Department of the Interior Survey Fact Sheet 017-98

US Geologicall Survey October 1998

Effects of the 1994 Retsof Salt Mine Collapse

in the Genesee Valley, New York

History of the Mine

The Retsof Mine was founded in 1885 as a

vertical shaft was excavated to a layer of rock

salt about 900 feet below land surface. During

the next 110 years, this mine became the

largest salt-producing mine in North America

and the second largest in the world. Before the

collapse in March 1994, the mine encompassed an underground area of more than

6,000 acres—nearly 10 square miles.

During the 17 months following the initial

collapse, mining operations were shifted to the

northern part of the mine in an attempt to

salvage mineable salt before the entire mine

became flooded. Mining operations ceased on

September 2, 1995, and the mine was completely flooded by December of that year, 21

months after the initial collapse.

An apparent earthquake of magnitude 3.6

(Richter scale) centered near Cuylerville, N.Y.,

woke residents on March 12, 1994, at 5:43

a.m. and was detected by seismographs 300

miles away. The U.S. Geological Survey

National Earthquake Information Center

confirmed that a seismic event had occurred

near Cuylerville. The Livingston County

Sheriff’s Department asked the Retsof Mine

whether the earthquake could have resulted

from mining activity. Mine personnel reported

that they had not heard anything, nor felt any

unusual ground motion, nor had they detonated

any explosives that weekend. Retsof officials

examined the mine and discovered that a 500-

by 500-foot section of shale ceiling rock had

collapsed some 1,200 feet below land surface,

in the southern part of the mine. Methane and

hydrogen sulfide gases were escaping, and

ground water was flowing from the collapsed

ceiling into the room. The mine, which had

been in operation since 1885, had always been

dry previously. This initial collapse was

followed by another collapse and complete

flooding of the mine, large ground-water level

declines, and land-surface subsidence that

affected many structures, roads, and utilities

in this part of the Genesee Valley.

Canaseraga Creek

Footprint of

Retsof Mine

Avon

Dansville

Fowlerville

Mt. Morris

Cuylerville Collapse

0 2 Miles

0 2 Kilometers

Flow

Fowlerville

Moraine

Line of 

geologic section

(see back page)

Genesee Valley 

flood-plain 

deposits

N

W

E

S

Sonyea

Genesee River

NEW YORK

Map area

Geneseo

A roadbed fractured above the mine collapse area.
The principal aquifer system in the

Genesee Valley is confined below the

lacustrine (lake) deposits. A 10-foot-thick

sand and gravel aquifer lies just below the

lacustrine deposits, and a 10- to 40-footthick basal sand and gravel aquifer directly

overlies the Hamilton Group shales in the

southern part of the valley and the

Onondaga Limestone in the central and

northern parts. The hydraulic connection

between the basal aquifer and the

underlying bedrock units throughout the

valley is poorly documented but is probably

greatest in the northern half of the valley.

The basal sand and gravel aquifer and

water-bearing fractures in the Onondaga and

Bertie Limestone bedrock are the major

sources of ground water that flooded the

Retsof Mine.

Hydrogeology of the Genesee Valley

The Genesee Valley study area includes the

Canaseraga Creek Valley from Dansville north

to Mt. Morris, and the Genesee River Valley

from Mt. Morris north to Avon. The Genesee

Valley was formed by preglacial stream and

glacial erosion of sedimentary bedrock that

dips southward at about 42 feet per mile. The

bedrock floor of the Genesee Valley consists

of Hamilton Group Shales from Dansville

north to near Sonyea; from there north to

Fowlerville, it consists of Onondaga

Limestone (see cross sections on back page).

As much as 750 feet of unconsolidated

sediments were deposited in the valley by

glacial processes and another 50 feet of gravel,

sand, and silt have been deposited on top of

the glacial sediments by valley-floor and

tributary streams since the glaciers receded

about 12,000 years ago.

