PERMIT RATIONALE FOR MODIFICATION
Relativity Space Inc
Hancock County
Stennis Space Center, Mississippi
Water NPDES No. MS0062677
June 26, 2025

I.	FACILITY INFORMATION

Facility Name:  Relativity Space Inc
Facility Address: Hockey stick shaped parcel by Propellant Road and Sutton Drive
Stennis Space Center, MS  39529    
Stennis Space Center, 
Permit No.:  MS0062677
SIC:  3761, 3764
Permit Writer:  Rajeev K. Gupta
EPD Branch:  Water Branch II

II.	NATURE OF BUSINESS

Relativity Space is engaged in the space flight hardware and launch vehicle design, assembly, processing and testing necessary or desirable to develop and operate a commercial service for delivery of cargo and payloads to space.  They are also involved in mission control and other associated engineering, technical, and administrative support activities associated with the provision of a commercial service for cargo and payloads to space.  

III.	EFFLUENT AND RECEIVING STREAM FLOW DATA

The facility has applied for NPDES Permit modification for one outfall.  The effluent from Outfall 001 consists of intermittent deluge water originating from engine stand testing and stage stand testing.  The facility has applied for a permit modification for an increase in the maximum daily discharge from 0.198 MGD to 0.600 MGD due to a proposed increase in engine testing at the site.  The receiving stream for the discharge will be an unnamed tributary to an access canal and thence to the East Pearl River.  The 7q10 of the unnamed tributary is 0.09 cfs and resides in the Pearl River Basin. The receiving stream is classified as fish and wildlife.   

	An Anti-degradation Review was completed for the flow increase.

IV.	303(d) and TMDL ISSUES

	East Pearl River does not appear on the 2024 State of Mississippi 303(d) List of Impaired 	Waterbodies.  This permit complies with the following applicable TMDLs:

•	Fecal Coliform TMDL for Pearl River Pearl River Basin, Hancock County (December 2000; ID# 500121503)
•	TMDL for Low pH in the Pearl River Pearl River Basin, Neshoba, Leake, Hinds, Rankin, & Hancock Counties (December 2000, ID 500121501)
•	TMDL Designated Streams in the Pearl River  Basin for Impairment Due to Sediment (March 25, 2009; ID 509032501)
•	TMDL Total Nitrogen and Total Phosphorus for the Pearl River (June 29, 2009)

V.	TYPE OF WASTEWATER TREATMENT

The facility has a 300,000 gallon retention pond with a geomembrane liner where sedimentation and settling take place prior to discharge.  The facility dechlorinates their water supply water prior to use.

VI.	EPA APPLICABLE CATEGORICAL GUIDELINES

	There are no categorical guidelines that apply to the proposed discharge.
	
VII.	DATA FROM APPLICATION FORM 2D

	Outfall 001

Parameter	1Max. 	
BOD5, mg/L	<6 	
COD, mg/L	< 15 	
TOC, mg/L	1.43	
TSS, mg/L	<2	
Ammonia, mg/L	<.101	
Flow, MGD	0.600 	
pH (min), SU	6.0 	
pH (max), SU	9.0	
Total Residual Chlorine, mg/L	0*	
Fluoride, mg/L	0.254	
Nitrate-nitrite, mg/L	1.08	
Nitrogen, Total Organic (as N)	1.08	
Oil and Grease, mg/L	<1.6	
Phosphorus (as P), Total	<0.5	
Total Zinc, mg/L	0.027	
Total Aluminum, mg/L	<0.1	
Total Cyanide, mg/L	0.007	
Chloroform, mg/L	0.0144	
Total Iron, mg/L	<0.10 	
Total Moly, mg/L	<0.005	
Total Titanium, mg/L	<0.01	
1Max = Estimated Maximum Daily Value
*Based on actual discharge data

	Summary of Supplemental Cyanide Samples for Outfall 001

Sample Date	Free Cyanide (mg/L)	Reporting Limit (mg/L)	
05/22/25	ND	0.002	
05/27/25	ND	0.002	

The State of Mississippi Water Quality Criteria for Cyanide is expressed as µg free cyanide (as CN)/L.  

