PERMIT RATIONALE FOR REISSUANCE
Exxon Mobil Corporation (EMC), Centreville Landfarm
Wilkinson County
Centerville, Mississippi
Water NPDES No. MS0039942
June 2024

I.	FACILITY INFORMATION

Facility Name: Exxon Mobil Corporation (EMC), Centreville Landfarm
Facility Address: 11152 Highway 24 East
		     Centreville, MS 39631
Permit No.:  MS0039942
SIC:  4953
Permit Writer:  Samar Patel
EPD Branch:  Chemical Branch

II.	NATURE OF BUSINESS

The facility is a hazardous waste landfarm that is currently in post closure care. The facility was previously utilized for landfarming treatment of sludge (DAF float) generated from wastewater treatment plant operations. Landfarming operations were terminated, and the facility stopped accepting waste materials on November 8, 1990, in response to land disposal restrictions. Exxon Mobil Corporation purchased the property in 2001 and completed closure activities required for the former landfarming operations. Closure activities included the installation of perimeter fences to enhance security and limit accessibility, and the placement of clean fill and vegetative cover in the former landfarming areas to limit direct exposure and control stormwater runoff from the facility. 

III.	EFFLUENT AND RECEIVING STREAM FLOW DATA

Outfall 001: The wastewater discharge consists only of collected storm water runoff from the landfarm. The storm water is collected from a series of smaller retention ponds, which flow to the main retention pond (South pond) and is then discharged to an unnamed tributary (7Q10 = 0) of Thompson Creek.  This Outfall has an average flow of 1.11 MGD and is located in the South Independent basin.

IV.	303(d) ISSUES

The receiving stream at the point of discharge is listed on the 303(d) list of impaired water bodies due to biological impairment in the State of Mississippi 2022 List of water bodies. The receiving stream is in the South Independent drainage basin. The receiving stream is classified as Aquatic Life Use Support at the discharge location. Historical monitoring data indicates that the facility will not impact or aid in impairment to Thompson Creek from their non-contact storm water only discharge (see item no. VII below).

V.	TYPE OF WASTEWATER TREATMENT:

None.   No treatment is provided since storm water does not come in contact with previously treated waste material.

VI.	EPA APPLICABLE CATEGORICAL GUIDELINES

N/A

VII.	DATA FROM APPLICATION FORM 2C

Parameter	Maximum Values1	
Oil and Grease	<2.00 mg/L	
BOD	2.32 mg/L	
TSS	12.2 mg/L	
Ammonia Nitrogen (NH3-N)	0.55 mg/L	
Nitrate Nitrogen	<0.100 mg/L	
Phenols	0.106 mg/L	
Zinc 	0.0107 mg/L	
Lead	<0.002 mg/L	
Chromium	<0.004 mg/L	
Mercury	0.0000013 mg/L	
pH	6.93 (min.) - 8.10 (max.) SU	

1 All samples were collected during controlled discharge events from a holding pond (South Pond) that has a retention period of greater than 24 hours. The values are based on the most recent three years (2020-2022) analytical results. 

VIII. 	WATER QUALITY LIMITATIONS BASED ON WASTELOAD ALLOCATION 
	
Wastewater consists entirely of storm water; therefore, waste load allocation is not required.

IX.	CATEGORICAL GUIDELINE LIMITATIONS CALCULATIONS

Not Applicable

X.	TOXICITY SCREENING

See Attachment A

XI.	PROPOSED FINAL LIMITATIONS 

1.	Average Permit Limitations

Outfall	Parameter	Categorical Limitation	Water Quality Limitation	Present Permit Limitation	Proposed Permit Limitation	Basis	
001	Flow (MGD)			R (MGD)	R (MGD)	EJ 	
	Oil & Grease			      --	Report (mg/l)	EJ	
	pH	--	6.0 SU	6.5 SU	6.0 SU	MSWQC	

2.	Maximum Permit Limitations

Outfall	Parameter	Categorical Limitation	Water Quality Limitation	Present Permit Limitation	Proposed Permit Limitation	Basis	
001
	Flow (MGD)			Report (MGD)	Report (MGD)	EJ	
	Oil & Grease			20 mg/L	20 mg/L	EJ	
	pH	--	9.0 SU	9.0 SU	9.0 SU	MSWQC	
EJ = Engineering Judgment; MSWQC = Mississippi Water Quality Criteria

3.	Operation of Rainfall Runoff Ponds

Historical monitoring data indicates that the facility storm water appears to be within water quality standards. However, continued Monitoring of the Rainfall Runoff Ponds (see item 4 below) shall serve as additional indicators of water quality.  

4.	Monitoring Requirements for Rainfall Runoff Ponds

The applicant will be required to continue monitoring the final storage pond (South Pond) by taking a grab sample at mid-depth and analyze for the following parameters:

a.	Ammonia Nitrogen
b.	Biochemical Oxygen Demand (5-day)
c.	Chlorides
d.	Nitrate Nitrogen
e.	pH
f.	Total Recoverable Chromium (EPA Method 200.8)
g.	Total Recoverable Lead (EPA Method 200.8)
h.	Total Phenols
i.	Total Suspended Solids
j.	Total Recoverable Zinc (EPA Method 200.8)
k.	Total Recoverable Mercury (EPA Method 1631E)

This facility must continue monitoring South Pond for the above parameters for the life of the NPDES permit and RCRA permit.  The EPA testing methods for the metals have been specified, in the permit, to ensure the most accurate testing results. The sampling frequency is once per year for the above parameters. 

5.	Acute Bioassay Requirements

The applicant will be required to continue their storm water Bioassay testing semi-annually for the life of the NPDES permit.  The continued Whole Effluent Toxicity (Bioassay) testing of the facilities storm water will serve as an additional water quality indicator.  

Historical testing for last five years (2019-2023) shows that the wastewater/storm water consistently passed the Bioassay testing with a 100% survival of Ceriodaphnia dubia and Pimphales promelas.

ATTACHMENT A

TOXICITY SCREENING

QW (for Outfall 001) = 1.11 MGD
Qr = 0 MGD

Since this discharge occurs approximately 5-6 times/year, a chronic water quality evaluation will not be performed. 
Zinc
XW (for Outfall 001) = 0.0107 mg/L* 10 = 0.107 mg/L (No. of Samples <12)
Xt = (0.107 * 1.11)/1.11 = 0.107 mg/L
AWQC = 0.065	
HHWQC (organisms only) = 26 mg/L

Toxicity Screening Summary (Acute Screening):       

Parameter 	Xt	AWQC    HHWQC	AWQC (Pass/Fail)	HHWAC (Pass/Fail)
Zinc		0.107	0.065	       26.0		Fail			Pass

Because the wastewater failed the acute screen for zinc because of a variability factor (i.e. number of samples less than 12) as calculated above, the permittee will be required to continue monitor the storm water in the final storage pond (South Pond) for zinc at a frequency of once per year as specified in the proposed permit (see item 4 above).