Ogallala (High Plains)

America's largest aquifer — and its slowest-moving disaster

States

SD, NE, WY, CO, KS, OK, NM, TX

Type

Unconfined sand & gravel

Status

Mined — withdrawal exceeds recharge by 6× regionally, 20×+ in Texas

The Ogallala — properly the High Plains Aquifer, of which the Ogallala Formation is the largest part — is the largest aquifer in the United States. It underlies 174,000 square miles of the Great Plains, from southern South Dakota to the Texas Panhandle, and contains roughly 2.9 billion acre-feet of water. It supplies about 30% of all groundwater used for irrigation in the United States. And it is being mined.

The geology is straightforward and the math is brutal. Average annual recharge is about half an inch. In much of the region, recharge is effectively zero. Withdrawal in the High Plains averages around six times the rate of recharge, and in the southern Texas High Plains the ratio is closer to twenty to one. The water you pump out is not coming back, on any timescale that matters to you, your children, or your grandchildren.

What it is, geologically

The Ogallala Formation is a wedge of unconsolidated sand and gravel, deposited during the Miocene as Rocky Mountain runoff washed eastward across what's now the Great Plains. The deposits are between 2 and 6 million years old. Saturated thickness — the depth of rock that's actually full of water — varies from 0 at the basin's southern fringes to over 1,300 feet in parts of Nebraska's Sand Hills. Most of the aquifer is unconfined: water sits in the pore spaces between sand grains, and the water table moves up and down with pumping and recharge.

Crucially, the modern climate doesn't supply much recharge. The water in the Ogallala accumulated during a wetter climate regime millions of years ago. Today's Great Plains average 12-22 inches of annual precipitation, most of which evaporates or is taken up by plants before it can percolate down to the aquifer. The water table, where it's still high, is the residue of an older world.

The recharge math

The simplest way to understand the Ogallala is by the ratio of withdrawal to recharge.

• Annual recharge, region-wide average: about 0.5 inches per year. In the southern High Plains (Texas Panhandle, eastern New Mexico): closer to zero.
• Annual withdrawal: roughly 21 cubic kilometers across the aquifer, almost all of it for irrigation.
• Net depletion since 1950: the aquifer has lost about 410 cubic kilometers of water — roughly the volume of Lake Erie.
• Decline at the worst spots: 100-150 feet of water-table drop since 1950 in parts of the Texas Panhandle and southwest Kansas.

The 2017 USGS Scientific Investigations Report 2017-5040 estimated that 30% of the Ogallala's pre-development saturated volume has already been removed, with most of the loss concentrated in the southern third of the aquifer.

Where depletion is worst

Texas High Plains is the canary. Some counties — Hale, Floyd, Lubbock, Castro, Parmer — have already crossed the threshold where economically viable irrigation is no longer possible. Center-pivot fields that produced cotton or corn in the 1980s are reverting to dryland farming or grassland. The Texas Water Development Board projects that over 70% of Texas High Plains irrigation capacity will be unavailable within 50 years at current trends.

Southwest Kansas is following the same trajectory. The Garden City and Liberal areas have seen 50-100+ feet of decline. Kansas's Local Enhanced Management Areas (LEMAs) are local groundwater districts that voluntarily cap pumping; results have been mixed.

Eastern New Mexico (Curry, Roosevelt, Quay counties) has some of the thinnest remaining saturated layers in the system — wells are increasingly going dry.

Nebraska is the exception. The Sand Hills region in north-central Nebraska sits on extraordinarily thick saturated layers (1,000+ feet in places) and gets meaningful recharge through the highly permeable sand-dune surface. Nebraska's Natural Resources Districts also impose stricter controls than most other Ogallala states. Nebraska's portion of the aquifer is, in places, actually rising.

What's at stake

The Ogallala doesn't just water private wells. It waters one of the largest concentrations of irrigated agriculture on Earth. About a fifth of US wheat, corn, and cotton, and most of US sorghum, comes off the High Plains. Cattle feedlots — the supply chain that finishes most American beef — depend on Ogallala-irrigated feed corn. The Ogallala's depletion is, indirectly, an event in the global food system.

For private well owners on the aquifer, the story is more immediate: your well's productive life depends on the saturated thickness underneath your property and on what your neighbors are pumping. Two equivalent wells two miles apart can have very different futures.

Water quality

The Ogallala's water is generally good — better, on average, than the deep sedimentary aquifers like the Denver Basin. Hardness is moderate to hard. TDS varies but is mostly below 1,000 mg/L. The major water-quality concerns are not naturally occurring; they're agricultural:

• Nitrates — from fertilizer and feedlot runoff. The shallow water table under irrigated fields is highly vulnerable. Some regions of Nebraska and Kansas exceed the 10 mg/L drinking water standard in significant fractions of wells.
• Arsenic — concentrated in pockets, especially the southern High Plains. Naturally elevated in some Texas counties (Pecos, Reeves) and in Curry County, NM. See the arsenic page.
• Pesticides — atrazine, glyphosate, and others detected in Ogallala wells, though usually at low concentrations.
• Hardness, iron, manganese — the usual sedimentary-aquifer suspects, though usually less severe than in deeper formations.

What this means if you're on an Ogallala well

If you're on a well that taps the Ogallala, the most important thing you can do is understand your local saturated thickness and your local rate of decline. The USGS National Water Information System has well-monitoring data; many states have their own dashboards (the Texas Water Development Board's well database is excellent; Kansas's WIZARD system is another good resource).

For testing: nitrates first, every year, especially if you're in irrigated farm country. Then a basic mineral panel. Then arsenic if you're in the southern High Plains.

Known contaminant concerns

Communities on this aquifer