New Jersey Zinc/Mobil Chemical

South Ditch Sediments Focused Remedial Investigation

Fact Sheet #5
February 1998

DePue, Illinois

This Fact Sheet Summarizes

  • the results of the south ditch sediments investigation and
  • the risk assessment based on those results.

In 1996, the DePue Group (Horsehead Industries, Inc.; Mobil Oil Corporation, and Viacom International Inc.) investigated sediments in the south ditch between the former New Jersey Zinc/Mobil Chemical site and DePue Lake. See map. This fact sheet discusses the results of that investigation and the risk assessment based on those results.

Why were the ditch sediments investigated?

In 1995, the DePue Group and the State of Illinois signed an interim consent order in which the DePue Group agreed to investigate properties that may have been affected by past industrial operations at the New Jersey Zinc/Mobil Chemical site. The consent order required the DePue Group to conduct an accelerated investigation of south ditch sediments, because limited sampling by the Illinois Environmental Protection Agency (Illinois EPA) and U.S. Environmental Protection Agency (U.S. EPA) indicated that the sediments contained high levels of metals.

The south ditch sediments investigation had two goals.

  1. The first goal was to determine the amount of unnatural sediment in the ditch and the concentrations of metals in these unnatural sediments. "Unnatural"was used as a loose reference to sediments that may have been deposited in the ditch as the result of past operations at the New Jersey Zinc/Mobil Chemical site.
  2. The second goal was to use the information collected about the ditch sediments to conduct a risk assessment. The purpose of the risk assessment was to evaluate the possible need for immediate action to protect the environment or people who may have contact with the unnatural ditch sediments.
Focused Remedial Investigation study area - South Ditch

Investigation Results

How were the ditch sediments investigated?

The depth and breadth of the unnatural sediment were determined by a combination of reviewing aerial photographs, visual observation of changes in sediment color and the use of a ten-foot probe. The concentrations of metals were measured by collecting both surface samples and samples at depth of the unnatural sediment. These samples were analyzed for the metals of most concern at the New Jersey Zinc/Mobil Chemical site and the analytical results compared with results from samples collected from Turner Lake (also known as Mud Lake.) Turner Lake was chosen for comparison, because it has not been affected by past operations at the New Jersey Zinc/Mobil Chemical site.

What are the results of the ditch sediment investigation?

Fourteen transects were drawn across the south ditch. The investigation determined that the unnatural sediment depth in the middle of each transect ranged approximately from three to seven feet with the deeper part being near DePue Lake. The estimated volume of unnatural sediment is 7,900 cubic yards. Table 1 compares the range of metal concentrations in south ditch unnatural sediment to the range of metal concentrations in Turner Lake sediment.

Table 1

Comparison of South Ditch Sediment and Turner Lake Sediment Metal Concentrations

MetalsRange of South Ditch Sediment Concentrations
Range of Turner Lake Sediment Concentrations
7.8 - 82.0
6.9 - 10.3
193 - 4,140
140 - 150
0.38 - 1.9
0.83 - 0.88
32.4 - 910
3.4 - 8.0
6.2u** - 78.2
34.5 - 47.7
8.1 - 70.2
9.7 - 10.1
144 - 97,700
36.1 - 46.1
125 - 3,440
36.9 - 55.7
433 - 3,130
524 - 621
0.20 - 4.6
0.17u** - 0.20u**
11.6 - 60.3
34.3 - 41.2
0.57 - 4.6
0.48 - 0.82
1.4u** - 144
1.0u** - 1.2u**
5.0u** - 38.0
23.8 - 28.6
3,840 - 204,000
163 - 370

* mg/kg = milligrams/kilogram or parts per million
** "u" after a number means that the metal was undetected at the detection limit of the equipment. The mumber preceding the "u" is the detection level of the equipment.

Risk Assessment

The purpose of the risk assessment was to evaluate the unnatural sediments pose an ecological or human health risk from short term exposure. Risks that might be from long term exposure to the ditch sediments will be evaluated in the overall investigation of the entire site.

Ecological Risk Assessment

The DePue Group evaluated the ecological risk by testing the effect of sediments on two benthic organisms (small organisms that live at the bottom of a stream) that would probably be found in an environment like the ditch if unnatural sediments were not present. Two organisms, midge larvae and scud, were chosen, because one or more of their life stages is spent within sediment. These organisms are important, because they are food for larger organisms such as fish.

Ecological risk assessment results.

The midge and scud were placed in containers with eight samples of south ditch sediment and two samples of Turner Lake sediment. One hundred percent of the midge in seven of the ditch samples and 85% of the midge in the other ditch sample died in four days. One hundred percent of the scud in all ditch samples died within four days. For comparison, 22% and 35% of the midge and 22% and 23% of the scud died within four days in two samples of Turner Lake sediment.

These ecological risk assessment results indicate that the south ditch sediment is acutely toxic to midge and scud and probably to other benthic organisms. Due to the lack of benthic organisms and therefore the lack of a food source for fish, it is unlikely that fish would live solely in the south ditch.

