Ohio State University Extension

Food, Agricultural and Biological Engineering

590 Woody Hayes Dr., Columbus, Ohio 43210

Mercer County Ground-Water Resources

AEX-490.54

E. Joseph Beiler
James M. Raab
Larry C. Brown
Karen T. Ricker

Water stored under the earth's surface is a plentiful, yet precious, resource in most areas of Ohio. Many human activities may affect the quality and quantity of this resource. However, the availability and quality of this resource are influenced directly by the properties of the geologic formations that hold water. The chemical and physical nature of these formations varies from area to area, creating a wide range of water yields and quality at different depths. This publication contains information about the ground-water resources underlying Mercer County. Its purpose is to help the reader better understand the factors that influence the quantity and quality of ground water. An overview of the county's water resources is provided in the publication Water Resources of Mercer County, AEX-480.54.

Much of the water resource and water quality terminology used in this publication is described in Extension Fact Sheets AEX 460 and 465. Extension publications are available through the Mercer County office of Ohio State University Extension.

Aquifers

Geologic formations (e.g., sand, gravel, limestone, sandstone) have the ability to receive, store and transmit water. In general, if a formation is capable of yielding enough water to support a well or spring, it is called an aquifer. The material from which the formation originally was made influences its ability to store and transmit water. For example, sand and gravel allow water to flow through easily. By comparison, shale, which originated from compacted layers of mud and clay, generally allows very little water to flow through it unless the shale is highly fractured.

The regionally extensive carbonate aquifer, which is composed of layers of limestone and dolomite, is the principal source of ground water in west-central Ohio, including Mercer County. Limestone is consolidated limy mud or calcareous sand, sometimes with fossilized sea shells and shell fragments. The main mineral in limestone is calcium carbonate, CaCO₃. Dolomite is similar to limestone, but its main mineral is calcium magnesium carbonate, (Ca,Mg)CO₃. Both limestone and dolomite are technically referred to as carbonate rocks, though they are commonly both called limestone. The limestone and dolomite formations, which underlie most of the western portion of Ohio, were deposited about 400 to 450 million years ago. These formations are covered by a layer of glacial till, which is an unsorted mixture of clay, silt, sand, gravel and boulders, deposited by glacial processes that occurred approximately 10,000 to 20,000 years ago.

Limestone formations usually are adequate sources of ground water because of their naturally formed solution channels, joints and fractures, which provide water storage capacity and pathways for water movement. The number of fractures and other openings in limestone varies greatly from one location to another and affects the amount of water that may be encountered when drilling a well. The position of such openings rarely can be determined from the land surface; therefore, there is always some uncertainty as to the production capability of a proposed well. Ground water also occurs in lenses (or pockets) of sand and gravel deposited by glacial processes. Some of these deposits occur in buried valleys associated with the ancestral Teays Valley. The Teays Valley is a remnant of an ancient drainage system that cut a valley into the limestone before the area was glaciated. In some areas of Mercer County, the valley has cut 350 feet into the bedrock (see Figure 2). Glacial till generally does not provide enough water to support a well. Contact the Ohio Department of Natural Resources (ODNR), Division of Geological Survey, for information on Ohio's geologic formations (Fountain Square, Columbus, OH 43224-1362).

Well Yield

The actual yield of a well, in gallons per minute (gpm), will vary considerably depending on the age and depth of the well, the diameter of the casing, well construction, pump capacity and age, and most importantly, properties of the geologic formation. The exact yield and depth of each well will depend on the properties of the geologic formation at the specific location of the well.

Ground-Water Availability

To support the development of ground-water availability assessments in Ohio, the Ohio Department of Natural Resources (ODNR), Division of Water, maintains a statewide database of more than 700,000 well logs. The Ground-Water Resources Section of the Division manages this valuable database, which includes some information collected by the U.S. Geological Survey (USGS) and the Ohio Environmental Protection Agency (Ohio EPA). Since 1948, well-log information has been collected to increase the understanding of the ground-water resources in Ohio. Geologists and hydrogeologists continue to study the state's ground-water resources. As a result, Ohio is one of only a few states that has been completely mapped for ground-water availability (mapped by river basin, from 1959 to 1962).

