Standard Guide for Estimation of LNAPL Transmissivity

Importancia y uso:

4.1 Application: 

4.1.1 LNAPL transmissivity is an accurate metric for understanding LNAPL recovery, is directly proportional to LNAPL recoverability and tracking remediation progress towards residual LNAPL saturation.

4.1.2 LNAPL transmissivity can be used to estimate the rate of recovery for a given drawdown from various technologies.

4.1.3 LNAPL transmissivity is not an intrinsic aquifer property but rather a summary metric based on the aquifer properties, LNAPL physical properties, and the magnitude of LNAPL saturation over a given interval of aquifer.

4.1.4 LNAPL transmissivity will vary over time with changing conditions such as, seasonal fluctuations in water table, changing hydrogeologic conditions and with variability in LNAPL impacts (that is, interval that LNAPL flows over in the formation and LNAPL pore space saturation) within the formation.

4.1.5 Any observed temporal or spatial variability in values derived from consistent data collection and analysis methods of LNAPL transmissivity is not erroneous, rather is indicative of the actual variability in subsurface conditions related to the parameters encompassed by LNAPL transmissivity (that is, fluid pore space saturation, soil permeability, fluid density, fluid viscosity, and the interval that LNAPL flows over in the formation).

4.1.6 LNAPL transmissivity is a more accurate metric for evaluating recoverability and mobile LNAPL than gauged LNAPL thickness. Gauged LNAPL thickness does not account for soil permeability, magnitude of LNAPL saturation above residual saturation, or physical fluid properties of LNAPL (that is, density, interfacial tension, and viscosity).

4.1.7 The accurate calculation of LNAPL transmissivity requires certain aspects of the LNAPL Conceptual Site Model (LCSM) to be completely understood and defined in order to calculate LNAPL drawdown correctly. The methodologies for development of the LCSM are provided in Guide E2531. The general conceptual site model aspects applicable to this guide include:

4.1.7.1 Equilibrium fluid levels (for example, air/LNAPL and LNAPL/water).

4.1.7.2 Soil profile over which LNAPL is mobile.

4.1.7.3 LNAPL hydrogeologic scenario (for example, unconfined, confined, perched, macro pores, and so forth).

4.1.7.4 LNAPL density.

4.1.7.5 Hydraulic conductivity for each soil type within the well screen interval.

4.1.7.6 Well screen interval in the vadose and saturated zones.

4.1.8 Incorporation of LNAPL transmissivity can further LCSMs by providing a single comparable metric that quantifies LNAPL recoverability at individual locations across a site.

4.1.9 Each of the methods provided in this document is applicable to LNAPL in confined, unconfined, and perched conditions. Any differences in evaluation are discussed in Section 5.

4.2 Purpose—The methods used to calculate LNAPL transmissivity have been published over the past 20 years; however little effort has been focused on providing quality assurance for individual tests or refinement of field procedures. In addition to summarizing the existing methods to calculate LNAPL transmissivity, this document will provide guidance on refined field procedures for data collection and minimum requirements for data sets before they are used to calculate LNAPL transmissivity.

4.2.1 Considerations—The following section provides a brief review of considerations associated with LNAPL transmissivity testing.

4.2.1.1 Aquifer Conditions (confined, unconfined, perched)—In general, each testing type is applicable to confined, unconfined, and perched conditions; however, consideration should be given to how LNAPL drawdown is calculated from well gauging data relative to formation conditions. Calculation of LNAPL transmissivity for confined and perched conditions is possible; however, the soil profile needs to be considered in combination with the fluid levels to accurately calculate drawdown. Drawdown values for perched and confined conditions can easily be overestimated without proper consideration. This results in LNAPL transmissivity being underestimated. The calculations of drawdown under perched and confined conditions are discussed within this document. Tidal influences or a vertical gradient on the water table also affect measurements and could distort the transmissivity results. Tidal influences are discussed in more detail in Appendix X1.

4.2.1.2 Well Construction—Any well being tested should be screened over the entire mobile interval of LNAPL. For locations where multiple discrete mobile intervals exist, it may be preferable to screen individual wells across each mobile interval. This will simplify the calculation of drawdown and derivation of LNAPL transmissivity. The interval of mobile LNAPL does not always correspond to the elevation of the air/LNAPL interface (for example, the mobile interval can be beneath the base of a confining layer under confined conditions). Appropriately screened wells can be substantiated based on vertical delineation of the entire LNAPL impacted interval (see Guide E2531).

4.2.1.3 LNAPL Type—No limitations have been identified for LNAPL type. However, the specific gravity of the LNAPL must contrast with that of the water to be measurable with an interface probe.

4.2.1.4 Well Development—In order to derive the most accurate LNAPL transmissivity value, appropriate well development should be conducted to ensure connectivity between LNAPL in the formation and the well (Hampton 2003) (3). Industry experience has observed that LNAPL can require up to several months following well installation to saturate the filter pack and establish connectivity within the well. Well development can help to reduce this time frame and should be completed in accordance with Guide D5521.

4.2.2 Analysis Method—An understanding of the analysis method and theory is necessary prior to the field testing to ensure that all appropriate dimensions and measurements are properly recorded.

4.3 Precision and Bias—At this time this document aims to provide methodologies for data collection and analysis to yield an accuracy of LNAPL transmissivity values within a factor of two (compared with the unknown actual value). This modest accuracy is reasonable based on the overall industry experience in implementing these procedures and the lack of comparison studies. The objectives initiated through development of this document are to provide improved guidance for more consistent data collection and analysis methodology, which in turn will provide a larger and more accurate data set on which to base future methodology revisions and improvements.

Subcomité:

E50.04

Referida por:

E3281-21A, E3361-22, E3282-22

Volúmen:

11.05

Número ICS:

13.080.05 (Examination of soil in general)