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Using Direct-push Methods for Aquifer Characterization in Dune-lake Environments of The Nebraska Sand Hills

¹ Department of Geosciences, University of Nebraska–Lincoln, Lincoln, NE 68588-0340
² School of Natural Resources, University of Nebraska–Lincoln, Lincoln, NE 68588-0517
³ Department of Geosciences, University of Nebraska–Lincoln, Lincoln, NE 68588-0340

	ABSTRACT

TOP ABSTRACT Introduction Rationale for Using Direct... Direct Push Techniques and... Planning and Field... Summary of The Nebraska... Conclusions REFERENCES CITED

 
The direct-push (DP) approach for characterization of the shallow unconsolidated sub-surface is a rapidly developing methodology that deploys hydrogeological, geotechnical, and geophysical tools in the sub-surface. It offers significant advantages as compared to techniques using traditional drilling and permanent piezometers, but requires real-time on-site decisions. Selection of the number of tests, sequence, location, and depth makes the planning stage crucial for successful and effective field studies. Whereas the analysis of various DP-based hydrogeological, geotechnical, and geophysical methods is well presented in the literature, recommendations for planning DP field applications are scarce. We illustrate applications of DP-based techniques (electrical conductivity profiling of the aquifer, groundwater sampling, slug testing, and soil core extraction) for evaluation of aquifer-lake interactions in a remote and poorly accessible area of the Nebraska Sand Hills with numerous saline lakes. In addition to the background data on hydrostratigraphy and groundwater salinity, we report an approach to combining and optimizing the DP techniques, including the work sequence, depth limitations, data quantity, and scheduling. The data and insights gained will be useful in designing characterization programs in other sand dune-lake environments.

Key Words: Eolian Sediments • Lakes • Direct Push • Slug Test • Groundwater Sampling • Soil Cores

	Introduction

TOP ABSTRACT Introduction Rationale for Using Direct... Direct Push Techniques and... Planning and Field... Summary of The Nebraska... Conclusions REFERENCES CITED

 
Direct-push (DP) methods for deployment of various sampling instruments in the subsurface present a viable alternative to traditional drilling. These techniques use more compact and lighter track-mounted equipment, which is mobile on hummocky off-road terrain, non-cohesive (e.g., sand) and slanted surfaces (e.g., dunes with slopes 15–30°), and soft and wet areas (e.g., wetlands). Currently, Geoprobe Systems® and AMS^TM are two leading manufacturers of track-mounted systems designed for deployment of various tools or samplers at given depths.

Recent publications have focused primarily on methodological DP issues (e.g., Christy et al., 1994, Cho et al., 2000; Butler et al., 2002; McCall et al., 2002; Schulmeister et al., 2003a, 2003b, 2004; and Sellwood et al., 2005). These studies of vertical profiling of the formation electrical conductivity (EC), hydraulic testing, coring, and water-chemistry sampling were performed in controlled environments and were focused on technical aspects and quality-control issues without time constraints. Little has been presented on field-work planning, which is important at this stage of DP proliferation, especially in uncontrolled environments.

We illustrate this situation by the design of an aquifer characterization program in a poorly accessible area of the Sand Hills, Nebraska, for studies of lake-aquifer interactions. Shallow saline lakes whose origins are traced to groundwater seepage exist in Asia, Africa, and the Americas (Yechieli and Wood, 2002). Pathways of lake salinization at the conceptual level sometimes can be studied using geochemical and mass-balance modeling (Wood and Sanford, 1990), but application of these models to specific lakes requires estimations of lake-aquifer exchange fluxes, which involves invasive procedures for measuring groundwater and formation parameters.

Typically, piezometers/wells are installed using various drilling techniques in eolian sand (Winter, 1986) or glacial till (Donovan et al., 2002; Filby et al., 2002), but their applications are hindered by high costs and logistical problems, especially in dune environments. Therefore, DP methods offer an attractive alternative for studying lake-aquifer interactions. Recently, this approach was applied in the Sand Hills in western Nebraska (Figure 1). This area is 400 km west of the base of operations (University of Nebraska–Lincoln [UNL]), up to 80 km from the nearest refueling and supply bases and outside of cellular phone communications. Such environments drastically reduce error tolerance in field-campaign planning, but published recommendations on the timing and depth for various DP techniques in such environments are cursory.

