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chipping at the horizontal dam
Wednesday, February 25 2009
Today I noted two indications from the water in the greenhouse well that suggest it has deep origins and isn't simply surface runoff. The first is its temperature: 42.6 degrees Fahrenheit. If it were surface runoff, it would be only a degree or so above freezing. But 42.6 degrees is only about six degrees colder than the water being pulled from the bottom of our house's drilled well, which is hundreds of feet deep. That six degrees could be accounted for entirely by the shallowness of the rock I'm pulling it from. Remember, as measured from the undisturbed surface, my well is only four and a half feet deep.
The other indication that the water doesn't have surface origins is that it smells slightly of sulfur (a problem our well water doesn't have). Water must have anærobic origins to become contaminated with hydrogen sulfide, which rules out surface water as an immediate source.
Though the greenhouse well is shallow, it taps a strata of rock that lies thirty to forty feet deep beneath the plateau to the northwest of the house. This plateau features a number of wetlands and other indications of underdeveloped drainage. It wouldn't surprise me if water from some of these wetlands gradually finds its way through cracks in the rock to a number of geologic layers, including the one my well taps.
One possible explanation for the concentration of water at the depth of my greenhouse well might be the rock layer that lies immediately under it. In an effort to create a collection basin, I've been repeatedly trying to chip away at it, but the rock (which appears to be bluestone) is proving much tougher than the toughest bluestone layers I've encountered to date. Even the most extreme blows with a post-hole-digging steel rod manage to dislodge chips only the size of a corn flake. So far I've been unable to find or exploit any cracks in it. Perhaps, unlike the layers of rock overlying it, this layer lacks a network of cracks, faults, and fissures. This would make it into a sort of horizontal dam, trapping a layer of water immediately above it.
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