Where does the Closed Basin water go?

Letter by Paul Martz

Geology – December 1998 – Colorado Central Magazine –

Where does the water go?

Editors:

After reading both parts of the article on water in the San Luis Valley, one significant question remains unanswered: Does the artesian aquifer (lower one) drain anywhere? The reason that knowing this is significant is because if it does — say south to the head of the Rio Grande gorge — then recharge has been occurring constantly from the already adjudicated surface waters. If the “missing” or unaccounted for million acre feet has been leaving via gravity, then obviously any pumping from that formation will withdraw additional water from the annual runoff budget.

Also, because artesian conditions exist not only within the Closed Basin, but beyond it, it is impossible for any surface water within that portion of the Basin overlying artesian conditions to percolate down into the lower aquifer. If that were possible, instead of having the Closed Basin, you would have a lake.

Thus any argument that pumping from the artesian aquifer will have any effect on the Closed Basin surface waters is specious. The only possible exception to this may lie along the fringes of the system where a variety of conditions could allow recharge of the artesian aquifer from present day surface waters.

The other major issue I take with the portrayal of the sub-surface geology of the Valley is the U.S.G.S. interpretation of the structure in the middle of it. Gold exploration drilling several years ago hit oil (an event commented upon in this magazine) in a block of Paleozoic rocks that are equivalent to what rests on top of the Sangres. Follow-up seismic work delimited a significant volume of prospective rocks right where the Valley is supposed to be miles deep. Just west and south of Villa Grove, Precambrian basement rocks are exposed on the surface. These two occurrences and a third north of U.S. 285 after it turns west towards Saguache past the Highway 117 turn-off, argue strongly that the sub-surface structure in the middle of the Valley is nowhere near as simple as illustrated. The significance of this is that there may not be anywhere near as much water stored beneath the Valley as has been suggested.

Before anyone starts withdrawing massive amounts of water from the artesian aquifer, a complete seismic survey of the entire valley to determine its actual structure should be undertaken. One only has to look at the south end of the San Joaquin Valley in California to find a basin with similar hydrology and significant impacts resulting from what has in effect been the mining of fossil water. Since both the state and the feds have muddied up the situation with the pumping from the artesian aquifer to supplement Rio Grande flows, they are the obvious parties to undertake such a study. Boyce might have an entirely valid claim to water beneath his lands, but then again, until the facts are known, he might not.

Paul Martz Consulting Geologist Poncha Springs