Title:
Introduction
Text: A piezocone has traditionally been used as a direct push sensor probe to measure water levels. The instrument consists of a porous element connected to a transducer that converts pore water pressures to water levels. A high resolution piezocone is an advancement over the conventional piezocone. It is a sensor probe capable of generating highly resolved hydraulic head values (i.e., plus or minus 1-inch of water level) while simultaneously collecting critical soil type information.
The high resolution piezocone data is used to measure the direction and rate of groundwater flow in three-dimensions. Mass flux can then be calculated using the groundwater flow rate data and contaminant concentration data.
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Title:
Theory: Hydraulic Conductivity/Gradient (1 of 2)
Text: The piezocone measures soil resistance to penetration and dynamic pore water pressure while being advanced into the ground. A pressure transducer mounted on the probe measures pore water pressure and is used to determine hydraulic head and estimates of hydraulic conductivity using the Parez and Fauriel relationship.
To obtain hydraulic conductivity estimates, the piezocone penetration is stopped at a target depth and excess pressure decay over time is monitored and eventually reaches the equilibrium pressure corresponding to the hydrostatic head for that depth. The rate of dissipation is directly related to hydraulic conductivity. The time required to reach the 50% dissipation pressure is called t50 and is used to calculate hydraulic conductivity (k) using the Parez and Fauriel mathematical relationship shown here.
In addition, the hydraulic gradient (i) is derived based on interpolation of the hydrostatic head distribution values.
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Title:
Theory: Mass Flux Calculation (2 of 2)
Text: This figure represents a 3-D model of contaminant mass flux distribution. The illustration depicts areas where high concentrations correspond with high mass flux (longer vector) and low concentrations correspond with low mass flux (shorter vector). However, there are also areas where high concentrations do not correspond to high mass flux. These zones imply that the high concentrations are located in low velocity soils such as silts and clays. Furthermore, several moderate concentration zones exhibit high mass flux, which implies that they are in high velocity soils.
The high resolution piezocone data can be used to develop a 3-D mass flux distribution for a given site. Mass flux is equal to the seepage velocity times the concentration at each location. The seepage velocity (v) is calculated based on the effective porosity (r) of the soil and the hydraulic conductivity (k) and hydraulic gradient (i) measured via the piezocone. This seepage velocity is then multiplied by the interpolated concentration value for each location, yielding a 3-D contaminant mass flux distribution for the site.
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Title:
Application
Text: Determination of mass flux using a high resolution piezocone can be beneficial at both the Remedial Action Operations (RAO) and Long-Term Monitoring (LTM) phases of a project.
For RAO applications, the high resolution piezocone data can be used to build a conceptual site model (CSM) that can assist in optimal placement and design of monitoring and injection/extraction wells. Identification of areas with high contaminant mass flux can enable proper source area determination and selection of the target treatment zone. It can also be used in designing for hydraulic containment of the contamination and in the selection of appropriate in situ remedial technologies.
For LTM applications, understanding contaminant flow direction, flow rate, and mass flux distribution together with certain physical subsurface parameters such as soil type distribution can be very useful in establishing a monitoring network and for generating time series analyses appropriate for demonstrating plume attenuation.
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Title:
Data Collection and Interpretation (1 of 2)
Text: During deployment, all of the high resolution piezocone readings collected from the subsurface are processed using the same system and software used for data acquisition. As the probe is advanced into the subsurface, continuous tracking of dynamic pressure, barometric pressure, soil type, and temperature occur. A water saturated zone is identified by a positive pore pressure. Final pressures measured by the probe can be used to develop 3-D hydraulic head models.
The figure shown here provides an example WinOCPT version of the upgraded piezocone output for a single push with five dissipation tests. Beginning with the left portion of the graphic, a soil type classification log, hydraulic conductivity log, hydraulic conductivity at specific depths, and a log of effective porosity estimates are displayed in columns with depths listed along the y-axes.
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Title:
Data Collection and Interpretation (2 of 2)
Text: Dissipation curves are generated using the piezocone dissipation analysis module to develop the hydraulic conductivity and pressure profiles. Click on the blue box to view example data.
The dissipation curves for specific depth tests are generally displayed along the lower right portion of the graphic. Final pressures and hydraulic conductivity values that are derived from the dissipation test are presented in the hydraulic pressure profile (HPP) in the upper right graph, along with calculated water depth below surface and corrected water table depth (relative to sea level). In addition, the HPP can be used to determine whether a fine layer is confining.
The data are then exported to a groundwater modeling system (GMS) for interpolation, finite difference analyses, generation of velocity fields, and 3-D flux determinations and visualizations.
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Title:
WinOCPT
Text: WinOCPT is a data acquisition and processing software used for the Site Characterization and Analysis Penetrometer System (SCAPS). SCAPS is a soil and groundwater analysis system that provides detailed information about the subsurface geology and contaminant distributions using various probes deployed with a Cone Penetrometer Test (CPT) system. Many of the SCAPS probes are equipped with soil type classification capabilities which are integrated with certain software.
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Title:
Pore Pressure Graphs
Text: Hydrostatic Pressure Profile (HPP):
The HPP represents final pressures measured for a single push depicted as depth on the X-axis, and pressure on the Y-axis. The HPP best fit line can be extrapolated to the zero pressure depth, which represents the water table.
Pressure Dissipation Curve:
The pore pressure dissipation curve represents observed changes in pressure over time as measured via a transducer in the high resolution piezocone for a specific depth.
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Title:
Performance Assessment
Text: As part of an Environmental Security Technology Certification Program (ESTCP) project, NAVFAC conducted an in-depth evaluation of the high resolution piezocone performance to benchmark the following criteria:
Ease of deployment in the field
Accuracy of data generated
High-resolution conceptual model generation
The deployment of the instrument at the ESTCP test sites demonstrated that the data obtained from a piezocone were accurate. For example, it was observed that the instrument provided the data within one inch of accuracy for head values (±0.08 ft) and hydraulic conductivity is measured accurately within one order of magnitude.
The data obtained can be easily integrated into different groundwater modeling software such as MODFLOW to generate 3-D renderings of the subsurface (as shown here); these provide a good representation of contaminant flow and transport.
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Title:
Advantages/Limitations
Text: Advantages:
Data are collected in real-time while the site investigation is still on-going.
These instruments provide a higher vertical spatial resolution of the soil stratigraphy.
Collection of physical parameters of soils such as porosity lower the time and resources spent getting a laboratory analysis conducted for similar parameters.
It is a more cost-effective way of collecting data than conventional trial and error method.
Limitations:
This technology is mostly applicable at sites where penetration using a cone penetrometer can be done.
The probe doesn't work very well with silt and clays for conducting porosity measurements because these materials press against the probe limiting its ability to directly observe the pore spaces between the particles.
In case of no dissipation data, CPT results are used to estimate hydraulic conductivity values. These results are not very accurate and provide the data within an order of magnitude accuracy.
Models employed to generate maps for the site generally interpolate the probe data that can instill some inherent biases into the data.
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