The Hydraulic Profiling Tool for Hydrogeologic Investigation of Unconsolidated Formations

The hydraulic profiling tool (HPT) has become one of the basic tools for investigation of soils and unconsolidated formations over the last 10 years. The HPT is advanced into the subsurface using direct push methods. Clean water is injected into the formation from a small screened port on the side of the probe as it is steadily advanced into the subsurface. A downhole pressure sensor detects the pressure required to inject the water into the formation while an up‐hole flowmeter monitors the water flow rate. An electrical conductivity (EC) array included in the lower end of the probe provides a simultaneous EC log of the bulk formation. The EC log, HPT pressure, and flow rate are logged and displayed onscreen as the probe is advanced. These logs enable the investigator to evaluate vertical changes in relative formation permeability at high resolution. Pressure dissipation tests may be performed at selected depths in coarse‐grained materials to determine the piezometric pressure in saturated formations. This enables the operator to define the piezometric profile and determine the piezometric surface without a well. Post processing of the log in the viewing software provides for calculation of the corrected HPT pressure (Pc) and estimation of hydraulic conductivity (Est. K) within limits (~0.1 to 75 ft/d). In clean, coarse‐grained materials the tandem EC log may be used to estimate groundwater specific conductance based on an Archie's Law model. Cross sections of HPT logs provide an efficient means to define hydrostratigraphy. When combined with contaminant logging tools such as the membrane interface probe (MIP) the HPT data may help to define contaminant migration pathways or contaminated low permeability zones that may result in back diffusion. The HPT can be a useful tool for many geoenvironmental investigations in unconsolidated formations.


Introduction
PHT3D is a computer code for general reactive transport calculations, coupling MODFLOW/MT3DMS for transport and PHREEQC for chemical reactions. It was developed by Henning Prommer in the 1990s and has been applied by him and his coworkers to various groundwater problems of practical interest. The resulting publications (http://www.pht3d.org/pht3d public.html) show an impressive applicability of the code and illustrate the underlying understanding of quite complicated interactions (e.g., Prommer and Stuyfzand 2005;Prommer et al. 2008Prommer et al. , 2009). In the original version, transport is calculated during a time step, an input file is written for PHREEQC for calculating reactions such as ion exchange and precipitation or dissolution of minerals, and these steps are repeated for subsequent time steps until finished. This loose coupling has the advantage that updates of the master programs can be installed without much effort. A disadvantage is that the calculation of the chemical reactions needs to be initialized time and again for each cell in the model, which adds another time-consuming step to calculations that are already computer-intensive. Another disadvantage is that surface complexation reactions need to be calculated first using the water composition from the previous time step and then reacted with the changed water concentrations. This procedure was not implemented in the original version of PHT3D, and surface complexation reactions could not be calculated.
Prommer and Post recently released the second version of PHT3D that resolves the shortcomings and works very well. The improvement is owing firstly to the implementation of total-variation-diminishing (TVD) scheme that MT3DMS uses for calculating advective and dispersive transport (Zheng and Wang 1999). Secondly, it is because PHREEQC is now being used for storing the chemical data of the model, including the chemical activities and the composition of surface complexes from the previous time step. In addition, the procedure to transport total oxygen and hydrogen has been adapted from PHAST (PHAST is the 3D reactive transport model developed by Parkhurst et al. 2004, based on HST3D and PHREEQC). This enables the user to obtain the redox state of the solution without having to transport individual redox concentrations of the elements (e.g., C being distributed over carbon-dioxide, C(4), and methane, C(-4)). The tighter coupling quickens the calculations twofold at least, but probably by an order of magnitude for the more interesting cases. In this review, the background of the new implementation is presented and illustrated with examples and compared with results from PHREEQC and PHAST.

