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Arsenic and Old Laws
A Scientific and Public Health Analysis of Arsenic Occurrence in Drinking Water, Its Health Effects, and EPA's Outdated Arsenic Tap Water Standard

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Chapter 1


NRDC has obtained new data showing that tens of millions of Americans are consuming tap water every day that poses unacceptable cancer risks. This chapter summarizes these new arsenic occurrence data, while subsequent chapters discuss in detail the health implications of arsenic contamination of drinking water and the need for a stricter standard for arsenic in tap water.

The source of these new data is an EPA database not previously made public, obtained by NRDC under the Freedom of Information Act. In preparing to develop an updated standard for arsenic in drinking water, EPA asked all states for data on the occurrence of arsenic in the tap water served by public water systems. Twenty-five states responded (see Figure 1, National Arsenic Occurrence Map), providing over 100,000 arsenic test results taken from 1980 to 1998 from over 23,000 public water systems. These water systems serve a total of about 99.5 million Americans, or 40 percent of the 1990 U.S. population. Because the database does not cover states in which approximately 60 percent of the U.S. population resides, the estimates of population affected by arsenic in their tap water likely are substantial underestimates. NRDC has deleted from consideration, as potentially unreliable, samples that exceeded 1,000 parts per billion.

These new data reveal startling new details about the extent of arsenic contamination in the tap water. Table 1 shows our best estimate is that over 56 million Americans in these 25 states consumed water from systems containing arsenic at levels presenting a potentially fatal cancer risk above the level that is EPA’s highest acceptable cancer risk (1 in 10,000). Even our extremely conservative "low average" analysis approach indicates that at a minimum, over 34 million people in these 25 states drank water posing these elevated cancer risks. Our estimates are based on detailed evaluations of the EPA-collected occurrence data and the National Academy of Sciences (NAS) total cancer risk estimates.[3] Table 2 notes the total potentially fatal cancer risk that would be associated with drinking two liters of water containing arsenic at a given level for a lifetime, based upon the NAS estimates. Chapter 2 includes a further discussion of these data on risks and health effects, and how these estimates were derived.

As is clear from Tables 1 and 2, tens of millions of Americans are consuming tap water every day at levels that may pose a serious potentially fatal cancer risk and other health risks. Appendix A lists each public water system in which arsenic was found in the 25 states reporting data. The national map is intended to show the general areas that are hardest hit by the highest levels of arsenic. However, to determine whether arsenic has been found in a particular public water system, according to EPA’s database, readers should refer to the table of water systems reported in Appendix A. The map cannot be used by itself to identify whether a particular water system has an arsenic problem, because often there are several water systems located immediately adjacent to each other, and the map was generated at a scale that cannot be used to identify precisely which water system contains a given level of arsenic.

Table 1: Arsenic Levels in Tap Water Systems in 25 States -- Low and Best Estimates

Average Arsenic Level
(in ppb)
Low Estimate* of Number of Water Systems Affected Low Estimate* of Total Population Served Best Estimate** of Number of Water Systems Affected Best Estimate** of Total Population Served
None detected 15,624 40,619,400 15,624 40,619,400
Detected, <1* 2,068 28,017,372 884 5,925,297
> 1 and <3 2,935 19,994,024 3,146 25,711,312
> 3 and <5 1,321 7,440,564 1,947 17,494,651
> 5 and <10 1,348 5,033,538 1,652 10,611,259
> 10 and <15 535 1,451,616 566 2,075,157
> 15 and <20 251 243,526 258 340,284
> 20 and <25 171 269,393 173 270,332
> 25 and <50 280 354,802 283 376,542
> 50 66 99,736 66 99,736
TOTAL 24,599 103,523,971 24,599 103,523,970
TOTAL at or above 1 ppb
(0.5 ppb presents the highest cancer risk EPA traditionally allows in tap water)
6,907 34,887,199 8,091 56,979,263
*The low estimate is based on the assumption that any nondetect, no matter what the reporting limit, contained no arsenic, even if other samples showed arsenic was present. This highly conservative analysis results in a large number of systems having average concentrations below 1 ppb, because all reported nondetects, no matter what the reporting limit, are averaged as zero. See the discussion in the text for more details on how these averages were calculated.

** The best estimate is the estimated mid-average level of each system, which is the average of the detected levels of arsenic and, for those systems for which there was at least one detect of arsenic, one-half the level of detection for all nondetects. See the discussion in the text for more details on how these averages were calculated.

Table 2: Lifetime Risks of Dying of Cancer from Arsenic in Tap Water
Based upon the National Academy of Sciences' 1999 Risk Estimates*

Arsenic Level in Tap Water
(in parts per billion, or ppb)
Approximate Total Cancer Risk
(assuming 2 liters consumed/day)
0.5 ppb 1 in 10,000
(highest cancer risk EPA usually allows in tap water)
1 ppb 1 in 5,000
3 ppb 1 in 1,667
4 ppb 1 in 1,250
5 ppb 1 in 1,000
10 ppb 1 in 500
20 ppb 1 in 250
25 ppb 1 in 200
50 ppb 1 in 100
*See note 3 and Chapter 3 for details on how we calculated total cancer risk based on an extrapolation of NAS's risk estimates, which assumed a linear dose-response and no threshold.


