High water table and pools: what Ohio homeowners should know

The water table is the level below which the ground is saturated. In much of Northeast Ohio, the one that matters for pools is a perched water table: water sitting on top of a dense clay layer or fragipan instead of draining down. It rises with fall rains and snowmelt and drops through summer.

The USDA's soil descriptions put numbers on it. Mahoning soils, which cover more than 450,000 acres of northeastern Ohio and northwestern Pennsylvania, can hold a perched or apparent seasonal high water table 6 to 12 inches below the surface from October through June in most years. Wadsworth soils hold one 7 to 11 inches down from October to June.

A pool's deep end sits several feet down. In soils like these, for much of the year, the bottom of a pool is well below the level where the ground is full of water.

It seeps in from the sides and bottom and collects at the lowest point. In a wet spring the hole can fill overnight. Standing water softens the base, washes fines into the stone and makes it impossible to set a shell or pour a floor properly.

Builders deal with it in a few standard ways. Aqua Magazine describes the two main approaches: well points, which are pipes driven around the excavation and pumped to draw the water table down before and during the dig, or a sump dug lower than the pool with a slotted pipe in gravel and a submersible pump pulling out the water that comes in. River Pools, a fiberglass manufacturer and installer, recommends starting the excavation from the deep end so water flows there to be pumped, and digging at least 12 inches below the pool depth for the drain pipe.

Which method a site needs depends on how much water there is and how fast it comes. That is why dewatering is priced after a site visit rather than built into a package price. Some yards need a pump for a few hours. Some need one running for days.

A sump pipe, also called a dewatering pipe or well point, is a vertical pipe set at the deep end of the pool, outside the shell, reaching the bottom of the excavation and surrounded by clean stone. The top is cut off at deck level and capped with a lid. River Pools describes using an 8 inch PVC pipe this way. Because the stone backfill lets groundwater move freely, the water level inside the pipe matches the water level around the pool.

That gives you two things for the life of the pool. You can look down the pipe and see where the groundwater sits. And if the pool ever has to be lowered or drained for a repair, you can drop a sump pump in the pipe and pull the groundwater down first. One Latham fiberglass dealer notes that Latham requires a permanent sump pipe on all its pools, or a gravity daylight drain as an alternative.

Keep the lid visible and accessible. When a patio or planting bed buries the sump pipe, it stops being useful at exactly the moment you need it.

It can, when the pool is empty or partly empty and the groundwater outside is high. A full pool weighs far more than the water pressing up on it. An empty fiberglass shell is light, and water pushing on its underside can lift it. A drained concrete pool can crack or heave. A vinyl liner can float up off its floor.

Aqua Magazine points out that a pool can pop even without a naturally high water table. If rainwater collects in the old excavation ramp and soaks in around the pool, the local groundwater around the shell can rise enough to cause trouble. That is one more reason the backfill, grading and sump pipe matter on every site, not only the obviously wet ones. We go into this in more detail in our guide on whether a fiberglass pool can pop out of the ground.

Rarely, and never casually. Most water problems can be solved without draining, including through partial water exchange. If a repair truly needs the pool emptied, River Pools' guidance is to find out first how much water is around the outside of the pool and to be able to remove it, which is what the sump pipe is for.

Timing matters in Ohio. Draining in late fall, winter or spring, when the local soils' perched water table is at its highest, is asking for trouble. Mid to late summer, after a dry spell, is the lowest-risk window. Even then, check the sump pipe, pump the groundwater down, keep pumping while the pool is empty, and refill promptly.

Normal winterizing is different. Closing a pool for an Ohio winter lowers the water only a little below the skimmer or return lines, not anywhere near empty. That is fine in any soil.

If a service company or a well-meaning neighbor suggests draining the pool to fix cloudy water or a stain, ask what else can be done first. With a fiberglass pool in wet ground, emptying it should be the last resort, planned with the sump pipe, not the first step.

A hydrostatic relief valve is a one-way valve in the pool floor, often in the main drain, that opens if groundwater pressure under the pool gets higher than the water pressure inside, letting groundwater in rather than letting it push the structure. They are common in concrete pools.

Their use in fiberglass pools is debated among installers. The pressure needed to open a valve may be more than enough to lift a light, empty shell, so many fiberglass installers rely on the stone backfill and sump pipe instead. For ICF concrete pools, relief at the floor is a standard part of the design conversation. We choose the approach per pool type and per site.

Groundwater that sits close to the surface does not only affect the pool shell. It keeps the soil under the deck wet, and wet soil in an Ohio winter freezes and heaves. A deck poured straight onto clay in a yard like this will move. The answer is a compacted aggregate base under the concrete, a slope that carries water away from the pool, and a plan for where that water goes once it leaves the deck.

The equipment pad needs the same thought. Pumps, filters and heaters should sit on a solid base a little above the surrounding grade, not in the low corner of the yard where water collects every March. It is a small detail that keeps electrical equipment dry and extends its life.

A few clues come before any digging:

• The USDA Web Soil Survey for your lot lists the depth to the water table and the months it is high.
• A sump pump in your basement that runs often in spring tells you the groundwater comes up near your foundation.
• Puddles that linger for days, mossy lawn and soggy low spots point to a perched water table.
• Nearby ponds, wetlands or creeks at about the same elevation as your yard.

We read these on the site visit, and the plan and quote reflect what we find. Occasionally a site is wet enough that moving the pool a few feet higher on the lot, or raising it partly out of the ground with a retaining wall, is the better answer.

A few minutes each spring, when the water table is at its highest, tells you how your pool's site behaves:

• Lift the sump pipe lid and note how high the groundwater sits. Write it down, so you learn what normal looks like.
• Make sure downspouts and sump pump outlets are not discharging toward the pool or deck.
• Walk the deck for new cracks, lifted edges or joints that have opened after the winter.
• Keep the water level normal. Opening the pool is not the time to lower the water for a cleaning.

If the pool needs work that means lowering the water, schedule it for late summer instead, when the ground is driest.

It changes how the pool is built more than which pool you pick. Fiberglass, ICF concrete and vinyl liner pools are all built successfully in wet Northeast Ohio ground. What matters is dewatering during construction, drained stone backfill, a working sump pipe and grading that sends surface water away from the pool.

Our fiberglass packages start at $68,400 installed. Dewatering, when a site needs it, is priced separately along with other site items like spoils haul-off and extra decking, so your quote reflects your actual ground. Financing through lenders we work with can cover the whole project, site work included.