ater defines Michigan -- our heritage, our history, our future. Although Michigan is renowned as the Great Lakes State, the vast bodies of freshwater that surround the two peninsulas and our inland lakes and streams provide drinking water for little more than half the state's residents.

It's the water we can't see -- liquid life that gathers below the ground, soaked up by sponge-like earth or trickling through prehistoric bedrock -- that sustains 43 percent of Michigan residents.

It's called groundwater. It's out of sight, and too often out-of-mind, but protecting it and preserving it is vital for the life we cherish. For those who grew up in rural Michigan, groundwater sprung from a backyard well, or quenched our thirst after a summertime hike brought us to a locally-famous artesian spring.

From tiny villages in the Upper Peninsula to the urban areas of Lansing and Kalamazoo, groundwater is harnessed to use when we drink, bathe, cook, clean and create.

But groundwater, for many of us our drinking water, is constantly being threatened by modern life in our homes, on our farms, and through businesses and industries. If we over-fertilize our lawn, dump used motor oil in a vacant lot, or don't take proper care of a septic tank, we can endanger the groundwater supply for our neighbors and ourselves.

Small amounts of hazardous products can contaminate large amounts of water. For example, one gallon of gasoline dumped on the ground or into an abandoned well has the potential to contaminate as much as 750,000 gallons of water.

Here, in the United States, we enjoy access to safe drinking water. Seven out of ten people in this world do not. But what we take for granted can easily be endangered. That's why the W.K. Kellogg Foundation, of Battle Creek, has funded Groundwater Education in Michigan, the GEM program, to help homeowners, farmers, businesses, elected officials and schools stress the importance of protecting our groundwater, and therefore, our drinking water.

Unfortunately, in recent years there have been more than 1,300 instances in Michigan where wells supplied by groundwater have been contaminated by bacteria, carcinogenic compounds, petroleum solvents, and other pollutants. The problem of groundwater contamination is particularly serious for owners of private wells because water testing is not conducted on a regular basis unless initiated by the homeowner. Thus, the residents may not be aware of contaminants in their drinking water.

In Pinckney in Livingston County, solvents sent down a floor drain in an auto parts factory contaminated well-water for several hundred residents. They drank bottled water until newer, deeper wells could be drilled. In addition, contamination also resulted from two aged gas stations and a bulk fuel storage facility.

In Eaton County's Grand Ledge, some 4,500 gallons of gasoline leaking from a service station's underground tank, contaminated a public well that provides drinking water for 3,300 homes and businesses. Clean-up costs have exceeded $2 million.

In Eben and Trenary, in central Upper Peninsula, the state public health officials supplied bottled water to residents for several years because petroleum contaminated shallow drinking water wells. Some residents in the U.P.'s Garden

Peninsula have wells contaminated by bacteria, which may have come from improperly designed or failing septic systems, runoff from livestock operations, and improper garbage disposal. At the Fayette State Park in the Garden Peninsula, water is trucked in for all uses.

In the village of Schoolcraft near Kalamazoo, groundwater was contaminated two decades ago by the release of 15 to 20 gallons of a chemical once commonly used to fumigate grain stores in silos. Although the slow-moving plume of contamination has yet to reach potential drinking water supplies, the State of Michigan has spent $2.5 million to clean it up before it reaches wells or lakes in the area.

What we dump today on our lawns, down a drain, or into a vacant lot can end up in someone else's drinking water the next day -- or years later -- through groundwater.

When it rains some water evaporates, and some water is taken up by plants, while some flows into lakes, rivers and wetlands. But some seeps into the ground to become groundwater, which totally fills the pores in the sand, gravel, clay, and bedrock beneath the earth's surface.

Groundwater is not an underground lake. The earth hidden beneath us more closely resembles a large sponge where groundwater seeps into the empty spaces. The layers of soil and rock underground holding useable amounts of freshwater is called an aquifer. An aquifer is the geologic material into which we drill to get water.

Groundwater is constantly moving. It sometimes flows upward, responding to pressure differences underground, to feed springs, wetlands, rivers and lakes. Groundwater can move several feet per day in very porous materials. Through clay, it barely moves at all. In limestone, the water can actually dissolve the rock along the cracks, creating cave passages through which groundwater can move very quickly.

Groundwater is a shared resource. It moves from one piece of property to another, respecting only the boundaries nature intended. Although groundwater is a hidden human resource, many Michigan residents see it, taste it, and feel it every time they turn on a faucet. And everyone depends on groundwater indirectly through the water cycle.

Here are some examples of what you can do to protect groundwater. We'll explore this in greater depth in upcoming articles:

1. At Home: Landscape your yard with plants that need a minimum of water and fertilizer. Use only the amount of fertilizers and pesticides that plants need. Use non-toxic chemical alternatives and limit the hazardous products you use when feasible.

2. In Your Community: Protect wetlands that serve as natural buffers against pollution, soil erosion and flooding. Support wellhead protection efforts.

3. On the Farm: Apply pesticides at recommended rates. Utilize soil testing to determine necessary amounts of fertilizers. Get involved with the Groundwater Stewardship Program, sponsored by the Michigan Department of Agriculture, in your county. Work through a Farm*A*Syst assessment of your farmstead.

4. Contact your Regional GEM Center, your local Soil Conservation District office, Extension office, or health department for more information about how to protect your groundwater.