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DRINKING WATER QUALITY INDICATOR
REPORTING OPTIONS FOR THE TAHOE BASIN
This research report was developed by Environmental Incentives for the Tahoe Monitoring and Evaluation Program
under a Southern Nevada Public Land Management Act funded project. For additional information please contact
Jane Rice (jrice@enviroincentives.com) or Eoin Doherty (edoherty@enviroincentives.com).
Abstract
The quality of drinking water is important to the health of Tahoe Basin residents and visitors. An array of
drinking water quality data sets exist in the Tahoe Basin that serve different regulatory and general water
supplier purposes. This study evaluates existing data sets available to inform future Tahoe Basin drinking
water quality status and trend reporting efforts. Criteria are used that determine the applicability and
limitations of different data sets for status and trend analyses, and are used to rate the data sets and
potential indicators within each data set. Example drinking water quality status and trend summaries are
provided for two of the highest rated indicators. Lastly, key findings resulting from the evaluation of
existing data sets and potential indicators are provided.
Contents
INTRODUCTION ............................................................................................................................................ 2
OBJECTIVES ..................................................................................................................................................... 2
RESEARCH QUESTION .................................................................................................................................... 2
HYPOTHESIS ..................................................................................................................................................... 2
ACRONYMS .................................................................................................................................................. 3
METHODS ...................................................................................................................................................... 4
DATA COLLECTION PROCESS ...................................................................................................................... 4
EVALUATION CRITERIA & RATING GUIDELINES .......................................................................................... 4
RESULTS ......................................................................................................................................................... 8
SUMMARY OF DATASET CONTENTS ............................................................................................................. 8
DATA SET EVALUATION .................................................................................................................................. 9
EVALUATION OF MOST PROMISING INDICATORS .................................................................................. 17
CONCLUSIONS ........................................................................................................................................... 18
KEY FINDINGS ................................................................................................................................................ 18
REFERENCES................................................................................................................................................ 20
APPENDIX A - EXAMPLE STATUS & TREND SUMMARIES ........................................................................... 21
DRINKING WATER QUALITY: RAW SURFACE WATER TURBIDITY ............................................................. 22
DRINKING WATER QUALITY: FINISHED WATER ARSENIC ......................................................................... 24
APPENDIX B – DATASET COLLECTION CONTACT INFORMATION........................................................... 26
APPENDIX C – ADDITIONAL INFORMATION ............................................................................................ 27
CANADIAN COUNCIL OF MINISTERS OF THE ENVIRONMENT WATER QUALITY INDEX ...................... 27
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INTRODUCTION
Drinking water quality is a vital driver of public health. Residents and visitors of the Tahoe Basin
consume local tap water with the expectation that it will not expose them to near or long-term health
risks. Although Lake Tahoe and surrounding waterways are specifically well known for their
transparency, the quality of drinking water should not be taken for granted. Particularly as the built
environment evolves, the regional climate changes, and treatment facilities age, it is beneficial to
understand the status and trends of drinking water quality in order to identify emerging risks and ensure
that treatment facilities are adequately removing harmful constituents.
Consumers can currently access treated drinking water quality information from their respective local
public water suppliers through mandated annual consumer confidence reports (CCRs). In addition, raw
surface water quality information is available for Tahoe Water Supplier Association (TWSA) public water
suppliers. However, there are dozens of water suppliers in the Tahoe Basin and there is currently no
aggregated drinking water quality reporting for the Tahoe Basin. Thus, it is difficult to understand how
water resources are responding over time to changing conditions basin-wide. In addition, stakeholders
have occasionally inquired about the state of drinking water quality in the basin and the value of a
related human health indicator through processes such as Pathway 2007.
This paper describes the drinking water data sets available in the Tahoe Basin, and evaluates the potential
applications and limitations of each drinking water quality data set to assist future efforts in identifying
the best data set and indicator for answering basin-wide drinking water quality questions.
OBJ E CT IV ES
This research paper aims to achieve the following:
1) Describe the drinking water quality data sets available in the Tahoe Basin.
2) Define the applicability of each data set and potential indicators for status and trend analyses.
3) Provide examples of status and trend summaries for the two indicators determined to be the most
applicable for status and trend analyses.
RE S E AR C H Q UES TI O N
What drinking water quality datasets exist and which indicators best inform the understanding of
drinking water quality status and trends in the Tahoe Basin?
HYP OT H ES IS
The hypothesis is that consumer confidence reports can be collected from the major water suppliers in the
Tahoe Basin and the data in the individual reports can be aggregated to serve as the data set for basinwide drinking water quality status and tend analysis. The data set will enable basin-wide status and
trend analyses using specific parameters of concern, and a standard drinking water quality index that
produces a composite drinking water quality score for basin-wide status and trend analyses.
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ACRONYMS
CCR
Consumer Confidence Report
CADPH
California Department of Public Health
IVGID
Incline Village General Improvement District
MCL
Maximum Contaminant Level
NVDEP
Nevada Department of Environmental Protection
SDWA
Safe Drinking Water Act
STPUD
South Lake Tahoe Public Utility District
SWTR
Surface Water Treatment Rule
TWSA
Tahoe Water Suppliers Association
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METHODS
The methods section describes the data collection process to create an understanding of why each data set
was pursued and how each was obtained. This section also explains the criteria used to evaluate the data
sets and the most promising potential indicators. Rating guidelines are provided for each criterion on a +2
to -2 scale.
DAT A C OL L E C TI ON P RO C ES S
Five different drinking water quality data sets were collected for this study. As indicated by the
hypothesis, the original assumption was that CCR data would be applicable to drinking water quality
status and trend reporting. CCRs from 2003 to 2011 were collected from nine different water suppliers in
the Tahoe Basin through downloading reports available on the internet and requesting reports via email
and phone calls. Obtaining data from the largest water suppliers was prioritized to achieve the objective
of representing the largest portion of consumers in the Tahoe Basin as possible.
During a preliminary analysis of CCR data, limitations to conducting status and trend analyses were
discovered. There were significant inconsistencies in the parameters and time periods reported from
supplier to supplier. The research and interviews conducted to compile CCR data alerted the project team
to additional drinking water quality data sets that were potentially applicable to basin-wide drinking
water quality status and trend reporting. The additional data sets were collected in the order that they are
listed below, after preliminary research of each unveiled limitations for status and trend reporting and
inspired the project team to seek a better option. The method to collect each data set is also provided in
parenthesis following the dataset owner and description.




Tahoe Water Suppliers Association (TWSA) raw surface water data (requested by phone and
received by email)
Incline Village General Improvement District (IVGID) raw water data for stream and lake
locations (requested by phone and received by email)
California Department of Public Health state drinking water quality data (downloaded from
webpage and received by mail in CD format)
Nevada Division of Environmental Protection state drinking water quality data (requested by
phone and received by email)
All data sets collected and corresponding contact information are listed in Appendix B.
