Cambrian-Ordovician

The deep aquifer beneath the Upper Midwest, with two characteristic problems few other aquifers share — radium and arsenic in the same wells

States

WI, MN, IA, IL, MO

Type

Confined sandstone and carbonate, multi-layered

Status

Stable in most of the system; long-term declines around major pumping centers (Madison, Twin Cities, Chicago)

The Cambrian-Ordovician aquifer system underlies most of the Upper Midwest — Wisconsin, Minnesota, Iowa, parts of Illinois and Missouri — and supplies the drinking water for millions of people including the cities of Madison, Minneapolis-St. Paul, Rochester (MN), Iowa City, and dozens of smaller communities. It also supplies a substantial fraction of private wells across rural WI, IA, and MN.

The aquifer's water is generally good — cold, clear, mineralized but drinkable, low in the agricultural contaminants that plague shallower Quaternary aquifers in the same region. But it has two characteristic problems that few other aquifers share, and they tend to occur in the same wells: radium (a naturally occurring radioactive metal regulated separately from radon) and naturally occurring arsenic in specific sub-regions of Wisconsin.

What it is, geologically

The aquifer system consists of Cambrian and Ordovician sandstones (Mt. Simon, Wonewoc, Jordan formations) interbedded with carbonate units (Prairie du Chien, Galena-Platteville). It is overlain across most of the basin by the Maquoketa Shale, which acts as a regional confining layer separating the deep Cambrian-Ordovician system from the shallower glacial-drift and surface aquifers. Where the Maquoketa is absent or thin (parts of southwestern WI, NE IA), the deep aquifer is unconfined and behaves more like an ordinary surface aquifer.

Wells targeting the deep aquifer are typically 500-1,500 feet deep, capped through the shallow drift, and produce reliably from the sandstone units. The water is older — recharge ages of thousands of years are common in confined parts of the system — and its chemistry reflects long contact times with carbonate and sulfide minerals.

Radium: the signature problem

Radium is a naturally occurring radioactive metal — a decay product in the uranium and thorium series, but distinct from uranium and from radon. The two regulated radium isotopes are Ra-226 (uranium-series, half-life 1,600 years) and Ra-228 (thorium-series, half-life 5.75 years). The EPA Combined Radium MCL is 5 picocuries per liter.

The Cambrian-Ordovician aquifer has elevated radium across a wide region — particularly the Mt. Simon and Wonewoc sandstones in eastern Wisconsin, northeastern Illinois, and parts of Iowa. The mechanism: uranium and thorium in trace amounts are present in the sandstone matrix; their decay produces radium; under the slightly reducing chemistry of the deep confined aquifer, radium dissolves into the groundwater.

The pattern of exceedances:

• Eastern Wisconsin (Waukesha, Brown, Outagamie counties): widespread combined radium above 5 pCi/L. Waukesha had to undertake a multi-decade infrastructure project — switching from groundwater to Lake Michigan via Great Lakes Compact diversion — primarily because of the radium issue.
• Northeastern Illinois (Kane, McHenry, DuPage counties): documented exceedances; many community systems treat or blend.
• Eastern Iowa: scattered exceedances, particularly in older confined wells.

Health effects: chronic radium ingestion accumulates in bone (radium is chemically similar to calcium) and can cause bone cancer and head sinus cancers at high cumulative exposures. The 5 pCi/L MCL is set to limit lifetime cancer risk; exceedances are considered serious by regulators.

Arsenic: the regional surprise

Eastern Wisconsin's Lower Wisconsin River valley and adjacent counties have one of the most documented natural arsenic problems in any sandstone aquifer in the country. The mechanism is unusual: arsenic-bearing pyrite in the St. Peter and Sinnipee sandstone units oxidizes when the water table drops (drought, pumping), releasing arsenic into solution. Wells in southeastern WI counties (Outagamie, Winnebago, Brown) have shown 20-30% exceedance rates for arsenic above the 10 μg/L MCL.

This is geologically distinct from the volcanic-source arsenic of the western US and the reducing-condition arsenic of the Mississippi Alluvial — but the human exposure outcome is the same. See arsenic.

Other water quality

• Iron and manganese — common in the deep confined aquifer where reducing conditions prevail. Many WI/MN homes have softeners and iron filters as standard equipment. See iron and manganese.
• Hardness — moderate to hard, especially where carbonate rocks dominate.
• Bacteria — generally low risk in the deep confined aquifer; significant risk in the shallower unconfined drift aquifers many private wells actually tap. See bacteria.
• Nitrate — generally low in the deep system but elevated in the shallow drift aquifers above it, particularly in Wisconsin's Karst Region (Door, Kewaunee, Calumet counties) where surface contamination reaches the aquifer rapidly. See nitrates.

The Karst exception

Northeastern Wisconsin's Karst Region — the Niagara Escarpment limestone exposed in Door, Kewaunee, and Calumet counties — is a separate hydrogeologic situation. Karst limestone with shallow soil cover means surface contamination from CAFOs (concentrated animal feeding operations) reaches private wells in days. Kewaunee County in particular has documented bacterial and nitrate contamination affecting a substantial fraction of private wells, and has been the subject of state and federal regulatory attention. If you're in this region, your concerns are different from the deep-aquifer concerns described above.

Known contaminant concerns

Communities on this aquifer