Significance and Use
5.1 Direct-push groundwater sampling and profiling are economical methods for obtaining discrete interval groundwater quality samples in many soils and unconsolidated formations without the expense of permanent monitoring well installation (1-10).4 Many of these devices can be used to profile groundwater quality or contamination and/or hydraulic conductivity with depth by performing repetitive sampling and testing events. DP groundwater sampling is often used in expedited site characterization (Practice D6235) and as a means to accomplish high resolution site characterization (HRSC) (11, 12). The formation to be sampled should be sufficiently permeable to allow filling of the sampler in a relatively short time. The zone to be sampled and/or slug tested can be isolated by matching sampler screen length to obtain discrete samples of thin saturated, permeable layers. Use of these sampling and hydraulic testing techniques will result in more detailed characterization of sites containing multiple aquifers. The field conditions, sampler design and data quality objectives should be reviewed to determine if development (Guide D5521/D5521M) of the screened formation is appropriate. The samplers do not have a filter pack designed to retain fines like conventional wells, but only a slotted screen or wire-mesh covered ports. So, obtaining low turbidity samples may be difficult or even impossible in formations with a significant proportion of fine-grained materials. With most systems turbidity will always be high so consult Guide D6564/D6564M if field filtration of samples is required. Discrete water sampling, combined with knowledge of location and thickness of target aquifers, may better define conditions in thin multiple aquifers than monitoring wells with long screened intervals that can intersect and allow for intercommunication of multiple aquifers (4, 6, 11-15). DP sampling performed without knowledge of the location and thickness of target aquifers can result in sampling of the wrong aquifer or penetration through confining beds. Results from DP explorations can be used to develop conceptual site models, guide placement of permanent groundwater monitoring wells, and direct remediation efforts. These devices are often used under dynamic work plans (11, 16) to complete site characterizations in a single mobilization. However, multiple sampling events can be performed to depict conditions over time or refine earlier work if needed.
5.2 Targeting Aquifer Sample Test Zones for Accurate Sampling—As with any investigation it is important to phase the investigation such that target intervals for groundwater sampling are accurately located. For sites that allow surface push of the sampling device, discrete water sampling is often performed in conjunction with the cone penetration test (Test Method D6067/D6067M) (4-6, 13, 14) or continuous soil sampling (Guide D6282/D6282M) which is often used for stratigraphic mapping of aquifers and to delineate high-permeability zones for sampling. Alternately, resistivity logging, or injection logging (Practice D8037/D8037M) may be used to assess formation permeability and lithology prior to the groundwater sampling or profiling activities to guide selection of sampling intervals (10, 15, 17). In such cases, DP water sampling is normally performed close to previous test holes. In complex depositional environments (12), thin aquifers may vary in continuity such that water sampling devices may not intersect the same layer at equivalent depths as companion HPT, cone penetrometer, or electrical resistivity profiling soundings.
5.2.1 When volatile organic contaminants (VOC) such as trichloroethylene (TCE) or benzene are present in the subsurface, logging with the membrane interface probe (MIP) (Practice D7352) may be performed prior to groundwater sampling. MIP logs identify where significant concentrations of many VOCs are present and may be used to guide selection of groundwater sampling locations, depths and intervals (17). When petroleum fuels are present in the subsurface laser induced fluorescence (LIF) (Practice D6187) or the Optical Imaging Profiler (OIP) (18) may be used to identify where significant petroleum contamination is present to assist in guiding selection of sample locations and depths.
5.3 Slug tests can be performed with several of the DP groundwater samplers (D7242/D7242M) to determine hydraulic conductivity over discrete intervals. Development of the screened interval should be conducted to assure that formation flow into and out of the device is representative of natural formation conditions. Development with a simple inertial pump to surge and purge the formation is often adequate. Other methods for development (D5521/D5521M) may be advised depending on field conditions and data quality objectives.
5.4 Water sampling chambers may be sealed to maintain in situ pressures and to allow for pressure measurements and permeability testing (Practice D7242/D7242M) (6, 13, 19). Sealing of samples under pressure may reduce the possible volatilization of some organic compounds. Field comparisons may be used to evaluate any systematic errors in sampling equipments and methods. Comparison studies may include the need for pressurizing samples, or the use of vacuum to extract fluids more rapidly from low hydraulic conductivity soils (8.2.3.1(2)).
