If your well is in eastern Oklahoma County, Cleveland County, Logan County or western Pottawatomie County, odds are it draws from the Central Oklahoma aquifer, usually called the Garber-Wellington. Knowing a little about it explains why wells here differ so much and what is worth testing for.
What the aquifer is
The USGS describes the aquifer as underlying about 3,000 square miles in Cleveland, Lincoln, Logan, Oklahoma, Payne, Pottawatomie and Seminole counties. Most of the usable water is in two Permian-age units, the Garber Sandstone and the Wellington Formation. An ancient river system laid them down as sandstone channel bars stacked between mudstone floodplain deposits (USGS Fact Sheet 2005-3111). The metro area over it covers parts of Canadian, Cleveland, Grady, Lincoln, Logan, McClain and Oklahoma counties and roughly 1.2 million people (USGS SIR 2013-5219).
In practice, sandstone holds and gives up water, and the mudstone between the layers mostly doesn't. A well that hits a thick sandstone can yield well, while one a short distance away may catch thin lenses. That is why drillers lean on nearby well logs. See well drilling.
Confined in the west, open to recharge in the east
On the western side of the aquifer, the sandstones are buried under the Hennessey Group, which confines them. Water there has been underground longer and in contact with the rock, so its chemistry changes more (USGS). Further east, where the sandstone is closer to the surface, rain recharges it more directly.
Dry taps or a pump that won't quit? One call gets you to a licensed pump installer near you. (405) 536-9841
The arsenic question
Small amounts of arsenic occur in nearly all the aquifer's rock. The USGS found it comes out into the water mainly where pH rises above 8.5, which happens most in the confined part. Concentrations as high as 232 micrograms per liter have been measured in the confined aquifer at Norman, against a federal drinking-water limit of 10. Shallow wells under about 300 feet rarely exceed that limit, and arsenic tends to rise with depth (USGS). For a private well, the takeaway is simple. If your well is deep, or you are on the west side of the aquifer, test for arsenic at least once.
Nobody tests a private well but you
Oklahoma DEQ puts it plainly: federal and Oklahoma governments do not regulate privately owned wells, and the owner is responsible for keeping the water safe (ODEQ). OWRB recommends testing for coliform bacteria and nitrates at least once a year (OWRB). DEQ also suggests testing after flooding, land disturbance or new construction nearby, after any repair to the well system, or when the water's odor, color or taste changes.
| Test | Why it matters here | Reference level |
|---|---|---|
| Coliform bacteria | Shows whether surface water or waste is getting in, and is the usual check after pump work | Should be absent |
| Nitrate | Septic systems, fertilizer and livestock. Boiling concentrates it instead of removing it | 10 mg/L |
| Arsenic | Natural in the Garber-Wellington, highest in deep and confined wells | 10 µg/L |
| Chloride, sodium, conductivity | DEQ flags these near oil and gas activity | Chloride 250 mg/L |
| Hardness, iron, manganese | Scale, stains and lost pressure as pipes narrow | Iron 0.3 mg/L; manganese 0.05 mg/L |
| pH | Low pH corrodes plumbing; high pH scales it (and frees arsenic in the aquifer) | 6.5–8.5 |
Reference levels from ODEQ's private well fact sheet and USGS. DEQ's Water Wellness panel covers several of these but doesn't list arsenic or coliform, so ask the lab what is included.
Protect the wellhead
OWRB's owner guide offers simple habits. Don't turn the area around the casing into a flower bed. Keep pesticides, fuel and other chemicals out of the well house. Where possible, keep contamination sources at least 50 feet away and downhill. Older wells are often cased only to shallow depth and are more prone to contamination (OWRB). If a test comes back bad, OWRB's disinfection steps call for chlorine at 100 ppm, left to stand at least two hours (ideally 24), then flushed, with a resample a week later. A licensed installer can shock the well and check the cap and seal while they're at it.
FAQ
Is there arsenic in Oklahoma City well water?
Some Garber-Wellington wells exceed the federal 10 µg/L limit, mostly deep wells in the confined western part of the aquifer, such as around Norman. Many shallow wells test fine. The only way to know for your well is a lab test.
How often should I test my well?
OWRB recommends testing for coliform bacteria and nitrates at least once a year. Also test after flooding, nearby construction or well repairs, or when the water changes.
Will a softener fix my water?
A softener handles hardness, not arsenic or nitrate. Test first so any treatment matches what is actually in the water.