resource estimation, grain size analysis

Source: ITD Source Plat BK-142-S extraction plan (5/95)

Thickness of Silt Unit: n.a.

Depth to base of Upper Gravel: n.a.

 

Appendix III - Unpublished Lithologic Logs

Well Logs for Pocatello City’s test wells drilled in the siting of Well 44

Test holes drilled with air rotary (Vollmer Drilling), 6" casing advanced, with foam additive

Lithologic changes were logged during drilling by sampling of rotary (air) cuttings

Depth range logged is estimated in feet below land surface to within 0.5 ft, considered accurate to within 1.5 ft

Samples were collected every 5 ft or where lithology changed significantly for subsequent evaluation

Logging personnel: J. Kaser (ISU) except where noted (J. Welhan, IGS)

Note: Intervals not containing bolded mention of fines appear to be silt/clay-poor in bagged samples

Note: all gravel/coarse sand clasts look to be of similar composition in all six test holes (mixed quartzite and metasediments; colors: pink, purple, green, white)

Test Hole 1 (TH-1)

0-10: Dark brown silt, silt clasts, dark brown silt loam

10-11: Dark sand

11-12: Dark gravel (rounded quartzite)

12-12.5:Dark gravel and sand

14-17: Dark gravel, found a white mollusk shell (fresh water oyster)

17-17.5: Dark sand and gravel, complete white grastropod shells

17.5-18.5: Dark gravel Stopped at 10 AM, J. Welhan logged remainder

20-29: med.-coarse gravel, less silty

 

Base of Upper Gravel

29-30: very sudden transition into silt-rich, coarse gravel

30-32: silt-rich, med.-coarse gravel

32-35: cleaner, still silty, coarse gravel (again, with sudden transition)

35-38: coarse, clean gravel, with some med.-coarse sand

38-40: thin clay seam

40-45: silty med. gravel

45-55: relatively clean, med.-coarse gravel, with sand

57: another silt layer, some clay, no sand or gravel

57-60: grading back into silty, med.-coarse gravel

60: silty med.-fine gravel, some sand

 

Test Hole 2 (TH-2)

0-12: Dark brown silt

12-15: Dark gravel and sand

15-19: Dark gravel

21-22: Brown silt and dark gravel

22-32: Dark gravel

Base of Upper Gravel

32-36: Brown clay and sand

36-36.5: Dark sand

36.5-38: Gray clay and gravel

38-39: Clay color change to a deep brown, dark gravel

39-40: sand, silty, clayey

40-43: Dark sand and gravel, some brown clay

43-47: Brown clay and gravel layers

47-49: Brown clay and dark gravel

49-51: Brown clay

51-53: Dark gravel and brown clay

53-55: Dark gravel and sand

55-56.5: Brown clay

56.5-57: Dark gravel

57-58: Dark sand with some gravel

58-58.5: Mostly dark sand and some dark gravel

59-59.5: Dark sand

59.5-60: Dark gravel and sand – water encountered

60-62: Brown clay. No water

62-63: Dark sand

63: drilling ceased at an obstruction. Cuttings contain various rounded rock lithologies (i.e. gray mudstone, yellow quartz, red-brown quartzite) mixed with sand. The casing could not be hammered past the obstruction.

 

Test Hole 3 (TH-3)

0-2: Dark brown topsoil

2-5: Brown clay and dark gravel, white gastropod shells

5-10: Dark gravel, some complete white gastropod shells

10-13: Dark gravel and sand, white gastropod shells

13-14: Mostly dark sand with some dark gravel

14-18: Dark gravel and sand, white gastropod shells

18-19: Dark gravel and sand

19-23: Dark gravel and sand, white gastropod shells

23-25.5: Dark gravel, drill moving slowly through

25.5: Brown clay, dark gravel and sand

25.5-28: Mostly dark gravel with brown clay and some sand

Base of Upper Gravel

28-29: Brown clay with gravel and sand

29-29.5: Brown clay

29.5-30: Brown clay, gravel and sand

30-31: conspicuous brown silt, some clay

31-33: brown clay and dark gravel

33-35: brown clay and dark sand

35-38: brown clay with some dark sand

38-38.5: brown clay (some clay chips found)

38.5-39: dark sand with some dark gravel, sand is brown to dark red

39-41: brown clay, brown to dark red sand, and dark gravel

41-43: gravel, pink and dark red quartzites or granite with black basalt or mudstone

43-46: brown clay, pink to dark red sand and dark gravel

46-49: brown silt/clay, dark sand and dark gravel, several thin layers of brown clay