EV AL U AT IO N CR IT E RI A & RAT I NG G U ID EL I NE S
During the preliminary analysis of drinking water quality data sets, several key factors emerged that
determine the applicability and limitations of each dataset for basin-wide status and trend analysis. These
factors led to the development of seven criteria to evaluate the data sets and specific water quality
parameters within each data set that are most applicable to indicator development for status and trend
reporting.
The first of the seven criteria captures whether the data applies to raw surface water or finished drinking
water, the second measures how representative the data is of the entire Basin, the third and fourth relate
to the robustness and completeness of the data set, the fifth and six criteria pertain to the potential value
added from producing status and trend reports with the data, and finally the seventh criterion gauges the
annual cost of replicating a status and trend analysis with the data.
Each criterion is described below with corresponding rating guidelines that are intended to ensure
consistent evaluation across data sets. All but one of the criteria are rated on a +2 to -2 scale, with
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descriptions provided for the +2, 0, and -2 ratings. The +1 and -1 ratings should also be used as
appropriate.
Finished vs. Raw Water
Finished water has been treated for distribution from suppliers to consumers and has direct public health
implications. In contrast, raw water is collected at supplier intake pipes and demonstrates source water
conditions, which dictate water treatment infrastructure investments for surface water suppliers. The six
TWSA filtration exempt suppliers, in particular, are required to report raw water data to ensure that their
treatment practices are sufficient without filtration. The distinction between finished and raw water data
is important for understanding potential applications of the data and for comparison to regulatory
standards. For example, treated water should only be compared to Safe Drinking Water Act maximum
contaminant levels (MCLs) because these standards pertain to treated water. Similarly, raw water data
should not be directly used to draw conclusions about the quality of water consumers are drinking
because the water has yet to be treated.
Rating guidelines do not exist for this criterion because finished and raw data are both useful depending
on the question being asked. Thus, instead of using a rating scheme, the data sets are simply categorized
by whether they contain finished or raw water quality data.
Drinking Water Consumers Served
The data sets obtained vary in the number of suppliers they represent, and in the relative size of
suppliers. Given that the status and trend indicator is intended to demonstrate the drinking water quality
in the Basin as a whole, this criteria aims to capture the degree to which each data set represents basinwide conditions, in terms of the number of consumers served. Therefore, data sets containing data from
several larger suppliers would rate higher than data sets containing data from only a few smaller
suppliers.
Drinking water suppliers provide estimates of the number of consumers they serve. The consumer served
estimates per supplier were summed across suppliers for a total per data set. Obviously, datasets that
only contain surface or groundwater suppliers can at most represent a subset of the Tahoe Basin
population. Any dataset that scores a “-2” in this category should automatically be eliminated.
RATING
+2
0
-2
STANDARD
Consumers served population > 40,000 people
Consumers served population = 30,000 people
Consumers served population < 20,000 people
Temporal Consistency
This criterion captures the consistency of parameters reported over time across suppliers in the data set.
Reporting inconsistencies occur because water suppliers are legally required to report on drinking water
quality parameters according to their unique compliance history, vulnerabilities identified during the
permitting process, size, type, location, and reporting schedule. Furthermore, the parameters reported in
any given year differ between surface water and groundwater suppliers because these two source water
types are particularly vulnerable to different constituents. For example, groundwater suppliers are more
likely to report arsenic levels because it is more likely to have high concentrations in groundwater as
opposed to surface water. Temporal reporting inconsistencies can skew trends over time.
RATING
+2
0
-2
STANDARD
All sites or suppliers report data consistently over time for all parameters.
At least 65% of the suppliers report data consistently over time for the
majority of parameters.
50% or less of the suppliers in the dataset report data consistently over time
for the majority of parameters.
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Number of Data Points
The number of data points included in status and trend analyses influences the statistical confidence of
the analysis. A greater number of data points used in analyses increase the statistical confidence of status
and trend determinations. Some of the data sets collected include sampling data based on frequent
monitoring, while others only include annual averages by supplier. Thus some datasets can be used to
report numerous indicators, including annual basin-wide highest values and annual basin-wide average
values across using all sampling values, while others should not be aggregated because averaging
average values that do not have equal weight can skew results.
RATING
+2
0
-2
STANDARD
At least monthly data points are included in the data set for each site or
supplier in each year.
Five or more data points are included in the dataset for each site or supplier
each year.
One data point is included in the dataset for each site or supplier each year.
Level of Concern
Given the resources required for data collection, analysis and reporting, it is important to capture the
level of concern associated with the data set to guide the decision of whether to produce a status and
trend report with that data. The “level of concern” was assessed by comparing the data in each data set
to the relevant regulatory standards, under the assumption that data near or worse than the standards
would trigger a higher level of public concern.
Finished water data was compared to Safe Drinking Water Act MCL standards, while raw water data
was compared to Surface Water Treatment Rule turbidity and bacteriological standards for filtration
exempt entities. Water suppliers that are filtration exempt do not filter their source water before
distribution, but apply other treatment techniques.
RATING
+2
0
-2
STANDARD
In 20 or more instances a parameter in the data set is at or worse than the
corresponding regulatory standard, in the past 10 years.
In 10 to 15 instances a parameter in the data set is at or worse than the
corresponding regulatory standard, in the past 10 years.
No indicator in the dataset is known to be at the corresponding regulatory
standard to date.
Existing Reporting
Existing reporting of a dataset influences the importance of investing in additional reporting using the
dataset. For example, if existing reports are thorough and comprehensive, then additional status and
trend reporting may not add significant value relative to the cost and other reporting needs.
RATING
+2
0
-2
STANDARD
No publicly accessible reporting is available that uses the dataset.
Annual and publicly accessible reporting is available that uses the data set,
however the reporting is targeted towards a technical audience or is not
aggregated for basin-wide evaluation.
Annual and publicly accessible reporting is available that uses the data set
and provides an effective presentation of basin-wide evaluation for the
same audience as status and trend summaries.
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Annual Reporting Effort
The resources required to collect, reformat, and analyze data for a status and trend report impact the
feasibility of regularly reporting on an indicator. In order to evaluate and compare the annual reporting
effort, hourly estimates were made for the summed collection, reformatting and analysis effort required
to produce status and trends reports for each data set.
RATING
+2
0
-2
STANDARD
It takes less than 4 hours to collect all data files necessary to compile the
basin-wide dataset and update defined status and trend analyses each year.
This typically means data is publically accessible online and does not
require reformatting or interpretation of codes, although it could require
consolidation of multiple files.