Arsenic contamination of tap water is not a problem limited to a few pockets of the nation, nor is it limited in scope to small water systems. Tables 3 through 5 present summary data showing some water systems in which the EPA and state data indicate serious arsenic contamination problems may be found.

In addition, using ArcView Geographic Information System (GIS) software, and the latitude and longitude coordinates for public water systems reported in EPA's Safe Drinking Water Information System (SDWIS), NRDC has developed 25 state maps showing the regional variations in arsenic levels in tap water. The larger the dot, the larger the population served water system. In addition, we used graduated red coloration to show the concentration of arsenic found in the water, from light pink (representing low concentrations of arsenic) to bright red (representing mid-level arsenic levels) to dark red (representing severe arsenic contamination). In addition, NRDC wanted to give readers a picture of where arsenic was being searched for but not found. We used separate maps with graduated blue-green coloration to represent nondetects, with light blue-green representing nondetects using low levels of quantification (for example 1 ppb), and darker blue-green representing nondetects using high limits of quantification (for example 10 ppb).

As is clear from these tables and the 25 state maps, although arsenic contamination of tap water has substantial regional variation, no state is immune to the problem. Moreover, many of the nation's larger cities have levels of arsenic that are substantially above the level presenting what EPA would consider an acceptable cancer risk (that is, 1 in 10,000 risk of fatal cancer).

Note: Only the national map is included in the online version of this report.

How Average Arsenic Levels are Calculated in This Report and in Appendix A

Arsenic levels can vary with time, and old samples often used cruder analytical techniques that could not detect low arsenic levels (below 10 parts per billion). We found that the so-called reporting limits for arsenic (that is, the lowest level of arsenic in the water that states require to be reported) in many states was 5 to 10 ppb in the 1980's and even in the early 1990's. Figure 3 shows that in some states, such as California, many water systems testing their water for arsenic were allowed to report as nondetected any level of arsenic below the state’s relatively high reporting limits.

In many cases, those reporting limits later were lowered, due to improved analytical methods, and arsenic started to be reported in the water of many more communities, as would be expected. This presented a problem for our analysis: when a water system had for years not reported arsenic, and then reported it when the reporting limit dropped, how should we calculate the arsenic level for that system? Additionally, a relatively small number of water systems had very inconsistent reported levels of arsenic over time, and we had to decide how to report their average levels as well. We decided that when a water system conducted multiple tests of its water, we would use two different averaging techniques to estimate the arsenic exposure for consumers of that water:

  • First, we calculated a very conservative low average, which assumes that when arsenic was not reported as detected, there was absolutely no arsenic in the water at that time, even if the limit of detection was high (for example, 10 ppb), and even if other tests showed that arsenic was present in the water at levels somewhat below the previous reporting limit. For example, if a water system did five tests when the reporting limit was 10 ppb from 1985 to 1990 and found no arsenic, and then tested twice in 1993 to 1995 when the reporting limit was 3 ppb, and it found 8 ppb both of those later times, the low average calculated for that system would be 2.3 ppb (that is, [0 ppb + 0 ppb + 0 ppb + 0 ppb + 0 ppb + 8 ppb + 8ppb] ÷ 7 measurements = 2.3 ppb).

  • Second, we based our best estimate on a calculated mid-average, which assumes that if at least some arsenic was detected in a water system at some time, then whenever arsenic was not reported as detected, it was present at a level of one half of the reporting limit. Using the same example, if a water system had five tests when the reporting limit was 10 ppb from 1985 to 1990 and found no arsenic, and then tested twice in 1993 to 1995 when the reporting limit was 3 ppb, and found 8 ppb both of those later times, the mid-average calculated for that system would be 5.8 ppb (that is, [5 ppb + 5 ppb + 5 ppb + 5 ppb + 5 ppb + 8 ppb + 8 ppb] ÷ 7 measurements = 5.8 ppb).

Figure 2: State Average Arsenic Concentrations for Systems Finding Arsenic
[Figure 2]
Based on best estimate of average arsenic levels for systems that found arsenic.
Systems with all non-detects excluded.

Figure 3: Number of Tap Water Arsenic Samples, and the Lowest Level of Arsenic Required to Be Reported, by State (Reporting Limits)
[Figure 3]

Tables 3, 4, and 5

Table 3: 46 Largest Water Systems With Arsenic Levels Over 5 ppb (Ranked by Largest Population First)
Table 4: Highest Average Arsenic Levels in Water Systems Serving Over 10,000 (Ranked by Largest Population First)
Table 5: 50 Public Water Systems of All Sizes With Highest Average of Arsenic Concentrations


3. As is discussed in Chapter 3, NAS estimated that, considering lung and bladder cancers death studies, the total cancer risk at the current tap water standard of 50 ppb "could easily" be 1 in 100. NAS, Arsenic in Drinking Water, at 8, 301 (1999). The NAS also noted that while there may be some indication that arsenic may not have a linear dose-response relationship at low doses, these data are "inconclusive and do not meet EPA’s 1996 stated criteria for departure from the default assumption of linearity." Ibid at 7. Thus, as discussed in Chapter 2, we assume, as did NAS, that dose-response is linear with no threshold, and that the total lifetime potentially fatal cancer risk of consuming two liters per day of arsenic-contaminated water poses the risks noted in Table 2. While NAS did not explicitly calculate risks posed by water with arsenic at levels below 50 ppb, its analysis was used to develop Table 2.

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