Water Works 32 - 7/9/03

The Aquifer Recharge Blues

We need to talk about how our water supply is replenished. To help, I will take you through an exercise that avoids most of the millions-and-billions abstractions with a schematic that should make the whole process a little easier to grasp. It got me over the hump, anyway.

Imagine our aquifer as a tight assembly of identical, stacked shale blocks. Each stack reaches 300 feet underground, and each block has dimensions of 3 feet on every side, which gives us 100 blocks in every stack with a combined volume of 100 cubic yards per stack. The top surface area of any stack will be 9 square feet, equal to a face of the top block. Since an acre equals 43,500 square feet (I’m rounding), that puts 4,833 stacks under 1 acre of land. At 100 blocks per stack, our single acre of aquifer has a total volume of 483,300 cubic yards.

Storage capacities for our Lockatong shale bedrock range from slightly more than 2 gallons of water per cubic yard of rock to slightly more than 4, so our aquifer will have a theoretical total capacity somewhere between 1 million and 2 million gallons.

Since every inch of rain drops 27,500 gallons of water on an acre of land, and 46 inches of rain fall here each year, that gives 1.3 million gallons of water a chance to recharge our 1 million to 2 million gallon aquifer annually. We can dispense with 50% of that rainfall right away. During growing seasons, most of it feeds plant growth, runs off, or is sucked back out of the soil by the atmosphere in normal hot, dry spells. Only about one-half of our rainwater has a chance to recharge our water supply in any year.

It’s only from mid-October through mid-April that a significant amount of water migrates to our aquifer from the soil, through fractures between the top blocks of our bedrock stacks. To represent those fractures – the verticals and bedding planes we met last week – let’s reduce the size of our blocks, opening space between all the blocks and stacks equal to a uniform 1% to 2% of the original volume of the entire structure, without changing its overall dimensions.

That space holds all the water in the aquifer and determines its capacity. Its relative scarcity also means at least 98% of our bedrock’s surface is completely impermeable. In winter, when the ground freezes, nothing gets through the soil until it thaws, leaving late fall and early spring the only periods when water reaches the small gaps in the shale. Then, prolonged rainfalls, rapid snow melts and anything other than nearly optimal conditions forces water to flow from the soil to the entranceways between our bedrock stacks faster than the openings can handle the flow, queuing the water, saturating the soil above the rock and sending any rainfall that follows into streams as runoff.

Quakertown soils moderate the effects of variations in precipitation by retaining moisture extremely well. Unfortunately, that encourages runoff during periods of high concentrations of water in the soils. Our aquifer also lacks a transitional layer of unconsolidated rubble between its soils and bedrock, common elsewhere, which would normally increase water retention and reduce soil saturation. Here, water first meets rock by leaching directly onto an almost solid slab of shale. If it doesn’t find one of the extremely scarce surface fissures quickly enough, the water tends to linger above the rock, again increasing the likelihood of soil saturation and runoff.

Most of the 1.3 million gallons of rain falling on our acre of land every year never infiltrates our buried shale, leaving only 55,000 to 110,000 gallons to refill the rocks under our single acre. That’s only 2 to 4 inches of rain, a recharge rate of 5.5% to 11% of its total volume for a 1 million gallon aquifer, and a proportionally smaller rate for the 2 million gallon king size model. You can make my schematic represent the entire Quakertown area by converting my acre into 5 square miles. Reality changes the numbers very little. Recharge rates rise down by Oak Grove and shrink toward the higher ground around Quakertown, closest to the highest concentration of large water extractions from our real aquifer.

Of course, “not very much” still beats “less than nothing.” Other conditions affect recharge too. When an aquifer is pumped at a rate too near its natural replenishment rate – when water is “mined,” in other words – a large amount of surface water runoff means a reduced aquifer water level. The impact of continuous water mining on an aquifer is comparable to the effect a chronic drought would have. It also compounds the impact of natural droughts. Both are normally avoided by building conservative margins of error into calculations of aquifer productivity. Mining more water from an aquifer already suffering from chronic drought should be considered reckless endangerment, but our NJDEP calls it policy. We will talk about a few other impacts of that policy when we reconvene.

Ron Gutkowski