Naturally Occurring Well Water Contaminants

An important consideration within the Basin and Range Province is how geologic forces have influenced the quality of water held within the aquifers. The Basin and Range could resemble an egg carton filled with sand, with many isolated basins and drainage systems that could not reach the sea, generating large inland seas—such as the Great Salt Lake in Utah—that concentrated the salts leached from the soils as water evaporated. Large evaporite deposits of salt are common within valley aquifers within the Basin and Range province, and elevated concentrations of chemical constituents such as boron, sodium chloride (salt) and calcium sulfate (gypsum) are often found in the deeper alluvium zones of the basin these aquifers.

In the Gila River Valley, for example, deep petroleum exploration boreholes have been drilled throughout the region. Although oil was not found, salt brines are now discharging to the land surface through improperly sealed abandoned boreholes, and the local water quality has been impacted. Thick layers of salt are found deep throughout the entire valley.

Today, the Wilcox Playa (near Wilcox) is an example of the formation of evaporite deposits. Because the basin is not drained, salts are accumulating on the land surface. However, the geologic barrier that stops the flow out of the Wilcox Basin is relatively recent in geologic time, and because of this only the shallow ground water is salty. Water quality in the deep aquifer of the Willcox Basin is excellent.

Figure 11: Major aquifers and
regions of saline ground water

Figure 11 shows those portions of the state where ground water has been reported to be saline, either due to deep layers of salt originating from the depositional setting, playa formation, or in agricultural areas where evaporation of irrigation water concentrates naturally occurring salts.

In addition to elevated total dissolved solids (see section 4), the most common constituents found in Arizona ground water in concentrations above drinking water standards are arsenic, fluoride, gross alpha radiation, and nitrate. Nitrate contamination, although it can be natural, is usually due to either agricultural practices (excessive fertilizer use and/or poor irrigation practices), or failing septic systems that allow contaminated waters to drain into the aquifer. Ammonium and phosphorus contamination, much less common in Arizona aquifers, are also linked to septic sewage water contamination. Naturally occurring ground water contaminants are dependent on aquifer geology, and are discussed below.

Arsenic

Three significant geologic sources of arsenic are found in Arizona, and elevated concentrations of arsenic are found in each of the three geologic provinces. In geologically ancient Arizona, magma pushed upward into the host rock and hardened into granitic plutons and mineralized veins of ore containing copper, silver, gold, and arsenic. In Arizona, regions of granite bedrock with valuable gold ore often contain elevated concentrations of arsenic. Gold prospectors have found new mine sites by measuring the concentration of arsenic in rivers and streams, using arsenic as a pathfinder as they move upstream following greater and greater concentrations of arsenic until the source is found – and gold is discovered. In addition, Basin and Range aquifers consisting of alluvium eroded from granite bedrock will also contain arsenic.

The geology of northern Arizona and southern Utah consists of layers of ancient sedimentary rock, including the Redwall Limestone and the sandstone formations that can be seen in the exposed cliffs of the Grand Canyon, (see Figure 5). These sedimentary rocks are also found layered across the Colorado Plateau province of northeastern Arizona, and many water supply wells tap these formations. An extensive cave system was formed over 325 million years ago within the Redwall Limestone, similar to the limestone caves of Kartchner Caverns near Benson. Over geologic time, the weight of overlying rock layers that had accumulated on top of the caves in the Redwall Limestone collapsed, resulting in thousands of feet of vertical collapsed chimneys or drain pipes that filled with rock rubble. These pipes acted as drains, allowing ground water which contained dissolved chemicals from the adjacent sedimentary rock to concentrate. Arsenic, various metals, and uranium were deposited and concentrated within these pipes, which are found throughout the Supai Sandstone formation (Kenny, 2003). Wells constructed within the Supai Sandstone in the Colorado Plateau have elevated levels of dissolved arsenic in the ground water, as well as uranium and other radioactive elements, discussed below.

Arsenic is also found in the Central Highlands geologic province of Arizona. Within the past 2 to 5 million years, the Verde Valley of Yavapai County was formed as earth crust shifts produced faults that separated the Colorado Plateau from the Basin and Range. The arsenic rich Supai Sandstone formation was eroded and re-deposited in the Verde Alluvium Formation, which now forms the aquifer of the Big Chino and Verde Valley. The highest concentration of arsenic in ground water in Arizona was found near Pauldin in the Verde Valley, with a concentration of 2,900 parts per billion in a private, domestic (exempt) well. The EPA drinking water MCL for arsenic is 0.010 mg/L, or 10 parts-per-billion.