Introduction/Background
The Environmental Protection Agency (EPA) established regulations to protect the environment and human health (Code of Federal Regulations Title 40) and this led to the investigation of soil and groundwater contamination at a scale not previously conducted. The early methods applied for site investigation of unconsolidated formations utilized existing rotary drilling and sampling technology (EPA 1985(EPA , 1986(EPA , 1987(EPA , 1991a(EPA , 1992. The inefficiency of many of these methods for geoenvironmental investigations and the generation of large quantities of often contaminated waste cuttings and decontamination fluids encouraged the development of direct push methods for investigation of unconsolidated formations and aquifers (Geoprobe 1992;McCall et al. 2006). Addition-ally, the importance of understanding contaminant fate and transport at finer resolution grew as the industry developed (EPA 1989(EPA , 1991b(EPA , 1995(EPA , 1998Mercer and Cohen 1990). It became apparent that use of small diameter, discrete interval, direct push sampling and logging methods could provide the detailed information needed about contaminant distribution and migration as well as important geochemical and hydrologic properties of contaminated formations (EPA 1997(EPA , 1998. This led to the development of several methods to measure or estimate permeability or hydraulic conductivity by slug testing direct push installed groundwater sampling devices (Cho et al. 2000;Butler et al. 2002;McCall et al. 2002;Sellwood et al. 2004;Geoprobe 2002Geoprobe , 2003, American Society of Testing and Materials [ASTM] D6001 2018a, ASTM D7242 2006a). While these slug testing methods may be used to assess hydraulic conductivity over discrete intervals ranging from a few inches to a few feet they are time consuming, especially when detailed vertical profiles are required.
Due to interest in small scale variations in permeability and its influence on contaminant migration Geoprobe ® began evaluation and testing of a 2-port permeameter (Geoprobe 2004). Two ports separated by a few inches were placed on the side of the permeameter above a short screen near the end of the probe. With probe advancement halted, water injection at the screen allowed for measurement of differential pressure at the two ports. This system could provide quantitative to semi-quantitative measurement of hydraulic conductivity (K) at selected intervals, typically within a few minutes for sandy materials. But in heterogeneous geologic settings the 2-port system experienced anomalies when the ports were in contrasting K materials. Due to this complication, Geoprobe began development of the HPT with a single port for injection of water and simultaneous measurement of injection pressure as the probe is advanced (Geoprobe 2006a). During this time period work was underway by several researchers to investigate profiling vertical variations in permeability of unconsolidated formations with various direct push probes and methods (Pitkin et al. 1999;Pitkin and Rossi 2000;Butler et al. 2002Butler et al. , 2007Dietrich et al. 2008;Liu et al. 2009;Dietz and Dietrich 2012). Typically, these systems were obtaining pressure measurements at the surface which would not allow for determination of in situ piezometric pressures at depth nor the ambient water level. The up-hole pressure measurements also had to be corrected for frictional losses in the tubing system to obtain accurate determinations of the downhole injection pressure (Dietrich et al. 2008). Geoprobe ® was able to develop a robust pressure sensor that could be placed in the connection tube just above the HPT probe to be advanced with percussion probing methods (Geoprobe 2006b). This eliminated the need to correct for frictional losses in the water supply tube and allowed for measurement of the ambient piezometric (hydrostatic) pressure at any depth below the piezometric surface with simple pressure dissipation tests. This enabled the operator to define both the piezometric profile and the piezometric surface given the ambient atmospheric pressure.
The logs of HPT injection pressure allow the investigator to evaluate the relative permeability of formations versus depth at a high degree of resolution (0.05 feet, ~15 mm intervals) as the probe is advanced at about 2 cm/s (Geoprobe 2006a(Geoprobe , 2010(Geoprobe , 2011a(Geoprobe , 2011bASTM 2006b;Interstate Technology & Regulatory Council [ITRC] 2019). The HPT port and downhole pressure sensor also have been combined with several contaminant logging tools such as the membrane interface probe (MIP) (McCall et al. 2014; ASTM D7352), optical imaging profiler (OIP) (McCall et al. 2018; ASTM WK66935 2020), laser induced fluorescence (LIF) probes (Horst et al. 2018) and groundwater profiling tools (McCall et al. 2017). The HPT probe has become one of the most widely used tools for evaluating the permeability and hydrogeology of soils and unconsolidated formations (Maliva 2016;Liu et al. 2018). When used in combination with various contaminant logging tools or groundwater profiling tools the HPT is often used to support high resolution site characterization (HRSC) needs for many geo-environmental projects (Quinnan et al. 2010;ITRC 2015ITRC , 2018ITRC , 2019Suthersan et al. 2015). The HPT alone, or with these combined tools are often used to assist with development of conceptual site models (EPA 2011;McCall et al. 2014McCall et al. , 2017McCall et al. , 2018Suthersan et al. 2015), estimation of mass flux (ITRC 2010), and to perform expedited site characterizations using the Triad approach (EPA 2001(EPA , 2010ITRC 2003ITRC , 2019 at contaminated facilities.

Equipment and Tools
The tools, equipment and accessories required to conduct HPT logging are summarized in the manufacturer's standard operating procedure (Geoprobe 2006b) and reviewed in ASTM Practice D8037 for hydraulic logging. In the field the HPT System is typically operated with a 2000-W gasoline powered generator supplying 120 V/60 cycle current. HPT systems are available for 220 V power supply for some international markets. The primary HPT system components are reviewed here.

The HPT Probe
The HPT probe (Geoprobe ® MN 226553) is 21.5-in. (546 mm) long and 1. 75-in. (44.5 mm) in maximum diameter ( Figure 1). A stainless-steel mesh covers the HPT injection port which is about 0.4-in. (10 mm) in diameter. The design of the probe helps to maintain good contact between the formation and screen face during advancement. Water pressure at the screen face prevents clogging. The screened port is replaceable in the field. The port is positioned 16-in. (406 mm) above the tip of the probe. This placement avoids pressure induced near the tip of a conical probe during advancement as observed in CPTu studies (Robertson et al. 1992). Experience indicates that the HPT probe provides inch-scale (2-3 cm) resolution of pressure change in the vertical direction for material with contrasting permeability (e.g., sand-clay). However, the port provides limited sensitivity in the horizontal direction so that lateral variability beyond a few centimeters from the port may be difficult or impossible to discern. This enables the HPT system to detect changes in small scale vertical features that may occur between logs obtained only a meter apart. Larger scale vertical features (0.3 m or above) are often observed to be continuous laterally over meters to hundreds of meters. This lithologic continuity (or lack of it) is primarily a function of the environment of deposition. Depositional processes such as cut-and-fill in stream beds can lead to abrupt horizontal as well as vertical lithologic changes (McCall et al. 2014;EPA 2017).