Estimates of the size, shape, geologic make-up and yields of aquifers are being mapped county by county. Most of Ohio's counties have a completed map. The map presented in Figure 1 is a generalized representation of the water-bearing formations underlying Mercer County (adapted from map by R. J. Kostelnick, 1982). This illustration is based on a hydrogeologic interpretation of the well-log data from Mercer County and surrounding areas. It should be used only as a guide to understanding the ground-water resources in the county. The section below provides a brief description of the types of aquifers illustrated on the map in Figure 1.

Figure 1. Ground-water resources of Mercer County, Ohio (modified from K.S. Crowell, ODNR Division of Water, by R.A. Roberts).

AREA A: Limestone and Dolomite, High-Yield Potential

Area A in Figure 1 is the primary and most productive aquifer in Mercer County. Yields of 400 gpm may be developed from wells encountering various solution cavities in the carbonate bedrock. Farm and domestic supplies of 20 to 40 gpm may be developed at depths of less than 150 feet. The bedrock is covered with 20 to 100 feet of glacial till consisting of thick clay, sandy clay and thin lenses of sand and gravel.

AREA B: Sand and Gravel in Teays Valley System

Delineated as Area B, deep sand and gravel lenses lie within the ancestral Teays Valley. These deposits are underlain by impermeable Silurian-aged (425 to 438 million years ago) shale. Sand and gravel lenses may yield over 25 gpm to screened wells. These buried valleys are remnants of old streams which had cut their courses deep into the bedrock before the area was glaciated. Later, with the coming of the glaciers, the valleys were completely filled with glacial till. Exploratory drilling is necessary to determine the presence of thick, coarse deposits capable of yielding large supplies. The larger yields are obtained in portions of the buried valleys where such deposits have access to recharge from surface streams.

Figure 2 is a generalized cross section (referenced in Figure 1 as the line X-X') of a portion of Mercer County. This cross section shows the range of depth to bedrock as well as the variation in composition of the glacial till in the vicinity of Grand Lake St. Marys. The cross section cuts across the Teays Valley and one of its tributaries. It shows one of the deepest areas extending over 400 feet below the surface of Grand Lake St. Marys.

Figure 2. Generalized cross section of Mercer County, Ohio (adapted from Underground Water Resources, ODNR Division of Water, by R. Roberts).

Ground-Water Levels

The water level in any well does not remain constant, but changes in response to several factors. Rainfall distribution and amount may affect ground-water recharge and discharge, and subsequently may affect the water level in area wells. Also, wells that are hydraulically connected to a stream may show fluctuations in the water level as the stream level changes. In some cases, depending upon the hydraulic properties of the geologic formation, the intense pumping of a well, or number of wells, may cause the water level in some nearby wells to be lowered.

The ODNR Division of Water monitors one well in Mercer County. It is located at New Idea in Coldwater, and is noted as Observation Well MR-2 in Figure 1. This well, along with other wells throughout west-central Ohio, is used to monitor the natural seasonal fluctuation of water levels in various aquifers. Observation Well MR-2 is 253 feet deep and the depth to limestone is approximately 37 feet. This well penetrates most of the carbonate aquifer. Continuous water-level measurements have been recorded at MR-2 since February 1967. The lowest level recorded on MR-2 at Coldwater was 81.6 feet below land surface in September 1988; the highest level recorded was 60.1 feet below land surface in February 1967.

Ground-Water Quality

Various state and federal agencies have participated in programs to determine the ground-water quality in Ohio. For six wells in Mercer County, water-quality data were available from the ODNR Division of Water. In Figure 1, these wells are noted as Chemical Analysis Sites 1 through 6. These sites are municipal and ODNR wells.

