A comprehensive Ottawa County groundwater study was completed in 2018 by the Michigan State University Department of Civil and Environmental Engineering and the Michigan State University Institute of Water Research.
The study was commissioned in 2011 due to years of anecdotal reports from residents and well-drillers related to groundwater quality issues, reports of wells becoming dry, and a projected increase in groundwater usage. These conditions were verified using scientific methods and then 3D models were created to show the aquifers under the county. After the 3D models were completed, they were modified in order to predict future conditions based on development projections.
The technical study results are provided below. Other pertinent information--such as the GIS mapping that was developed as part of the study and various techniques and strategies to alleviate groundwater issues-–can also be found throughout this webpage.
Explanation of Key Concepts
1. Basic Michigan Geology
Michigan’s bedrock is mostly covered by materials deposited during consecutive glacial events in this region’s history. In the lower peninsula of Michigan, this bedrock material consists of several sedimentary rock formations that were created during different periods of time. Examples of the types of sedimentary rocks are shales, limestones, and sandstones. The deposition of these formations occurred in a massive basin, known as the Michigan Basin. Because of the basin structure, each glacial event created a formation on top of the other, with the oldest located at the bottom of the basin at a depth of approximately 16,000 feet (4,900 meters) deep, near what is now Gladwin County.
Fig. 1: (Click to Enlarge) Formations that make up the bedrock geology of the Michigan Basin region. (Note that the Marshall Formation underlies the majority of Ottawa County.)
2. Marshall Formation
The type of rock deposited depended upon the conditions of the time it was being made. In the case of the major formation underlying Ottawa County, the Marshall Formation, Michigan was covered by a shallow sea, similar to the Mediterranean Sea. Water running into this sea carried with it eroded materials, such as granite, that in turn wound up as sand being deposited at the bottom of this sea. As time progressed, this sea became separated from the surrounding oceans and began to evaporate because of changing weather conditions. This process of evaporation left behind salt deposits that were mixed in with sand being deposited into the shallow sea. After millions of years, these deposits were compressed into a rock formation that we today call the Marshall Formation.
Fig. 2: A cross-section view of the Michigan Basin Bedrock Formations.
Fig. 3: Locations of shallow seas that once covered parts of North America.
Groundwater is stored in aquifers, which are water-bearing formations underneath the land surface that readily transmit water. Groundwater is extracted via well pumping. A glacial aquifer (also known as a drift aquifer) refers to material that was deposited by retreating glaciers. Much of this material consists of organic material, sand, gravel, and small rocks. Water is typically recharged to this aquifer from precipitation, such as rain or snowmelt. In Michigan, the glacial aquifer is located above the bedrock aquifer.
A bedrock aquifer refers to a layer of water-bearing rock. Examples of bedrock aquifer materials include sandstone or limestone. Bedrock aquifers are typically recharged through the glacial aquifer. Ottawa County has three bedrock aquifers beneath it: the Michigan Formation, the Marshall Formation, and the Coldwater Shale Formation.
Aquitards are formations that do not easily transmit groundwater. A thick, continuous glacial clay layer underlies central portions of Ottawa County, which restricts freshwater recharge to the bedrock aquifer.
5. Static Water Level
Static water level can be defined as the depth below ground level where water stands in a well when it’s not being pumped.
Drawdown is the change in groundwater level as groundwater is extracted via water pumping. Changes in groundwater levels may be small (a few feet) or very large (hundreds of feet). Depending on the intensity and duration of pumping and composition of the aquifer, water levels may recover within a day, week, months, years or never.
Fig. 4: Static water level in an aquifer and the change as drawdown occurs.
Upwelling is the movement of water from deeper depths to replace the water in the aquifer in the zone of influence of a water withdrawal site (vertical movement). In the case of Ottawa County, upwelling of chloride concentrations vertically in the Marshall formation is caused by the slow movement of groundwater horizontally from the surrounding region.
8. Chloride (Cl) Concentrations
The term chloride concentrations refers to the existence of the element chlorine (Cl) in groundwater. Chloride is the common element in all of the salts that make-up the brine within the underlying bedrock aquifer. Examples of salts that occur in the groundwater are sodium chloride (NaCl) and calcium chloride (CaCl).
EPA established a drinking water standard of 250 mg/L for chloride. The standard, which is not regulated or enforced, serves as a guideline to manage non-threatening water issues like taste and odor. High chloride levels not only make drinking water salty tasting, but may negatively affect irrigated crops. Field sampling conducted as part of this study found concentrations of chloride as high as 1,855 mg/L and as a low as 1 mg/L. Background chloride concentrations in Michigan are typically 10-30 mg/L.
9. Water Cycle
Fig. 6: An example water cycle found on Earth.
The water cycle is the movement of water throughout the world. Here are some of the important components:
- Precipitation - Commonly known as rain, snow, sleet, or hail. Precipitation is the formation of water droplets from water vapor, which then falls to the surface of the Earth.
- Recharge - This is the process of water percolating into the ground from the surface. This typically happens after a rainstorm or snow melts on unfrozen ground.
- Discharge - Discharge is the movement of water from an aquifer to a body of water, typically a lake or river in Michigan. This occurs because of changes in elevation, with gravity moving the water from a higher elevation to a lower elevation.
- Evapotranspiration - This is the process by which water enters the atmosphere from the land. Examples include evaporation from wet soil or water from plants.
- Runoff - Runoff is the term used to describe the surface movement of water after a precipitation event (such as a rainstorm). Runoff will continue until it either reaches a nearby body of water or infiltrates through the soil.