HPT Pressure Sensor
The HPT pressure sensor (Geoprobe ® MN 210091) is approximately 4-in. (101 mm) long × 0.75-in. (19 mm) in diameter ( Figure 1). The sensor is installed just above the probe, in the connection tube ( Figure 2) so pressure measurements are made down hole. This provides for measurement of the pressure required to inject water into the formation at the given flow rate and depth. Placement of the transducer downhole eliminates the need for correction of friction losses in the supply line. Placement of the transducer downhole also allows for measurement of the ambient piezometric/hydrostatic pressure at depth during pressure dissipation tests. Each pressure sensor is calibrated from atmospheric pressure (14.7 psi/101 kPa) to 80 psi (552 kPa) using air pressure and an NIST certified pressure transducer. The calibration points are best fit to a straight line over the pressure range and each transducer is independently calibrated ( Figure S1). This calibration has proven effective for typical HPT logging over the 100 psi (690 kPa) pressure range to distinguish changes in relative formation permeability. A few operators have conducted transducer specific calibrations over a smaller pressure range for better resolution over a specific range. Note that a noisy power supply or poor grounding of the electrical system can significantly degrade the sensor signal and increase baseline noise of the system resulting in poor quality pressure logs and dissipation test data.

EC Array
The HPT probe is equipped with a 4-electrode Wenner array ( Figure 1) that can be operated as a top, middle, or bottom dipole if needed or desired. An engineering grade plastic is injected to isolate each electrode in the assembly. The Wenner array is approximately 2.5-in. (6.3 cm) in length and measures a formation volume proportional to its length. This results in some averaging of the formation response so that small (cm-scale) features may not be resolved but contrasting features in the 2 to 4-in. (5 to 10 cm) scale typically are resolved. The dipole arrays measure a smaller volume but provide resolution of centimeter-scale features, such as a narrow sand lens in clay or vice versa. Dipole array logs typically show more small-scale variability as compared to the Wenner array probe due to the greater volume averaging of the longer Wenner array. Thus, dipole array logs often appear "noisier" at the small scale as compared to Wenner array logs, but the large-scale features are typically consistent. All things considered the Wenner array is usually preferred as it samples a larger volume of the formation to provide a more representative result with less noise.
Each EC array of the same model probe has the same design and geometry. So, a model specific calibration is used for all probes of that model. The Wenner array used on the HPT probe is calibrated over a range of 0 to 500 mS/m. This is the EC range of typical soils and unconsolidated formations in freshwater environments. Probe specific calibrations could be performed by an operator for a different EC range if needed (e.g., high EC for brines).

Trunkline and Connections
The HPT trunkline ( Figure 2) is a polymer clad cable typically 150 feet in length (MN 214095) and it is prestrung through all drive rods before logging is started. The trunkline includes electrical wires for power/communication between the downhole probe and uphole instruments. Also, a 0.25-in. (6 mm) OD nylon tube for water supply downhole is included in the trunkline. A connection tube (MN 219594) is installed above the HPT probe and the HPT sensor is installed inline between the trunkline and probe ( Figure 2). Electrical connections for both the EC array and pressure sensor as well as the water supply connections are made inside the connection tube and a strain relief bushing assembly reduces stress on the wire and tubing connections. At the upper end, the trunkline is connected to the HPT con-troller and field instrument (FI) to provide electrical power and water supply to the probe downhole. The trunkline also returns data signals to the HPT Controller and FI from the pressure sensor and EC array on the probe.

String Potentiometer and Depth Tracking
A string potentiometer (MN 214227) is weighted with a steel plate and placed on the ground surface next to the mast of the direct push machine. The string is attached to the mast of the probe unit so that it accurately tracks movement of the mast and HPT probe, as the probe is advanced into the ground. A cable attaches the string potentiometer to the FI for power supply and data transmission. The length of the rods used for tool advancement (e.g., 4, 5 feet, or 1 m) is entered into the system software so that tracking of the mast position and top-of-rod is done automatically after initial placement of the probe with injection port at ground surface. The string potentiometer allows for tracking depth and plotting of down hole signal at the correct depth in 0.05 foot (~15 mm) increments.

Field Instrument, Computer and Software
The Model FI6000 field instrument (FI) is the analog to digital interface for the HPT system ( Figure 3). The FI provides conditioned alternating current to power the EC array. Simultaneously, the FI receives analog data from the HPT sensor, line pressure sensor, flowmeter, EC array and string potentiometer. The FI converts the analog data to digital format for output to the field computer equipped with acquisition software. Typically, the electrical conductivity, HPT pressure and HPT flow rate are plotted onscreen versus depth as the probe is advanced. The HPT line pressure also may be plotted onscreen as a QC check on the performance of the downhole HPT transducer. The speed of probe advancement also may be plotted onscreen to keep rate of advancement near the desired 2 cm/s (~4 feet/min) rate. Experience has shown that a faster rate of advancement can result in low biased HPT pressures and so high biased estimated hydraulic conductivity (Est. K) values. Having the log data plotted onscreen as logging progresses allows for rapid assessment of the data and may be used to guide the selection of depths where pressure dissipation tests are run.
For HPT logging a modest laptop computer with 4 MB of ram running on Windows 7 to Windows 10 operating system is sufficient (computer and operating system requirements may change over time). Both the Acquisition and Viewing software are installed for data acquisition and viewing logs in the field. The viewing software allows for plotting of the log data versus depth and postacquisition processing operations on the acquired data. Some post acquisition processing operations will be reviewed below.