VIII. 	WATER QUALITY LIMITATIONS BASED ON WASTELOAD ALLOCATION 

A WLA was requested and received for the existing permit.  However, since the discharge was new and the expected concentration of pollutants was uncertain, the WLA was not utilized.  

VII.	TOXICITY SCREENING

See Attachment A

XI.	PROPOSED FINAL LIMITATIONS 

Average Permit Limitations

Parameter	Categorical Limitation	Water Quality Limitation	Present Permit Limitation	Proposed Permit Limitation	Basis	
Flow (MGD)	---	---	---	Report	TJ	
pH	---	6.0 SU (min)	---	6.0 SU (min)	MSWQS	
Oil and Grease	---	---	---	10 mg/L	TJ	
TR Chlorine	---	0.012 mg/L	---	0.012 mg/L	MSWQS	
TR Iron	---	1.1 mg/L	---	Report	TJ	
TR Al 	---	0.153 mg/L	---	Report	TJ	
TR Zn	---	0.071 mg/L	---	Report	TJ	

Maximum Permit Limitations

Parameter	Categorical Limitation	Water Quality Limitation	Present Permit Limitation	Proposed Permit Limitation	Basis	
Flow (MGD)	---	---	Report	Report	TJ	
pH	---	9.0 SU (max)	---	9.0 SU (max)	MSWQS	
Oil and Grease	---	---	---	15 mg/L	TJ	
TR Chlorine	---	0.0208 mg/L	---	0.0208 mg/L	MSWQS	
TR Iron	---	---	---	Report	TJ	
TR Al 	---	0.274 mg/L	---	Report	TJ	
TR Zn	---	0.071 mg/L	---	Report	TJ	

MSWQS = Mississippi Water Quality Standards
TJ = Technical Judgement
TR = Total Recoverable

Flow shall be measured once per week through estimation.  When the original permit was issued, Flow was required to be measured twice per month using a Totalizer.  The facility was unsuccessful in measuring the effluent using a Totalizer and began estimating the discharge.  As a part of this permit modification, the facility has requested this change be made in the permit.  Since it is believed in most cases due to freeboard in the pond this is a conservative approach, this change is being made. pH, TRC, and Oil and Grease shall be monitored twice per month with a grab sample of the effluent. Total Residual Chlorine limits are in the permit as the facility’s water supply contains Total Residual Chlorine and the facility dechlorinates their water supply prior to use.  

Please see Attachment A for an explanation of all monitoring requirements and permit conditions for Total Rec. Iron, Total Rec. Aluminum, and Total Rec. Zinc.

ATTACHMENT A

TOXICITY SCREENING

Qr (7Q10) = 7Q10 of receiving stream (Acute and Chronic Screenings) = 0.0576 MGD 
Qr (MAF) = Mean annual flow of receiving stream for Human Health (HH) screenings.
Qw	= Maximum 30 day flow = 0.600 MGD.  
Xr	= Receiving stream concentration.  
Xw 	= Historical effluent data from the application.
Xt	= The calculated instream concentration based on historical effluent data from the application

IWC (%) = Qw /(Qr + Qw)*100 	

Xt = (Qr * Xr) + (Qw * Xw)			
                    (Qr + Qw)					

Xr is 0 mg/L as no in-stream metals data exists for the unnamed tributary.

If number of samples reported in application <12 for a pollutant then Xt = Xw *10

Xt = (Qw * Xw) / (Qr + Qw) (For Human Health Screening)
				
Xt for HH screening is much lower than Xt for acute and chronic screening and is not being calculated in this rationale since using the higher Xt is being more conservative and still passes HH screening.

IWC (%) = 0.600 mgd/(0.600 mgd + 0.0576 mgd) = 91.2%

Total Recoverable Zinc:
Xt = (0.0576 *0) + (0.6 *0.027 *10)  =  0.246 mg/L
			(0.6 + 0.0576)

Toxicity Screening Summary
						
Outfall 001- (units for all data are mg/L)
Parameter	Xt	CWQC	Pass/Fail	Xt	AWQC	Pass/Fail	Xt	HHWQC	Pass/Fail	
TR, Zn	0.246	0.065	Fail	0.246	0.065	Fail	0.246	7.4 	Pass	

The facility failed the toxicity screening for Total Recoverable Zinc.  As part of the previous permit, the facility was to required to sample for Total Recoverable Zinc, Total Residual Chlorine, Total Recoverable Aluminum, and Total Recoverable Iron once per discharge event with a grab sample of the effluent until 12 samples were collected for each parameter. The facility submitted this data on a Form 2C which was required as part of the permit on August 30, 2024.  The results for Aluminum, Iron, and Zinc indicated toxicity screening thresholds  were being exceeded  The facility believes that metals in the discharge are primarily not present in the dissolved form.   

