The town of Harpswell is 81% water, so it seems odd to say that the town has a water shortage. Odd but true. Of course the reason is that the 81% refers to salt water, aka the Atlantic Ocean, and the shortage refers to fresh drinking water, or technically, potable groundwater. A full understanding of the complex issues involved in accessing drinking water requires years of study, but perhaps a simplified description will increase awareness of a potentially urgent situation.

Stories about climate change and sea level rise fill the news almost every day, and the news is usually pretty grim. In a community so closely tied to the ocean, emotionally and physically, we have no choice but to take such warnings seriously. One concern is that as more homes are built on our narrow islands and peninsulas, the finite water supply will be at risk. As if a warning was needed, according to the National Drought Mitigation Center, parts of five Maine counties were officially catalogued as having severe drought conditions this summer. Harpswell was one of the towns affected. Whatever the future holds, it will help us all to have a better understanding of drinking water.

In grade school we learned how water evaporates from oceans, rivers and streams, filling the air with moisture. This eventually returns to the earth’s surface in the form of rain and snow, creating a cycle that has been going on for eons. This is great, of course, but bear in mind that what goes up here might come down there — the dew on your grass might next show up as a storm at sea. In a word, it’s complicated.

In a property that touches or is close to the ocean, there is a constant push and pull between fresh groundwater and the heavier salty ocean water. In many cases, the natural angle of the landscape as it slopes to the shore causes the flow of groundwater to push against the salt water, keeping it out. If too much fresh water is pumped out, this vacancy can allow the salt water to move into the water-bearing rocks under the landscape, potentially creating salty water that is not fit to drink.

One of the terms we hear in news stories about water is “aquifer” which Wikipedia defines as “an underground layer of water-bearing permeable rock, rock fractures or gravel, sand and silt.” Those news stories often refer to vast aquifers in the midwest that sprawl beneath several states. The rocky geology of coastal Maine that was formed by glacial advance and retreat does not have vast underground lakes. Instead, imagine this: picture a bucket filled with mixed sand and gravel. Put a hose into the bucket and you’ll find that there is still room to add water which seeps down through these layers, sliding sideways as gravel and gravity dictate, eventually collecting in gaps between the rocks. Where the gravel includes large rocks, there are generous spaces that fill with water. Where the gravel is finer, the spaces between the rocks are smaller.

Now, in your mind’s eye, imagine pushing a rigid tube into the bucket, cutting through or pushing aside gravel as it is pressed into the bucket. If you’re lucky, the end of the tube finds a generous space between chunks of gravel and you have created the model of a productive well. As you suck water out of that space, surrounding water drips in to replace what was taken. This is the situation in hundreds of wells in Harpswell. Unfortunately there is no economical way to map the structure of rock deep underground, which explains why locating where to drill a well involves guesswork and luck. Two houses not far apart might have wells with very different yields.

The strength of a well is measured in gallons per minute, a number derived by drawing enough water to fill, for instance, a five-gallon bucket while measuring the time it takes to fill. If the bucket fills in a minute, you have a flow rate of gallons per minute, or 5 gpm. Before you run for a bucket, understand that a one-minute test would not give an accurate understanding of a well. The New Hampshire Water Well Association recommends a flow rate of 4 gallons per minute for a four hour period. That’s equivalent to 960 gallons of water flowing steadily for four hours. A family of four typically uses about 360 gallons per day (80-100 gallons per person per day) — a well producing five gallons per minute will probably provide what is needed. This of course assumes that the well refills as quickly as it is drawn down, and that is not always the case.

Which brings us to the topic of recharging; the ability of a well to refill efficiently. A strong well will refill as quickly as water is taken out, though bear in mind that even a slower refresh rate might be sufficient because the well is refilling overnight when less water is being used. Many Mid-Atlantic states consider a well adequate if it produces 1 gallon per minute when complete, and consistently over specific time periods. The fact is that 1 gpm will produce 1,440 gallons per day.

Recently I spoke with Bert Temple, on member of family whose business for a couple generations has focused on well drilling, maintenance and repair. Bert mentioned that a question he gets frequently has to do with the pace of surface water making its way into the water supply. He often hears, “We had a lot of rain last week, so how come my well’s gone dry?” Bert said it can take a couple of months for surface water like rain to seep down into the subterranean pocket that will refill a well. The positive side of the situation is that even in a dry spell, there is water from several months ago oozing downward through the rocks.

I learned something new from writing this article, and I hope you did too!