49-50.5: gravel

50.5-51.5: brown clay and dark gravel

51.5-52: dark gravel and dark sand

52-53: brown clay, dark gravel and dark sand

53-55: brown clay, dark sand and dark gravel

55-57: brown clay, dark gravel and dark sand

57-59: brown clay with minor amounts of dark sand

59-62: brown clay, dark sand and dark gravel

62-63: brown clay

63-68: brown clay, dark gravel and dark sand Note: drill and casing is moving slower starting at 65-ft

69: Drilling halted due to obstruction (difficult drilling)

69-71: ‘hardpan’ gravel and sticky clay was penetrated with difficulty

71-72: dark gravel

72-74: brown clay and dark gravel

74-75: brown clay and sand with minor amounts of gravel

75-77: pink gravel (pink to red quartzites, some gray and black slate or mudstone) and coarse sand

77-79: pink gravel and sand (Note: Gravel from 75-100' looks clean, low silt/clay)

79-99: pink gravel and coarse sand

 

Test Hole 4 (TH-4)

0-3: Dark brown topsoil

3-8: Brown topsoil

8-10: Brown clay, and dark gravel

10-16: Dark gravel and sand, white shell fragments

16-20: Dark gravel and sand with white shell fragments

20-21: Dark gravel and sand with little amounts of clay

21-23: Dark gravel and sand

22-24: Brown clay, dark gravel and sand

24-26: Pink and red gravel

26-30: Pink and red gravel, pink and red coarse sand

30-30.6: Brown clay, pink-red gravel and sand

30.6-35: Pink and red gravel, pink and red coarse sand

35-36: Cobble or boulder obstruction. Drill hammered through obstruction; gray slate- or mudstone-like cuttings.

36-37: pink and red gravel, pink and red coarse sand.

37-38: Pink and red sand mostly, with minor amounts of pink and red gravel

38-39.6: Pink and red gravel, pink and red coarse sand, and brown water.

39.6-45: Pink-red, white, gray, and black gravel

45-47: Pink-red, white, gray, and black gravel and coarse sand

47-48: Dark brown clay, pink-red, white, gray, and black gravel and coarse sand

48-54: Pink-red, white, gray, and black gravel and coarse sand. Higher volume of dark brown water

54-55: Light brown-dark orange silt, some clay mixed with fine to coarse sand. No water flowed into casing here

55-55.5: Pink-red gravel and sand.

55.5-56: Some brown-orange clay stuck to pink-red gravel and coarse sand

56-59: Pink-red, white, and gray-black gravel (mostly quartzites) and coarse sand

59-62: Mostly coarse sand with some pink-red, white, and gray-black gravel

62-62.5: Orange-brown clay, coarse sand and some pink-red, white, and gray-black gravel, no water

62.5-64: Pink-red, white, and gray-black gravel and coarse sand (mostly pink-red quartzites)

64-65: Mostly coarse sand and some pink-red, white, and gray-black gravel

65-70: Pink-red, white, and gray-black gravel and coarse sand.

70-71: Brown clay, pink-red, white, and gray-black gravel and coarse sand; less water

71-72: Pink-red, white, and gray-black gravel and coarse sand; less water

72-74: Pink-red, white, and gray-black gravel and coarse sand; water (dark brown) is less dirty

74-74.5: Coarse dark sand

Base of Upper Gravel

74.5-75: Brown clay and sand, no water

75-75.5: Pink-red, white, and gray-black gravel and coarse sand, some water

75.5-75.6: Brown clay, pink-red, white, and gray-black gravel and coarse sand

76-77: Pink-red, white, and gray-black gravel and coarse sand, some water

77-78: Brown clay, and coarse sand

78-79: Pink-red, white, and gray-black gravel and coarse sand

79-81: Brown clay, fine gravel and coarse sand, no water

81-84: Brown clay and gravel, no water

84-89: Pink-red, white, and gray-black gravel and coarse sand, no water

89-93: Brown clay, pink-red, white, and gray-black gravel and coarse sand

93-95: Brown clay and coarse sand, no water

95-99: Mostly coarse sand with some brown clay, and pink-red, white, and gray-black gravel

99-107: Some brown clay, fine pink-red, white, and gray-black gravel and coarse sand, no water

107-115: Pink-red, white, and gray-black gravel and coarse sand, no water (Note: 105-115 interval looks clean)

115-118: Fine pink-red, white, and gray-black gravel and coarse sand

118-119: More clay with mostly coarse sand and some fine pink-red, white, and gray-black gravel, no water

 

Test Hole 5 (TH-5)

Upper portion of hole not logged; driller states that topsoil extends to 10 ft bls and that cuttings from 10 to 59 ft bls were composed of silt/clay gravel and lots of water.

