Groundwater Model

This model will calculate the number of months required to remediate a contaminated aquifer using the soil flushing method. Details describing the formula and operation of this model can be found by clicking on the help button at the bottom of the screen.

To get started, fill in the labeled fields and click the associated buttons to perform the calculations. The appropriate value will be placed in the text box next to each button. Please take care to perform the calculations in the order given. Each calculation is dependent upon the previous calculation(s).

Depth (feet):
Radius (feet):

Porosity (0-1):

Total Organic Content (TOC) (unitless--%):
Soil-Water Coefficient (Koc) (ml/g):

Soil Bulk Density (g/ml):
Regulated Analyte Concentration (ug/l):
Actual Analyte Concentration (ug/l):

Pump Rate (gallons/day):

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Help On Using this Groundwater Model

Table of Contents (for Help on Calculations)

For help on a particular Aquifer Volume Calculation field, please click below:

The aquifer volume calculation uses the standard formula (Pi x radius-squared x height) to calculate the volume of a cylinder. This model assumes that a volume shaped as a cylinder will be able to encompass the entire contaminated aquifer. To ensure reliable estimations, be sure to select the radius and height (e.g., depth) of the cylinder to be large enough to cover the entire impacted site.

To determine the height (depth) of the cylinder, one must first understand how deep beneath the surface the impacted aquifer resides. This measurement must be determined from field measurements during soil and groundwater sampling or monitoring well installation events. The depth of the sample and / or monitoring well should be recorded in the geologist's field notebook. Laboratory analyses must be compared to the depths of the samples or to the depths of the monitoring wells. When selecting the depth, ensure that your choice is greater than the deepest impacted sample or monitoring well to ensure worst case scenario is considered.

These values will vary from site to site, however the values for depth may typically range from 1 to 200 feet. These values are all site-dependent, so the ranges are only a very rough guideline. Typical values may be "50 ± 50". Also, be sure that the units are in "feet."

To determine the radius, one must first understand the size of the plume along the x-y axis (e.g., as one would vision the plume in aerial view). Once the aerial view is mapped to scale, a circle can be drawn around the impacted site. The circle must be large enough to encompass the entire aquifer, so draw the circle larger than the impacted area to ensure that the worst case scenario is considered. Although a circle may not have the best fit for the particular aquifer, as long as the circle is larger than the aerial extent of contamination, the worst case is considered. The radius of this circle can then be measured for use in the radius field of this model. Be sure that the radius measurement is in "feet."

These values will vary from site to site, however the values for radius may typically range from 1 to 500 feet. These values are site-dependent, so the ranges are only a very rough guideline. Typical values may be "50 ± 50". Also, be sure that the units are in "feet."

For help on a particular Pore Volume Calculation field, please click below:

Pore volume (gallons) is the actual amount of space in which groundwater could potentially reside. Thus, the pore volume is the porosity times the aquifer volume.

Aquifers typically reside in sediments that have openings between grains called pore spaces. The porosity of the earth material is the percentage of rock or soil that is void of material. Porosity is a percentage and it is unitless. The mathematical equation defining porosity is:

Where n is the porosity; Vv is the volume of void space in a unit volume of earth material; and V is the volume of earth material including both voids and solids. Pore volume is the actual amount of space in which groundwater could potentially reside. Thus, the pore volume is the porosity x aquifer volume. Because porosity is simply a percentage, values will range from 0 to 1 with typical values around "0.25 ± 0.25".

For help on a particular Distribution Coefficient Calculation field, please click below:

The distribution coefficient for an analyte is used to determine the retardation factor of groundwater in soil. An organic compound may remain dissolved in water or adsorbed onto the soil organic carbon, depending on the total organic content (TOC) of the soils. Retardation happens when the analyte is adsorbed onto the soil. When adsorption occurs, remediating the groundwater will take much longer because some of the contaminant adheres to the soil and is not removed with the groundwater. The distribution coefficient in our model is calculated by multiplying the soil-water partition coefficient (Koc) by the total organic carbon content (TOC) of the soil.

The total organic content must be determined during laboratory analysis of soils. Typical values for TOC are usually numbers around "0.00005 ±0.000025".  TOC is reported as a percentage from the lab and is unitless. Please be sure to use the actual value (e.g., if reported as 0.001%, use the number 0.00001).