5.5 DP groundwater profiling tools (7, 8, 10, 20, 21) allow the investigator to sample groundwater at multiple depths during incremental advancement of the device. Clean water is injected through the screen(s) or port(s) of these tools to keep the screens open and rinsed as advancement proceeds. Concerns for cross contamination and contaminant drag down must be considered. Some tools have an inline pressure transducer either above grade or down hole to monitor pressure required to inject water into the formation during advancement. The pressure injection log may be used to guide selection of permeable zones for sampling. When the injection flow rate is also measured, estimates of formation permeability may be calculated.
5.6 Degradation of water samples during handling and transport can be reduced if discrete water sampling events with sealed screen samplers are combined with real time field analysis of potential contaminants. In limited studies, researchers have found that the combination of discrete sealed screen sampling with onsite field analytical testing provide accurate data of aquifer water quality conditions at the time of testing (4, 6). DP water sampling with exposed screen sampling devices, which may require development or purging, are considered as screening tools depending on precautions that are taken during testing.
5.7 In difficult driving conditions, penetrating to the desired depth to make sure of sealing of the sampler screen may not be possible. If the screen cannot be inserted into the formation with an adequate seal, the water-sampling event would require sealing in accordance with Practice D5092/D5092M to isolate the aquifer. Selection of the appropriate equipment and methods to reach required depth at the site of concern should be made in consultation with experienced operators or manufacturers. If there is no information as to the subsurface conditions, initial explorations consisting of penetration-resistance tests, such as Test Method D6067/D6067M, resistivity profiling, or DP logging with the injection logging system (Practice D8037/D8037M) to perform trials can be performed to select the appropriate testing system.
5.7.1 Typical penetration depths for a specific equipment configuration depend on many variables. Some of the variables are the driving system, the diameter of the sampler and riser pipes, and the resistance of the materials.
5.7.2 Certain subsurface conditions may prevent sampler insertion. Penetration is not possible in hard rock and sometimes not possible in softer rocks such as claystones and shales. Coarse particles such as gravels, cobbles, and boulders may be difficult to penetrate or cause damage to the sampler or riser pipes. Cemented soil zones may be difficult to penetrate depending on the strength and thickness of the layers. If layers are present that prevent DP from the surface, then rotary or percussion drilling methods (Guide D6286/D6286M) can be employed to advance a boring through impeding layers to reach testing zones.
5.7.3 Driving systems are generally selected based on testing depths and the materials to be penetrated. For systems using primarily static reaction force to insert the sampler, depth will be limited by the reaction weight of the equipment or anchoring stability and penetration resistance of the material. The ability to pull back the rod string is also a consideration. Impact or percussion soil probing has an advantage of reducing the reaction weight required for penetration. Penetration capability in clays may be increased by reducing rod friction by enlarging tips or friction reducers. However, over reaming of the hole may increase the possibility of rod buckling and may allow for communication of differing groundwater tables. Hand-held equipment is generally used on very shallow explorations, typically less than 5 m [15 ft] depth, but depths on the order of 10 m [30 ft] have been reached in very soft lacustrine clays. Intermediate size driving systems, such as small truck-mounted hydraulic-powered push and impact drivers, typically work within depth ranges from 5 to 30 m [20 to 100 ft]. Larger DP machines may be capable of reaching 60 m [200 ft] depending on subsurface conditions. Heavy static-push cone penetrometer vehicles, such as 20 ton trucks, typically work within depth ranges from 15 to 45 m [50 to 150 ft], and also reach depth ranges on the order of 100 m [300 ft] in soft ground conditions. Guide D6286/D6286M shows depth ranges of other drilling equipment to attain greater depths.
Note 1: Users and manufacturers cannot agree on depth ranges for different soil types. Users should consult with experienced local producers and manufacturers to determine depth capability for their specific site conditions.
5.8 Combining multiple-sampling events in a single-sample chamber (profiling) without decontamination (Practice D5088) is generally discouraged. In this application, purging of the screen or sampling chamber should be performed to make sure of isolation of the sampling event. Purging should be performed by removing several volumes of fluid until new chemical properties have been stabilized or elements are flushed with fluid of known chemistry. Purging requirements may depend upon the materials used in the sampler and the sampler design (Guide D6634/D6634M). Rinsate samples may be collected and analyzed to assess concerns with carryover of contaminants from overlying zones that are heavily contaminated. Monitoring water quality parameters (pH, specific conductance, dissolved oxygen, oxidation-reduction potential, etc.) to stability is often used to document when representative water is being purged from a sampling interval (Practice D6771).