View larger version (42K): [in this window] [in a new window]   Figure 1. Study area in the Sand Hills, Nebraska. (a) Sites of 11 test holes across the water table and an additional hole on the dune crest for eolian sediment dating using the optically stimulated luminescence technique (OSL). Map indicates TDS (g/L) and pH for several lakes in the area and an outline of the buried Blue Creek paleovalley, after McCarraher (1977). (b) Surface profile along the cross section A-A'-A''

	Rationale for Using Direct Push Methods in The Nebraska Sand Hills

TOP ABSTRACT Introduction Rationale for Using Direct... Direct Push Techniques and... Planning and Field... Summary of The Nebraska... Conclusions REFERENCES CITED

 
The Sand Hills are the largest sand dune field (about 50,000 km²) in the Western Hemisphere (Loope and Swinehart, 2000). Numerous lakes occur in topographic depressions under west-east regional groundwater flow (Winter, 1986). In Sheridan and Garden counties alone there are approximately 400 lakes with surface areas larger than 4 ha. The concentration of total dissolved solids (TDS) in lake water ranges from 0.3 g/L to more than 100 g/L, and pH ranges from 8.4 to >10, as shown in Figure 1a (see McCarraher, 1977; Gosselin, 1997). Although several hypotheses are available, causes of the wide variations in lake salinity within this large geographic area have not been determined conclusively (Bleed and Ginsberg, 1998, p. 122).

Aquifer characterization in a sub-area of about 400 km² northwest of the Crescent Lake National Wildlife Refuge (CLNWR) in Garden County, Nebraska (Figure 1a), was proposed for several reasons (see Tcherepanov and others, 2005, and Loope and others, 1995, for references). This portion of the Sand Hills has numerous shallow groundwater-fed lakes (mostly not deeper than 1 m) in interdunal depressions (Figure 1b); vegetation is sparse, and sediments are represented mostly by sands and silts that are expected to be relatively uniform. Holocene eolian sand deposits overlie Quaternary and/or Pliocene alluvial sands and silts with low organic content. Vegetated dunes on average are 40-m high, 825-m long, and 1220-m wide. The area hosts a hydrologically, chemically, and biologically diverse group of lakes, which offers broad opportunities for the interpretation of the origins of the large variability in lake salinity.

Hydrogeologically, this is one of the most studied areas of the Sand Hills (see a summary by Gosselin, 1997). In the first extensive hydrogeological field study of the Sand Hills, Winter (1986) analyzed water-level fluctuations and the configuration of the water table using an array of piezometers. LaBaugh (1986) studied the limnological and chemical characteristics of lakes and groundwater in the same area. Both studies hypothesized that the major source of salinity differentiation is the origin of lake water, i.e., deep flow system stream tubes discharge more saline groundwater into the lakes (see Tóth, 1963), and the lake salinity is determined by position with respect to the regional groundwater flow system. Gosselin and others (1994) sampled several lakes and wells and showed that geochemical evolution of fresh shallow groundwater in lakes by evaporation may lead to the observed lake water chemistry without invoking the deep flow concept. Loope and others (1995) described the geological controls on modern groundwater in an ancient buried Blue Creek valley in the area (Figure 1a), which might control mass transport. Tcherepanov and others (2005) used Landsat Thematic Mapper or Enhanced Thematic Mapper Plus to identify potential zones of the groundwater seepage to the lakes from spatial temperature variations and to assess the groundwater flow direction.

To develop a basic understanding of mass and solute fluxes in the lakes and the aquifer, several baseline questions must be addressed: (1) Is groundwater fresh in the immediate vicinity of the saline lakes, and does solute concentration vary with depth? (2) What is the distribution of formation hydraulic conductivity near lakes? (3) Is it possible to differentiate eolian and fluvial sediments and organic matter near lakes based on hydraulic and geophysical methods? In addition, some critical aquifer data are lacking in the CLNWR region of the Sand Hills (distribution of hydrofacies, hydraulic conductivity, and groundwater TDS), and systematic presentation of such information will aid in developing models of the origins of the Sand Hills lakes and possible consequences of climate change on lake-groundwater interactions.