Human Risk Assessment

The human health assessment was conducted by evaluating the effect metal concentrations found in the south ditch sediment would have on a child trespasser playing in the ditch and a construction worker digging in the ditch sediment. Both of these assessments assumed a short term exposure to the concentrations fo metals found solely in the ditch. Since diet, occupation, and lifestyle (e.g. some people smoke and others do not) vary from person to person, the amount of metals that a person may be exposed to outside of the ditch vary and so were not included in this risk assessment.

The child trespasser scenario.

In order to estimate the quantity of metal entering the body of a child playing in the ditch, a number of reasonable assumptions about the maximum exposure of the child were made including the following:

*The child is between six and 12 years old and weighs 70 pounds.

*The child plays in the ditch area no more than four hours a day, 50 days per year for six years.

  • Exposure occurs through breathing and ingestion (e.g., putting dirty hands in the mouth). Exposure through the skin is not a great concern because metals in soil generally are not absorbed through the skin.
  • The ditch is dry for the total time the child is in the area. By assuming the ditch is dry, the worse case scenario is being assumed, because dry mud might become airborne making it more likely that a child would inhale metal contaminated dust. Under current conditions, the ditch is seldom dry.
  • The child is not running (because the northern part of the ditch has steep slopes and the southern portion is marshy and often covered with water) so the inhalation rate is for light not heavy activity.
Conclusion for child trespasser.

Using calculations approved by the U.S. EPA and the Illinois EPA, the maximum levels of metals in the south ditch sediment exceed child trespasser screening levels for arsenic, copper, and lead. See Table 2. What this means is that although there may be no immediate threat from a small number of exposures, there is concern for a child trespasser who meets the description and who is exposed to the unnatural sediment for four hours a day for 50 days per year for six years, assuming that the ditch is dry during the exposure period.

Table 2

Comparison of Maximum Metal Concentrations in Unnatural Ditch Sediment with Screening Levels for Five Metals

MetalsMaximum Concentrations in South Ditch SedimentsScreening Levels2
Frequency of Screening Level Exceedance in All Unnatural South Ditch Sediment Samples
 0 - 6 inches from surfaceAll depthsChild TrespasserConstruction WorkerChild TresspasserConstruction Worker

1 mg/kg = miligram/kilograms or parts per million
2 The DePue Group developed the child trespasser screening levels using Illinois EPA and U.S. EPA approved calculations. The construction worker screening levels were developed by the Illinois EPA for residential and industrial/ commercial properties and have been adopted into the Illinois Administrative Code. They are referred to as Tiered Approach to Corrective Action Objectives or TACO. Screening levels for a child are different from construction worker screening levels, because it is assumed that the child's exposure is for four hours a day/50 days a year for six years, whereas the construction worker's exposure is for 30 days out of a six-week period during one year. The construction worker's exposure also differs in that, unlike the child, the worker may be excavating or in other ways disturbing the sediment causing more soil particles to be airborne and thus more metal bearing dirt to be inhaled and/or ingested.

Construction worker scenario.

This scenario is characterized by short term direct contact with soil by workers excavating or in other ways disturbing the unnatural sediment.

Conclusion for construction worker scenario.

The DePue Group obtained the screening levels for a construction worker from an Illinois EPA table calculated to protect construction workers exposed to these metal concentrations 30 days out of a 42day period during one year. The metal concentrations in the south ditch unnatural sediment exceeded the Illinois EPA arsenic, cadmium, copper, lead, and zinc screening levels for construction workers, See Table 2. This means that there is concern for construction workers who may work in the unnatural ditch sediments without adequate health and safety protection.

Next Steps

The DePue Group is completing a study of possible alternatives (called a feasibility study) for the ditch and will propose one of these alternatives as the remedy. When the Illinois EPA and the DePue Group agree on a remedy, this proposed remedy (called a proposed plan) and the feasibility study will be submitted to the public for comment. The Illinois EPA will make the final decision on the remedy after carefully considering all public comments

For Additional Information

Repository and future administrative record file location: The project repository, which contains the Phase 1 investigation work plan and other project documents, is located in the Selby Township Library in DePue. The local location for the administrative record file will also be at the Selby Township Library. The administrative record is a file of documents upon which site decisions about remedies will be based.


If you have questions about the project you may contact:

Kurt Neibergall
Office of Community Relations
Illinois EPA
1021 N. Grand Ave. E.
Box 19276
Springfield, Illinois 62794-9276
Phone: (217) 785-3819
Richard Lange
Project Manager
Illinois EPA
1021 N. Grand Ave. E.
Box 19276
Springfield, Illinois 62794-9276
Phone: 815/223-6836

The DePue Group has asked that the name of their contractor be placed on Illinois EPA's fact sheets so the public can contact the DePue Group if they so desire. The DePue Group's contractor is:

James Frank
Frank & Cowles Inc.
Environmental Consultants & Engineers
7226 N. Walnut Street Rd.
Springfield, II 62707