HPT Flow Module
The model K6003 HPT flow module ( Figure 3) contains a metering pump providing water flow rates between approximately 50 and 500 mL/min at pressures up to 120 psi (~830 kPa). The pump provides for down hole injection through the screened port into the formation via the supply tube in the trunkline. A bypass valve in the flow module set at 120 psi (830 kPa) prevents damage to the pump and downhole transducer. An inline flowmeter monitors flow rate and flow rate data are sent to the FI and computer software. The flowmeter is not designed to monitor flows below 100 mL/ min. A shut-off valve on the front of the controller enables the operator to stop water flow during pressure dissipation tests and other operations. A pressure gauge on the front of the controller gives read-out of the active line pressure while an in-line pressure transducer (250 psi/~1700 kPa) provides line pressure data to the FI and computer. Line pressure data can be a useful quality assurance (QA) check on system and downhole HPT pressure transducer performance. A water supply line and a bypass flow line connect to the back of the HPT controller by bulkhead fittings. Both lines are placed in a clean water supply during operation. The project manager should specify the quality of water required to meet project objectives (e.g., tap water, deionized water, VOC or PFAS free water, etc.). Typical injection flow rates range between 200 and 300 mL/min, this means about a cup of water is injected for every foot (~30 cm) of log depth. An HPT log to 60 feet (~20 m) depth usually requires about 5 gal (20 L) of clean water. This includes maintaining flow as the probe is retracted to prevent clogging of the screen.
The HPT flow module is now being built without Teflon or Teflon containing components. This has been requested by investigators working at sites where polyfluorinated alkyl substances (PFAS) may be of concern. Older HPT flow modules may still contain Teflon materials/components. It is recommended that a rinsate sample be collected through each HPT flow module/system prior to use during a PFAS site investigation.

Probing Tools and Machine
The HPT probe is typically advanced with 1.75″ diameter drive rods that are 4 feet (MN 220746) or 5 feet (MN 220032) in length. One-meter length rods (MN 230326) also are available. A slotted drive cap (MN 220125) is used to advance the tools with the trunkline in place. Either a slotted pull cap (MN 221276) or a rod grip tool (MN 220912) is used to retract the tools once logging is completed. A simple rubber rod wiper (MN 600341) and weldment (MN 204387) may be used to remove soil and mud from the tools as they are recovered. This significantly reduces clean up, decontamination effort, waste handling and potential worker exposure at contaminated facilities. Several direct push machines are available to advance the HPT probe to depth. One of the most commonly used is the model 7822 Geoprobe ® equipped with a model GH60 series hydraulic hammer for percussion advancement of the tools. The HPT may also be advanced by push-only methods with a CPT machine, or similar.

Field Procedures System Set Up
Field procedures for HPT logging are outlined in the manufacturer's standard operating procedure (Geoprobe 2006b) and reviewed in ASTM Practice D8037 for Direct Push Injection Logging. Some of the primary steps in the field process are outlined here.
The trunkline is prestrung through the number of drive rods needed to achieve the desired depth and the HPT probe is installed (Figure 2). Next the lap top computer, model FI6000 Field Instrument and model K6300 HPT flow module are prepared for logging ( Figure 3). Once the system is assembled the HPT pump is used to purge the trunkline and HPT probe with clean water to remove all air from the plumbing system. Then the EC QA test is conducted, and the HPT transducer QA test is performed using the HPT reference tube (MN 212689). The pre-and post-log QA test sequence usually requires about 5 min to complete. All QA test data are saved to the log file for later review and verification of probe and instrument performance ( Figure S2). An important point to note here is that the atmospheric pressure, as observed by the HPT pressure sensor at the time the log was run, is determined during the pre-and postlog reference tests. This information will be critical for several post-acquisition data functions.

Logging
Once the QA tests are completed the HPT probe, with flow rate set, is placed under the machine mast and the probe is plumbed and advanced until the HPT screen bisects the ground surface. The HPT probe is advanced into the formation ( Figure S3) with the hydraulic press and hammer (as needed) at a rate of approximately 2 cm/s (4 feet/min). The EC log, HPT pressure, flow rate and line pressure are viewed onscreen as the log is run. The probe rate of penetration (ROP/ speed) also may be plotted onscreen. If the probe is advanced too rapidly the observed injection pressure may be biased low due to poor coupling with the formation. This can have a detrimental impact on the data quality and log interpretation. Additionally, avoid unnecessary movement of the trunkline while logging as this may cause pressure anomalies.