	Direct Push Techniques and Their Coupling

TOP ABSTRACT Introduction Rationale for Using Direct... Direct Push Techniques and... Planning and Field... Summary of The Nebraska... Conclusions REFERENCES CITED

 
Several DP techniques are available for addressing the questions mentioned above: Groundwater samples can be drawn from different depths using a sheathed screen; distribution of formation hydraulic conductivity can be obtained from hydraulic tests (slug tests in particular), and soil core analyses can be utilized to corroborate these data; differentiation of sediments (clay/silt/soil/sand/peat) can be obtained by corroborative data from vertical profiling of formation EC, slug tests, and cores.

A Geoprobe Systems® DP machine (model 6610DT) and tools (Screen Point 16 [SP-16] system for slug tests and groundwater sampling [Figure 2a] and soil conductivity probes [model SC300 on Figure 2b and model SC400 on Figure 2c] for formation EC profiling) were used in the Sand Hills expedition (see Geoprobe Systems® Tools Catalogue, 2003). Planning and implementation involved optimization of a sequence of four techniques (EC profiling, slug tests, water-chemistry sampling, and soil core collection) during the cycle of downward-upward rod string movement to minimize the number of DP cycles at one location. Considering the inherent properties of the existing core retrieval system working on a downward run, optimization was applied only to three techniques.

View larger version (19K): [in this window] [in a new window]   Figure 2. Geoprobe® tools (a) system for slug testing SP16, (b) EC probe SC300, and (c) EC probe SC 400 (modified from Geoprobe Systems Tools Catalogue, 2003)

Coupling several techniques at a given elevation during downward or upward runs in the DP cycle requires stopping the DP operation, changing probes, and implementing additional quality control measures at each depth if one collects a vertical profile of some parameters. So, the advantages may be alleviated in specific cases because of additional technical, timing, or reliability problems. Following the above-mentioned studies, we optimized the sequence of these four techniques into three DP cycles (Figure 3).

View larger version (20K): [in this window] [in a new window]   Figure 3. A schematic diagram of three DP cycles: (a) cycle of delineation of the formation texture using EC on downward run; (b) sheathed screen deployment on the downward run and screen exposure on the upward run for slug test and water sample collection; and (c) cycle of core collection on downward run

1. EC profiling on the downward run (Figure 3a) precedes the other data collection procedures and provides basic data on hydrostratigraphic conditions, such as the sequence of fine/coarse units, anomalies, bedrock depth, and penetration speed (e.g., Schulmeister et al., 2003a) .
   
2. Groundwater samples for TDS analyses and slug test data are collected on the upward run. For this purpose, a sheathed screen is deployed and opened at the maximum target depth for water sampling and hydraulic conductivity data collection (Figure 3b). The screen is exposed, and fine sediments are removed by purging (development). Several criteria are used to determine development completion: at least three volumes of rod-string inner space, stabilization of temperature and electrical conductivity of effluent, and minimization of turbidity in the effluent. Development is followed by water-sample collection and the slug test. Note that the time for switching the tools (from the ball valve system used for water-sample collection to the pneumatic slug-test system) may not be insignificant. By retracting the rod string on the upward run (Figure 3b), the procedures can be repeated at higher elevations. At the deepest elevation, skin occurrence due to fine sediments on the screen is minimal because of the presence of the sheath. Possible smearing of fine sediments or incomplete formation collapse near the screen during the upward rod movement would reflect the consistent trend in hydraulic conductivity. Typically, this was not the case. In such conditions, the small-diameter (3.8 cm) borehole collapsed rapidly. Screen development further facilitated the formation collapse. Recently, Sellwood and others (2005) used even larger rod diameters (5.7 cm) on the upward run. TDS data collected by this approach are not prone to a sampling bias (Schulmeister et al., 2004).
   
3. Core collection from each depth requires a separate DP cycle (Figure 3c). The sequence of core collection depends on familiarity with the site and EC profiling data. In cases when EC data are inconclusive, core collection should follow the formation EC profiling for better delineation of stratigraphy if cores can be processed on site before the water sample collection and slug test.
   