It takes 15 hours to compile all data files necessary and update defined
status and trend analyses each year. For example, more than 50% of the
data files necessary to compile basin-wide data can be accessed online, and
the remainder is available by request via email or phone, and the data
requires additional but minimal reformatting and interpretation of codes.
It takes at least 25 hours to compile all data files necessary and update
defined status and trend analyses each year. For example, phone calls
and/or emails with multiple contacts and substantial reformatting and
interpretation/ de-coding are necessary.
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RESULTS
The results section first presents summary information on each of the five data sets to provide an
understanding of the relative scope and context of each data set. Next, each of the data sets are separately
evaluated with a narrative description and tabular evaluation summary, using the criteria defined in the
methods section. Lastly, the most promising indicators related to each data set are evaluated using the
criteria.
S U M MA RY O F D ATAS ET C O NT EN TS
The information provided in the table below conveys the scope of each dataset in relation to status and
trend reporting. The “Timeframe” column captures the years for which data was collected for this
evaluation. The “Reporting Universe” column indicates which types and how many suppliers or systems
are included in the dataset. The “Parameters” column summarizes the breadth of potential indicator
options within each dataset. The “Surface Water or Groundwater” column indicates whether the
suppliers in the dataset source their drinking water from surface water or groundwater. The “Indicator
Options” column presents the indicator options that were preliminarily identified as most applicable to
status and trend reporting, based on the evaluation criteria defined above.
Dataset
Consumer
Confidence
Reports
TWSA Raw
Water
IVGID Raw
Water
NVDEP
CADPH
Timeframe
Reporting
Universe
Parameters
Surface Water or
Groundwater
Indicator
Options
2003-2011
All public water
suppliers required to
report; Sample
consists of 9 water
suppliers
Over 25 different
bacteriological and
chemical
parameters
Surface water and
groundwater
Arsenic
Nitrate
Turbidity
TDS
1997-2012
Tahoe Water
Suppliers
Association surface
water suppliers (10)
Turbidity and
bacteriological
parameters
Surface Water
Turbidity
Bacteria
2003-2013
Incline Village
General
Improvement
District water
quality sampling
Turbidity and
bacteriological
parameters
Surface Water
Turbidity
Bacteria
2003-2012
NV Tahoe Suppliers
(16)
Hundreds of
chemical and
bacteriological
parameters
Surface water and
groundwater
Many
chemical or
bacteria
All CA Suppliers
Hundreds of
chemical and
bacteriological
parameters
Surface water and
groundwater
Arsenic
Nitrate
TDS
pH
2000-2013
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DAT A S ET EV AL UAT I ON
The data sets are evaluated below for applicability to basin-wide status and trend reporting using the
criteria defined in the Evaluation Criteria & Rating Guidelines section above. For each data set, a
narrative evaluation that highlights unique qualities of the dataset or aspects of the evaluation rationale
that require further explanation are provided along with a table containing the rating for each criteria and
supporting rationale.
Consumer Confidence Reports
Public water suppliers are required to produce consumer confidence reports (CCRs) annually to inform
their customers of their drinking water quality under the Safe Drinking Water Act (SDWA). CCRs report
finished water quality data because the SDWA strives to protect the health of public water supply
consumers, as opposed to protecting the original source water of the drinking water supply. The CCR
data set is the most representative of basin-wide drinking water quality conditions because reports were
collected from nine suppliers, including some of the largest in the Tahoe Basin. However, as
demonstrated by the indicator-specific analysis at the end of the results section, the representativeness of
the CCR data set decreases when focusing on a particular parameter, because not all suppliers report on
all water quality parameters in all years, which lowers the percentage of consumers served for a specific
parameter.
The temporal inconsistency in reporting across suppliers is captured in the screen shot below. Water
suppliers report on different parameters in different years, according to their unique compliance history,
size, type, and source water vulnerabilities, which results in unique combinations of water quality
parameters reported over time and across suppliers. Temporal inconsistency hinders the ability to
aggregate data across suppliers for status and trend analysis.
CCRs typically
report parameter
data in terms of
annual averages,
while maximum and
minimum annual
values are included
for certain
parameters by some
water suppliers.
Given that the
sampling data and
the number of
samples used to
derive the annual average are not provided, only one value per year would be incorporated into the
status and trend reporting for each supplier, which lowers the statistical confidence of the analysis.
Furthermore, without information on sampling frequency the calculation of an index score is not
possible. For more information on the drinking water quality index methodology that was considered for
this analysis, see Appendix C.
CCRs inform consumers and public health officials of the status of drinking water in relation to
maximum contaminant levels (MCLs), and must include any violations that occurred in the reporting
period. MCLs are defined under the Safe Drinking Water Act for the purpose of protecting public health,
and drinking water treatment technologies are designed to prevent these standards from being exceeded.
Therefore, it is to be expected that the CCR data remain well below MCLs, indicating a low level of
concern and less potential value added by investing in status and trend reporting with this data.
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Existing reporting efforts address consumers’ needs to understand the quality of their drinking water, but
do not contribute to a holistic understanding of drinking water quality in the Tahoe Basin as a whole, nor
do they provide trend information, which could be particularly informative for changes in treatment
technology needs or for highlighting emerging public health concerns.
The collection of CCRs required a significant level of time and effort. The collection process typically
involved researching water supplier websites to obtain contact information, multiple rounds of email
and/ or phone correspondence, and converting the reports into a usable format for analysis. In a few
instances, water suppliers either refused to send the reports or charged a fee to process and fax the
reports. While this initial data collection process required more than 100 hours, on an annual basis only
additional data from the most recent year would need to be added to the data set, which would require
approximately 25 hours to update status and trend results. Additional time would be needed to expand
the data set sample beyond the current sample of nine suppliers.
CCR Summary Evaluation
Criterion
Rating
Rationale
Finished vs. Raw Data
N/A
Finished
Drinking Water Consumers
Served
+2
9 suppliers serving approximately 57,000 people.
Temporal Consistency
-1
The majority of parameters do not contain data from 50% or more of the
suppliers over time.
Number of Data Points
-1
Annual averages are provided for each supplier in each year; maximum
and minimum values are provided by some suppliers for some parameters;
violations are also noted, but rarely occur.
Level of Concern
-1
Data rarely approach or exceed standard. More often, data are an order of
magnitude below the standard.
Annual Reporting Effort
-2
CCRs for most recent year are typically uploaded to supplier website, but
phone calls to suppliers are required in some instances and it is expected
approximately 25 hours will be required. Reformatting into consistent
format or common database requires additional effort.
Existing Reporting
0
CCRs are provided to consumers to inform them about the quality of their
water in relation to public health standards, but trend and basin-wide
information is not provided.
