Occurrence of Ground Waters 

of Low Hardness and of 

High Chloride Content in 

Lyon County, Minnesota

GEOLOGICAL SURVEY 

CIRCULAR 423
Occurrence of Ground Waters of Low

Hardness and of High Chloride Content 

in Lyon County, Minnesota

By H. G. Rodis and Robert Schneider

Prepared in cooperation with the Division of Waten, 

Minnesota Department of Conservation

GEOLOGICAL SURVEY CIRCULAR 423

Washington, D. C. 

I960
United States Department of the Interior

FRED A. SEATON, SECRETARY

Geological Survey

THOMAS B. NOLAN, DIRECTOR

Free on application to the U.S. Geological Survey, Washington 25, 0. C.
CONTENTS

Page Page

Introduction......................... 1 Occurrence of ground water....._.__ 1

Geologic conditions................... 1 Conclusions........._............. 2

ILLUSTRATION

Plate 1, Map showing approximate extent of areas where ground waters of low hardness

and of high chloride content occur in Lyon County, Minn......... Inside back cover

III
Occurrence of Ground Waters of Low Hardness and of 

High Chloride Content in Lyon County, Minnesota

By H. G. Rodis and Robert Schneider

INTRODUCTION

The ground water in Lyon County and elsewhere in southwestern Minnesota is generally hard and low in chloride. It is the purpose of this report to describe briefly the 

occurrence in Lyon County (pi. 1) of waters 

of low hardness and of high chloride content. 

The waters are found largely in Cretaceous 

sandstone.

The data were collected in conjunction with 

a countywide study of the geology and groundwater resources that is being made as part 

of a statewide investigation in cooperation 

with the Division of Waters, Minnesota Department of Conservation. The work is under 

the direct supervision of the junior author, 

who is district geologist for Minnesota.

Hardness and chloride determinations were 

made with field-testing kits at the time data 

were obtained on most of the farm wells in 

the county. Tests were made on wells that 

were reported to yield relatively soft or 

"salty" water.

GEOLOGIC CONDITIONS

The basement rock in northern, northeasttern, and eastern Lyon County is Precambrian granite that has been deeply weathered. 

It is covered by a thin, extensive regolith 

which, in this area, consists of colluvial 

deposits and weathered materials, some of 

which were transported and reworked during 

the Cretaceous.

Upper Cretaceous strata overlie the granitic basement rock and are covered with 

glacial drift. They are mostly flat lying and 

are composed largely of thick beds of soft 

bluish-gray shale alternating with thin beds

of loosely consolidated sandstone. The strata 

were deposited in a basin whose axis bisects 

the county from northwest to southeast. The 

Cretaceous strata are about 450 feet thick 

near the center of the depositional basin and 

gradually pinch out against its flanks. In the 

vicinity of the northeast flank of the basin, 

thin, discontinuous beds of sandstone occur 

at depths of about 100 to 300 feet. The sandstones are composed of subangular to subrounded quartz grains and are not more than 

a few feet thick. In places, they are in contact with the underlying regolith.

OCCURRENCE OF GROUND WATER

The Cretaceous sandstones are the principal sources of ground water in the report 

area. The glacial drift, being very thin in 

this area, is relatively unimportant. A few 

domestic wells of small yield obtain water 

from the regolith at depths of 300 to 550 feet.

Water in the Cretaceous aquifers occurs 

under artesian conditions. Static water levels in most farm wells range from 20 to 70 

feet below the land surface, but there are 

some flowing wells along the southwest edge 

of the area. The sandstones containing water of low hardness and of high chloride content generally occur between depths of about 

100 and 300 feet.

The accompanying map shows the approximate extent of areas in Lyon County where 

soft and moderately hard waters occur. Soft 

water is defined in this report as having a 

hardness of less than 70 ppm (parts per 

million), and moderately hard water, 70 to 

120 ppm. Soft and moderately hard waters 

occur in areas totaling about 130 square 

miles, including about 20 square miles in 

which soft water alone is found. The chloride

1
GROUND WATERS OF LOW HARDNESS AND HIGH CHLORIDE, LYON COUNTY, MINN.

content of water in these areas ranges from 

about 50 to 500 ppm, the average being about 

250 ppm.