Sellwood and others (2005) published an alternative technique to optimize EC profiling and slug testing after our study was complete. A custom-built dual-tube assembly is used to operate or exchange different probes in the test hole. This methodology couples EC on the downward run and slug tests on the upward run. However, during replacement of the EC probe with the screen for the slug test, the outer tube must stay open under the water table for several minutes; potential inflow of heaving sand to the system may lead to test failure. So, more experiments are warranted to assess features of this technique in various environments, and we focused on the three-cycle approach using standard components from the Geoprobe Systems® Tools Catalogue (2003).

	Planning and Field Implementation

TOP ABSTRACT Introduction Rationale for Using Direct... Direct Push Techniques and... Planning and Field... Summary of The Nebraska... Conclusions REFERENCES CITED

 
After consideration of topography, geology, project resources, and land access, 11 holes were proposed for preliminary characterization of the area. Our initial target depths (30 m) were based on regional information (Mason et al., 1997). Such depths could yield data on the eolian/fluvial system interface that is expected in the area. The field operations planning is summarized in Table 1 for a crew composed of a Geoprobe Systems® operator with more than 10 years experience and two or three assistants.

The 11 locations tested from June 21 to July 1, 2004, for a total of 11 days are shown in Figure 1a. Table 1 also summarizes actual time and DP depths. These test holes were located in topographically low positions to maximize penetration of the saturated zone. Distances from the lake strandlines varied from 25 m to more than 200 m. The water table was encountered at depths of 1 m in all tested locations. After tests in the saturated zone and fine lacustrine sediments, a more significant effort went into cleaning rods after each cycle as compared to eolian or fluvial environments. Often, a thin layer of a fluid lacustrine mud covered the outer surface of the extracted rods. On several occasions, this mud penetrated the contacts between rods. The electric connectors of cables leading to the EC probes were watertight on all occasions, but safety requirements led to frequent cleaning of the cable and inner rod surfaces. In the absence of penetrating mud, a simple rod wiper can be used on the outer surface.

Distances between adjacent sites were only a few kilometers. However, mobilization/demobilization of equipment ranged from half an hour to an hour. Longer mobilization/demobilization times were incurred when the track-mounted unit had to be unloaded from the trailer and driven up long dune paths or on slopes on the order of 10° and larger. Net testing time reached 110 hours over 11 consecutive days. Deviations between the initial estimate and the actual number of samples, and between numbers of hours per test, are due to various factors. These factors included unconsolidated sands and gravels that were thinner in some locations than anticipated and considerably more resistant to direct push at depths below 10 m, possibly due to dense Ogallala Group sands. Using 3.81-cm (1.50-in.) rods with 1.59-cm inner diameter, our penetration depths were on the order of 10 m, which was less than the planned depth but at least an order of magnitude greater than any lake depths in the area.

Formation EC procedures required more time because of cleaning the rods and pulling cable through the rod string (stringing). Probes SC-300 (dipole array) and SC-400 (in dipole array configuration) were used; one of each had to be replaced with spare units in the course of work, which involved total penetration depths on the order of 300–400 m per probe life (their total penetration has not been rated, and similar situations must be envisioned when planning the field work). In certain situations, a set of additional pre-strung rods with a rod rack could be helpful to minimize the rod stringing time. However, with seals leakage during the downward run, cable extraction, and the inside rod surfaces and cable cleaning, time savings from using pre-strung rods might be minimal. Several rods (about 20 percent) bent, and treads were deformed (rods were rated for penetration depth or any other indicators of longevity). Spare rods or even multiple sets may be helpful. However, estimates of the minimal safe number of rods needed may be feasible after systematic data collection on rod performance.

An example of a vertical formation EC profile typical for the area is shown in Figure 4 (hole 3 on Figure 1a). The EC values and the DP penetration speed clearly indicate finer sediments in the top 2 m and relatively uniform sand below, which is consistent with regional stratigraphy. The EC values are similar to the values given for sand by Sellwood and others (2005).