Average
-.5
TWSA Raw Water Data
The Tahoe Water Suppliers Association (TWSA) collects raw water turbidity and bacteriological data for
surface water suppliers. This data collection is required for the six filtration exempt TWSA suppliers
under the Surface Water Treatment Rule (SWTR). See Appendix C for more information on filtration
exemption. Surface water monitoring is required under the SWTR to ensure that raw turbidity and
bacteriological levels are low enough at the intake of filtration exempt suppliers to ensure that the water
will be safe for consumption without filtration. If raw water samples exceed the Surface Water Treatment
thresholds, then suppliers are required to install filtration equipment.
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The TWSA raw water data set only includes the 10 TWSA surface water suppliers, which together serve
roughly 27,000 people in the Tahoe Basin. Thus, the South Tahoe Public Utility District (STPUD), which
serves a significant portion of the population in the Tahoe Basin, is not represented in the data set.
Furthermore, TWSA samples cannot be considered representative of all Tahoe surface water suppliers
because significant variation occurs in water quality depending on the location and depth of their intakes.
The TWSA dataset contains frequent sampling information. Exact sampling frequency is dependent on
the flow of raw water relative to the community demand of the water supplier, but approximately 10 to
15 bacteriological samples are collected per month and turbidity samples are collected daily.
Temporal consistency is high because all of the suppliers report consistently on turbidity and bacteria
over time. The screenshot below illustrates the consistency in TWSA turbidity reporting, especially
beginning in 2002, which facilitates aggregation across suppliers.
The level of concern is low
because the data do not reach
or exceed standards and Lake
Tahoe public water systems do
not have a history of significant
microbial contamination.
Turbidity measurements at
intakes predominantly stay
below 1 NTU, with exceptions
resulting from extreme climatic
events, while the standard for raw water is 5 NTU under the surface water treatment rule.
The TWSA raw water dataset serves as the basis for an extensive and detailed annual report produced by
TWSA; however, the report does not contain indicators that aggregate data across suppliers and it is
targeted toward a technical audience. Replicating status and trend reports on an annual basis with TWSA
data is expected to require minimal time and effort.
TWSA Raw Data Summary Evaluation
Criterion
Rating
Rationale
Finished vs. Raw Data
N/A
Raw
Drinking Water Consumers
Served
-1
More than 27,000 people served by suppliers for turbidity and
bacteriological data sets; however, the data set only contains surface water
supplier data and omits groundwater suppliers.
Temporal Consistency
+2
Data consistently reported over time and across suppliers. Begin analysis
in 2002 or 2003 for highest representation across suppliers.
Number of Data Points
+2
Turbidity daily max; Bacteriological 10/ month
Level of Concern
-1
Data rarely approach or exceed standards.
Annual Reporting Effort
+1
Data for each additional year could be obtained from TWSA. Data is stored
in a format that can easily be reorganized for status and trend analysis.
Existing Reporting
-1
TWSA produces a very comprehensive report on this data annually, as a
compliance requirement for filtration exemption. Reporting is not targeted
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towards the general public.
Average
.33
IVGID Stream and Lake Data
The Incline Village General
Improvement District (IVGID) monitors
raw water quality data at stream and
lake locations surrounding the Incline
Village Beach. The following water
quality parameters are included:
temperature, turbidity, total coliform,
fecal coliform, total dissolved solids, and
dissolved oxygen. A map of the
monitoring locations accompanies the
data, and is depicted in a screenshot to
the right. The stream sampling sites are
approximately 50ft from the lake and the
lake sampling sites are collected by
wading from shore to waist depth.
Although this dataset could be useful for
a status and trend analysis specific to the
geographic area monitored by IVGID, it
cannot be considered for a status and
trend analysis of drinking water quality
in the Tahoe Basin because it is only
indicative of conditions in the Incline
Village geographical area. As stipulated
in the criteria rating determinations, any
dataset that scores a “-2” in the consumers served criterion should automatically be eliminated because it
cannot be considered representative of the Tahoe Basin -wide conditions. A screen shot of the data is
provided below to demonstrate the consistency in reporting across sites, in contrast to the other datasets.
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IVGID Summary Evaluation
Criterion
Rating
Rationale
Finished vs. Raw Data
N/A
Raw
Drinking Water Consumers
Served
-2
Less than 18,000 people served by IVGID.
Temporal Consistency
+2
Number of Data Points
+2
Level of Concern
-1
Data do not exceed standards in 5 or more instances.
Annual Reporting Effort
+2
Data for each additional year moving forward could be obtained from
IVGID. Data is stored in a format that can easily be reorganized for
status and trend analysis.
Existing Reporting
+2
Average
.83
4 sampling locations, consistent monitoring and reporting over time.
4 samples per month at 4 sampling sites.
IVGID does not produce a report on this data.
State Drinking Water Quality Data
Public water suppliers are required to submit drinking water quality data to the state under the Safe
Drinking Water Act. The data is evaluated for compliance with monitoring, reporting, and notification
requirements. Each supplier’s monitoring and reporting requirements are dictated by preliminary testing,
research, and analysis during the permitting process. The California Department of Public Health
(CADPH) collects drinking water quality data for the state of California and the Nevada Division of
Environmental Protection (NVDEP) collects drinking water quality data for the state of Nevada. The
datasets managed by each state are evaluated separately below, followed by a discussion on aggregating
the two datasets for basin-wide status and trend reporting.
CADPH
The CADPH data set contains hundreds of chemical parameters, but does not include bacteriological data
because this information is collected at the district level. While both finished and raw water data are
reported in the data set, these two types of data cannot be aggregated for a status and trend analysis;
therefore the evaluation was conducted with the finished water data only, because this subset contained a
greater pool of data for conducting analyses. For the purpose of this evaluation, the total of 130 drinking
water systems in the data set was narrowed to the systems serving 500 or more consumers, in order to
focus on systems that are more representative of Tahoe Basin conditions in terms of consumers served.
After data sorting, the remaining subset consisted of 26 water systems, which supply water to
approximately 35% of the Tahoe Basin population.
The number of samples reported per parameter for any one supplier varied drastically and there were
few suppliers reporting all parameters. For example, the samples reported per supplier ranged from one
to about 220, with an average of about 30 samples reported per supplier. Several parameters were only
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reported on by one supplier, and most were
not reported on by more than three
suppliers. This trend is demonstrated by
the screenshot below; only one supplier,
identified in column “A,” reported on the
two parameters listed in column “K.”
Given the limited amount of data reported
by any supplier on any one parameter, and
the limited amount of data reported for any
one parameter across suppliers, it is
infeasible to select an indicator from this
dataset that would be representative of basin-wide drinking water quality. Additionally, the parameters
that contained relatively more data, such as temperature, were not particularly relevant to drinking water
concerns in the Tahoe Basin. Even if adequate data were to be obtained for a particular parameter across
suppliers and over the ten year time span, an initial analysis suggests that exceedances rarely occur and
there is little variation in parameter levels over the last decade.