Pressure Dissipation Tests
Pressure dissipation tests (Figure 4) may be run at various depths as the HPT log is acquired. It is most advantageous to perform dissipation tests in saturated coarse-grained media, as the pressure will dissipate quickly to the ambient piezometric pressure. Dissipation tests performed in fine-grained zones may require hours for full dissipation of the pressure. As such it is usually not advantageous to conduct dissipation tests in fine-grained materials. Due to the water column in the trunkline, HPT dissipation tests may not be modeled like CPT dissipation tests to obtain a point estimate of hydraulic conductivity (see Est K below). Conditions permitting, one or more pressure dissipation tests should be performed during each log. If permeable zones are separated by low permeability layers (aquitards) it may be wise to perform a separate dissipation test in each isolated permeable zone to assess the potential for vertical gradients or hydraulically isolated and/or perched water zones.
To perform a pressure dissipation test the operator halts probe advancement at the desired depth. Then a dissipation test time-file is started in the software. The HPT injection flow is turned off. Allow the pressure to drop and then stabilize for 2 or 3 min before restarting the HPT injection flow and ending the test. Dissipation test data is saved at depth in the log file for later review and use in plotting the potentiometric profile and other parameters. Once the dissipation test file is ended the operator may continue with logging operations.

Probe Retraction
Once at the desired depth or probe refusal, the HPT probe and tool string are incrementally retracted to the surface using the rod grip system or slotted pull cap. The HPT injection flow is continued during retraction to keep the screen clean. It is best to retract the rods at a moderate speed to prevent over pressuring and intrusion of clay into the screen and probe plumbing. When the probe is recovered it is cleaned and run through the post log QA tests to assure that the probe and system are operating properly, and all log data is valid. For contaminated sites appropriate decontamination procedures should be used (ASTM D5088 2019). Following the post log QA test the operator mobilizes the HPT equipment and probe unit to the next log location and the process is repeated. A 60 foot (~20 m) log and tripping the tool out can often be completed in 1.5 to 2 h with a 2-person team. Grouting the boring will require additional time and often is completed by a second team of workers with dedicated grouting tools and equipment. This increases the efficiency of the logging process when multiple logs are required at a site.

Postacquisition Log Review, Analysis, and Interpretation
While many professionals in the geo-environmental industry are utilizing HPT logs, they often depend on field technicians to provide printed or pdf copies of the logs. As such they are depending on the field technicians to make significant decisions about log processing and interpretation. The viewing software allows the data user to perform several post acquisition operations (select dissipation pressures, plot piezometric profiles, plot corrected HPT pressure, etc.) as well as presentation options. The end user can also review the log QA test data with the viewing software to verify the log data is valid and acceptable. Some of the software post processing features for HPT logs are reviewed here.

Log Viewing Software and the HPT Log
The HPT logs are saved as .zip format files. The log viewing software is available for download at no charge (https:// geoprobe.com/direct-image-viewer). There are some online resources that provide an introduction to HPT logs and log review (https://geoprobe.com/videos/di-viewer-reviewingoihpt-log, https://youtu.be/UMVQdeXflP8?t=43), technical bulletins that provide guidance on processing and interpretation of HPT log data (Geoprobe 2010(Geoprobe , 2011a(Geoprobe , 2011b and related publications (McCall et al. 2014(McCall et al. , 2017.
There are a few basic rules for EC log and HPT pressure log interpretation. These are: HPT pressure: Increasing pressure indicates decreasing permeability, and vice versa. (Dilatant sands may cause pressure anomalies.) EC Log: In freshwater formations, increasing EC often indicates increased clay content and decreased permeability, and vice versa. However, it is important to note: 1. not all clays or fine-grained materials may exhibit elevated EC 2. brines/ionic contaminants/ionic remediation injectates may impart an elevated EC to clean sands/gravels which are typically low EC materials.
In fresh-water environments EC logs may provide a good indication of lithology and to a lesser degree permeability (Schulmeister et al. 2004). However, EC logs alone are a bit more complicated to interpret, but when used with tandem HPT pressure logs the EC data can provide some valuable insight into lithology and water quality even at sites where brine impacts are present (McCall et al. 2017).
An HPT log obtained in the Smoky Hill Alluvial aquifer from central Kansas provides a useful example ( Figure 5). Higher EC and HPT pressure in the upper 35 feet of this formation indicate the presence of generally finer-grained, lower permeability materials. However, lower HPT pressure and EC readings over approximately 20 to 24 feet below grade indicate a sandy, more permeable zone in this interval. Continuous soil cores were obtained to a depth of about 50 feet adjacent to this log location using the MC5 sampler (Geoprobe 2011a, 2011b; ASTM D6282 2018b). Photographs of core segments at the 15 foot interval and 23 foot interval reveal a hard, silty-clay and silty fine-sand respectively ( Figure 6A,B), confirming interpretation of the log. Below 35 feet the EC log is consistently lower suggesting the presence of coarser grained materials with higher permeability. The HPT pressure is generally lower below 35 feet but shows more variability than the EC log, and a "baseline" rise in pressure as the probe is advanced below the piezometric surface. A photograph of segments of three cores collected between 34 and 49 feet reveal this interval is primarily a mixture of sand, gravel and silt ( Figure 6C). While increases and decreases in EC and HPT pressure often correlate, they do not always do so. For example, over the 45 to 50 foot interval the EC is low, conversely the HPT pressure increases. This may be due to increased silt con-   An important part of log quality control is to conduct soil/sediment sampling as done here. Once the site soils and unconsolidated sediments are correlated to the log the investigator can proceed with logging across the site. When significant changes are observed in the log data collect samples, targeting the intervals of interest. Use sampling only as needed, let the logs work for you. Sometimes a groundwater sample may be needed to evaluate the specific conductance of the groundwater over a discrete interval. This may be done with direct push groundwater samplers (Geoprobe 2006a; ASTM D6001 2018a) over discrete intervals in 1 to 2 hours.

