Groundwater Quality in the Colorado Plateaus Aquifers

From a U.S. Geological Survey fact sheet of April 2021 (National Water-Quality Assessment Project).

Groundwater provides nearly 50 percent of the nation's drinking water. The USGS National Water-Quality Assessment (NAWQA) Project assesses groundwater quality in the aquifers most important for drinking water — among them the Colorado Plateaus aquifers.

The aquifers

The Colorado Plateaus aquifers underlie about 141,000 square miles of southwestern Wyoming, eastern Utah, western Colorado, northwestern New Mexico and northeastern Arizona. Groundwater is the main source of drinking water for many of the 1.2 million people living above them.

• Use. They rank 28th in the nation as a source of groundwater for public supply — about 102 million gallons a day in 2000 — and supplied about 81.9 million gallons a day for irrigation.
• Land. Mostly natural land cover (97 percent), with small areas of pasture and farmland (1.9 percent) and urban land (1 percent). The largest urban areas are Flagstaff, Arizona (metropolitan population about 134,000), and Grand Junction, Colorado (about 147,000).
• Geology. Four distinct regional aquifers — the Uinta-Animas, Mesaverde, Dakota–Glen Canyon and Coconino–De Chelly — in rocks of Tertiary to Permian age. Each is made of several local aquifers, mainly sandstones and conglomerates, with siltstone, carbonate rocks and shale; fractures, faults and solution cavities in carbonate rock locally let water move more freely. The rocks are thousands of feet thick, faulted and variably uplifted or sunk, and mountains and deep canyons add to the complexity. The climate is arid to semiarid at lower elevations and cool and wet in the mountains to the east and north.

The study

In 2017 the USGS sampled 60 public-supply wells: 15 in Arizona, 10 in New Mexico, 19 in Utah, 11 in Colorado and 5 in Wyoming. Because the region is sparsely settled and wells are unevenly spread, the study area was defined by drawing 20-kilometre (12.4-mile) buffers around existing public-supply wells and laying an equal-area grid within them; it is the depth zone used for public supply. Wells were 100 to 3,636 feet deep (median 565 feet). The results are the percentage of the study area with high, moderate or low concentrations — their accuracy depends on how many wells there are and how they are distributed, not on the size of the area.

The Colorado Plateaus aquifers, the buffered study area and the sampled wells. Base from U.S. Geological Survey digital data and U.S. Census Bureau urbanized areas. Credit: U.S. Geological Survey.

How water quality is judged

Samples of untreated water are compared with drinking-water benchmarks. Water delivered to consumers may differ, because well water can be treated or blended first. Human-health benchmarks exist for 28 of 55 inorganic constituents and properties, and 170 of 310 organic constituents.

• High: above a human-health benchmark or a secondary (aesthetic) standard.
• Moderate: above one-half of the benchmark for inorganic constituents, or one-tenth for organic constituents, which are generally less common and at lower levels relative to their benchmarks.

Results

In 84 percent of the study area, no contaminant measured was at a high concentration.

Overview of water quality, as percentages of the study area (totals may not reach 100 because of rounding). Credit: U.S. Geological Survey.

• Arsenic was the trace element most often found at high (8 percent) and moderate (14 percent) concentrations. Manganese was high in 3 percent and moderate in 3 percent, and cobalt high in 2 percent; fluoride, iron, uranium, molybdenum and strontium were moderate in 2 to 7 percent. (Health-based screening levels were added in 2018 for aluminum, cobalt and iron.)
• Radioactivity in groundwater comes mostly from the decay of uranium and thorium in aquifer minerals; gross-alpha activity and radium were the only radioactive constituents at high levels.
• Nitrate, which at higher levels usually reflects fertiliser, septic systems or human and animal waste, was never high.
• VOCs at moderate levels were carbon tetrachloride, dichloromethane and chloroform.

Taste, colour and nuisance

Eleven constituents with secondary (non-regulatory, aesthetic) standards were analysed; one or more was high in about 48 percent of the study area and moderate in about 30 percent.

Total dissolved solids come naturally from weathering of rock, but can also rise from road salt, fertilisers and other chemicals. Low-oxygen groundwater can release iron and manganese from minerals. The secondary standard for pH is 6.5 to 8.5: in about 2 percent of the area pH was below 6.5 (acidic and potentially corrosive), and in about 22 percent above 8.5 (alkaline).

Benchmarks for constituents found at high levels

MCL: EPA maximum contaminant level (regulatory). HBSL: health-based screening level (non-regulatory). SMCL: EPA secondary maximum contaminant level (non-regulatory).

About the studies

NAWQA has assessed groundwater quality since 1991 through three kinds of study: Land Use Studies (shallow observation wells), Major Aquifer Studies (intermediate-depth domestic wells) and Principal Aquifer Studies (deeper public-supply wells). This summary is from a Principal Aquifer Study. Its analyses go beyond regulatory compliance monitoring, measuring concentrations far below health benchmarks and constituents that trace where groundwater comes from and how it moves.

Sources

• Degnan, J., and Musgrove, M., 2021, Groundwater quality in the Colorado Plateaus aquifers, western United States: U.S. Geological Survey Fact Sheet 2021–3012. https://doi.org/10.3133/fs20213012 — data in Kingsbury and others (2020).
• The background, the map and the charts are taken from the fact sheet's PDF; the imported text lacked them.
• Rewritten in hubnx's own words.