Water Works 31 - 7/2/03

A Game of Cards in the Basement

As foretold, hydraulic head at well pumps is still our topic.

Before we proceed let’s clarify the process of moving water to those pumps. At the risk of conjuring Hearing Officer Miri and his enchanted calendar into our tale again, looking for some action, the best example I know for the structure of an aquifer like ours is a deck of cards. Several decks of cards, with a few bent cards in each deck – bringing Officer Miri to mind most vividly – is even closer to what we would see if we inspected a cross-section of our bedrock. Instead of mains and pipes to move water, our system uses fractures, blocks and planes.

That’s common to what geologic maps call New Jersey’s Piedmont region, especially here in the southern foothills of the once-towering Precambrian Highlands, some of the oldest rock on the face of our planet. The Hunterdon Plateau and Sourland Mountain are the last bastions of epochs that followed, like the frontier battlements and walls of an ancient stone stronghold that even in its decline would have dwarfed the younger Continental Divide. Our own Triassic survivor weathered forces that ground weaker Lockatong kin and even older rock into the valleys around us. The books tell us about “resistant” Lockatong argillites, but the familiar views from Quakertown of our apparent peers miles away, the Highlands to our north and the tops of the Pennsylvania bluffs across the Delaware, are all you need to tell you that ours is the toughest chip on the Lockatong block.

Argillites are formed through chemical bonding and hardening. Strictly speaking, our Lockatong formation is a “chemically cemented, sedimentary silt mudstone,” which may not sound like much. To put “cemented” in perspective think “Bonneville Flats,” which should have a familiar ring if you ever had an interest in really fast cars. For the uninitiated: until someone found a more perfect stretch of Nevada in the eighties, Bonneville, Utah was the Mount Everest of drag racing, where men in rockets on wheels went to challenge the world land-speed record. The prehistoric salt lake bed there is so hard it takes a two-handed, half-inch drill to put a hole in it. At 600 miles per hour, crack-proof pavement is a must. Our local ingredients differ, but the cement that binds our old sedimentary silt was mixed, poured and hardened the same way as the cement that binds Bonneville’s salt.

All of which means our water-bearing rock is extremely water-resistant; a seeming natural contradiction best modeled by marked playing cards. Here is how it works. In each of our decks of cards layers of several flat cards, separated from other layers of flat cards by the few bent cards, represent individual blocks of sealed shale. Imagine several decks of cards in a formation one deck tall, with the edges of the cards in each deck nearly meeting the edges of adjacent decks and the bent cards running a bit out of vertical alignment with each other, every few decks, and you capture the general scheme.

The spaces between the decks of cards represent fractures called “verticals.” The bent cards in the decks represent smashed rock above and below the shale blocks, called “bedding planes.” Now make your layers of flat cards a few inches to a few feet deep and several times longer than their widths, so that the horizontal faces of the shale blocks measure up to hundreds of square feet, and open the verticals and bedding planes only a few millimeters. Never let all the gaps exceed two percent of the volume of the entire formation, fill the spaces with water, and that’s our aquifer.

Every well here is an engineered fracture through a series of bedding planes, fed by the vertical fractures in the top face of the aquifer rock and the occasional bedding plane that makes its way up to the surface. Like our cones of depression, all wells and fractures are not created equal. Deeper wells with more pulling power usually win the competition for water from particularly rich flows, “de-watering” the upper reaches of the aquifer and reducing the pressure that delivers water to other well pumps. At about 300 feet down, the shale becomes so compressed that fracturing stops, and deeper wells become no more than extended underground storage tanks bored into the solid, waterless foundation of the aquifer.

Hang on to that notion of a waterless aquifer. It will soon prove handy, when I drain an acre of ours to make room for a little more math than usual. Fear not. The NJDEP may be ineducable, but our track record here at Water Works assures you that whenever we’re screaming our way around a learning curve, you’re trapped in a nightmare you can trust. It probably won’t hurt much, even if you’re more math-challenged than I am, as you will learn when we meet again.

Ron Gutkowski