Pressure Dissipation Tests, Potentiometric Profile, and Water Level
At least one pressure dissipation test should be run with each log when formation conditions permit. The dissipation tests allow the operator to determine the ambient hydrostatic pressure in the formation at the selected depth. Not all dissipation tests may reach the ambient piezometric pressure, so each test should be evaluated. Two dissipation tests from depths of 34.5 and 39.5 feet (obtained as the log presented in Figures 5 and 7 was run) are useful examples (Figure 4). The pressure in the dissipation test run at a depth of 34.5 feet in finer grained materials did not stabilize after 20 min of dissipation. Even a late-time pressure selected from this test would result in an incorrect piezometric profile and determination of an incorrect water level (above-grade). Conversely the dissipation test run at a depth of 39.5 feet in sandy materials stabilized at the local piezometric pressure in only a few minutes. This stabilized dissipation pressure is plotted on the log at the depth of measurement (Figure 7, red triangle). Back calculation from this depth with the stabilized dissipation pressure to atmospheric pressure indicates a static water level (piezometric surface) of approximately 21.5 feet below grade (red circle). This agreed with the water level in a nearby well. Three additional dissipation tests (red triangles) were run during this log at increasing depths. These also are plotted at corresponding depths on the log and help to define the piezometric profile at this location (inclined red line). The vertical red line above the piezometric surface indicates the atmospheric pressure as observed by the pre-log HPT reference test.

HPT Corrected Pressure, Estimation of Hydraulic Conductivity and K Limits
As noted above, the HPT port is positioned 16-in. (406 mm) above the tip of the probe to avoid pressure anomalies induced near the tip of a conical probe during advancement. If we assume that excess pressure due to probe insertion has dissipated by the time the port passes a given depth then the total HPT pressure (P tot ) measured as the log is run includes the atmospheric pressure (P atm ) and the piezometric pressure (P piezo ) as well as the pressure required to inject water into the formation matrix (Figure 7). So, we can calculate the corrected HPT pressure (P c ) at each depth increment in the log with this simple equation: Once the piezometric profile is defined the viewing software can quickly calculate the corrected HPT pressure log based on this simple formula. The P c log provides us with a clear view of the relative permeability of the formation versus depth at the inch-scale, even below the piezometric surface ( Figure 7B).
From Darcy's Law we know that the hydraulic conductivity (K) is a function of the flow rate (Q) divided by the pressure (P) generated as the water flows through the granular material of the formation.

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The HPT log data provides us with both the flow rate (Q) and the pressure (P c ) required to inject the water into the formation matrix at the given flow rate. Using this relationship, an empirical model was developed to estimate the hydraulic conductivity of the formation. This model is based on several co-located HPT logs and discrete interval slug tests in an alluvial aquifer (McCall and Christy 2010;Geoprobe 2010). An empirical model was used because the HPT log is a dynamic process with the probe moving as the water is injected and the pressure measured. The slug tests were performed using an SP16 groundwater sampler (Geoprobe 2006a; ASTM D6001 2018a) and pneumatic slug testing methods ASTM D7242 2006a;Geoprobe 2014). A screen interval of 1 foot (~30 cm) was used for most of the co-located slug tests. The HPT pressure and flow data were averaged from the co-located logs for the interval screened for each slug test (e.g., 33 to 34 feet). A semilog plot of the paired data ( Figure 8) reveals an exponential relationship between the K measured by slug testing and the HPT Q/P c ratio with a correlation coefficient (R 2 ) of about 0.83. Paired HPT Q/P c and slug test data from five other sites across the central US are plotted over the model curve. All sites were unconsolidated, granular aquifers. In some instances, the HPT logs and slug test data at the other  Figure 8. Semilog plot of the co-located HPT log data and discrete interval slug test data (blue diamonds) used to define the estimated hydraulic conductivity (Est. K) model for the HPT system. Paired HPT and slug test data from five sites across the central US are also displayed. At some locations slug tests were run several meters from the paired HPT log. This may have increased scatter in some of the data points. P* = average corrected HPT pressure and Q = average HPT flow rate over the screen interval of the co-located DP sampler or well. Wichita, KS = low permeability stream margin deposits with SP16 sampler: Clarks, NE = Platte River alluvium with ¾″ prepack screen DP wells: Monona, WI = glacial outwash with SP16 sampler: 5th St, Salina, KS = Smoky Hill alluvium with SP16 sampler: Halstead, KS = Arkansas River alluvium, with SP16 Sampler. Note: Some of the data points from the five field sites are near or above the upper K limit of the model. five sites were obtained several meters apart, leading to some scatter in the data. Regardless, the data from the other sites follows the trend of the model curve.
As the plot suggests the Est. K model has limits at both higher and lower K-values. At the higher end, the model is limited due to several factors. The primary factor is the limit of sensitivity at the low end for the 100 psi range pressure sensor. Frictional losses in the plumbing below the sensor also may impact the upper K limit. At the low-pressure/ high K end of the model very small changes in pressure (P c ) begin to yield very large changes in the calculated K-value. To be conservative we recommend using an upper bound for the Est. K model of 75 ft/d. Therefore, if an Est K value near or above 75 ft/d is given with the model other means (discrete interval slug tests, etc.) should be used to verify the K reported. The actual K could potentially be much higher than the reported Est. K. Conversely, on the low-K end of the model a lower boundary of 0.1 ft/d is recommended. We have limited slug test data below this level and observation suggests that flow bypass along the rods or formation fracturing may begin to occur near this lower boundary (loss of Darcian flow) at typical injection flow rates of 200 mL/min. Indeed, recent work (Liu et al. 2018) at reduced flow rates has shown when K is below approximately 0.1 ft/d excess pressure due to probe insertion may not be fully dissipated at the port. Thus, K estimates below this level with this simple empirical model would not be accurate. A more rigorous analytical model for estimation of K with HPT flow and pressure data is under development (Borden et al. 2020 in review). While the Est. K model presented here is limited to a relatively small range of permeability (3+ orders of magnitude) it provides an effective way to distinguish between low K and high K zones in a formation to support planning for further investigation and remediation efforts at many sites.