View larger version (12K): [in this window] [in a new window]   Figure 4. Formation electrical conductivity (EC) profile and speed of penetration from Hole 3 indicating typical stratigraphy of the area. Boxes indicate tested intervals and corresponding hydraulic conductivity K and TDS

Samples were collected in 1-L polyethylene bottles and preserved for TDS analyses in the Water Sciences Laboratory, UNL. Because of the nature of study, we were interested in the TDS averaged over a few meters vertically, rather than high-resolution data (sub-meter scale). The general question of this study was whether we could identify locations of elevated concentrations in groundwater. Sellwood and others (2005) did not report preferential flow or the hydraulic conductivity bias even for larger-diameter rods (5.4 cm). Our smaller-diameter rods (3.8 cm) make our setting even less prone to preferential flow artifacts.

To rapidly assess TDS of the groundwater on-site, a calibration curve of EC vs. TDS was constructed (Figure 5a) using data on the regional lake water chemistry from Gosselin (1997, his Table 1). The resulting linear regression was used to directly estimate TDS from on-site measured groundwater EC. Elevated EC values would indicate a possible source of saline groundwater, which would be of major interest for assessing hypotheses of the lake salinity and could heavily influence choices of the test hole locations.

View larger version (7K): [in this window] [in a new window]   Figure 5. Calibration curve TDS (g/L) as a function of electrical specific conductance EC (mS/cm) in the study area (a) for lake water from pre-trip data after Gosselin (1997), (b) for groundwater from after-trip data in Table 2

View larger version (9K): [in this window] [in a new window]   Figure 6. Underdamped and overdamped slug test responses in Hole 2 at depth 7.2 m and 3.6 m

The average total time per hole was about 9 hours including necessary repairs. All test sequences in one location were completed within 1 day or divided between 2 consecutive days. In favorable conditions, this time could be reduced; however, the average penetration depth remained the major time-controlling parameter for a given test hole.

	Summary of The Nebraska Sand Hills Data Set

TOP ABSTRACT Introduction Rationale for Using Direct... Direct Push Techniques and... Planning and Field... Summary of The Nebraska... Conclusions REFERENCES CITED

 
The strategic positioning of the holes with respect to the strandlines of several lakes (from a few meters to several hundred meters) allowed for several important data generalizations on the lake hydrogeological settings.

1. Difficulties of DP penetration in several locations suggest some lithological transition at depths varying from 10 m to 15 m below the ground surface. Noticeable heating of the drive cap, slanted rod strings, and minor rod deformations supported the operator observations. A contact between the eolian sands and Ogallala Group in western Nebraska and western Kansas (McCall, 2004) may explain the change in density/penetration rate at depth. An outcrop in a quarry in the area (N41°38.930', W102°34.682') exhibits clear contact between the sequence of younger sediments (soil, loess, and alluvium) and underlying clearly pronounced Ogallala sediments at about 10-m depth. All Ogallala samples indicated significant cementation and induration. However, if the dune crest elevation was high enough above the water table, the penetration was unobstructed, as was observed in the OSL hole (Figure 1a) that was tested for a related project in the area. Several DP-based cores were collected from this hole. The deepest core in the unsaturated zone at this hole was 25 m below the ground surface. Preliminary visual analyses of all cores did not reveal systematic textural changes in most holes. Additional work may be required to confirm that the Ogallala Group sediments are reached.
   
2. Formation EC data, cores, and slug tests in sediments at the lake margins did not indicate the presence of peat or gyttja (mainly plant and animal residues and mud deposited from standing water) in any location. This differs from conditions beneath some lakes, which were observed in vibracores (Mason et al., 1997). The absence of peat or gyttja may suggest that these lakes emerged after a regional water-table rise. However, this climate paleorecord could be "erased" during flooding-dewatering cycles related to periodic drought conditions.
   
3. The DP methodology produced a significant number of groundwater samples over the large area. The dramatic difference between the TDS ranges of groundwater and surface water is apparent. Groundwater in samples from all depths was fresh or near fresh (Table 2). TDS for these samples varied in range from 0.13 g/L at Site 2 to 1.7 g/L at Site 3. By comparison, values of TDS from the sampled lakes were more diverse and varied from 0.32 g/L (Lake Island) to 122 g/L (Alkali Lake) by the latest data (Bennett and others, 2006), with mean 17 g/L. The absence of high groundwater salinity values is in contrast with the hypothesis of lake salinity originating directly from saline groundwater discharge (e.g., LaBaugh, 1986). Considering the broad range of the test hole locations, these data make a hypothesis of evaporative origins (e.g., Gosselin, 1997) more favorable. Figure 5b presents a correlation between the EC of field groundwater samples and TDS determined in the laboratory. A comparison of EC-TDS correlation characteristics for lake water (Figure 5a) and groundwater (Figure 5b) shows that lake salinity data are good proxies for groundwater.
   
