Groundwater Quality in the Stream Valley Aquifers, Nebraska to Oklahoma

By James A. Kingsbury, U.S. Geological Survey

Groundwater supplies nearly 50 percent of the Nation's drinking water. The USGS National Water-Quality Assessment (NAWQA) Project assesses aquifers that are important drinking-water sources; the Stream Valley aquifers are among them.

The aquifers

• Where: sand and gravel in the valleys of the Arkansas, Missouri and Red River drainages — about 41,200 square miles across parts of Oklahoma, Nebraska, Kansas, Missouri and Colorado, including Omaha, Kansas City and Tulsa.
• Who depends on them: about 4.5 million people live above them, and about 167 million gallons per day were pumped for public supply in these states in 2000 (Sargent and others, 2008; Kingsbury and others, 2021).
• Land above: 54 percent undeveloped, 39 percent farmland, 7 percent urban.
• Shape: usually only a few miles wide, but long; typically no more than 100 feet thick, up to 160 feet along large rivers (Ryder, 1996; Miller and Appel, 1997).
• Water: usually unconfined (at the water table), and connected to the streams; recharged by rain and by water from the adjacent rivers.

How the study worked

In June–September 2018, the USGS sampled 59 public-supply wells, spread randomly over an equal-area grid so they represent the whole study area (Belitz and others, 2010). The wells were about 30 to 200 feet deep, averaging about 85 feet. The water was tested untreated, for many constituents from natural and human sources.

Judging the results: against drinking-water benchmarks — federal health limits where they exist, otherwise non-regulatory health or aesthetic ones. Of 55 inorganic constituents and properties and 317 organic ones, 24 and 157 have health benchmarks.

Overview

Share of the study area with high, moderate and low concentrations. USGS.

• Inorganic constituents with health benchmarks: high in about 46 percent, moderate in about 27 percent.
• Organic constituents: detected infrequently, usually at low levels; moderate in about 8 percent, never high.

Inorganic constituents with health benchmarks

• Trace elements and major and minor ions: 34 analyzed, 22 with health benchmarks (screening levels added aluminum, cobalt and iron in 2018; Norman and others, 2018). High or moderate in 63 percent of the study area, eight constituents in all.
• • Manganese: high in 22 percent, moderate in 8 percent.
  • Arsenic: high in 10 percent, moderate in 24 percent.
• Radioactive constituents: 8 analyzed, 4 with benchmarks. High in about 12 percent, moderate in 17 percent — most often gross-alpha radioactivity.
• Nutrients: 5 analyzed; nitrate — from fertilizer, septic systems and waste — was the only one elevated: high in 7 percent, moderate in 5 percent.

Arsenic in river-valley aquifers

Arsenic above 10 µg/L has been found in alluvial aquifers elsewhere in the country (Saunders and others, 2005; Sharif and others, 2008) and around the world (Ahmed and others, 2004). The Stream Valley aquifers — mostly Holocene alluvium — are another example: about one-third of the study area had moderate or high arsenic, with no clear pattern, though more often in the Missouri River Basin. In other alluvial aquifers, arsenic has generally come from natural sources and aquifer chemistry — such as high dissolved organic carbon and iron.

Taste, color and scaling

(Not in the overview charts.) Of 11 constituents with secondary (aesthetic) benchmarks, one or more was high in about 63 percent and moderate in about 22 percent of the study area.

• Total dissolved solids (salinity): high in 48 percent, moderate in 34 percent.
• With little oxygen, groundwater releases natural iron and manganese: high relative to aesthetic limits in about 27 and 36 percent.
• pH fell outside 6.5–8.5 in about 2 percent.

Organic constituents

• VOCs: 85 analyzed (51 with benchmarks); none high; two solvents and the disinfection byproduct chloroform moderate in about 5 percent.
• Pesticides: 225 compounds analyzed (119 with benchmarks); found at low levels across much of the area; atrazine and one of its breakdown products moderate in 5 percent.

Selected benchmarks: arsenic 10 µg/L; uranium 30 µg/L; nitrate (as N) 10 mg/L; gross-alpha 15 pCi/L; manganese 300 µg/L (health) and 50 µg/L (aesthetic); total dissolved solids 500 mg/L (aesthetic).

About the program

NAWQA has assessed groundwater since 1991 with Land Use (shallow), Major Aquifer (household wells) and Principal Aquifer (deep public-supply wells) studies. Principal Aquifer Studies compare untreated water with health and aesthetic benchmarks and let aquifers be compared, with analyses far below health limits and tracers of where water comes from — work not usually part of regulatory monitoring. Data: Kingsbury and others (2020).

Sources

Based on Groundwater Quality in Selected Stream Valley Aquifers, Western United States, by James A. Kingsbury, USGS Fact Sheet 2021–3011, U.S. Geological Survey; a work of the United States government in the public domain. The overview chart and the arsenic section are taken from the fact sheet's PDF, which the import had left out; its two maps, drawn on a licensed Esri background, are not reproduced.