Finally, the cost of producing an annual status and trend report with this data is expected to be high. The
data files are difficult to use because of their large size and they require experience using specific
software (Microsoft Access). This preliminary analysis required significant time and effort to not only
obtain the data from the state, but also to received supporting materials required to translate codes
contained in the data sets. There are no known reports that use this state data to report on drinking water
quality status and trends in the Tahoe Basin.
CADPH Summary Evaluation
Criterion
Rating
Rationale
Finished vs. Raw Data
N/A
Raw & Finished
Drinking Water Consumers
Served
0
26 water systems; approximately 34% of the population, but proportion
diminishes significantly when selecting any single parameter for indicator
analysis.
Temporal Consistency
-2
Dataset often only contains parameter data from one supplier, due to
variations in reporting requirements and schedules.
Number of Data Points
-2
Level of Concern
-1
Very high variability between water suppliers. Data is reported monthly
for some parameters, and once or not at all for other suppliers.
Data rarely approach or exceed the standard.
Annual Reporting Effort
-2
Due to difficulty unzipping files, data CD had to be sent by mail. There are
5 different databases with supplemental information that must be crossreferenced with the 4 different databases that provide reporting data for 4
time periods. De-coding and cross-referencing databases requires
considerable effort. Coordination with CA DPH staff was required for
interpretation of codes and for data access.
Existing Reporting
+1
None known
Average
-1
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NVDEP
The dataset obtained from the Nevada Division of Environmental Protection contains both bacteriological
and chemical water quality parameter data from 2003 to 2012 for all Nevada Tahoe water suppliers, of
which there were sixteen, serving nearly 27,000 people. The data set includes groundwater and surface
water suppliers, finished and raw water samples, and hundreds of different chemical water quality
parameters.
Temporal inconsistency in
reporting is high. Only a
small group of suppliers
report on each parameter
in any given year, and not
many samples are
reported for each
supplier. For example, in
2012 only three suppliers
reported on arsenic, and
two of those suppliers
provided just one sample
each, as indicated in the
screenshot below. This
data set does not offer substantial data to aggregate and provide an accurate representation of basin wide drinking water quality conditions. Furthermore, raw water and finished water data cannot be
aggregated together, which further limits the data available for specific parameter analyses.
Obtaining and evaluating this Nevada data required three phone calls with the Nevada Division of
Environmental Protection. The data was, however, easier to manage than the California dataset because it
was preselected for the Tahoe Basin and required less interpretation of terms and codes. There are no
known reports that use this state data to report on drinking water quality status and trends in the Tahoe
Basin.
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NVDEP Summary Evaluation
Criterion
Rating
Rationale
Finished vs. Raw Data
N/A
Raw & Finished
Drinking Water Consumers
Served
0
16 suppliers, approximately 27,000 people served.
Temporal Consistency
-2
High variation in the number of samples reported per supplier and a
low number of samples reported overall for any parameter in a given
year.
Number of Data Points
-1
High variability between suppliers. Parameter values may be reported
between 1 and 380 times per year, or not at all, depending on the supplier.
Level of Concern
-1
Data do not typically approach or exceed the standard.
Annual Reporting Effort
0
Data must be sent by NVDEP staff and coordination with staff is
required for de-coding and interpreting information.
Existing Reporting
+1
Average
-.5
None Known
CA & NV Data Aggregation
The original objective was to aggregate the two datasets to create a dataset that was representative of the
Tahoe Basin as a whole, instead of just of one state or the other. In addition to the aforementioned data set
limitations for both California and Nevada data individually, the ability to aggregate data from the two
states is limited due to the Nevada dataset predominantly contains raw water data, whereas the
California dataset predominantly contains finished water data.
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EV AL U AT IO N O F M O S T P R OM IS IN G I N D I CA TO RS
The following water quality parameters were selected from each data set based on a preliminary analysis, and then were specifically evaluated by each
criterion, with the results documented in the table below. The criteria “Annual Reporting Effort” and “Existing Reporting” are constant for each
parameter within each dataset, while the remaining criteria vary depending on the proposed indicator. The two indicator options that were selected for
the development of example status and trend summaries, provided in Appendix A, are highlighted in yellow below. The indicators were selected based
on relatively high net rating averages and in order to offer one option for raw water data and one for finished water data.
Datasets
Indicator Options
Criteria & Rating
Net Rating Average
Finished or
Raw
Consumers
Served
Temporal
consistency
Number of
Data Points
Annual
Reporting Effort
Existing
Reporting
Level of
Concern
Arsenic
Finished
+1
0
-1
-2
0
+2
0
Turbidity
Finished
+1
0
-1
-2
0
-1
-0.5
Total Dissolved
Solids
Finished
0
-1
-1
-2
0
-1
-0.67
Nitrate
Finished
0
-1
-1
-2
0
-1
-0.67
Turbidity
Raw
-1
+2
+2
+1
-1
-1
0.33
Bacteria
Raw
-1
+2
+1
+1
-1
-1
0.167
Nitrate
Finished
-2
-2
-1
-2
+1
-1
-1.17
pH
Finished
-2
-2
-1
-2
+1
-2
-1.3
TDS
Finished
-2
-2
-1
-2
+1
-1
-1.17
Arsenic
Finished
-2
-2
-1
-2
+1
-1
-1.17
NV
Many
Finished &
Raw
0
-2
-1
0
+1
-1
-0.5
IVGID
Turbidity
Raw
-2
+2
+2
+1
+1
0
-0.67
Bacteria
Raw
-2
+2
+2
+1
+1
0
-0.67
CCR
TWSA
CA
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CONCLUSIONS
The quality of drinking water is important to public health. An array of drinking water quality data sets
exist in the Tahoe Basin that serve primarily regulatory purposes. This study evaluates existing data sets
for potential to inform future Tahoe Basin drinking water quality status and trend reporting efforts. The
study defines criteria that determine the applicability and limitations of different data sets for status and
trend analyses, rates the data sets and potential indicators within each data set using these criteria, and
provides two example drinking water quality status and trend summaries.
The hypothesis was that CCRs could be collected, aggregated, and analyzed for Tahoe Basin drinking
water quality status and trend reporting. However, the criteria evaluation did not identify an indicator
option that is highly applicable to basin-wide drinking water quality status and trend reporting. While
the hypothesis proved to be largely incorrect, several findings emerged through this study that are useful
for understanding drinking water quality monitoring and reporting in the Tahoe Basin and should
inform future analyses.