Total Recoverable Aluminum

In 2018, EPA recommended new total recoverable aluminum (TR Al) criteria that is calculated using pH, hardness, and dissolved organic carbon (DOC).  As per the permittee’s request, the permittee shall sample TR Al, pH, DOC, and hardness with a grab sample of the effluent following discharge events at a frequency sufficient to obtain a total of 24 samples.  Sampling shall occur no less frequently than six times per month during periods where a discharge occurs.  No more than one sample shall be taken during batch discharge events.  Sampling shall begin the month following the effective issuance date of the permit.  All monitoring results shall be submitted to the Mississippi Environmental Quality Permit Board no later than the 28th day of the month following the completed collection of 24 samples.  The NPDES permit may be modified to include TR Al limits after TR Al criteria is developed and screening is accomplished with the additional data.  In the interim while the site specific data is being collected, TR Al will be required to be monitored twice per month with a grab sample of the effluent.  

Total Recoverable Zinc

As allowed by the State of Mississippi Wastewater Regulations, the facility has requested to establish limits by developing a Chemical Translator using site specific data to determine the dissolved fraction of Zinc.  The facility has requested use the methodology as allowed in 11 Miss. Admin. Code Pt. 6 Ch.1., Rule1.2.6.C(2)(b)(2)(ii).  As a result, the facility shall collect 20 paired samples of effluent for dissolved and total recoverable Zinc using EPA’s “clean” analytical techniques and sampling procedures following discharge events at a frequency sufficient to obtain a total of 20 paired samples.  Sampling shall occur no less frequently than five times per month during periods where a discharge occurs.  No more than one sample shall be taken during batch discharge events.  The permittee shall calculate the dissolved fraction for each sampling.  Sampling shall begin the month following the effective issuance date of the permit. All monitoring results shall be submitted to the Mississippi Environmental Quality Permit Board no later than the 28th day of the month following the complete collection of the 20 paired samples.  While the paired sampling is being collected, TR Zinc will be required to be monitored twice per month with a grab sample of the effluent.  

Total Recoverable Iron

Total Recoverable Iron sampling shall be conducted in accordance with Relativity Space’s  sampling plan dated March 20, 2025. The NPDES permit may be reopened to include Total Recoverable Iron limits based on review of the sampling data.  While the Iron study plan is being completed, Total Recoverable Iron will be required to be monitored twice per month with a grab sample of the effluent.  

ATTACHMENT B

Water Quality Calculations

7Q10 of unnamed tributary  = 0.09 cfs = 0.0576 MGD 
Qw= Maximum 30 day flow = 0.600 MGD.  

Instream Waste Concentration (Chronic and Acute) = 100 * (0.600/(0.600 +0.0576) = 91.2 %

Parameter			Acute Screening Value (mg/L)   Chronic Screening Value (mg/L)
Total Res. Cl(TRC)			0.019					0.011
Total Rec. Iron			----					1			
Total Rec. Al				0.250					0.140
Total Rec. Zinc			0.065 					0.065

TRC, Chronic or Avg limit: 0.011/0.912= 0.012 mg/L
TRC, Acute or Max limit: 0.019/0.912= 0.0208 mg/L

TR Iron, Chronic or Avg limit: 1.0/0.912= 1.1 mg/L

TR Al, Chronic or Avg limit: 0.140/0.912= 0.153 mg/L
TR Al, Acute or Max limit: 0.250/0.912= 0.274 mg/L

TR Zn, Chronic or Avg limit: 0.065/0.912= 0.071 mg/L
TR Zn, Acute or Max limit: 0.065/0.912= 0.071 mg/L