5.9 Bottom-up profiling by driving a DP groundwater sampler to the base of the formation and retracting incrementally, while the screen is exposed, for sampling at decreasing depths should be avoided as this may lead to cross contamination and inaccurate contaminant distribution information. Slug tests should not be performed by bottom-up profiling as there is poor or no control on the length of formation being tested under these conditions.
5.10 Screens designed and deployed in dual tube use are generally designed for use inside the dual tubing and overdriving the screen past the casing can damage the sampler screen and subsequent exposed screen samples would be subject to cross contamination. Use equipment according to manufactures instructions.
Note 2: The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Practitioners that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.
Practice D3740 was developed for agencies engaged in the testing and/or inspection of soils and rock. As such, it is not totally applicable to agencies performing this field practice. However, users of this practice should recognize that the framework of Practice D3740 is appropriate for evaluating the quality of an agency performing this practice. Currently there is no known qualifying national authority that inspects agencies that perform this practice.
Scope
1.1 This guide covers a review of methods for sampling groundwater at discrete points or in increments by insertion of groundwater sampling devices using Direct Push Methods (D6286/D6286M, see 3.3.2). By directly pushing the sampler, the soil is displaced and helps to form an annular seal above the sampling zone. Direct-push water sampling can be one time, or multiple sampling events. Knowledge of site specific geology and hydrogeologic conditions is necessary to successfully obtain groundwater samples with these devices.
1.2 Direct-push methods of water sampling are used for groundwater quality and geohydrologic studies. Water quality and permeability may vary at different depths below the surface depending on geohydrologic conditions. Incremental sampling or sampling at discrete depths is used to determine the distribution of contaminants and to more completely characterize geohydrologic environments. These explorations are frequently advised in characterization of hazardous and toxic waste sites and for geohydrologic studies.
1.3 This guide covers several types of groundwater samplers; sealed screen samplers, profiling samplers, dual tube sampling systems, and simple exposed screen samplers. In general, sealed screen samplers driven to discrete depth provide the highest quality water samples. Profiling samplers using an exposed screen(s) which are purged between sampling events allow for more rapid sample collection at multiple depths. Simple exposed screen samplers driven to a test zone with no purging prior to sampling may result in more questionable water quality if exposed to upper contaminated zones, and in that case, would be considered screening devices.
1.4 Methods for obtaining groundwater samples for water quality analysis and detection of contaminants are presented. These methods include use of related standards such as; selection of purging and sampling devices (Guide D6452 and D6634/D6634M), sampling methods (Guide D4448 and D6771) and sampling preparation and handling (Guides D5903, D6089, D6517, D6564/D6564M, and D6911).
1.5 When appropriately installed and developed many of these devices may be used to perform pneumatic slug testing (Practice D7242/D7242M) to quantitatively evaluate formation hydraulic conductivity over discrete intervals of unconsolidated formations. These slug tests provide reliable determinations of hydraulic conductivity and can be performed after water quality sampling is completed.
1.6 Direct-push water sampling is limited to unconsolidated formations that can be penetrated with available equipment. In strong soils damage may result during insertion of the sampler from rod bending or assembly buckling. Penetration may be limited, or damage to samplers or rods can occur in certain ground conditions, some of which are discussed in 5.7. Drilling equipment such as sonic drilling (Practice D6914/D6914M) or rotary drilling (Guide D6286/D6286M) can be used to advance holes past formations difficult to penetrate using typical Direct Push equipment. Some soil formations do not yield water in a timely fashion for direct-push sampling. In the case of unyielding formations, direct-push soil sampling can be performed (Guide D6282/D6282M).
1.7 Direct push water sampling with one-time sealed screen samplers can also be performed using cone penetrometer equipment (Guide D6067/D6067M).
1.8 This guide does not address installation of permanent water sampling systems such as those presented in Practice D5092/D5092M. Direct-push monitoring wells for long term monitoring are addressed in Guide D6724/D6724M and Practice D6725/D6725M.
1.9 Units—The values stated in either SI units or inch-pound units [presented in brackets] are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with the standard. Reporting of test results in units other than SI shall not be regarded as nonconformance with this standard.
1.10 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026, unless superseded by this standard.
1.11 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.12 This guide offers an organized collection of information or a series of options and does not recommend a specific course of action. This document cannot replace education or experience and should be used in conjunction with professional judgment. Not all aspects of this guide may be applicable in all circumstances. This ASTM standard is not intended to represent or replace the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied without consideration of a project's many unique aspects. The word “Standard” in the title of this document means only that the document has been approved through the ASTM consensus process.
1.13 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