In the north-central part of the county- the 

map shows the approximate limits of an area 

of about 50 square miles in which the chloride content of the ground water from Cretaceous rocks ranges from 500 to 2,000 ppm 

and more. The area overlaps only in small 

part with that of water of low hardness. 

Variations in chloride concentration are 

shown by isochlors lines connecting points 

where equal chloride concentrations occur. 

The areas of maximum concentration (more 

than 2,000 ppm) are in the vicinity of sec. 2, 

T. 113 N., R. 43 W., and sees. 2, 3, 4, and 5, 

T. 113 N., R. 42 W.

Most of the waters of relatively high chloride content in north-central Lyon County 

are found at about the same depths as the 

waters of low hardness to the east and northeast (100 to 300 feet), but locally they may 

also occur in the underlying regolith at 

greater depths. Except for the waters of 

highest chloride content, which are used for 

stock, most of the waters in this part of the 

county are used for both domestic and stock 

supplies.

Although the map appears to indicate rather 

definite limits for the areas shown, relatively 

soft water and water of high chloride content 

have been found locally outside these areas. 

Examples of this are the wells at the following locations: sec. 11, T. 109 N., R. 41 W.; sec. 

1, T. 110 N., R. 42 W.; sec. 35, T. Ill N., R. 

41 W.; sees. 16 and 28, T. 112 N., R. 42 W.; 

and sec. 6, T. 113 N., R. 43 W.

Despite the high artesian head, there is a 

considerable lowering of the water level when 

farm wells are pumped at rates of 4 to 7 gallons per minute. Apparently the sandstones 

have a low coefficient of transmissibility.

CONCLUSIONS

The report area was near the margins of 

the Cretaceous sea where marine, near shore, 

and continental sediments were deposited. It 

is probable that the strata originally contained water ranging from fresh to salty. 

When the Cretaceous seas receded, fresh 

water infiltrated and moved underground, diluting the water in the more permeable strata. The water apparently was softened by 

natural ion exchange where it came in contact 

with clay minerals which were derived from 

the weathered granitic rock mass to the 

northeast and which became saturated with 

absorbed sodium where the strata contained 

salt water.

Because the chemical data shown on the 

accompanying map were obtained by field 

methods of analysis, the quality and occurrence of the ground water canbe described in 

only a general way. To describe the quality 

in detail, would require numerous laboratory 

analyses.

Ground waters of the same quality probably 

occur in the Cretaceous aquifers of adjacent 

areas in Redwood and Yellow Medicine Counties. Elsewhere in southwestern Minnesota 

where the same geologic environment prevails, it is a reasonable as sumption that similar ground-water conditions may be found.

INT.DW,,D.Q.$O-
UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

CIRCULAR 4225 

PI ATE i

96005 96° 00' 55' 50' 45' 40' 95" 35'

EXPLANATION

R.43W.

r Area of soft (hardness <70 ppm) and 

moderately hard (hardness 70-120 

ppm) water

44°35'

30'

Area of soft water (hardness <70 ppm)

N lOOOv_

Isochlor, dashed where inferred; figure 

is chloride concentration, in parts per 

million

WELLS

Figure near well is depth, in feet below land 

surface. Depths shown to tenths of a foot 

are measured; all others are reported. Where 

no figure appears, depth is unknown. "F" 

indicates flowing well

Yields soft water

Yields moderately hard water

Yields water containing more than 500 

ppm of chloride

Yields soft water containing more than 

^00 ppm of chloride

Yields moderately hard water containing 

more than 500 ppm of chloride

25'

95'35

MAP SHOWING APPROXIMATE EXTENT OF AREAS WHERE GROUND WATERS OF LOW HARDNESS AND OF HIGH CHLORIDE

CONTENT OCCUR IN LYON COUNTY, MINNESOTA

SCALE 1:125000