KEY FI ND IN GS
Indicators using existing datasets are not highly representative of basin-wide drinking water
quality conditions. The lack of representativeness results from monitoring and reporting variability for
each supplier that limits the ability to aggregate the data for basin-wide reporting. Reporting
inconsistencies are due to the Safe Drinking Water Act regulatory framework, which requires unique
monitoring and reporting for each supplier.
Drinking water quality data infrequently exceed standards, which suggests that periodic
status and trend reporting would likely be sufficient. Drinking water quality data are typically
well below the relevant standards of the Safe Drinking Water Act or the filtration exemption of the
Surface Water Treatment Rule, which indicates a low level of concern for the public and regulators.
Drinking water quality reporting should highlight maximum data values in order to protect
public health and mirror the structure of regulatory standards. Drinking water quality standards
are values that are not to be exceeded in order to protect public health, whereas average values are used
to evaluate other resources. In several instances maximum values are not provided by suppliers, which
hinders reporting consistency, aggregation ability, and comparison of the data to standards.
A valuable data set could be created if local water suppliers committed to monitoring a short
set of priority drinking water quality parameters consistently. Reporting inconsistency across
suppliers is the primary issue with the existing data sets evaluated; however a dataset with consistent
data on a short set of priority drinking water quality parameters could be valuable. The existing
monitoring network could likely be leveraged with minimal additional investment.
Finished CCR arsenic data and raw TWSA turbidity indicators provide the most viable options
for basin-wide drinking water quality indicators of the options evaluated. These indicators were
selected because they scored among the highest in the evaluation and because one is representative of
raw water, while the other is representative of finished water. However, both should be reviewed with an
understanding of their limitations. Despite the relatively high temporal consistency CCR arsenic data, the
existing temporal inconsistency created false trends post-aggregation. Additionally, the surface water
suppliers tended to not report annual maximum values. For these reasons, the South Tahoe Public Utility
District (STPUD) data was the only CCR data used for the example status and trend report. The TWSA
turbidity only represents a subset of surface water suppliers, while more than half of the consumers in
the Tahoe Basin are served by ground water suppliers, and the TWSA already provides a robust annual
report.
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Drinking water quality data from the South Tahoe Public Utility District (STPUD) could be
very useful for some reporting purposes because STPUD is the largest drinking water
supplier in the basin and maintains an extensive monitoring network, however the data
source was not collected or evaluated in this research project because it presents a small
geographical area of the basin. STPUD is the largest drinking water supplier in the Tahoe Basin; it
serves over 2.4 billion gallons of water annually from 16 active drinking wells to approximately 14,000
homes and businesses. Despite STPUD’s size, the data was not requested because the intention of this
project was to aggregate data sources from several water suppliers, geographically distributed across the
Basin, to create a holistic picture of basin-wide drinking water quality. Additionally, an STPUD drinking
water quality indicator would only be representative of groundwater sources, and not surface water
sources. Finally, STPUD staff explained during a phone interview that the particular vulnerabilities of
individual groundwater wells to different constituents vary considerably, which would likely present
challenges to aggregation and indicator selection.
Monitoring emerging pollutants of concern may be useful if drinking water quality
regulations do not adapt rapidly enough. There may be benefits to monitoring unregulated
parameters that pose risks to public health, such as new herbicide chemicals.
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REFERENCES
Bergsohn, Ivo and Power, Terry. South Lake Tahoe Public Utility District. Interview. May 29, 2013
Dunbar, Madonna. Executive Director, Tahoe Water Suppliers Association. Interview. April 9, 2013
Office of Groundwater and Drinking Water. U.S. Environmental Protection Agency. Safe Drinking Water
Act Amendments of 1996 (Section By-section Summary). 1996. Web.
<http://water.epa.gov/lawsregs/guidance/sdwa/summ.cfm>.
Office of Water . U.S. Environmental Protection Agency. Comprehensive Surface Water Treatment Rules
Quick Reference Guide: Unfiltered Systems . 2004. Web.
<http://water.epa.gov/lawsregs/rulesregs/sdwa/mdbp/upload/2005_05_11_mdbp_qrg_mdbp_surfacewate
rtreatment_unfiltered.pdf>.
Office of Water. U.S. Environmental Protection Agency. Surface Water Treatment Rule . 2012. Web.
<http://water.epa.gov/lawsregs/rulesregs/sdwa/swtr/index.cfm>.
Region 2 Water . U.S. Environmental Protection Agency. Filtration Avoidance. 2010. Web.
<http://www.epa.gov/Region2/water/nycshed/filtad.htm>.
Tahoe Water Suppliers Association and US Army Corps of Engineers. Lake Tahoe Source Water Protection
Risk Assessment Final Report 2008. 2008. Web. <http://www.tahoeh2o.org/RiskAssess08.pdf>.
Tahoe Water Suppliers Association. Watershed Control Program 2009 Annual Report. 2009. Web.
<http://www.tahoeh2o.org/>.
Tahoe Water Suppliers Association. Watershed Control Program 2012 Annual Report. 2012. Web.
<http://www.tahoeh2o.org/>.
Williams, Rebecca. Tahoe Water Suppliers Association. Interview. April 11, 2013
Young, Marvin. Drinking Water Office, USEPA Region 9. Interview. April 8, 2013
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APPENDIX A - EXAMPLE STATUS & TREND SUMMARIES
The example status and trend summaries below are intended to provide a basic understanding of two of
the most highly rated indicators in the evaluation conducted by this study. There are important
limitations to each indicator, however, and additional detail would need to be added to the reports in
order to fulfill the status and trend reporting guidelines.
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Drinking Water Quality: Raw Surface Water Turbidity
Reporting Icon
Trend
6
y = 0.0702x + 2.0936
R² = 0.0407
Turbidity (NTU)
5
Map
TWSA Service Boundaries
4
3
Filtration
Exemption
Standard
2
1
Annual Maximum
0
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
Status: Considerably better than target
Trend: Moderate decline
Confidence: Moderate
Median Annual
Average
Year
The red triangles represent the annual maximum turbidity value of any one TWSA supplier in a
given year. In 2012 the annual maximum value was 3.55 NTU, which is 29% better than the
standard for filtration exemption of 5 NTU. As indicated by the black trend line, annual
maximum turbidity values have moderately declined over the last decade, relative to the
filtration exemption standard. The yellow square data points represent the average annual raw
water turbidity values of Tahoe Water Suppliers Association (TWSA) surface water suppliers
from 2002 to 2012. Each average annual value was calculated by averaging daily maximum
values for each supplier in each year, for an annual average per supplier. Then supplier annual
averages were averaged across suppliers for each year.
