Let’s take a close look at the San Luis Valley’s confined aquifer. What is the reality? Does confined aquifer water, deeply buried and out of sight under much of the Valley, have any connection to the unconfined aquifer or to surface streams? How is water recharged to the confined aquifer? Even today, after 120+ years of use and study, some people think of the confined aquifer as just a big bathtub full of water-saturated sand and gravel, with lots of water available for the taking. In reality, the Valley’s hydrologic system, composed of surface water (streams, ditches, drains, wetlands, etc.), the unconfined aquifer, and the confined aquifer, is complex, dynamic, and interactive.
The Confined Aquifer: How should we think about it?
The confined aquifer is present beneath most of the floor of the Valley (the San Luis Hills is an exception.) The confined aquifer is composed of many – often dozens – of layers of unconsolidated sediments (sand, gravel, clay). In some areas, particularly in Conejos and Costilla Counties, layers of volcanic rock also are part of the confined aquifer. From well and test drilling data, along with seismic and other geophysical data, the deepest part of the confined aquifer may be as much as 4,000 feet or even deeper in the northeast part of the Valley. However, this does not mean groundwater at that great depth is economically feasible to pump, or that it is isolated from the rest of the Valley’s hydrologic system. In other areas of the Valley, such as near the mountain fronts and in the Conejos River valley, the confined aquifer is much shallower: in many places 1,000 feet deep or less.
About 70% of the San Luis Valley floor is underlain by a series of interbedded clay, sand, and gravel layers. Geologists call this series of layers the Alamosa Formation. The clay layers, taken together, make up what is called the “confining clay series”. The confining clay series serves to partially separate and slow the movement of water from the confined aquifer, located within and below the clay series, to the overlying unconfined aquifer and the surface streams and other features.
Even though nearly half of the Alamosa Formation is made up of thin clay layers, there is sufficient sand and gravel that wells in this layer in many places are good water producers. It is important to realize that in most areas the clay layers don’t form a single, continuous pancake. Most of the clay layers are thin – often just a few feet thick – and discontinuous due to stream erosion cutting through the layers back in geologic time. The clay layers, primarily horizontal or gently sloping, have low permeability, which means water moves vertically through these layers slowly and with difficulty.
Due to their thinness and lack of continuity, the confining clays are “leaky” in most areas of the Valley. Thus the clay layers limit but do not entirely prevent the vertical movement of groundwater between aquifer layers. This means that in most areas water under confining pressure finds its way from the confined aquifer upward into the unconfined aquifer. The area from Alamosa north to Mosca, roughly along Highway 17, may be an exception. In this area, the Alamosa Formation clay is hundreds of feet thick with very little sand or gravel, and forms a thick, soft, layer that tends to ooze like warm peanut butter when it is drilled.
Around the edges of the Valley, ordinarily within about a mile of the mountains, the clay layers thin out and finally disappear. This means there is typically no low-permeability layer to prevent water from moving downward from the surface streams. Near the edges of the Valley floor where streams enter from the mountains, stream water percolates downward through mostly sand and gravel, and thus recharges the unconfined and confined aquifers. Groundwater in the confined aquifer moves relatively slowly toward the center of the Valley (the “sump” area) and southward, driven by gravity, filling up the pore spaces between the sand, gravel and clay particles.
The water-saturated pore spaces between the sediment grains, or the water-saturated fractures of volcanic rock layers, is the reservoir of confined groundwater. Toward the middle of the valley the confining pressure pushes water upward, and water moves through and around the discontinuous confining clay layers. This means that there is a continuous hydrologic connection between the confined aquifer and the unconfined aquifer. In many areas, the streams and other surface water features are in hydrologic connection with the underlying aquifer layers. Thus the entire hydrologic system is dynamic with constantly moving water, is interactive between aquifers and the surface system, and is sensitive to the effects of well pumping from one layer to another.
Below the Alamosa Formation is a deeper and geologically older zone that has less clay and more sand and gravel. Geologists call this the Santa Fe Formation. The clay-rich Alamosa Formation and the underlying Santa Fe Formation are all part of the confined aquifer, particularly in the Closed Basin. About 40% of the Valley, mostly in Conejos and Costilla Counties, but also in parts of Rio Grande and Saguache County, is underlain by sediments interlayered with hard volcanic rocks such as basalt and cemented volcanic ash called “tuff”. To envision these rocks, think of the roadcut along the Gunbarrel just south of Saguache. Where the volcanic rocks are fractured and water-saturated, and are shallow enough, they are productive aquifer layers. But where the volcanic rock layers are massive and unfractured, they, too, tend to act as aquitards: layers that impede or slow down the vertical movement of water between rock and sediment layers.
Deeper than about 2,000 to 3,000 feet across much of the northern Valley, but varying with location, the permeability (which is directly related to well productivity) declines rapidly, and the water quality is much poorer. Hydrogeologists used to call this deeper, less productive zone the “passive” part of the confined aquifer, to distinguish its slow water movement from the more productive and better water quality zone above, sometimes called the “active” confined aquifer. These terms are still useful to describe how conditions change with increasing depth in the confined aquifer. How do we know that “active” and “passive” confined aquifer zones exist? We know because we have data from geophysical surveys and deep test drilling for oil and gas that show significant permeability and water quality changes at about this depth range.
How should we think about drawdown in the Confined Aquifer?
As a practical matter, what happens when a confined aquifer well is pumped? How should we envision the drawdown effects? (A well in any confined aquifer, not just the confined aquifer in the Valley.) Basically, two things happen. As compared to an unconfined aquifer well, it means that the cone of depression (the funnel-shaped depression in the water table centered at a pumping well) spreads outward much faster and much further than it does from pumping an unconfined aquifer well, all other factors being equal. It also means the drawdown measured in a confined aquifer well actually reflects a confining-pressure change that affects the layer being pumped, but also affects shallower layers above the layer that is being pumped. Is this magic? Or smoke and mirrors by glassy-eyed hydrologists? No, this is just the physics of how confined aquifers work. Confined aquifer well tests by the RGDSS team in the early 2000’s, and also by the U.S. Geological Survey and the Colorado Division of Water Resources, showed repeatedly that pumping impacts move outward from a confined aquifer well very rapidly, often causing drawdown (i.e. measurable decline in the confined aquifer pressure) up to a mile or more away within only a few days of pump startup. At several test locations, drawdown was measured in layers much shallower than the pumping zone, demonstrating the “leakiness” of the confining clay layers.
Standing at a point on the Valley floor and looking any direction at many, many miles of flat, open farming and grazing land, it may seem like fiction, or a water user’s bad dream, that pumping a confined aquifer well can affect a stream or another well, even a shallower well, a long distance away. But, due to the physics of confined aquifer drawdown, and the “leakiness” between the aquifers, these pumping impacts do happen, and cause drawdown and stream depletions many miles away.
By Eric J. Harmon, P.E., Hydrogeologist