Archie's Law and Estimation of Groundwater Specific Conductance
Recent work (McCall et al. 2017) has demonstrated that direct push EC logs follow Archie's Law (Archie 1942(Archie , 1950 in saturated, clean, coarse grained formations. Therefore, when Pc is low and flat in an HPT log the corresponding EC log can be used to estimate the groundwater specific conductance with the following equation: This model has been added to the log viewing software so that under appropriate conditions (low P c ) a log of estimated groundwater specific conductance can be calculated and plotted with the viewing software. This may be useful in assessing road salt or brine impacts, tracking landfill leachate plumes or assessing seawater intrusion along shallow coastal aquifers. Further work is recommended to verify this model. Alternately, site-specific models relating bulk formation EC and groundwater specific conductance could be developed.

Log Cross Sections, Hydrogeologic Interpretation, and Conceptual Site Models
The log viewing software enables the project manager to construct simplified cross sections (Figure 9). While the logs may be plotted at elevation, adjustment of horizontal spacing is not possible in the log viewing software. There are several third party two-dimensional (2D) and three-dimensional (3D) software programs that provide more advanced 2D and 3D plotting and viewing capabilities of HPT and EC logs (www.rockware.com/, www. mpassociates.gr/software/environment/evs.html, etc.). The cross section of HPT pressure logs reveals the variability and complexity of vertical and lateral changes in such an alluvial formation. The upper 20 feet consists primarily of higher pressure/lower permeability fine-grained materials. There is a lower pressure/higher permeability zone over the 20 to 25 foot zone that is pinching out from west to east across the section. Below approximately 25 to 26 feet there is another zone of higher pressure/lower permeability that extends down to 40 to 45 feet across the section. Saturated, sandy materials dominate the formation from a depth of about 42 to 60 feet below grade. Shale bedrock is encountered below 60 to 65 feet where refusal occurs. In log BWHP02 there are abundant clay lenses between about 47 and 55 feet as evidenced by the elevated HPT pressure spikes over this interval. Conversely, in log BWHP03 the HPT pressure is generally low with minimal pressure peaks (clay lenses) between about 42 and 61 feet below grade. This interval at this location would be an optimal location for a water supply well.
Obtaining this level of detail from conventional drilling and soil coring is difficult and time consuming. These five HPT logs were obtained in 1 day. To conduct continuous split spoon sampling by hollow stem augers to 60 feet through saturated sands often requires 4 to 8 h for a single boring. Sample recoveries can often be less than 50% in the saturated sands in these formations. It is apparent that the HPT logs can provide an increase in efficiency compared to conventional drilling/sampling methods. The HPT logs also minimize the variability often associated with visual/ manual interpretation of core samples (e.g., was it a siltysand with clay or a sandy-clayey-silt?).

