Groundwater Quality in the Biscayne Aquifer, Florida

Groundwater supplies nearly 50 percent of the nation's drinking water. The U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Project checks the quality of groundwater in aquifers that are important drinking-water sources — among them the Biscayne aquifer of south-east Florida.

The aquifer

• Area and people. It underlies about 4,000 square miles, home to about 4 million people, and is their main source of drinking water: about 700 million gallons a day were pumped for public supply in 2000.
• Where. The study area covers much of Broward and Miami-Dade Counties, including Miami and Fort Lauderdale.
• Land above it. Mostly developed: about 63 percent urban and 9 percent farmland; 28 percent undeveloped.
• Rock. An unconfined aquifer at the surface, of shallow, highly permeable limestone and some sandstone. Because it is so shallow, its link with surface water is central to how it works.
• Water management. A system of canals and levees conserves fresh water, controls floods and holds back saltwater creeping in along the coast.
• Flow. Canals and rain recharge it, and water generally flows west to east into the Atlantic, except where heavy pumping has changed its direction.

How it was sampled

The USGS sampled 40 public-supply wells placed at random in an equal-area grid, so that together they represent the whole study area. The results estimate the percentage of the area with high, moderate or low concentrations; accuracy depends on the number and spread of wells, not the size of the area. The wells were about 20 to 150 feet deep, averaging about 91. Samples were taken from June to September 2016 and tested before any treatment for many natural and man-made substances.

How "high" and "moderate" are defined

Concentrations are compared with drinking-water benchmarks: EPA's regulatory maximum contaminant levels (MCLs), non-regulatory health-based screening levels (HBSLs), and EPA's non-regulatory, aesthetic secondary levels (SMCLs). Human-health benchmarks exist for 28 of 55 inorganic constituents and properties and 170 of 310 organic ones.

• High: above a human-health benchmark or an SMCL.
• Moderate: above half a benchmark for inorganic constituents, or above one-tenth for organic ones, which are generally rarer and lower relative to benchmarks.

Water that reaches customers may differ, because it may be treated after pumping.

Overview

Share of the study area with high (yellow), moderate (green) or low or undetected (blue) concentrations. USGS.

• Inorganic constituents with human-health benchmarks: high in about 3 percent of the study area, moderate in about 17.
• Organic constituents, from products used in homes, businesses, industry and farming, were found infrequently and usually at low levels: moderate in about 5 percent, never high.

Inorganic constituents

Inorganic constituents by group. USGS.

• Trace elements and major and minor ions (34 analysed, 22 with health benchmarks): strontium was the only one at high or moderate levels — about 3 and 7 percent.
• Radioactivity, mostly from the natural decay of uranium and thorium (8 analysed, 4 with benchmarks): moderate in about 5 percent, all from radium-226 plus radium-228.
• Nutrients (5 analysed; nitrate and nitrite have benchmarks) — mainly from fertiliser, septic systems and human and animal waste: nitrate was moderate in 5 percent.

Taste, colour and scaling

Eleven constituents with SMCLs were measured. At least one was high in about 45 percent of the study area and moderate in about 55.

• Total dissolved solids (TDS), a measure of salinity: moderate throughout the study area.
• Iron, released from minerals where groundwater has little oxygen: high in about 45 percent, moderate in about 13.

Benchmarks

ppb: parts per billion (micrograms per litre); ppm: parts per million (milligrams per litre).

Organic constituents

Organic constituents. USGS.

• Volatile organic compounds (85 analysed, 51 with benchmarks): none high; vinyl chloride was moderate in about 5 percent.
• Pesticides (225 compounds and breakdown products analysed, 119 with benchmarks): found at low levels across most of the area, never moderate or high.

A shallow, vulnerable aquifer

• Although most wells were less than 150 feet deep, man-made contaminants were low compared with health benchmarks.
• But every well had at least one organic compound, a sign that the aquifer is vulnerable to contamination. Earlier studies found the same.
• Iron and TDS were moderate to high relative to their SMCLs. Iron tended to be low in the far north and south of the study area, but moderate to high values turned up throughout it.
• High iron and dissolved organic carbon point to reducing conditions, which may help keep nitrate low in this shallow aquifer through denitrification.

About these studies

NAWQA has assessed groundwater since 1991 with three kinds of well network: Land Use Studies (shallow observation wells), Major Aquifer Studies (mid-depth domestic wells) and Principal Aquifer Studies (deeper public-supply wells), like this one. They compare untreated groundwater with benchmarks and between principal aquifers, and include analyses not usually part of regulatory monitoring — at far lower concentrations than benchmarks, and of substances that trace where groundwater comes from and how it moves.

Sources

• Kingsbury, J.A., Groundwater quality in the Biscayne aquifer, Florida: U.S. Geological Survey Fact Sheet 2019–3056. https://doi.org/10.3133/fs20193056 · https://pubs.usgs.gov/publication/fs20193056
• Data: Arnold and others, 2020b, U.S. Geological Survey data release.
• The pie charts, the benchmark values and the closing section on iron come from the fact sheet's PDF. Its two maps are drawn over a commercially licensed shaded-relief base and are not reproduced here.
• Rewritten in hubnx's own words.