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By James A. Kingsbury, U.S. Geological Survey, National Water-Quality Assessment Project. Fact Sheet 2019–3057, March 2020.
The aquifer
- Area: about 49,000 square miles — much of southern Missouri, northwestern Arkansas, and small parts of Kansas and Oklahoma — above which about 4 million people live, including in St. Louis and Springfield.
- Use: about 212 million gallons a day for public and domestic supply in 2010.
- Land above: 58 percent natural, 35 percent agricultural, 7 percent urban.
- Three aquifers, separated by confining units, mostly limestone and dolomite:
- Springfield Plateau (top): wells usually yield 20 gallons a minute or less;
- Ozark: the main water source, and where most sampled wells were;
- St. Francois (bottom): used mainly where it's near the surface in Missouri.
- Karst: the carbonate rock dissolves, so sinkholes, springs and caves are common. Recharge is mostly rain, with sinkholes channeling it in.
The survey
From April to August 2016, the USGS sampled 60 public-supply wells in the Ozark and St. Francois aquifers, placed at random on an equal-area grid (one substitute outside the boundary where no well existed). The wells were about 370 to 3,420 feet deep, averaging about 1,180.
Benchmarks: untreated water was compared with drinking-water benchmarks; 28 of 55 inorganic constituents and 170 of 310 organic ones have health benchmarks. High means above a benchmark; moderate means above half of it for inorganics, or one-tenth for organics.
Results
Overall: inorganic constituents with health benchmarks were high in about 7 percent of the study area and moderate in about 13 percent. No organic constituent reached high or moderate levels.
Share of the study area with high, moderate and low levels. USGS.
| Group | Tested (with health benchmarks) | High | Moderate | Notes |
|---|---|---|---|---|
| Trace elements and ions | 34 (22) | about 3% | about 4% | only arsenic and strontium high; benchmarks updated in 2018 to add aluminum, cobalt and iron |
| Radioactive constituents | 8 (4) | about 3% | about 10% | only gross alpha and radium-226 plus radium-228 high |
| Nutrients | 5 (2) | none | none | |
| VOCs | 85 (51) | none | none | several detected, all low |
| Pesticides | 225 (119) | none | none | about 10 detected, all low |
Taste and nuisance (non-health limits): of 11 constituents, one or more were high in about 7 percent and moderate in 50 percent of the area — dissolved solids high in 3 percent, moderate in 50; fluoride high in 2, moderate in 3; manganese high and moderate each in about 2 percent; pH above 8.5 (alkaline, affecting taste and leaving deposits) in about 2 percent.
What stands out
Earlier studies found high nitrate in springs and shallow groundwater here, but these much deeper public-supply wells showed no nitrate or other mainly man-made constituent at high or moderate levels. Only natural constituents were high. Radium was the most frequent at moderate and high levels — mostly in the west — but in only 7 percent of the study area.
About these studies
Since 1991, NAWQA has studied groundwater through land-use (shallow), major aquifer (domestic) and principal aquifer (deep public-supply) studies. Principal aquifer studies compare untreated water with benchmarks, allow comparisons between aquifers, and include measurements far below benchmarks and tracers of groundwater movement. Data: Arnold and others (2020).
Sources
Based on Groundwater Quality in the Ozark Plateaus Aquifer System, Central United States, by James A. Kingsbury, USGS Fact Sheet 2019–3057, U.S. Geological Survey; a work of the United States government in the public domain. The background section and the chart are taken from the fact sheet's PDF, which the import had left out; its maps are left out because their shaded relief is from Esri.
Licença: CC0 1.0 (domínio público) · Adaptado de pubs.usgs.gov
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