ML20147D278

Forwards NRC Staff'S Analysis of Potential Effects of Constr Dewatering at Lamprey Pond on Well Field at Hampton Water Works Re Util Conclusion That Constr of Nuc Pwr Plant Will Have No Effect on Water Levels

ML20147D278
Person / Time
Site:	Seabrook
Issue date:	10/03/1978
From:	Brenner L NRC OFFICE OF THE EXECUTIVE LEGAL DIRECTOR (OELD)
To:	Burt L MASSACHUSETTS, COMMONWEALTH OF
References
NUDOCS 7810140011
Download: ML20147D278 (5)

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.'Uny October 3,1978

,gocux q Laurie Burt, Esq. ROoy Assistant Attorney General m Commonwealth of Massachusetts $5"40 Environmental Protection Division -

One Ashburton Place C COCT d  :-

19th Floor Boston, liassachusetts 02108 ,w % g f-

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In the Matter of  %(/

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7 Public Service Company of New Hampshire, et al. Nq (Seabrook Station, Units 1 end 2)

Docket Nos. 50-443, 50-444

Dear Ms. Burt:

At the time the parties agreed that the Iamprey Pond site would be featured in the Staff's analysis as the representative New Hampshire .

coastal alternative site, the Comnonwealth of Massachusetts requested furt.her information on the Staff's analysis of potential effects of construction dewatering at Lamprey Pond;qpon the Hampton water supply wells. ~'-

The requested Staf f analysis, consist'ing of four pages, is enclosed.

If you have any questions regarding the enclosed cnalysis, please give me a call at (331) 492-7268 by October 16, 1978.

Sincerely, O c

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, b Lawrence Brenner Counsel for NRC Staff Enclosure '

"Ef fects of Construction Dewatering at Lamprey Pond On Well Field At Hampton Water Works" cc w/ enc 1: Alan S. Rosenthal , Esq. , Chai rman Norman l'oss , Esq.

Dr. John H. Do E. Tupper Kinder, Esq.

Michael C. r ror, Esq. Karin P. Sheldon, Esq.

Ivan W. Smilo, Lsq. Atomic Safety and Licensing -

Joseph F. Tubridy, Esq. Board Panel Dr. Marvin M. !! ann Atomic Safety and Licensing Dr. Ernest O. Salo~ Appeal Board Dr. Kenneth A. McCollom Docketing and Service Section Robert A. Backus , Esq.

Thomas G. Dignan, Jr., Esq.

John A. Ritsher, Esq.

3 B \ Y 14- p% 1 t

EFFECTS OF CONSTRUCTION DEUATERING AT LAl1 PREY POND OM WELL FIELD AT HAMPTON WATER WORKS The Hampton Water Works well field located about 2500-3000 feet west of the proposed Lamprey Pond site, consists of eight wells averaging about 55 feet in depth. Approximately 1.2 to 3.4 million gallons per day are withdrawn for use. The staff has conducted analyses of the effect of construction dewatering at Lamprey Pond on the well field at Hampton Water Works. Based on these analyses, we conclude that it is highly unlikely that construction dewatering at a proposed Lamprey Pond site will have an effect on the water supply wells at the Hampton Water Works.

This conclusion is based on several factors: (1) the foundation conditions at Lamprey Pond are similar to those at Seabrook, (2) groundwater occurs in the Lamprey Pond area in localized pockets with relatively impermeable rock and till inhibiting the flow between pockets and toward the proposed plant excavation, (3) it is highly unlikely that the radius of influence due to dewatering would extend to the wells, (4) procedures are available to mitigate any adverse effects at the Hanoton Water Works wells, should they occur.

(1) Similar Founda tion Conditions to Snabrook Dur ing our site visit to Seabrook we observed the foundation excavation for Unit 2. We noted that groundwater (dewatering) inflows to the excavation were very small. The low inflows are

_2-due to the presence of relatively impermeable rock and till which underlie the site. Based upon a study of geologic maps of the area, it is expected that the foundation excavation at Lamprey Pond would be similar to the Seabrook site. There may be n: ore overburden material, but the Lamprey Pond site area is probably underlain and surrounded by types and thickness of rock and till similar to Seabrook. The rock and till would act as barriers to groundwater fl ow. Since these hydraulic barriers exist between the site and the Hampton wells there would be no etfects on water levels.

1 (2) localized Pockets of Groundwater Based on telephone conversations with the manager of the Hampton Water Works and our review of groundwater resources in the area, groundwater occurs in the Water Wo-ks area in localized pockets.

These pockets consist primarily of sand and gravel surrounded by relatively impemeable rock and till. The rock and till inhibit the flow of groundwater from one pocket to the other and would inhibit groundwater flow toward the plant site excavation.

(3) Radius of Influerice of Dewatering Even if it is assumed that conditions (with no rock and till in-hibiting flows) exist in the area, and using a conservative estima te of pemeability for a sandy gravel, the influence of dewatering

on the local groundwater table would extend a maximum of about 1000 feet f rom the excavation. Since the wells are located approximately 2500-3000 feet away from a lil:ely plant site ex-cavation, there would be no effect en water levels in the wells.

The independent staff conclusion was reinforced by statements of the manager of Hampton Water Works, who indicated that there is very little decline in water levels in observation wells located only 200 feet away from the pumping wells. This in-dicatcs relatively low soil permeabilities and further sub-stantiates the opinion that dewatering at a distance of 3000 feet from the walls will have no adverse effect at the walls.

(4) Mi tiaation to be Engloy_ed.

flotwithstanding the above analyses and conclusions, if it is found that dewatering the plant excavation has an effect on the wells, there are several things which could be Jone to eliminate or decrease the adverse effects. One alternative would be to drili an entirely new and different well field (or another water supply source) et another location to replace the existing well fi^1d. Another solution would be to place some type of more impermeable barrier (such as a orout curtain or sheet piling)

_4 around the foundation excavation, so as to limit the inflows and areal extent of the dewatering.

(5) Sulgg,ry In conclusion, the flRC Staff feels that the construction of a nuclear power plarit at a Lamprey Pond site will have no effect on water levels at the Hampton Water Works wells. If, for any reason, water levels in the wells are affected by plant de-watering, mitigation procedures could be employed. However, based on our analyses, it is our professional opinoin that mitigation discussed in paragraph 4 will not be necessary.

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