By Timothy M. Mathany, Carmen A. Burton and Miranda S. Fram, U.S. Geological Survey, with the California State Water Resources Control Board
Groundwater supplies more than 40 percent of California's drinking water. To protect it, the State created the Groundwater Ambient Monitoring and Assessment (GAMA) Program, whose Priority Basin Project assesses groundwater quality statewide and makes the results public. The Bear Valley and Lake Arrowhead Watershed study areas in southern California make up one of its study units.
The study unit
The two study areas and their rocks: sedimentary deposits (yellow), granitic (pink) and metamorphic (purple). USGS.
| Bear Valley | Lake Arrowhead Watershed | |
|---|---|---|
| Setting | an alluvium-filled valley around Big Bear and Baldwin Lakes — the Bear Valley groundwater basin | mostly granitic bedrock, in parts of six watersheds around Lake Arrowhead |
| Water source | wells, usually 250–550 feet deep, in sedimentary deposits, sometimes reaching the metamorphic and granitic rock below | more than half springs or horizontal wells; vertical wells usually 230–500 feet deep, tapping fractures in granite |
| Recharge | mainly winter precipitation in the surrounding mountains; municipal pumping has lowered groundwater levels (Flint and Martin, 2012) | — |
| Urban land | 58 percent | 21 percent |
The study unit covers about 112 square miles (290 square kilometers) of the San Bernardino Mountains; natural land is mostly forest.
Samples: in 2010, the USGS sampled 38 wells and springs across the two areas, and data for 78 other public-supply sources (2007–2010) came from the State Water Resources Control Board's Division of Drinking Water.
How results are judged
The project assesses untreated groundwater — not the drinking water that reaches taps, which can differ after contact with plumbing, the air, or treatment. For context, concentrations are compared with drinking-water benchmarks: federal and California regulatory limits where they exist, otherwise non-regulatory health and aesthetic ones.
| Inorganic constituents | Organic constituents | |
|---|---|---|
| High | above a benchmark | above a benchmark |
| Moderate | above half a benchmark | above one-tenth of a benchmark |
Overview
Share of the groundwater resources used for public supply with high (yellow), moderate (green) and low or undetected (blue) concentrations. USGS.
| Bear Valley | Lake Arrowhead | |
|---|---|---|
| Inorganic — high | about 9% | about 25% |
| Organic — high | about 1% | none |
Inorganic constituents with health benchmarks
- Trace elements — from the minerals in rock and sediment: high in about 9% of Bear Valley samples (fluoride and arsenic); none high at Lake Arrowhead.
- Radioactive constituents — mostly from the decay of uranium and thorium in aquifer minerals: high in about 25% of Lake Arrowhead (uranium and adjusted gross alpha activity); none high in Bear Valley.
- Nitrate — usually high only from human sources such as fertilizer, septic systems and waste: not high in either area.
Uranium, granite and the age of the water
Uranium was above the federal limit in 25 percent of Lake Arrowhead samples, and never high in Bear Valley.
- Why: fractured granite, like Lake Arrowhead's aquifer, often contains uranium-bearing minerals that dissolve into groundwater. The metamorphic rocks around Bear Valley — and the sediments that came from them — usually don't.
- Older water, more uranium: in the granite, uranium was inversely related to tritium, a radioactive form of hydrogen made in the upper atmosphere that enters water as it falls and then decays — so it dates the water (Mathany and Burton, 2017). Water above 2.0 tritium units was mostly recharged in the last 60 years; water below that is older, hundreds to thousands of years, or mixed. High uranium turned up mostly in older water — dissolving much uranium may take long contact with the minerals.
Uranium against tritium by aquifer rock. Samples left of the dashed line are pre-modern or mixed-age water; to the right, water recharged since 1952. USGS.
Aesthetic and proposed benchmarks
(Not in the overview charts.)
- Total dissolved solids (a measure of salinity): never above California's recommended or upper benchmark in either area.
- Manganese or iron — released from minerals where groundwater has little oxygen — above aesthetic benchmarks in about 3% of Bear Valley samples and about 23% of Lake Arrowhead's.
- Radon-222, a radioactive gas from uranium, is mainly a concern for indoor air; its proposed water benchmark reflects how groundwater levels predict indoor levels (National Academy of Sciences, 1999). It exceeded the proposed benchmark in about 4% of Bear Valley and about 62% of Lake Arrowhead.
Organic constituents
The project's lab methods detect VOCs and pesticides far below health benchmarks — traces that help show how water moves from the land into aquifers.
- VOCs — in many household, commercial and industrial products: high or moderate in about 8% of both areas. These were the solvents trichloroethene, carbon tetrachloride and 1,1-dichloroethene, used in dry cleaning, degreasing metal, paint stripping, fire extinguishers and lubricants. Solvents and trihalomethanes — formed when water is disinfected, and entering groundwater from irrigation or leaky water lines — appeared at low levels in both areas; gasoline oxygenates at low levels in Bear Valley.
- Pesticides: never high or moderate; herbicides at low levels in both areas.
Benchmarks named here:
| Constituent | Benchmark | Value |
|---|---|---|
| Arsenic | Federal MCL | 10 µg/L |
| Fluoride | California MCL | 2 mg/L |
| Uranium | Federal MCL | 30 µg/L |
| Gross alpha activity | Federal MCL | 15 pCi/L |
| Nitrate (as nitrate) | Federal MCL | 45 mg/L |
| Trichloroethene | Federal MCL | 5 µg/L |
| Carbon tetrachloride | California MCL | 0.5 µg/L |
| 1,1-Dichloroethene | California MCL | 6 µg/L |
| Iron | California SMCL | 300 µg/L |
| Manganese | California SMCL | 50 µg/L |
| Total dissolved solids | California SMCL | 1,000 mg/L |
| Radon-222 | proposed Federal MCL | 4,000 pCi/L |
MCL: maximum contaminant level; SMCL: secondary (aesthetic) maximum contaminant level.
About the project
The GAMA Priority Basin Project assesses groundwater used for drinking-water supply — here, public supply, which usually draws from deeper in aquifers than household wells; springs tap fractured granite, and their water varies by place. Assessments are under way in more than 120 groundwater basins and areas across California. The project uses a statistical network of wells to assess status, and combines water-quality, hydrologic and geographic data to understand what controls quality. Its analyses include measurements far below health benchmarks and tracers of groundwater's sources and movement — not usually part of regulatory monitoring.
Sources
Based on Groundwater Quality in the Bear Valley and Lake Arrowhead Watershed, California, by Timothy M. Mathany, Carmen A. Burton and Miranda S. Fram, USGS Fact Sheet 2017–3037, U.S. Geological Survey; a work of the United States government in the public domain. The study-unit description, the uranium section, the benchmark table and the three figures are taken from the fact sheet's PDF, which the import had left out.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
1
0
0
0
Comments
