By Jeannie R.B. Barlow and Kenneth Belitz, U.S. Geological Survey, National Water-Quality Assessment Project. Fact Sheet 2016–3077, September 2016.
Groundwater supplies nearly 50 percent of the nation's drinking water. The USGS National Water-Quality Assessment (NAWQA) Project studies the aquifers that matter most — including the Coastal Lowlands aquifer system.
The aquifer
- Area: 99,000 square miles along the Gulf Coast, home to about 15 million people in Texas, Louisiana, Mississippi, Alabama and Florida, including Houston, New Orleans, Baton Rouge and Mobile.
- Use: fourth in the nation for public supply and fifth for private domestic supply — about 1 billion gallons a day in 2000.
- Land above: 67 percent undeveloped, 24 percent farmland, 9 percent urban.
- Rock: loose to partly consolidated sand, silt and clay, Oligocene to Holocene in age, thickening coastward from a few tens of feet to as much as 14,000 feet near the southern Louisiana coast.
- Water flow: recharged by rain on higher ground and the landward edge; flows toward the coast and the Mississippi River, discharging to streams, wetlands and the Gulf. Earlier studies split it into five permeable zones, but with no continuous confining layers it was treated here as one unit.
The survey
In 2013, the USGS sampled 60 public-supply wells spread across the aquifer — 27 in Texas, 19 in Louisiana, 9 in Mississippi, 1 in Alabama and 4 in Florida. They ranged from about 115 to 2,540 feet deep (average about 660), open over long intervals (about 100 feet). The results estimate the share of the depth zone used for public supply with high, moderate or low levels of each constituent.
Benchmarks: untreated groundwater is compared with drinking-water benchmarks for context — legal health limits where they exist, otherwise non-regulatory health or taste limits. Of 55 inorganic constituents and 317 organic ones measured, 24 and 157 have health benchmarks.
| Level | Inorganic | Organic |
|---|---|---|
| High | above the benchmark | above the benchmark |
| Moderate | above half of it | above one-tenth of it (organics are usually rarer and lower) |
Results
Overall: inorganic constituents with health benchmarks were high in about 12 percent of the study area and moderate in about 18 percent. Organic constituents were not found at high or moderate levels.
Share of the study area with high (yellow), moderate (green) and low (blue) levels. USGS.
| Group | Tested (with health benchmarks) | High | Moderate | Notes |
|---|---|---|---|---|
| Trace elements and ions | 34 (19) | about 10% | about 12% | only arsenic and manganese high |
| Radioactive constituents | 8 (4) | about 3% | about 10% | only gross alpha and radon-222 high; mostly from natural uranium and thorium |
| Nutrients | 5 (2: nitrate, nitrite) | none | about 2% (nitrate) | high levels usually mean fertilizer, septic systems or waste |
| VOCs | 90 (38) | none | none | |
| Pesticides | 227 (119) | none | none |
Taste, color and nuisance (non-health limits): of 11 constituents, one or more were high in about 42 percent and moderate in 28 percent of the area.
- Dissolved solids (salinity): high in 27 percent, moderate in 30 percent.
- Chloride high in about 10 percent, moderate in 15; sulfate high in 3, moderate in 3.
- Fluoride high in about 2 percent, moderate in 10.
- Manganese high in about 20 percent; iron high in 13, moderate in 5 — both released from minerals where water is low in oxygen.
- pH outside 6.5–8.5 in 27 percent, usually below 6.5 — acidic and potentially corrosive.
By state
Inorganic constituents with health benchmarks were high or moderate in 30 percent of the whole area — but 55 percent in Texas, 16 percent in Louisiana, and zero in Mississippi, Alabama and Florida. Arsenic (19 percent), gross alpha (4 percent) and radon-222 (4 percent) exceeded benchmarks only in Texas; manganese (5 percent) only in Louisiana.
High, moderate and low levels by state (top), and the constituents responsible (bottom). USGS.
About these studies
Since 1991, NAWQA has studied groundwater through land-use studies (shallow observation wells), major aquifer studies (domestic wells) and principal aquifer studies (deep public-supply wells). The principal aquifer studies compare untreated groundwater with benchmarks, allow comparisons between aquifers, and include measurements far below health benchmarks and tracers of groundwater sources and movement that routine compliance testing doesn't. Data: Arnold and others (2016).
Sources
Based on Groundwater Quality in the Coastal Lowlands Aquifer System, South-Central United States, by Jeannie R.B. Barlow and Kenneth Belitz, USGS Fact Sheet 2016–3077, U.S. Geological Survey; a work of the United States government in the public domain. The background section and both charts are taken from the fact sheet's PDF, which the import had left out; its map is left out because its shaded relief is from Esri.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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