4. Hydraulic conductivity (K) was representative of fine sands of eolian origin. K ranges from 0.3 m/d to 15 m/d with mean K = 5.4±4.9 m/d and median K = 3.46 m/d (Table 2). Higher conductivity values are more characteristic of coarser sands (possibly of fluvial origin). To identify systematics in hydraulic conductivity, grain size analyses were performed on samples from various cores (Table 3). The Hazen (1911) method was used for independent estimation of hydraulic conductivity: K = C (d₁₀)², where K is in m/d, C = 40 for fine grain sand, and d₁₀ is the effective grain size in mm (e.g., Fetter, 2001). Figure 7 shows the correlation between the slug test data and the Hazen method; K from grain-size analyses are biased toward the lower values, which is common for these techniques (Cardenas and Zlotnik, 2003; Zlotnik et al., 2000). These data correspond well to previously obtained results for eolian sands from constant-head infiltration tests by Sweeney and Loope (2001) and air injection tests by Goss and Zlotnik (2006) in other locations of the Sand Hills. Vertical trends in K in most holes were not detected.
   

View larger version (12K): [in this window] [in a new window]   Figure 7. Correlation between hydraulic conductivity data obtained using DP from slug tests (KST) and from grain-size analyses (KGSA)

	Conclusions

TOP ABSTRACT Introduction Rationale for Using Direct... Direct Push Techniques and... Planning and Field... Summary of The Nebraska... Conclusions REFERENCES CITED

DP methods of aquifer characterization offer additional flexibility as compared with more traditional drilling techniques, because the sampling methodology is inseparable from the technique used to deploy sampling tools at various depths. In particular, our study in the Nebraska Sand Hills became feasible only because of developments of these DP capabilities. However, these methods may be more involved technologically and limited in penetration depth even in lake-dune environments. Therefore, planning the successful implementation of fieldwork requires more attention to technological details. This involves analyses of the sequence and coupling of tests in addition to specific problems of individual geotechnical, hydraulic, and geophysical methods used in the investigations.

New data on aquifer parameters, including hydraulic conductivity, lithology, and texture, and groundwater TDS of the Nebraska Sand Hills collected at such large scale are unique. Data collected relatively rapidly with DP were readily applicable to support the hypothesis of the evaporative nature of origins of high lake salinity in this semi-arid climate. Low groundwater salinity and high lake salinity are consistent with high evaporation from lakes that exhibit strong lake-groundwater interactions in a permeable aquifer. The collected data set is useful both in understanding the hydrological system of the Nebraska Sand Hills and as an example for designing and optimizing time and resources for future aquifer characterization programs in sand dune-lake environments, which commonly exist in various remote and poorly accessible areas of the world.

	ACKNOWLEDGMENTS

 
This study was supported by grants from National Science Foundation Biocomplexity in the Environment Program (BE03-22067), the U.S. Environmental Protection Agency Science to Achieve Results program (R828635), the U.S. Geological Survey (Program 104B, 2004–2006), and the University of Nebraska–Lincoln (UNL) Water Center. We are grateful to J. Healy, Kansas Geological Survey, for advising on DP procedures with self-fusing tape; D. Wedin, UNL, for directing the undergraduate support; A. Beringer, Creighton University, and J. Leach, UNL for field assistance. We thank J. Cooper, G. deWitt, T. Dietlein, M. Eldred, and J. Parker for granting access to their land and sharing historical observations on lake dynamics. Reviews by E&EG Co-Editor A. Fryar (University of Kentucky), B. Riha (Savannah River Technology Center), W. Anderson (Appalachian State University), and one anonymous reviewer led to significant improvements of the manuscript.

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TOP ABSTRACT Introduction Rationale for Using Direct... Direct Push Techniques and... Planning and Field... Summary of The Nebraska... Conclusions REFERENCES CITED

 

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