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California Environmental Health Tracking Program

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Last Edited: 8/11/2010

Drinking Water Quality: Measures and Limitations

There are major limitations in using drinking water quality data in its current state for environmental health tracking and for learning about the relationship between drinking water and health.

Because water quality monitoring databases are not designed to give a precise measure of contaminant levels that are delivered to consumer populations, these levels must be estimated from the available data.  This type of data is typically dependent on time and geography- often times these data do not exist in accessible, digital, centralized, or complete formats for the entire State.

Secondly, the measures presented here do not necessarily indicate the quality of the water served to the customers:  

  • Violations of a Maximum Contaminant Level (MCL) are not always explained by contaminants in the water as variability of testing can play a role
  • The location where a water sample was taken can affect the data as water quality may vary between different parts of the distribution system
  • Sampling locations may change over time
  • Water systems have different testing frequencies based on their size and/or contaminant being sampled
  • The required reporting of water quality tests is often based on annual and/or system-wide averages of individual sampling results.  For example, Community Water Systems (CWS) may be required to sample at four different locations, but report only the average.
  • Required sampling may be too infrequent to capture high levels or short-term variability of contaminants in drinking water.  For example, disinfection byproducts (DBP) and nitrate testing occurs quarterly to annually, while arsenic sampling ranges from annually to every nine years.
  • Water quality can change after a sample has been taken

The differences outlined above make it is difficult to interpret MCL violations for health surveillance purposes as well as limit comparisons between systems, between counties, and year to year.


Geographic boundaries of water systems are not known / inaccurate population estimates

First, there is limited information on the spatial extent of community water systems.  This means we do not know the geographic boundaries of water systems, and therefore, do not know exactly how many people each water system services. As mentioned in the Drinking Water Monitoring section, all Community Water Systems (CWS) in the state estimate the number of people they serve according to regulations; however, often the estimates are inaccurate. For example, when summed across all CWS, the estimated CWS population slightly exceeds the total Census population of the state.

Since not everyone in the state is serviced by CWS, it means we are overpredicting the number of people potentially exposed to water contaminants.

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Water quality data is collected for regulatory purposes

Since the water quality data is collected for the purposes of monitoring and enforcement of the Federal and California Safe Drinking Water Acts, it is not designed for population exposure or health assessment purposes. Therefore, more accurate methods of estimating population served by Community Water Systems (CWS)  need to be developed along with identification of the geographical water system service boundaries to allow studies of the potential risk of adverse health effects from exposure to drinking water contaminants.

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Changes in regulations and reporting requirements over time /  variability in monitoring requirements

When examining trends in drinking water quality, it is important to keep in mind that regulations and reporting requirements change over time.  Therefore, an increased number of Community Water Systems (CWS) reporting violations over the years often represents a stricter regulation, not a worsening water quality.  For example, the maximum contaminant level (MCL) of arsenic dropped from 50 micrograms per liter (µg/L) to 10 µg/L in 2006.  Therefore, we would expect the number of CWS with an arsenic MCL violation to increase in 2006 and beyond compared to 2005 and before.

Furthermore, monitoring requirements may vary depending on the size of the population served by the CWS.  This, in combination with changing regulations and reporting requirements, can make comparisons difficult.

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Data coding limitations

The information obtained from water quality data is limited by how the data is coded and extracted.  The Water Quality Monitoring Database (WQMD) and Permits, Inspection, Compliance, Monitoring and Enforcement Database (PICME) contain codes to let us identify active Community Water Systems (CWS).  Yet, there are no codes to determine where the water quality sample has been taken.  For example, was the sample taken at the entry into the distribution system or within the system?  This information is important for determining compliance.  For instance, the compliance point for DBP sampling is within the distribution system, but for arsenic and nitrate the compliance point is at the entry point to the distribution system.  As a result, we have to make a series of assumptions based on available data codes and query logic. This made is difficult to ascertain consistent results for every water system captured in the database.  Consequently, for the drinking water data on this site,  it is unclear whether the water quality sampling points and subsequent analytical results meet the US EPA definition for compliance.  There may also be data entry errors in the database.

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Lack of data on unregulated drinking water supplies

Sampling information for State or Federal unregulated drinking water supplies, such as private drinking water wells, is generally not collected or available electronically for California.  Since 10-15% of the California population receives drinking water from unregulated sources, we have an incomplete picture of drinking water quality.  The California Environmental Health Tracking Program is collaborating with the California Department of Water Resources to improve the completeness and availability of private well data. 

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Water consumption and other behavior patterns are largely unknown

Water consumption patterns- that is, how much water and what kinds (bottled, filtered, tap) are consumed- within a population are largely unknown.  It is important to note that bottled and vended water is not subject to the US EPA drinking water regulations.  In California, bottled and vended water are regulated as food, under California Department of Public Health’s Food and Drug Branch purview.  Human behaviors (e.g., showering and bathing time, consumption of tap water, use of bottled water, and exposure to water at workplaces or other locations outside the home) greatly influence exposure, complicating efforts to estimate exposure from tap water measurements.

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System-wide measures are not always representative for the population

Variability in locations of monitoring stations, sample scheduling, and contaminant-specific water quality within a water system interplay and make it difficult to interpret system-wide concentation calculations, particularly for large urban water systems. For systems which have more than one source supplying the distribution system or have contaminants that do not behave conservatively (e.g. DBPs), water quality may vary in different parts of the distribution system and these variations are lost in system-wide calculations. Many of these issues could be overcome if it was possible to isolate discreet geographic zones within a water system (e.g. pressure zones) where homogeneous water quality is expected for the majority of the time and specific monitoring sites are linked to these zones.

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Incomplete sampling information / variability in sampling frequency

Sampling may be too infrequent to capture high levels or short-term variability. Differences in the frequency of sampling between Community Water Systems (CWS) for a specific contaminant limit comparisons between systems and between counties. 

Furthermore, physical, biological, and chemical processes, natural or man-made, can have an effect on contaminant levels between the time and place where drinking water is monitored and consumed.

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Inaccurate or missing data

Gaps in the data are a major limitation.  For example, in California, sampling results related to DBPs are significantly incomplete.  Sampling results tend to be maintained at the regional district offices, often in hardcopy, and are not routinely reported to the State electronically.  Of 3,136 Community Water Systems (CWS), DBP samples were available for only 273 systems, or approximately 9%.  Therefore, DBP measures based on concentration levels in sampling results are not shown on this site.  DBP violations data, however, are completely reported and are displayed on this site.

Arsenic samples were available for only about 47% of community water systems and nitrate samples for 50% of CWS for the compliance periods during 1999 to 2006.  The lack of sampling results for the remaining CWS could be due to monitoring waivers or errors in selection of sampling stations or compliance period sampling points. This issue is expected to be resolved possibly by the end of 2009, when drinking water quality databases are migrated to a new system.

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Possible solutions: The Safe Drinking Water Information System and Water System Area Reporting

Some of the above issues, particularly surrounding coding and incomplete data, are expected to be resolved possibly by the end of 2009, when drinking water quality databases are migrated to a new system called the Safe Drinking Water Information System, or SDWIS-State. In addition, the CEHTP is working towards developing a water system service area reporting application in which water system personnel and State primacy personnel can collaboratively contribute to a spatial database over the Internet. This will help in overcoming many of the geographic-related issues described above.

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