59-61: A lot of dark brown groundwater, silt/clay, dark gravel

61-64: Pink-red, gray-black fine gravel and coarse sand

Base of Upper Gravel

64-65: Brown clay and coarse sand

65-67: Appearance of more pink-red gravel

67-69: Mostly coarse sand with some fine gravel and a little brown clay. Sample bag 67-70 contains the fraction.

69-71: Coarse dark sand and brown clay. No water

71-72: Brown clay and dark gravel. No water

 

Test Hole 6 (TH-6)

0-10: Dark brown topsoil

10-12: Dark gravel

12-16.5: Sand and brown clay, white shell fragments.

Note: drilling is rapid to this point.

16.5-17: Brown clay and dark gravel

Stopped ant 4 PM to add 3-ft and 20-ft casing extensions.

17-18: Dark brown clay and dark gravel.

18-19: Dark gravel.

19-23: Fine dark gravel and coarse sand.

23-24: Water and rock debris spraying up from the outside of the casing due to reaching a water-bearing layer. Not much water. Cuttings are mostly coarse sand but appear to be chips from gravel. Pink-red (quartzites), gray to black gravel (slate or mudstone), with some white gravel (quartz).

24-29: Dark gravel and sand. No water

29-30: Dark gravel coarse to fine sand. Some water.

30-40; Pink-red, gray-black, and white gravel.

40-47: Mostly red-pink gravel and coarse sand. Little water.

47-55: Mostly coarse sand and fine red-pink, black-gray, and some white gravel.

55-65: Mostly coarse sand and fine red-pink, black-gray, and some white gravel, with some brown clay.

65-66: Mostly coarse sand and fine red-pink, black-gray, and some white gravel., with some hard brown clay clasts.

66-68: Mostly coarse sand and fine red-pink, black-gray, and some white gravel, and brown clay.

68-69: Mostly fine red-pink, black-gray, and some white gravel, with some coarse sand.

Base of Upper Gravel

69-70: Mostly coarse sand and fine red-pink, black-gray, and some white gravel, and brown clay.

70-72: Mostly fine red-pink, black-gray, and some white gravel, with some coarse sand.

72-74: Mostly coarse sand and fine red-pink, black-gray gravel with some white gravel, and brown clay.

74-76: Fine red-pink, black-gray, and some white gravel, with some coarse sand. Some brown clay and hard dark brown fine gravel clay balls.

76-79: Red-pink, black-gray, and some white gravel, with some coarse sand. Little water.

79-92: Red-pink, black-gray, and some white gravel, with some coarse sand. No water.

92-98: Mostly coarse sand and fine red-pink, black-gray gravel with some white gravel.

98-99: Red-pink, black-gray, and some white gravel, with some coarse sand, with a minor layer of brown clay.

99-103: Red-pink, black-gray, and some white gravel, with some coarse sand and some brown clay.

Note: 80-100 interval looks clean

Hildreth Well 4

Drilled by Cushman Drilling Inc.; bagged samples were collected by Monty Staples every 5 ft

J. Welhan arrived when drill bit was at 165 ft bls

Log is based on examination of bagged samples and driller's description of drilling conditions

1-7ft: silt, topsoil

7-45: fine-med. sand and gravel

45-55: med.-coarse sand and gravel Note: water at about 30' bls

55-75: fine-med. sand and gravel

Base of Upper Gravel

75: hard drilling, clay zone , possibly indurated Note: 10 ft discrepancy between bagged

75-90: transition zone samples and driller’s notes; depth of contact

Note: Hole stayed open below 90' overnight is approximate and gradational

90-120: silt-rich, fine-med. sand and gravel Note: several clay zones 90-145 ft

120-145: med.-coarse gravel, silt-rich

145-165: med.-coarse gravel with sand, much less silt and clay

165-215: same as above Note: hole 90-200 ft stayed open overnight

 

IDWR lithology filed by Cushman Drilling for Hildreth Well 4:

0-5: hard pan clay

5-10: sandy clay and gravel

10-20: sand and gravel

20-70: sand and gravel

Base of Upper Gravel

70-110: compacted gravel

110-145: clay and gravel

145-160: clay and some gravel

160-170: compacted gravel

170-174: clay

174-200: clay and gravel

200-215: brown clay