Groundwater Quality in the Rio Grande Aquifer System

From a U.S. Geological Survey fact sheet by MaryLynn Musgrove and Laura M. Bexfield, National Water-Quality Assessment Project, 2017.

Groundwater provides nearly 50 percent of the nation's drinking water. The USGS National Water-Quality Assessment (NAWQA) Project assesses groundwater quality in aquifers that are important sources of drinking water, and the Rio Grande aquifer system is one of them.

The aquifer

The Rio Grande aquifer system underlies 29,000 square miles of Colorado, New Mexico and Texas.

The land above it is mostly undeveloped (93 percent), with a little farmland (4 percent) and urban land (3 percent). About 2 million people live there, including in Albuquerque, New Mexico, and El Paso, Texas.

The system is made of connected basin-fill aquifers in about 20 alluvial basins along the Rio Grande Valley and nearby valleys. The fill — gravel, sand, silt and clay of Tertiary to Quaternary age, loose to moderately consolidated — ranges from as little as 2,000 feet thick near El Paso to as much as 30,000 feet in the San Luis Valley of southern Colorado. The groundwater is mostly unconfined, though some basins have confining layers.

• Climate: semiarid to arid; rain falling in the valleys mostly evaporates or is taken up by plants.
• Recharge: mainly along mountain fronts, from mountain streams soaking in and water flowing from fractured bedrock; in some basins, also from the Rio Grande losing water and from irrigation.
• Discharge: mostly evapotranspiration, especially in closed basins, with natural discharge to the Rio Grande in places. In some basins, pumping for irrigation or public supply has become large enough to change the direction groundwater flows.

How it was sampled

In 2014 the USGS sampled 60 public-supply wells spread across the system: 6 in Colorado, 45 in New Mexico and 9 in Texas. Because public-supply wells are uneven in a sparsely populated region, the study area was drawn as 5-kilometre (3.2-mile) buffers around existing wells, and an equal-area grid laid over it. The "study area" here means the depth zone used for public supply within that buffered area.

The wells were 25 to 2,000 feet deep (643 feet on average) and open to the aquifer over long intervals (about 295 feet on average). The approach estimates what share of the study area has high, moderate or low concentrations of each constituent; its accuracy depends on the number and spread of wells, not the size of the area.

The Rio Grande aquifer system, the study area and the sampled wells. USGS.

How the results are judged

The samples are untreated groundwater, compared with drinking-water benchmarks for context; water reaching consumers may be treated first. Federal regulatory health benchmarks are used where they exist, otherwise non-regulatory health benchmarks and aesthetic benchmarks. Of 55 inorganic constituents and properties and 317 organic constituents analysed, 24 and 157 respectively have human-health benchmarks.

• High: above a human-health benchmark or a secondary maximum contaminant level (SMCL).
• Moderate: above half a benchmark for inorganic constituents, or above one-tenth for organic ones — a lower bar because organic constituents are generally less common and found at lower levels.

Results

Water from 70 percent of the wells had no contaminant at high concentrations.

Overview of water quality: share of the study area with high, moderate and low concentrations. USGS.

Inorganic constituents with health benchmarks

One or more were high in about 30 percent of the study area and moderate in about 28 percent.

• Trace elements and ions: 34 analysed, 19 with health benchmarks; high in about 27 percent, moderate in about 25 percent. Arsenic was the most frequent — high in 18 percent, moderate in 22 percent. Fluoride, strontium and uranium were each high in about 3 percent, and moderate in about 8, 2 and 7 percent. Manganese and molybdenum were never high, but moderate in about 2 and 3 percent.
• Radioactive constituents: 8 analysed, 4 with health benchmarks; high in about 5 percent (gross-alpha activity and radon) and moderate in about 10 percent. Most radioactivity in groundwater comes from uranium and thorium isotopes naturally present in aquifer minerals.
• Nutrients: 5 analysed, 2 with health benchmarks. Nitrate was high in about 2 percent; no nutrient was moderate. Besides soils, nutrients come from fertiliser, septic systems and human and animal waste.

Organic constituents

None with health benchmarks was high or moderate.

• Volatile organic compounds: 90 analysed, 38 with health benchmarks — none high or moderate.
• Pesticides: 227 pesticide compounds analysed, 119 with health benchmarks — none high or moderate.

Taste, colour and scaling

Some constituents affect how water tastes, looks or smells, or cause staining and scaling. Of 11 with SMCLs, one or more were high in about 42 percent of the study area and moderate in about 28 percent.

• Total dissolved solids, a measure of salinity, were high in 35 percent and moderate in 32 percent. Chloride, fluoride and sulfate were high in about 10, 12 and 7 percent, and moderate in 8, 10 and 23 percent.
• Manganese was high relative to its SMCL in about 3 percent; iron was never high. Both were moderate in about 2 percent. Low-oxygen groundwater can release them from minerals.

Arsenic from north to south

Inorganic constituents with health benchmarks were high or moderate across 58 percent of the study area, and arsenic, at 40 percent, was the most widespread. Its pattern changed from north to south: high or moderate in 17 percent of the Colorado samples, 36 percent in northern New Mexico, 34 percent in southern New Mexico, and 78 percent in Texas. The differences may reflect the rocks and sediments that make up the aquifers, the paths groundwater follows, the aquifers' chemistry and other factors.

Arsenic: low is 5 micrograms per litre or less, moderate above 5, and high above the health benchmark of 10. USGS.

Benchmarks for the constituents found at high levels

MCL: the Environmental Protection Agency's regulatory maximum contaminant level. HBSL: a non-regulatory health-based screening level. SMCL: the EPA's non-regulatory secondary maximum contaminant level.

About the project

NAWQA has studied groundwater quality since 1991, through shallow land-use studies, intermediate-depth studies of domestic wells, and deeper Principal Aquifer Studies of public-supply wells, like this one. Its data include measurements far below health benchmarks and tracers of where groundwater comes from and how it moves — information not usually collected for regulatory monitoring.

Sources

• MaryLynn Musgrove and Laura M. Bexfield, Groundwater Quality in the Rio Grande Aquifer System, Southwestern United States, U.S. Geological Survey Fact Sheet 2017–3047. https://doi.org/10.3133/fs20173047 — the description of the aquifer and the sampling, the pesticide results, the benchmark table, the arsenic section and three figures recovered from the PDF, which the imported page had lost.
• Data: Arnold and others (2017a, 2017b); the fact sheet also cites Anning and others (2007), Belitz and others (2010), Bexfield and others (2011), Burow and Belitz (2014), DeSimone and others (2014), Robson and Banta (1995), Thiros and others (2014), Toccalino and others (2004, 2014) and Wilkins (1998).
• Rewritten in hubnx's own words.