Combining HPT with Contaminant Logging Tools
For many geo-environmental projects soil and/or groundwater contamination can be a driving force behind the investigation. When volatile organic contaminants (VOCs) such as trichloroethylene (TCE), perchloroethylene (PCE) or carbon tetrachloride are present in unconsolidated materials the Membrane Interface Probe (MIP) is often used to define the vertical and lateral extent of the source area and dissolved groundwater plume (EPA 2004;Christy 1996;ASTM D7352). By including the EC and HPT sensors on the MIP probe the investigator also obtains information about the local geology and its possible control on contaminant migration (McCall et al. 2014). At a site dominated by fine-grained materials the combined MIP and HPT probe (MIHPT) was used to run a transect of logs across the plume down gradient from a former fire training area. The EC and HPT pressure in one of the logs (Figure 10) identified the lowest HPT pressure zone observed along the transect of MIHPT logs. This occurred at a depth of 34 to 36 feet (~10.4 to 10.9 m) just above bedrock. Additionally, high responses were observed with the photoionization detector (PID) and halogen specific detector (XSD) over this same depth interval. A Screen Point16 groundwater sampler (ASTM D6001; Geoprobe 2006a) was installed and screened over the 34 to 36 feet interval adjacent to this location. After development, the SP16 was sampled with a small diameter bladder pump (EPA 2003;McCall 2005). Analysis of the sample indicated the presence of TCE (19,500 μg/L), carbon tetrachloride (48,700 μg/L) and chloroform (3200 μg/L) for a total VOC concentration of 71,400 μg/L in this zone. Pneumatic slug testing (ASTM D7242, 2006; Geoprobe 2014) over this interval found the hydraulic conductivity to be about 35.5 ft/d (1.25 × 10E−2 cm/s), close to the HPT Est. K value determined for this zone. The combined MIP detector data and HPT pressure data clearly and rapidly identified the primary migration pathway for the VOC contaminants at this site. MIHPT logs may also help identify contaminated finegrained zones that can behave as secondary sources by back diffusion (Parker et al. 2008).
An HPT sensor and EC array also have been combined with the Optical Imaging Profiler (OIP) and laser induced fluorescence (LIF) tools to assist with investigations for nonaqueous phase liquids (NAPL) (ITRC 2019). When combined with the OIP, images may be captured with visible light as logging proceeds to provide photographic documentation of lithology ( Figure S4). The EC and HPT sensors also have been included on groundwater profiling tools to guide selection of sampling locations/depths and understanding of geologic influence on contaminant migration (McCall et al. 2017).

Summary and Conclusions
The HPT probe has become a widely used direct push logging system for the investigation of soils and unconsolidated formations. The small diameter probe (1.75-in./44.5 mm OD) is advanced with percussion probing machines or CPT machines to depths exceeding 20 m to 30 m in amenable formations. Refusal can occur in dense tills, thick caliche zones, on large cobbles or boulders and at bedrock. A 20 m depth log can often be completed in about 1.5 h. Injection of clean water through a small screened port on the side of the probe is used to provide a log of injection pressure versus depth at the inch-scale. The pressure log provides an indication of relative formation permeability versus depth. The operator can identify low permeability (elevated pres-sure) and high permeability (low pressure) zones that can behave as aquitards or transmissive zones, respectively. The HPT probe includes an electrical conductivity (EC) array which simultaneously provides a log of bulk formation EC at the inch-scale. Several factors influence the bulk formation EC including mineralogy, the presence of brines or other ionic contaminants or remediation injectates. However, by evaluating both the HPT pressure and EC logs together the investigator can obtain useful information about the formation lithology and potentially the quality of contained fluids or distribution of ionic remediation injectates. Targeted sampling of the formation under investigation should be conducted to ground truth log interpretation at each site (not each location). Single HPT-EC logs assist in defining the vertical lithology and permeability at one location to guide sampling, well placement or remediation activity (fluid injection, etc.). Multiple logs in a cross section or 3D grid can help to define site-wide hydrostratigraphy at high resolution in a time and cost-effective manner previously not available. Third party software packages are available to create 2D and 3D visualizations of the HPT pressure and EC data.
Pressure dissipation tests may be conducted below the piezometric surface by halting probe advancement and turning off the injection flow. These tests are usually performed in transmissive zones to quickly define the piezometric pressure at selected depths. Back calculation with the viewing software can define the piezometric profile and piezometric surface. Pressure dissipation tests conducted at multiple depths may help identify vertical gradients or perched water zones. When the piezometric profile is defined the total HPT pressure below the piezometric surface can be corrected (Pc) to more clearly define the relative formation permeability. Additionally, the corrected HPT pressure and the HPT injection flow rate at each depth below the piezometric surface may be used to estimate the hydraulic conductivity (Est K) of the local formation with a simple empirical model. The Est K model is effective over a modest but useful range of hydraulic conductivity (~0.1 to 75 ft/d).
When quantitative values of K are required, especially outside of this range, other techniques should be used, such as discrete interval slug testing with DP piezometers. Slug testing in long screened wells (e.g., 5 to 20 feet screen intervals) or pumping tests cannot provide a useful comparison to the high-resolution HPT data. This is due to gross averaging of significant variations in K over large volumes of the formation by these methods.
When combined with contaminant logging tools (e.g., MIP, OIP, LIF) at contaminated facilities the HPT pressure logs can help define high permeability contaminant migration pathways and low permeability layers that may impede contaminant migration. Alternately, sharp increases in HPT pressure may identify formation contacts where DNAPLs will pool. Later, the contaminated low permeability layers may behave as secondary sources where contaminants back diffuse into the adjoining transmissive zones. Additionally, the HPT-EC logging system should prove to be a valuable tool for the investigation and mapping of many Quaternary Age deposits. Whether combined with contaminant logging tools to investigate contaminated facilities or used alone to assess lithology and hydrostratigraphy the HPT probe and system can be a powerful tool for investigation of unconsolidated formations.