Because the solubility of arsenic in water is a function of its mineral form, water pH, and oxygen content, any change in the chemistry of an aquifer may increase or decrease arsenic concentrations. An example is with the introduction of oxygen as ground water elevations dropped due to drought in the Verde Valley. The change in geochemistry resulted in arsenic concentrations increasing, and consequently in arsenic poisoning of livestock (Foust et al., 2003)

Click here to access AZ Cooperative Extension's Arsenic in Arizona Ground Water.

Radioactive Elements

Radioactivity is the release of energy from within atoms. Certain atom structures are inherently unstable and spontaneously break down (decay) to form more stable atoms. For example, the potassium-40 isotope decays very slowly (half-life of 1.25 billion years) but eventually becomes the element argon. Because potassium is a significant component of clay minerals, it is generally true that all clay, including clay soils, and bricks and pottery made from clay soils, and living organisms (animals and plants) that contain potassium, are all slightly radioactive.

In Arizona, the most common source of radioactivity is dissolved uranium and dissolved radon gas. Remember the collapsed chimneys or pipes above the Redwall Limestone Formation acted as drains, allowing ground water which contained dissolved chemicals from the adjacent sedimentary rock to concentrate. In addition to arsenic, uranium was deposited and concentrated within these pipes. Uranium mines are found throughout the Supai Sandstone Formation (Kenny, 2003). The water from wells within the Supai Sandstone in the Colorado Plateau show elevated concentrations of uranium, sometimes exceeding the MCL of 0.030 mg/L, or 30 parts-per-billion.

Radioactive minerals containing the elements uranium and thorium (760 million and 4.46 billion years half-life, respectively) are also found in some Arizona granites. These elements are unstable and decay, eventually becoming a new element called radium (half-life of 1,620 years), which then decays to the element radon (half-life of 3.8 days). Radon is strongly radioactive as it emits high energy alpha particles. Unfortunately, the radon element is an odorless, colorless, tasteless gas that dissolves in ground water and may migrate upward though the soil to eventually dissipate to the atmosphere. If radon gas is trapped within a structure, such as a basement, the concentration of radon gas within the closed structure may exceed health standards. The EPA estimates that 1 in 15 U.S. homes contain a high level of the gas and is considered to be the second leading cause of lung cancer in the country (epa.gov/rado/radontest.html). The MCL for radon is 300 pCi/L.

‘Gross alpha’ is a measurement of the amount of radioactivity in water whether it is due to the decay of uranium, radium or radon, and is a gross measurement of overall radioactivity. ‘Gross alpha’ is a common naturally occurring “contaminant” in Arizona bedrock aquifers (such as the Supai Sandstone or granite) or in alluvial aquifers composed of eroded granite. The MCL for ‘Gross alpha’ is 15 pCi/L.

Fluoride

Fluoride is a common mineral that is concentrated in volcanic materials, and mineral particles that contain fluoride are common in some sedimentary rocks. In Arizona, the highest fluoride concentrations are found in Cochise County (Hem, 1970); Mohave, Graham, and Greenlee counties (ADEQ, 2005); and, along the lower Gila River in Yuma county. Most of the elevated concentrations are associated with confined aquifers. Ground water from confined aquifers usually has not had the opportunity to mix with recently recharged water high in dissolved oxygen. Therefore, the low oxygen environment and long resident time in confined aquifers allows for fluoride naturally present in the aquifer geology to dissolve into the ground water. Although fluoride at high concentrations may be harmful, it is essential for strong teeth and bones; many municipal water supply systems add fluoride to the water in a process called fluoridation. Excessive concentrations in drinking water results in tooth mottling and discoloration. The MCL for fluoride is 4.0 mg/L.

Elevated levels of other naturally occurring constituents have been found in wells across Arizona. For example, naturally occurring hexavalent chromium (CrVI) has been found in Paradise Valley north of Phoenix and in the Detrital Valley near Kingman (Robertson, 1975). Hexavalent chromium is known to cause cancer. Lithium is found in the brine ground water of the Gila Valley near Safford. Selenium and boron are also found in geologic settings with evaporite deposits, and these elements have been detected in ground waters near Kingman. Each of these constituents have known health impacts and should be avoided in high concentrations. The mineral-rich geology of our state results in elevated levels of elements such a copper, silver, zinc, manganese, and sulfate minerals, occasionally being encountered in ground water near mining districts. Iron is found in nearly all ground water and is responsible for iron-bacterial fouling of some wells.