Interpretation and Commentary




Importance –Lake Tahoe is a primary drinking water source for many Tahoe residents and visitors. The quality of drinking water
must be closely monitored in order to protect public health and identify any treatment shortcomings and needs. Six of the ten
TWSA surface water suppliers are filtration exempt and therefore must verify that their raw source water turbidity levels are below
5 Nephelometric turbidity units (NTU) at the intake pipes, under the Surface Water Treatment Rule, or else they will be required to
invest in filtration systems. High turbidity levels can interfere with disinfection and are correlated with higher incidence of disease
causing microorganisms and gastrointestinal infections. Turbidity is a measure of the optical properties of water that cause light
rays to be scattered and absorbed.
Status – The annual maximum of raw water turbidity across water surface water suppliers is 3.55 NTU in 2012, which is 29%
better than the filtration exemption standard of 5 NTU, which must not be exceeded in order to maintain the filtration exemption.
Trend – The trend from 2002 to 2012 is a 1.4% change per year relative to the standard of 5 NTU. This trend value indicates that
raw water turbidity has essentially not changed over the last decade.
Confidence – The overall confidence score is “Moderate” because although the trend confidence is low, the status confidence is
high.
o Status Confidence: Status confidence is high. Non-filtering water suppliers must conduct raw water turbidity and
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


PAGE 23
coliform monitoring under the federal Surface Water Treatment Rule (40 CFR S. 141.71). Samples are collected at the
first pump station of drinking water intake pipes before treatment occurs. The sampling frequency corresponds to the
flow of raw water in relation to community demand. Each month, water suppliers submit maximum and minimum
turbidity and bacteria values to Nevada or California, respectively. Any violations in monitoring practices must be
reported to state authorities as well.
o Trend Confidence: Although the p-value indicates high confidence with a low value of 0.00, the R-squared value
indicates low confidence because it is well below the threshold of 0.5, at 0.04. The R-squared value outweighs the pvalue for an overall low trend confidence.
Human and Environmental Drivers: High turbidity typically results from erosion, waste discharge, urban runoff, and the
proliferation of microscopic organisms, plankton, and algae.
Monitoring Approach: Samples are collected at the drinking water intake pipe, prior to treatment. The frequency of sampling
depends on the flow of raw water relative to community demand; however, TWSA suppliers all provide daily maximum values.
Monitoring Partners: The data was collected and provided by TWSA. All TWSA surface water members collect turbidity data.
Supporting Information
REFERENCES
1.
2.
3.
4.
Dunbar, Madonna, and Rebecca Williams. 2012 Watershed Control Program Annual Report. Rep. Tahoe Water Suppliers
Association, Nov. 2012. Web. Apr. 2013. <http://www.tahoeh2o.org/>.
Anderson, C., 2005, Turbidity (ver. 2.1): U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chap.
A6., sec. 6.6, July 2006, accessed [April 2013], from http://pubs.water.usgs.gov/twri9A6/.
"Drinking Water Contaminants: National Primary Drinking Water Regulations." United States Environmental Protection Agency,
03 June 2013. Web. 30 July 2013. <http://water.epa.gov/drink/contaminants/index.cfm>.
Guidelines for Drinking Water Quality, 4th Ed. Rep. World Health Organization, 2011. Web. Mar.-Apr. 2013.
<http://www.who.int/water_sanitation_health/publications/2011/dwq_guidelines/en/index.html>.
ADDITIONAL INFORMATION
1.
TWSA produces an Annual Report that includes analysis of turbidity and bacteriological data from their water provider
members. The report is produced as part of the filtration exemption requirements, although only six of the eleven providers are
currently filtration exempt. Annual report posted online: http://www.tahoeh2o.org/AnnualReport12.pdf
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Drinking Water Quality: Finished Water Arsenic
Trend
Arsenic (ppb)
Reporting Icon
Status: Somewhat better than standard
Trend: Rapid Improvement
Confidence: Moderate
20
18
16
14
12
10
8
6
4
2
0
y = -1.225x + 19.703
R² = 0.6449
Annual Maximum
MCL *
Annual Average
Year
Map
The graph above depicts the status and trend of average annual and annual maximum
arsenic levels reported from the South Tahoe Public Utility District (STPUD) in consumer
confidence reports (CCRs). The Maximum Contaminant Level (MCL) that cannot be
exceeded is 10 ppb. As of 2011, the annual maximum arsenic was 8ppb, which is 20%
better than the MCL. The data is of finished water that is distributed to customers and
has been treated to ensure safe levels of constituents like arsenic. Annual maximum
arsenic levels have been rapidly improving over the last decade, at a rate of 12.25% per
year relative to the MCL.
STPUD serves over 2.4 billion gallons of water annually from 16 active drinking wells to
approximately 14,000 homes and businesses. STPUD’s source is groundwater.
South Tahoe Public Utility District
* In 2001, EPA adopted a new standard for arsenic in drinking water of 10 parts per
billion (ppb), which replaced the previous standard of 50 ppb. While the standard went
into effect in 2006, water systems were not considered to be in violation immediately.
For the first 1-3 years of the new standard, water systems were required to monitor for
arsenic to determine compliance levels. The dashed blue line indicates that the
standard was previously above 20 ppb, while the green line indicates the standard level
as of 2006.
Interpretation and Commentary




Importance –The quality of drinking water must be closely monitored in order to protect public health from harmful constituents,
like arsenic. Arsenic occurs naturally in the environment and can be found in drinking water, particularly in groundwater sources. If
arsenic is consumed it can cause skin damage, circulatory system issues, and increased risk of cancer. As of 2001 the maximum
contaminant level (MCL) of arsenic allowed in drinking water is 10 ppb. It is important to understand the status and trend of
arsenic in water supplies to ensure that treatment technologies are updated as necessary to protect the public.
Status – The 2011 average annual value of arsenic was 8 ppb, which is 20% below, or somewhat better than the maximum
contaminant level of 10 ppb. Drinking water treatment technologies are designed to ensure that the levels of various water quality
parameters do not exceed designated maximum contaminant levels.
Trend – The absolute per cent change per year relative to the 10 ppb standard is 12.25%, which indicates rapid improvement
over the last decade.
Confidence – The overall confidence score is moderate because the trend confidence is moderate and the status confidence is
high.
o Status Confidence: Status confidence is high because monitoring and reporting practices are mandated by federal
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

PAGE 25
regulations and overseen by state authorities for compliance. Since the effective date of the new MCL of 10 ppb,
surface water suppliers have been required to collect annual samples and ground water suppliers were required to
collect one sample between 2005 and 2007. If the initial groundwater sample is less than the MCL, ground water
systems are only required to sample every 3 years. If a sample is above the MCL, the supplier must collect quarterly
samples until the levels are reliably and consistently below the MCL. Samples are required at each entry point of the
distribution system, unless State regulations specify otherwise.
o Trend Confidence: The overall trend confidence is moderate because the p-value is low, 0.00, and the R-squared value
is moderate, 0.645.
