Official Government Website

Groundwater Quality Facts


Arsenic has recently become a concern for many groundwater users in Idaho. In 2001, the U.S. Environmental Protection Agency (EPA) reduced the primary drinking water standard for total arsenic (AS) from 50 micrograms per liter (µg/L) to 10 µg/L. Beginning January 23, 2006 public water systems must comply with the new standard.

Arsenic is naturally found in the earth’s crust. Trace amounts are also found in plants and animals. The majority of industrially used arsenic is found in wood preservatives. Arsenic is also used in some pesticides. Sources of Arsenic in Idaho’s groundwater include mining wastes, industrial wastes, and arsenical pesticides.

sources of arsenic in our groundwater

Long-term exposure arsenic in drinking water increase the risk of cancer of the bladder, lungs, skin, kidneys, nasal passages, liver and prostate. It has also been linked to such things as cardiovascular disease, diabetes, anemia, and skin damages. Health risks increase with higher doses and longer exposure periods.

Click the image thumbnails to view the areas of concern for arsenic in Idaho:


Nitrate is an oxidized form of nitrogen that typically comes from inorganic fertilizers, decaying organic matter, wastewater from commercial operations, animal manure, and human sewage. Only rarely does nitrate originate from geologic formations. It is an odorless, colorless, and tasteless compound. It is highly mobile in water and can cause health problems if it enters drinking water.

The EPA has set a maximum contaminant level (MCL) for nitrate in public drinking water of 10 mg/L. IDWR considers groundwater nitrate levels above 2 mg/L to be impacted.

Reporting RangeConcentration
Low End< 0.5 to 1.99 mg/L
Impacted2 to 10 mg/L
Exceeds Drinking Water Standard> 10 mg/L

The health risk of nitrate include Blue Baby Syndrome—an oxygen deficiency that can induce illness and death in infants. Those at risk include babies less than 6 months old and pregnant women. Health officials recommend that water with greater than 10 mg/L nitrate not be used for drinking, cooking, or formula preparation for infants under six months and for pregnant women.

Effective methods of treating water for nitrate include:

  • Reverse Osmosis: Nitrate is removed by forcing water through a membrane.
  • Distillation: Water is purified by capturing the steam of boiled water and condensing it. Nitrate is left behind when the water turns to vapor.
  • Ion Exchange: Nitrate ions are exchanged for another ion.
WARNING: Boiling, filtering, and softening are not treatments for removing nitrate. Contact a licensed professional for treatment options.

In 1995, 100 sites were selected to be sampled annually (sample collection at one site was discontinued in 2000). Of the annual sites, 27 sites showed evidence of nitrate increases and 16 sites had nitrate decreases. The following graph is an example of trends in nitrate concentrations at Annual Sites.

Based on the nitrate study information from 1990-2004, nitrate has impacted groundwater quality in many places in Idaho:

ver: 3.0.0a | last updated:
Jump back to top of page button