Anatomy of the Chicot Aquifer
Anatomy of the Chicot Aquifer
With attention turned to national and local threats to drinking water, we thought it appropriate to get to the bottom of how our beloved aquifer works.
Lafayette is said to have among the best tasting drinking water in the country, a perk owed entirely to our fair city’s geographic situation above the Chicot Aquifer, a massive groundwater system that stretches almost the entire heel of Louisiana’s geographic boot.A lawsuit filed against the Union Pacific Railroad Company has raised the specter of contamination in Lafayette’s drinking water by way of decades of spilled gas, oil and cleaning solvents in and around an old rail yard along the Evangeline Thruway. Pollutants like phase separated hydrocarbons or volatile organic compounds (VOCs) like benzene, toluene, ethylbenzene and xylene — known collectively in water-quality-speak as BTEX — have been documented throughout the old rail yard at depths as low as 42 feet, and in quantities often above screening standards established by the Louisiana Department of Environmental Quality.
As it happens, this toxic plume sits directly above a portion of the Chicot Aquifer, which begins at depths as shallow as 35 feet. Thanks to gravity, these things have a tendency to migrate downward, which on the surface at least appears to threaten the safety of our water supply. With all that in mind, we thought it might be a good idea to give a very, very, very broad overview of how the aquifer works in the context of public and residential water wells.
Forgive the wonkiness of what proceeds. Despite being about water, this piece is sure to be very dry.
So what, exactly, is the Chicot Aquifer?
Although we’re all very familiar with the name, it’s not altogether obvious that everyone really understands the basic anatomy of an Aquifer; we certainly didn’t before we researched this piece. For the purposes of this article, we’ll keep information restricted to the physiology of our particular groundwater system, but please note that not all aquifers are alike in composition.
The Chicot Aquifer is essentially a massive, subsurface collection of sand soaked with fresh water. It spans roughly 9,000 square miles and services 15 parishes via wells that penetrate its soggy belly.
Its bottom boundary is over 1,000 feet below sea level, resting on top of a saltwater sand system called the Evangeline Aquifer.
For much of its span, the Chicot Aquifer is capped by a dense layer of clay that sits around 35 feet beneath the surface and limits the ability of water and other liquids to migrate into the upper sandy reaches of the aquifer itself. Such layers of clay and dense pockets of sand, gravel or silt form erratic, heterogenous layers scattered throughout the Aquifer’s topography. We like to think of it as a gigantic, silty lasagna.
As we pump water out of the Aquifer — over 600 million gallons per day system-wide, and 21 million gallons per day in Lafayette Parish — the system is slowly replenished by rainwater in clayless portions of the Aquifer near Alexandria called a Re-Charge Zone. Collected rainfall is conveyed southward by gravity due to the slightly higher elevation of central Louisiana as it angles toward the Gulf of Mexico.
As you could imagine, it takes a long time for water dropped in Alexandria to meander its way to wells in Lafayette. Because of the epic tardiness of water trickling through miles of sandy dirt, the age of Aquifer water can be measured in years, even hundreds of years in the cases of some aquifers elsewhere in North America.
So how does that old water get into my Nalgene bottle?
It’s important to note that very little of the silty lasagna that makes up the Chicot Aquifer is uniform. Water accumulates in varying degrees of density and quantity, which means that not all depths or areas are suitable to “produce” water for pumping. Wells tap productive zones after probe wells determine sufficient quantity and quality of water to make for a productive well. Residential or private wells can often hit just over 100 feet or shallower.
Public water wells, like those operated by Lafayette Utilities System, often tap zones located 500 feet beneath the surface. Pipes with filter screens are stuck deep into these pockets of produceable water and surrounded with a fist of rocks called a gravel pack that filters out the sand as the water seeps into the pipe. A motorized pump draws the water to the surface where it is tested, cleaned and distributed into the public water supply.
LUS operates at least 15 different public water wells scattered around the parish at varying depths. Water Well #16, which the lawsuit against Union Pacific alleges is perilously close to the rail yard’s toxic plume, is tapped at 580 feet. As another random example, Water Well #8 is tapped at 612 feet.
So, should I be scared?
Look, nothing in life is 100 percent safe, and that unfortunately includes your drinking water. But to evaluate reasonable risk, it’s best to speak in terms of probability and safeguards.
The Louisiana Department of Health and Hospitals tests these wells on a three-year schedule for a litany of hazardous substances, including heavy metals, microbes, chlorination by-products and the hydrocarbons and VOCs associated with that dirty old rail yard.
While three years may seem like a long gap when testing something as sensitive as the water you drink, consider the length of time it takes for water to migrate any distance through a collection of particulates and solid mass. Even without the thick layer of clay capping it, spilled contaminants in most cases need to travel hundreds of feet before striking productive water supplies, a process that could take years. So, in a general sense, spilled contaminants do threaten public drinking water, but the likelihood of water poisoning going unnoticed— like what’s happened in Flint, Mich. — is considerably low.
In the case of the rail yard, we’re dealing with chemicals that have been spilled over the course of decades, which potentially could be enough time to threaten the water we drink. Nearby Water Well #16 was most recently tested in 2015 with no activated alerts thus far.
This is not to say that all that nasty gunk shouldn’t be cleaned up. It absolutely should. Disturbance of the natural layers of resistance abound these water systems, including the wells themselves. To get to produceable water you gotta dig. When you dig you create avenues for surface water to flow downward. Given enough time, it is certainly possible that the contaminants could enter our water supply, so it’s certainly a smart policy to nip that problem in the bud.
Construction of the I-49 Connector could also further disturb that system with excavations and borings that break the clay layer and plunge into the aquifer itself, an obvious cause for concern. It’s well documented that churning and digging of soil that accompanies massive construction projects like that can create vertical conduits for whatever lies closer to the surface. According to environmental documents produced for the interstate’s construction, the Louisiana Department of Transportation and Development has committed to construction methods at least sensitive to the Aquifer system in its path.
There’s a dramatic difference between being scared and being concerned. Given the foregoing information, we think the latter is the appropriate emotional response at this point.
To be sure, no contamination should be considered safe, but it’s best to have realistic perspective of what exactly that means.
If you’d like to know more about your water system, LUS publishes water sampling data here. You can also access DHH information on the Lafayette water system here, and general information about aquifers via the US Geological Surveyhere.