Human and Environmental Drivers: Arsenic occurs in drinking water as a result of erosion of natural arsenic deposits. Runoff from
glass and electronic production wastes also contributes to the occurrence of arsenic in water sources.
Monitoring Approach: Monitoring is conducted in compliance with Safe Drinking Water Act regulations, discussed above. Supplier
data is reviewed by the state to ensure that monitoring requirements are not breached, in tandem with oversight for other types of
violations.
Monitoring Partners: The data was collected and reported by the South Tahoe Public Utility District and reported in their annual
Consumer confidence reports, which are published online: http://www.stpud.us/h2oquality.html. Consumer confidence reports are
required annually of public water suppliers that serve the same people year round.
Supporting Information
REFERENCES
1.
2.
3.
4.
Consumer confidence reports- STPUD 2011
US EPA, Office of Water. Water on Tap: What you need to know. December 2009.
http://water.epa.gov/drink/guide/upload/book_waterontap_full.pdf
US EPA. Arsenic rule . (2012, March 06). Retrieved from
http://water.epa.gov/lawsregs/rulesregs/sdwa/arsenic/regulations.cfm
Website: http://water.epa.gov/drink/contaminants/basicinformation/arsenic.cfm Basic Information about Arsenic in Drinking
Water.
ADDITIONAL INFORMATION
2.
Maximum contaminant levels (MCLs) are enforceable standards established under the Safe Drinking Water Act for water
quality parameters. MCLs are set as close to public health goals as possible, while taking into account cost, benefits, and
detection and treatment capabilities. The public health goals are intended to prevent health risks with an adequate margin of
safety, assuming exposure is long term. The public health goal or maximum contaminant level goal (MCLG) is 0.004 ppb for
arsenic.
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APPENDIX B – DATASET COLLECTION CONTACT INFORMATION
Consumer confidence reports
WATER SUPPLIER
CONTACT OR WEBSITE
Round Hill General Improvement District
Patti Acri (pacri@rhgid.org)
Incline Village General Improvement District
Rebecca Williams
Talmont Resort Improvement District
Libby (530.583.1889)
Tahoe Cedars Water Co
Rob (530.525.7555)
McKinney Estates
530.525.5987
Zephyr Water Utility District & Cave Rock/ Skyland
Douglas County (775.782.9989)
Ward Well Water Co
Vince Bruno (530.583.5766)
Tahoe Park Water Co
Rick Dewante (503.583.3938)
South Lake Tahoe Public Utility District
http://www.stpud.us/h2oquality.html
Tahoe Water Suppliers Association
DATABASE
CONTACT
TWSA surface water purveyors raw water data
Rebecca Williams
Incline Village General Improvement District
DATABASE
CONTACT
IVGID stream and lake raw water data
Rebecca Williams
NV Division of Environmental Protection
DATABASE
CONTACT
NV-Tahoe Water Suppliers
Judy Neubert
CA Department of Public Health
DATABASE
CA – Water Suppliers
CONTACT
Paul Collins
Robert Springborn
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APPENDIX C – ADDITIONAL INFORMATION
Filtration Exemption
Filtration exemptions are governed by the Surface Water Treatment Rule, which seeks to prevent
waterborne diseases cause by microbes in surface waters. States may allow unfiltered water systems with
surface water sources to use treatment other than filtration. In order to qualify for alternative treatment, a
water system must have an uninhabited, undeveloped watershed in consolidated ownership and have
control over access to and activities in the watershed. Under the Surface Water Treatment Rule, the
system cannot be the source of waterborne disease outbreak, must meet source water quality limits for
coliform and turbidity and meet coliform and total trihalomethane maximum parameter levels (MCL).
Disinfectant residual levels must also be maintained. The alternative treatment must ensure greater
removal or inactivation efficiencies of pathogenic organisms than would be achieved by the combination
of filtration and chlorine disinfection required by section 1412 (b)(7)(C). [1412(b)(7)(C)] Section 106.
Filtration exempt entities must implement a watershed control program in order to minimize microbial
contamination of the source water. The watershed control program includes a Sanitary Survey and
Watershed Control Plan every 5 years with annual updates. Furthermore, filtration exempt suppliers
must complete turbidity and total or fecal coliform analyses on raw drinking water, at the intake pipe
prior to treatment. The sample frequency is dependent on the flow of raw water relative to community
demand. If any of the criteria to avoid filtration are not met, systems must install filtration treatment
within 18 months of the failure.
TWSA Surface Water Filtration Exempt and Non-exempt Operators
TWSA OPERATOR UNDER FILTRATION EXEMPTION
(OZONE AND CHLORINE RESIDUAL FOR DELIVERY)
TWSA OPERATORS USING FILTRATION TREATMENT
(FILTRATION AND CHLORINE RESIDUAL FOR
DELIVERY)
Incline Village General Improvement District
Tahoe City PUD; McKinney Quail System
Kingsbury General Improvement District
Skyland Water Company
Edgewood Water Company
Cave Rock Water System
Zephyr Water Utility District
Round Hill General Improvement District
Glenbrook Water Company
Lakeside Park Association
North Tahoe Public Utility District (ultra-violet
disinfection and chlorine residual for delivery)
CA NA DI AN C OU N C IL O F MI NIS TE RS OF T H E E NV I RO NM E NT WA TE R Q U AL I TY
IN DE X
The water quality index was introduced in the 1990s by Water Quality Guidelines Task Group of the
Canadian Council of Ministers of the Environment. They used the British Columbia Water Quality Index
as a starting point for their index development. The newly developed CCME WQI has been employed by
various provinces and in various ecosystems all across Canada to assess water quality. This index was
selected as the model that the UNEP Water Program used in developing a global water quality index.
The index is calculated as follows:
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
Scope- Number of parameters that do not meet objective at some point.
o F1= (Number of parameters with some exceedance/ Total number of parameters
evaluated) X 100
Frequency- Frequency with which parameters do not meet objectives.
DRINKING WATER QUALI TY I NDICATOR REPORTI NG OPTIONS
TAHOE MONITORING & EVALUATION PROGRAM – DRINKING WATER QUALI TY INDICATOR REPORTI NG OPTIONS


o F2= ( Number of exceedances/ Total number of samples) X 100
Amplitude- Amount by which objectives are not met.
o Excursion = (Failed test value/ Objective value) -1
o NSE= Sum of excursion values/ Total number of tests
o F3= NSE /( .01NSE + .01)
WQI = 100 – ( square root( F1^2 + F2^2 + F3^3)/ 1.732)
o 1.732 is a constant
DRINKING WATER QUALI TY I NDICATOR REPORTI NG OPTIONS
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