Water Works 30 - 6/23/03

Mother Nature’s Plumbing System

Our last episode left a gigantic mound of water apparently defying gravity in a field above Quakertown by actually flowing uphill through the bedrock there. We rejoin it this week as it resumes the relationship we usually expect between water, a hill, and the earth’s mass and slowly finds its way down again, under the hollows and slopes on the north face of the Hunterdon Plateau, heading for Capoolong Creek.

Before I explain how the water flowed up that hill, I should explain the flow of the next several installments of this narrative. My plan is to begin with a look at how our public water supply and delivery system functions. Then we will examine the mess the NJDEP made of Quakertown’s aquifer long before its recent proposal to make matters worse. Finally, we will use the NJDEP’s argument for increasing water allocations here as a case study, to underscore what we learned.

By then you should know enough so you won’t lose time getting up to speed when the state shows up in your neighborhood with plans for your drinking water someday. If you live anywhere in the more than half of Hunterdon County that is still largely farmed, that could be sooner than you think, unless a developer shows up first with the NJDEP’s new math in hand to practice long division on your town’s zoning. Either way, trust the Boy Scouts: be prepared.

If you joined us for the Reality Check tour, you remember that the water flowing up that hill over the Capoolong begins flowing there from the top of its watershed on the south side of Quakertown. The higher elevation of its starting point gives it the equivalent of a running start called “hydraulic head,” or usually just “head.” That’s because gravity constantly pulls any column of water, including any portion of our aquifer, downward and outward, trying to wrap it around the center of the earth in effect. Prevented from doing that by the impassable, unfractured rock below the aquifer’s water-bearing rock, the water becomes a pile of stored energy, exerting an amount of force proportional to its elevation, in this instance enough to push itself up a smaller hill.

New York City’s aqueduct system relies on the same principle to send water 150 miles from the Catskills to Manhattan, including several uphill stretches on the way, with enough pressure left to climb a 4-story building when it arrives.

If I piped water to my house from an elevation high enough to produce sufficient head, I could dispense with my in-house pressure maintenance system, which produces the same result artificially. If you have a private well you also have a storage tank and pump like mine somewhere in your basement. They maintain an acceptable level of pressure in the household pipes, so your family can use a dishwasher and a shower at the same time, without a sudden drop in water pressure at one end of the system leaving someone lathered in more ways than one. Its effects are less dramatic, but the head of water above your well contributes to keeping that shower running too.

When any well pump draws water it lowers pressure in the surrounding aquifer, which causes water nearby to flow toward the pump, creating a sort of negative shock wave in the immediate neighborhood while the aquifer re-establishes an equilibrium. Since a column of water is virtually incompressible, and the weakest point in the system of forces is the top of the column, when the aquifer finally stabilizes itself the result is a cone-shaped depression in the water directly above the well being pumped. That conical depression – often called the well’s zone of interference – spreads its base outward at the surface of the aquifer. It lowers the aquifer’s water level to the greatest extent above the point of the cone and least at its outer limits, while depressing levels in between along a virtually straight line on the face of the cone.

Several wells pumping in close proximity to each other create overlapping cones of depression. Deeper, larger-capacity wells in the same vicinity create even larger cones encompassing smaller ones, a normal state of affairs under most circumstances, but with limitations. The cone of depression of a large-capacity well can extend more than a mile. (i) It can reach beyond the boundaries of watersheds, and even change the natural direction of water flowing in them.

Pumping any well always reduces the head available to other wells in its range of influence. Because a reduction of hydraulic head reduces water pressure, a reduction of the head available to smaller wells in a larger well’s zone of interference always reduces the pressure available to the pumps in those wells.

Which is where all the trouble starts, along with our next installment.

Ron Gutkowski