Hexavalent Chromium Contents in The Phoenix Metro Area’s Drinking Water

Drinking water contamination by industrial and domestic wastewater discharge and even natural rock exposures are the source of major concerns in the main cities in the world. Contamination by industrial activities in those cities like Phoenix with limited annual rainfall is a big problem because part of the tap water, particularly during the time of high demand, is provided from the groundwater. Table 1 represents the source of water in different cities of Phoenix Metro area in 2017. Surface water includes water from the Salt, Verde, and Colorado rivers and groundwater is provided from different number of wells. As we can see in this table, a large part of water in Chandler, Scottsdale, Tempe, and Mesa is provided by underground source. However, the proportion of groundwater in the city of Phoenix drinking water represents a big discrepancy with these other cities in the Metro area.

Table 1: Sources for Drinking Water Reported by Cities’ Water Services Departments 2017[1]. Number of Wells and groundwater share to supply drinking water in the city of Phoenix compare to other cities is interesting.

City	Surface Water (%)	Groundwater (%)	Number of Wells
Chandler	73	27	31
Scottsdale	78	22	23
Tempe	79	21	10
Mesa	80	20	31
Glendale	83	17	No data
Phoenix	98	2	20

Contaminants like arsenic, lead, radon, uranium, hexavalent chromium (Cr(VI)), nitrate, and asbestos could be a source of carcinogen in humans even at low contents. Cr(VI) in drinking water of Phoenix Metro area was the subject of a report, entitled “Erin Brockovich’ toxin found in Phoenix Metro drinking water”, in Azcentral news website in September 2016 (https://www.azcentral.com/story/news/local/arizona-water/2016/09/20/metro-phoenix-drinking-water-carcinogen-erin-brockovich/90523622/). Phoenix Water Services director Kathryn Sorensen responded to this report (https://www.phoenix.gov/news/waterservices/1458). She rejected any contamination and claimed since Cr(VI) comes mainly from groundwater and that groundwater makes up only approximately 2% of Phoenix’s water supply, thus the average level of 7.85 μg/L, as reported by Azcentral news website, is misleading.

However, I want to take another look at the Cr(VI) contents of drinking water in the Phoenix Metro area based on the information annually provided by Water Services Department of the other cities in the Metro area.

The average amounts of Cr(VI) vary considerably across Phoenix Metro area. Based on Table 2, drinking water in Mesa has the highest average content of Cr(VI) which is 7.6 μg/L ranging from 0.03 to 24 μg/L, probably depending on the entry point or different time of sampling during the year. Average Cr(VI) contents in Chandler and Scottsdale are 5.3 and 4.8 μg/L, respectively. These chromium levels are the average of samples from all entry points to the distribution system, not just groundwater (wells) as claimed by Phoenix Water Services director Kathryn Sorensen. If we accept the source of Cr(VI) is industrial and it comes just from groundwater, its high levels are related to those entry points with higher proportion of groundwater in drinking water. Therefore, it is necessary to eliminate those wells with highest Cr(VI) levels. Unfortunately in water quality report of Chandler there is no information about the content of Cr(VI). Cr(VI) in city of Phoenix drinking water is the lowest among all cities in the Metro area which could be related to the lower proportion of ground water.

Table 2: The Quantity of Cr(VI) in the Water Quality Reports by Cities’ Water Services Departments 2017. All values in μg/L (ppb).

City	Min.	Max.	Ave.
Mesa	0.03	24	7.6
Chandler	0.03	19	5.3
Scottsdale	No data	No data	4.8
Tempe	Not detected	Not detected	No data
Phoenix	0.1	1.6	0.4
Glendale	No data	No data	No data

Unfortunately, in the water quality report of Tempe they only showed the average value of Cr(VI) as less than 10 μg/L which is the Maximum Contaminant Level (MCL) of Cr(VI). MCL is the highest level of a contaminant defined by state of California’s Water Resources Control Board. Based on the Tempe report the total chromium range is between Not Detected (lower than the detection limit of analysis instrument) to 7.9 μg/L. The range of Cr(VI) in this report is also mentioned as Not Detected which is in contradiction with maximum content of total chromium. Total chromium is the combination of Cr(VI) and Cr(III) and based on the reports of Chandler, Phoenix, and Scottsdale total chromium in the Phoenix Metro area is mainly consist of Cr(VI). Therefore, the maximum Cr(VI) in Tempe should be close to 7.9 μg/L. If we consider the maximum values of Cr(VI) in the Mesa and Chandler, which are 24 and 19 μg/L, respectively, in association with minimum and average values, we could say there are probably some areas in these cities with Cr(VI) much greater than the MCL. As we said total chromium is the combination of Cr(VI) and Cr(III). Therefore, Cr(VI) content is always equal or lower than total chromium, but in the city of Mesa’s report the average content of Cr(VI) (7.6 μg/L) is higher than total chromium (6.5 μg/L).

Taking into consideration the above values with another parameter known as the California’s Maximum Contaminant Level Goal (MCLG) is more interesting. The goal for the level of Cr(VI) in drinking water where there is no known or expected risk to health is 0.2 μg/L. Based on this, only city of Phoenix meets almost this goal. All of the other cities in the Metro area are well above this standard.

One known source of Cr(VI) is from groundwater. The above tables show there are large discrepancies between cities in both water sources and quality. While 27% of water in Chandler is provided by wells, only 2% is provided by wells in the city Phoenix. Groundwater provides in average 21% of water sources in almost all of the Phoenix Metro areas except for the city of Phoenix. However, groundwater makes up maximum 20% of drinking water in the city of Mesa but its Cr(VI) content was more than other cities in 2017 which shows its groundwater has the lowest quality.

There is also discrepancy on how this information is reported. While other cities provided detailed information about the sources of water (i.e. how much gallons form Colorado, Salt, Verde rivers and wells), unfortunately city of Phoenix’s Water Quality Report stated the sources just as surface and underground percentage. Average Cr(VI) contents of city of Phoenix water is much lower than other cities. Remarkably even its maximum content in city of Phoenix water is much lower than the average of other cities which may shows those cities should use city of Phoenix policies and procedures.

Reaching this health goal may cost a lot to rectify the problem. For example Cr(VI) can be adsorbed by amorphous aluminum, iron oxides, and organic complexes. On the other hand eliminating water sources with Cr(VI) contents above the MCL would be another solution. However, the last one would put more stress on taking more water from the rivers, which we know is near impossible on high demand days.

As we know total chromium in water includes Cr(III) and Cr(VI) and in oxidizing and high pH condition of groundwater total chromium is mostly in the form of Cr(VI). Therefore, Cr(VI) is equal or less than total chromium which means Cr(VI) never will be greater than total chromium. In Mesa’s 2017 Water Quality Report Cr(VI) is greater than total chromium (Table 3) which shows their labs have accuracy problem and also they can not interpret their analytical data.

Table 3: Total chromium and Cr(VI) in the Mesa’s 2017 Water Quality Report. All values in μg/L (ppb).