PREDicT™ – Precision Recoverability Evaluation for DNAPL via TransmissivityDecember 14, 2017
PREDicT™ is a patent pending quantitative evaluation of the vertical distribution and recoverability of non-aqueous phase liquids (NAPLs). The process can be applied to a variety of products including dense non-aqueous phase liquids (DNAPLs) at former Manufactured Gas Plant (MPG) sites in unconsolidated aquifers and both light non-aqueous phase liquids (LNAPLs) and DNAPLs in fractured rock. PREDicT™ includes transmissivity testing and high resolution mobile NAPL interval definition. The method can precisely identify the geometry of the multiple zones in the subsurface that contain mobile/recoverable NAPL and the recoverability of that NAPL.
This information can be integrated into the conceptual site model (CSM) to:
- support low-threat risk closures; and
- improve the effectiveness and efficiency of a hydraulic remedy.
The article discusses the above and provides an overview of the PREDicT™ process for MGP DNAPL sites.
Support Low-Threat Risk Closure
MGP DNAPL poses a perceived concern due to its presence within monitoring wells or other observations of exposed subsurface. Historically, these concerns have been addressed via the excavation or dredging of shallow MGP DNAPL. More recently, in situ stabilization has been utilized to address the presence of shallow to moderately deep MGP DNAPL in both unconsolidated aquifers and waterways. However, there are still MGP DNAPL sites where measurable DNAPL is present. These sites often pose more challenging conditions where the MGP DNAPL impacts are in developed areas with limited accessibility, too deep, or within fractured environments and cannot be addressed via conventional methods.
A risk-based approach has been gaining acceptance for remediation for LNAPLs that can provide a pathway to improve the remedial outcomes at these challenging MGP DNAPL sites. In this case the risk presented by the LNAPL is managed by “saturation-based risk” (the risk presented by the presence of LNAPL itself) and “composition-based risk” (the risk presented by the resulting dissolved- and vapor-phase impacts from the NAPL source). For sites where no composition-based risk is present and the only remaining risk is the presence of LNAPL within a well, the need to hydraulically recover the LNAPL and to understand when said recovery is complete is measured through LNAPL transmissivity as well as other lines of evidence. Many States within the USA (Figure 1) are now adopting LNAPL transmissivity as a metric to deem when hydraulic recovery is complete, or more broadly, indicating when saturation-based risks are addressed.
FIGURE 1. Transmissivity accepted in most States as a line-of-evidence for ceasing hydraulic recovery (Hawthorne, J. Michael; Andrew J. Kirkman; and Lisa Reyenga (2016) Magnitude of Potential Errors in LNAPL Transmissivity Calculations in Complex Confined and Perched LNAPL Conditions. Battelle Tenth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, May 22-26, 2016, Palm Springs, California).
The same approach can be taken with MGP DNAPL. The DNAPL transmissivity can be calculated via PREDicT™ to demonstrate that the saturation-based risk has been addressed. Where no composition-based risks are remaining, site closure with an institutional control may be appropriate.
Improve the Effectiveness and Efficiency of a Hydraulic Remedy
Where additional hydraulic recovery is warranted, the results from PREDicT™ focus resources to the remaining concerns at a given Site. Wells are identified that contain MGP DNAPL that require additional recovery, and providing critical design information for recovery operations to enhance their design via targeting the depth intervals with recoverable MGP DNAPL within unconsolidated soils or within fractured media. PREDicT™ enhances the ability to understand MGP DNAPL mobility, and improves CSMs, and aids in understanding:
- how much NAPL can be pumped out of the subsurface;
- if one can shut down unproductive NAPL recovery wells;
- if installing or operating a NAPL recovery pump makes sense prior to its installation; and
- the progress and effectiveness of the NAPL recovery remediation efforts.
Transmissivity is a fundamental hydrogeologic property that applies to both groundwater and NAPLs. Transmissivity is an established universal metric for the recoverability of groundwater from aquifers, essentially measuring the rate at which groundwater can flow through a one-foot wide strip of an aquifer under a unit gradient in a unit amount of time (blue column, Figure 2). However, transmissivity is not limited to groundwater. It can be used to measure the flow potential for any liquid in the subsurface that exhibits Darcian flow. As documented in ASTM E2856, transmissivity may be measured for LNAPL via multiple methods (orange strip in the right-hand block, Figure 2b), and accounts for the different density and viscosity of the LNAPL, as well as the relative permeability resulting from two liquids (groundwater and LNAPL) competing to flow through an aquifer. Transmissivity may also be calculated for MGP DNAPL (orange strip in the left-hand block, Figure 2a) using a modification of the methods identified in ASTM E2856.
FIGURE 2. Transmissivity for groundwater, LNAPL, and DNAPL
One of the common methods to measure transmissivity for groundwater and NAPLs is the baildown test. Baildown testing is similar to slug testing for groundwater wells. NAPL is removed from a well, inducing drawdown in the NAPL and flow into the well proportional to the NAPL recoverability in the soil or rock around the well. As the NAPL recharges into the well, fluid interface elevations are monitored over time until the NAPL in the well and formation are at equilibrium. For MGP DNAPL within unconsolidated formations, the identification of mobile NAPL intervals (MNI) are shown on Figures 3 and 4, respectively. The PREDicT™ method identifies these through a patent-pending method but simply uses the drawdown versus discharge (DvD) responses following NAPL evacuation to identify multiple MNIs perched on finer grained soil or fractured bedrock intervals.
FIGURE 3. Baildown testing within unconsolidated aquifer showing two MNIs.
The well acts as a sump and MGP DNAPL enters the well from all MNIs causing an exaggeration of the NAPL thickness relative to where the NAPL is actual present within the formation; thus, exaggerating the height of MGP DNAPL in the well versus the impacted portion of the formation (Figure 3). If the well is evacuated of NAPL, then the well will fill from all MNIs initially and as each MNI is reached then the transmissivity of that given MNI can be calculated from the resulting measures similar to the ASTM E2856 method. An idealized DvD is shown on Figure 4 with respect to the MGP DNAPL recovering into the well. For highly transmissive MGP DNAPL the testing may take hours; whereas, for sites with NAPL reaching the threshold of recoverability may take weeks to months to have the MGP DNAPL thickness within the well return to pre-testing equilibrium conditions.
FIGURE 4. Discharge versus drawdown interpretation of baildown testing.