The results from some of the chemical tests performed on these Mercer County wells are given in Table 1. The chemical constituents listed are total dissolved solids, hardness (as CaCO₃), calcium, magnesium, iron, chloride, sulfate, and fluoride. For comparison purposes, secondary drinking water-quality standards for these chemical constituents also are shown. These standards are established by the U.S. Environmental Protection Agency (USEPA) for public water systems for aesthetic reasons (taste, odor, appearance, etc.), and are not enforceable. These chemical constituents do not pose a risk to human health (see notes in Table 1). For private wells, there are no legally enforceable drinking water-quality standards other than total coliform, which is an indicator of bacteriological quality.

Ground water, whether obtained from bedrock or glacial deposits, may require some treatment. In some areas, water containing calcium carbonate (CaCO₃, i.e. hard water) in concentrations greater than 180 ppm, and iron concentrations greater than 0.3 ppm, may require treatment for some uses (see notes in Table 1). Wells drilled into shale or limestone may produce water that contains objectionable quantities of hydrogen sulfide (rotten egg odor). In general, the probability of obtaining sulfur in objectionable amounts increases with the depth drilled. Ground water in the southern part of the county, obtained from both the glacial till and the bedrock, is hard and may contain iron in varying amounts.

The information in Table 1 can be used as a guide to what one might expect from an existing or new well developed in similar geologic material in the county. This information provides a general representation of the quality of the water at the time of sampling, which was not the same for all wells. In most cases, the data provided in Table 1 was taken from a water sample obtained just after the well was put into operation. Even though all six of these wells were developed in the limestone underlying Mercer County, and these wells are in the range of 240-294 feet deep, some variation exists in the concentrations of each of these chemical constituents. Just as well yields differ, water quality will vary depending on aquifer properties at the specific location of each well. One should not forget that many human activities also affect the quality of ground water (see AEX 465).

Table 1. Chemical constituents of selected Mercer County wells1.
Well No.	1	2	3	4	5	6	WQ Std2
Well Depth (feet)	280	240	290	241	294	294
Capacity (gpm)	1000	175	125	225	400	100
Depth to Bedrock (feet)	30	25	42	39	65	98
Water-Bearing Formation3	LS	LS	LS	LS	LS	LS
Chemical Constituents4
Total Dissolved Solids	1230	711	1280	1440	1344	1320	500
Hardness (as CaCO3)	884	610	910	860	796	750	none5
Calcium	189 1	40	200	nt6	nt	nt	none
Magnesium	100	63	100	nt	nt	nt	none
Iron	nt	nt	nt	1.5	2.5	5.0	0.3
Chloride	8	6	9.6	10	8	13	250
Sulfate	566	230	700	nt	nt	nt	250
Fluoride	1.2	1.0	1.5	nt	nt	nt	2
1 Data on wells 1-3 taken from map by R. J. Kostelnick, 1982; wells 4-6 from Underground Water Resources map G; general location of each well is shown on Figure 1.
2 USEPA Secondary Water Quality Standard.
3 LS-Limestone.
4 Units are parts-per-million, ppm; Comments as per Interpreting Your Water Test Report (1988); Total Dissolved Solids: Concentrations above 500 ppm may cause adverse taste and deteriorate domestic plumbing and appliances. Use of water containing 500 ppm is common. Hardness: Primary concerns are that more soap is required for effective cleaning, a film may form on fixtures, fabrics may yellow, and scales may form in boilers, water heaters, and cooking utensils. Calcium and Magnesium: Main constituents of hardness. Primary concerns with hardness are that more soap is required for effective cleaning, a film may form on fixtures, fabrics may yellow, and scales may form in boilers, water heaters, and cooking utensils. Iron and Manganese: Iron concentrations greater than 0.3 ppm and manganese concentrations greater than 0.03 may cause brown or black stains on laundry, plumbing fixtures and sinks. Metallic taste may be present which may affect the taste of beverages made from the water. Chloride: High concentrations may result in an objectionable, salty taste to water and the corrosion of plumbing in the hot water system. Sulfate: Concentrations in excess of 250 ppm may have laxative effect on persons unaccustomed to the water. Also affects the taste of water and will form a hard scale in boilers and heat exchangers. Fluoride: At concentrations greater than 1.5 ppm, fluorosis (mottling) of teeth may occur. USEPA Primary Standard is 4 ppm.
5 No USEPA Secondary Standard.
6 nt = not tested.

