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Development of a Dilution-Attenuation Factor (DAF) for the Impact to Groundwater (IGW) Pathway

Posted on March 4th, 2014

In November 2013, NJDEP released Version 2.0 of the guidance document Development of a DAF for the IGW Pathway . In this revised version of the guidance, a default DAF of 20 is utilized for New Jersey remediation cases. This change was based on revised NJDEP default values of the input data in the DAF equation to account for representative parameters, which better characterize New Jersey aquifers (i.e., Kirkwood-Cohansey).

In November 2013, NJDEP released Version 2.0 of the guidance document Development of a DAF for the IGW Pathway[1]. In this revised version of the guidance, a default DAF of 20 is utilized for New Jersey remediation cases. This change was based on revised NJDEP default values of the input data in the DAF equation to account for representative parameters, which better characterize New Jersey aquifers (i.e., Kirkwood-Cohansey).  In the DAF equation:

 

     where

K = aquifer hydraulic conductivity (m/yr);

i = hydraulic gradient (m/m);

I = infiltration rate (m/yr);

L = length of area of concern parallel to ground water flow (m), and

d = mixing zone depth (m), ;

     where da is the aquifer thickness (m).

The following default values are now used in the DAF equation:

L = 100 feet,

I = 11 in/yr,

K = 142 ft/day or 51,865 ft/yr (previously 27 ft/day or 9,842 ft/yr),

 i = 0.003 (previously 0.01) and

da = 11.5 ft.

For Sites at which the aquifer is well characterized, a site-specific DAF may now be calculated. In order for the Department to efficiently review the proposed site-specific DAF, it is recommended that the person responsible for conducting the remediation use the spreadsheet provided by the Department[2] and submit, along with the spreadsheet, supporting documentation for all input parameters used to determine the site-specific DAF value, which include the following:

  • length of the soil source in the flow direction, L;
  • monitoring well construction information, boring logs and cross-sections to determine the aquifer thickness, da;
  • measurements of depth to groundwater and groundwater elevation contours to determine flow direction and hydraulic gradient, i; and slug test or pump test results to determine hydraulic conductivity, K

The change in the DAF affects other guidance documents from NJDEP webpage, some of which have already been updated (i.e., Development of IGW SRS Using the Soil-Water Partition Equation[3], Development of IGW SRS Using Synthetic Precipitation Leaching Procedure[4]). Others will be updated in the upcoming months (Guidance for Using SESOIL and AT123D Models to Develop Site-Specific IGW SRS[5], Protocol For Addressing Extractable Petroleum Hydrocarbons[6]).

 

Liliana Cecan, Ph.D., P.E.
JMS Senior Project Manager

 


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