The Koc values are published by the United States Environmental Protection Agency (USEPA) in its Superfund Public Health Evaluation Manual dated October 1, 1986. This factor is called the soil-water partition coefficient (Koc). Additional information can be found at: http://www.epa.gov/superfund/oerr/soil/toc.htm#p5. Typical values for Koc range from 0 to 5500000 and are often numbers around "100 ± 50". These values vary quite a bit by analyte, so it is best to refer to the document listed above for this analyte-specific value. Be sure to use the units "ml/g -- milliliters per gram" for this field.

For help on a Number of Pore Volumes Calculation field, please click below:

The number of pore volumes is based on the porosity of the soil, the volume of the aquifer, the soil bulk density, the initial and final contaminant concentrations, and has no units. Soil bulk density, regulated analyte, and actual analyte concentrations are described below. Other values that are required for the number of pore volume calculation are listed in the previous sections. This calculation is the crucial portion of the model and is based on an equation developed by Gradient Corporation (http://www.gradcorp.com).

Number of pore volumes =

The function is logarithmic, therefore, although the number of months may be great, much of the contaminant will be removed within the relatively first few months of remedial efforts.

A brief description of each of the three input variables (e.g., Soil Bulk Density, Regulated Analyte Concentration, and Actual Analyte Concentration) follows:

Soil bulk density is the weight of the rock divided by the total volume, including the porosity. Bulk density is determined during laboratory analysis from a calibrated gamma-gamma log. Bulk density units are grams per milliliter (g/ml).

The Regulated Analyte concentration is the final analyte concentration and is the concentration of the chemical upon completion of remediation. This value is dependent on federal, state, and local government regulations and will vary from one chemical to another. This value must also have units in micrograms per liter (ug/l). For the state of Michigan, final concentrations are determined through the Michigan Department of Environmental Quality.

Actual analyte concentration is the initial analyte concentration and is the level of contamination of a particular chemical. This value is determined through laboratory analysis and must have units in micrograms per liter (ug/l).

For help on a particular Month Calculation field, please click below:

The number of months required to remediate a contaminated aquifer is equal to the number of pore volumes (gallons) divided by the pump rate (gallons per day) divided by 30 days per month. For simplicity, the number of days in a month has been set to equal 30.

Information regarding the number of pore volumes can be found by clicking on the question mark icon next to the Number of Pore Volumes Button in the Groundwater Model page.

The pump rate is determined during pump tests conducted at the site. This value must have the units "gallons per day." Pump rate values can range from 500 to 9000, with typical values at around 7000 to 7500 gallons per day. Information for calculating the number of pore volumes is described in the number of pore volumes help section.

This groundwater model uses a formula originally developed by Gradient Corporation (presented at the Hazardous Wastes and Hazardous Materials conference April 12-14, 1989: "Determining the Effectiveness of Soil Washing" by Brian L. Murphy, Ph.D.; James D. Doherty, Ph.D.; and Neil S. Shifrin, Ph.D.) and was implemented using a Microsoft Excel spreadsheet during my employment with McLaren/Hart in 1995. I've converted part of the spreadsheet model to this web-based version for a course project at The University of Michigan - Dearborn in 1998. To view the slide show for the course presentation, Click Here. The model is used to estimate the amount of time required to remediate a groundwater aquifer.

Remedial efforts would simply include removing the contaminated groundwater from the aquifer by pumping the water out of a well. Technically, this is called soil flushing and is a much less costly method of achieving desired cleanup levels in a reasonable amount of time than more common remedial methods.

Typical values and ranges are included under the help for each of the form fields. However, these values must be taken lightly. Most of the values can vary drastically from site to site, so the most important thing to remember is to ensure the data is converted to the appropriate units for each field.

Please remember, this is only a model and the final number of pore volumes and months required to remediate the aquifer are strictly estimates.

The fields MUST BE completed in the order presented (e.g., in top-down order). If a value is not completed in the proper order, the user will receive either a browser-generated JavaScript error or "NaN" as an answer. If the user does not enter a value in a required field, a JavaScript Alert dialog box will appear when the user clicks the calculation button associated with the field. The JavaScript Alert dialog box will contain a message indicating which field must be completed prior to attempting a calculation. If several required fields are skipped, the user may obtain a series of JavaScript Alert dialog boxes and a final browser-generated error message.

Help specific to each of the groups of fields can be found through the table of contents for the help system.