Summary

Mercer County's ground-water resources are valuable assets to the county's citizens and industry. The availability and quality of these resources are directly influenced by the properties of the geologic formations underlying the county. The productive limestone, and sand and gravel formations that underlay much of Mercer County, have the potential to provide excellent water adequate for domestic, agricultural, industrial, and many municipal uses. By understanding the physical and chemical nature of these resources, better decisions can be made about ground-water protection, management and use. This publication provides an overview of the county's ground-water resources. It should be used as a guide, and not as a substitute for detailed information and professional advice when drilling a well.

Where to Get More Information

The Mercer County office of Ohio State University Extension can provide other publications on the county's water resources. Your Extension agent, the Mercer County Health Department, and Ohio EPA Northwest District Office-NWDO (347 North Dunbridge Rd. Bowling Green, OH 43404) can provide information on potential sources of ground-water contamination, well-water testing and drinking-water quality. The ODNR Division of Water-Ground-Water Resources Section (Fountain Square, Columbus, OH 43224) is an excellent source of information on ground water. Some of the information in this publication was summarized from the map, Ground-Water Resources of Mercer County, and other information available through the Division of Water. This map is much more detailed than that given in Figure 1, and the Ground-Water Resources Section can provide detailed information on ground-water availability and wells. The USGS, Ohio District (975 W. Third Ave. Columbus, OH 43212), also provides information concerning ground water in Ohio.

Bibliography

Ground-Water Resources of Mercer County. 1982 R. J. Kostelnick. ODNR Division of Water. (map).

Interpreting Your Water Test Report. 1988. D. Lundstrom and S. Fundingsland. AE-937, No. 13-AENG-10. North Dakota State University Extension Service.

Nonpoint Source Pollution: Water Primer. 1993. R. Leeds and L. C. Brown. AEX 465. Ohio State University Extension.

Ohio Ground-Water Quality. USGS National Water Summary-Ohio. 1986. U.S. Geological Survey Water-Supply Paper 2325.

Ohio Ground-Water Resources. USGS National Water Summary-Ohio. 1984. U.S. Geological Survey Water-Supply Paper 2275.

Surface and Ground Water Terminology. 1990. L. C. Brown and L. P. Black. AEX 460. Ohio State University Extension.

Underground Water Resources (maps of various river basins). 1958-1962. ODNR Division of Water.

Water Resources of Mercer County. 1994. E. J. Beiler, K. T. Ricker and L. C. Brown. AEX-480.54. Ohio State University Extension.

Water Testing. 1988. K. Mancl. AEX 314. Ohio State University Extension.

Acknowledgments

This publication was produced through the Ohio Water Resources Education Project, in cooperation with: ODNR Division of Water; Ohio EPA; USGS, Ohio District; and Ohio Department of Health (ODH). Project leaders are Larry C. Brown and Karen T. Ricker. Partial support for this publication was provided by these cooperating agencies and programs: Ohio State University Extension, Mercer County; Mercer County Commissioners; Mercer Soil and Water Conservation District; Overholt Drainage Education and Research Program; and the Ohio Management Systems Evaluation Area Project (USDA Extension Service Grant No. 90-EWQI-1-9018).

The project leaders acknowledge the following reviewers: Jim Will (USDA-Soil Conservation Service, Mercer County); Ken Strickland (Wright State University, Lake Campus); Scott Golden (Environmental Health, ODH); Steve Hindall (USGS, Ohio District); and Tim Fishbaugh (Ohio EPA, NWDO).

A special thanks to Peg Wiehe (Secretary, Ohio State University Extension, Mercer County), Michelle Roby, Ross Roberts, and John Humphreys (Agricultural Engineering Undergraduate Assistants) for help in illustration and manuscript preparation, and Kim Wintringham, Associate Editor (Section of Communications and Technology, Ohio State University Extension), for editorial and graphic production.