Sea

level

600

600

10 5 0

Horizontal distance from collapse (in miles)

15 5 10

Elevation (in feet)

Shales, siltstones

and sandstone

Brackish water

Fresh water

Fowlerville

moraine

Dansville

Geneseo Avon

Sonyea

Hamilton 

Group 

shales

Onondoga limestone Bertie limestone

Salt layer

Basal

aquifer

Shallow aquifer

Glacial deposits

Alluvial (floodplain)

Lacustrine (lake)

Lacustrine (lake)

Mixed

Basal

Till

Area above collapse

Retsof salt mine

South North

Water from the basal aquifer

entered the mine through the

collapsed area.

Horizontal distance from collapse (in miles)

1 2 0 1

Fresh water

Retsof salt mine

Area above collapse

West East
Hydrogeologic Effects of the Collapse

The hydrogeologic effects of the mine

collapse and the subsequent dewatering of

overlying aquifers in the valley include, but

are not limited to: (1) the loss of potable

ground-water supplies through lowered water

• Ground-water level declines—After the

mine collapse, water levels declined in several

domestic wells along the edges of the Genesee

Valley, and in wells that tapped the deeper

aquifers. Shallow wells that tapped the

unconfined alluvial aquifer were unaffected.

The rate of water-level decline varied locally;

for example, water levels declined 20 feet or

more along Wheelock Road (about 1 mile

south of Cuylerville) within 5 days of the

collapse, a decline of more than 100 feet was

reported in one well in Mt. Morris during the

next 2 months, and declines of 50 feet or more

were reported in the Fowlerville area during

the next 6 months. The largest water-level

decline—greater than 400 feet—was reported

near the mine-collapse area. The loss of water

from the basal aquifer to the mine affected

water levels within 10 miles north and south

of the collapse area.

• Subsidence—Land subsidence caused by

the collapse has resulted in the formation of

two large (about 200 feet wide and 50 feet

deep) sinkholes and the collapse of the State

Route 20A bridge near Cuylerville. Several

homes and barns, as well as agricultural lands

and public utilities, were damaged by

subsidence. In addition to the formation of 2

large sinkholes above the mine’s southern end

as a direct consequence of the March and

April 1994 collapses, subsidence from three

other causes has occurred in the valley:

1. Subsidence due to the slow but steady

closure of the mine opening. This process

generally occurs above any mined-out area,

and mining engineers expect the land surface

above the Retsof Mine to subside 8 to 9 feet

over the next century or two. Differential

(nonuniform) subsidence is expected along the

margins of the mine, where stresses from

adjacent subsidence may create enough tension

to rupture the subsurface or surface. Horizontal

movement and tilting of the land surface in the

direction of the mine also can be expected in

these areas. These areas may continue to be

prone to structural damage over time.

2. Subsidence due to the dissolution of

remaining salt. Engineering reports indicate

that the magnitude of subsidence in the area of

rapid freshwater inflow to the mine was

greater than expected, and that the rate of

subsidence was much greater than would be

seen in a dry-mine situation. This type of

subsidence, although rapid, progressed only in

the area of freshwater inflow. Once the mine

was filled with saturated saltwater, no new

freshwater entered, and all subsidence of this

type quickly decreased.

3. Subsidence due to the rapid lowering of

ground-water levels in the principal aquifer,

followed by a slow recovery. This type of

subsidence is due to the compaction of finegrained layers generally within the confined

aquifer below the lacustrine unit as ground

water is drained, causing the removal of

buoyant support between the individual silt

and clay particles. Although this process

caused a much smaller degree of land

subsidence than the previously mentioned

processes, the large declines in ground-water

levels after the collapse were enough to cause

some measurable elastic and inelastic

compaction (consolidation) of the sediments,

and this compaction resulted in small-scale

land subsidence within an area larger than the

underlying mine. This subsidence was

greatest in areas where the fine-grained

sediments are thickest.

• Release of natural gas from the aquifer—

Soon after the mine began to flood, the decline

of water levels in the basal aquifer allowed

natural gases (hydrogen sulfide and methane)

to escape from the depressurized ground

water, just as carbon dioxide gas comes out of

solution, forming gas bubbles, when a bottle

of carbonated soda is first opened. These gases

escaped from test wells near the collapse, from

some domestic wells along the eastern valley

wall, and at land surface from fractures in the

bedrock near the northwestern part of the

mine. The gases were flared-off (burned) at

several wells to reduce the odor and protect

the health and safety of nearby residents.

levels in wells, (2) short- and long-term land

subsidence, and (3) impaired air quality as a

result of the release of methane and hydrogen

sulfide gases.
USGS Efforts

The collapse of the Retsof mine has caused

major changes in the ground-water-flow

system of the Genesee Valley. During the first

few weeks of the mine collapse, the USGS

provided hydrogeologic expertise to State and

local officials as they assessed the immediate

consequences. The USGS also assisted in

developing a ground-water level monitoring

network, collected surface- and boreholegeophysical data to characterize the hydrogeologic framework of the valley, and recently

developed a numerical model of groundwater-flow in the aquifer system to simulate

drainage of ground water into the mine and the

This fact sheet can be found on the WorldWide Web at: http//:ny.usgs.gov

Sources of Technical Information

Alpha Geoscience, 1996, Geologic and

hydrogeologic investigation of the Genesee

River Valley: Albany, N.Y., Alpha Geoscience,

prepared for Akzo Nobel Salt Inc., Clarks

Summit, Pa., 31 p.

Boyd, J. T., 1995, Impact analysis - Retsof

Mine - Akzo Nobel Salt Inc., Livingston

County, New York: Pittsburgh, Pa. John T.

Boyd Co., Report No. 2455, 45p.

Dunn Corporation, 1992, Hydrogeologic

report for the Akzo ash processing plant:

Albany, N.Y., Dunn Corporation, prepared for

Akzo Salt, Inc., Clarks Summit, Pa., 35 p.

Muller, E.H., Braun, D.D., Young, R.A.,

Wilson, M.P., 1988, Morphogenesis of the

Genesee Valley: Northeastern Geology, v. 10,

no. 2, p. 112-133.

Muller, E.H., Young, R.A., Rhodes,D.D.,

Willette, P., Wilson, M., and Fakundiny, R.H.,

1981, Surficial geology of the Genesee Valley:

New York State Geological Survey, Open-File

Report 2044, 174 p.

Mullins, H.T., Wellner, R.W., Petruccione,

J.L., Hinchey, E.J., and Wanzer, S., 1991,

Subsurface geology of the Finger Lakes

region, in New York State Geological

Association 1991 guidebook - 63rd Annual

Meeting, Oneonta, N.Y.: p. 1-54.

Shannon and Wilson Inc., 1995, Task 1

Report - Retsof Mine collapse - Technical

Assistance Grant Committee, Retsof, New

York, Report No. W-6983-01: Seattle, Wash.,

Shannon and Wilson, Inc., 42 p.

Van Sambeek, L. L., 1996, Dissolutioninduced mine subsidence at the Retsof Salt

Mine, in Solution Mining Research Institute

meeting papers: Cleveland, Ohio, October

20-23, 1996, p. 289-309.

Williams, J.H., 1996, Application of

borehole geo-physics at the Retsof salt mine

collapse site, western New York, in Bell, R.S.,

and Cramer, M.H., (eds.), 1996, Symposium

on the Application of Geophysics to

Engineering and Environmental Problems:

Proceedings, April 28 - May 2, 1996,

Keystone, Colorado, p. 813-821.

recovery of water levels in the aquifer. The

model is based on data supplied by Livingston

County, State agencies, consultants, and the

local citizens’ Technical Assistance Group, as

well as that collected by the USGS. The model

is being used to delineate areas affected by

water-level declines and to estimate the time

required for the ground-water system to return

to pre-collapse conditions. Model results will

also be used to identify the factors that control

the rate of drainage and recovery of the

aquifer system, and the areas where additional

hydrogeologic data are needed to improve the

accuracy of the model.

by William M. Kappel, Todd S. Miller, and Richard M. Yager

 For More Information contact:

Subdistrict Chief

U.S. Geological Survey

903 Hanshaw Road

Ithaca, N.Y. 14850