Total Chromium and Hexavalent Chromium Occurrence Analysis - Web Report #4414 Subject Area: Water Quality

 
Total Chromium and Hexavalent Chromium Occurrence Analysis - Web Report #4414 Subject Area: Water Quality
Total Chromium and Hexavalent
      Chromium Occurrence Analysis

Web Report #4414

Subject Area: Water Quality
Total Chromium and Hexavalent Chromium Occurrence Analysis - Web Report #4414 Subject Area: Water Quality
Total Chromium and Hexavalent
Chromium Occurrence Analysis

      ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Total Chromium and Hexavalent Chromium Occurrence Analysis - Web Report #4414 Subject Area: Water Quality
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                    ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Total Chromium and Hexavalent
Chromium Occurrence Analysis

Prepared by:
Chad J. Seidel and Christopher J. Corwin
Jacobs Engineering, Denver, CO 80202

Rajiv Khera
U.S. Environmental Protection Agency
Office of Ground Water and Drinking Water, Washington, DC 20460

Sponsored by:
Water Research Foundation
6666 West Quincy Avenue
Denver, CO 80235

Published by the

                   ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
DISCLAIMER

   This study was funded by the Water Research Foundation (Foundation). The Foundation
assumes no responsibility for the content of the research study reported in this publication or
 for the opinions or statements of fact expressed in the report. The mention of trade names
  for commercial products does not represent or imply the approval or endorsement of the
            Foundation. This report is presented solely for informational purposes.

                                      Copyright © 2012
                                by Water Research Foundation

                                    ALL RIGHTS RESERVED.
                   No part of this publication may be copied, reproduced
                         or otherwise utilized without permission.

                                     Printed in the U.S.A.

                   ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
CONTENTS
1.      Introduction ............................................................................................................................. 1

2.      Description of Data ................................................................................................................. 1
     2.1. National Chromium and Boron Occurrence Survey (NCBOS) ....................................... 2
     2.2. EPA’s Second Six Year Review ICR Contaminants Monitoring Data (6YR2) .............. 2
     2.3. CDPH Data ....................................................................................................................... 6

3.      Analysis of Total and Hexavalent Chromium Occurrence ..................................................... 8
     3.1. Comparison of Chromium Occurrence between Datasets ............................................... 8
     3.2. Analysis of Chromium Occurrence by Source Type, Region, and System Size ........... 10
     3.3. Analysis of Paired Total and Hexavalent Chromium Samples ...................................... 14

4.      Spatial Distribution of Chromium Occurrence ..................................................................... 16

5.      Co-occurrence with Chromium ............................................................................................ 19

6.      Estimated Extent of Potential Future Regulation ................................................................. 20

7.      Conclusions ........................................................................................................................... 23
8.      References ............................................................................................................................. 24

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                              ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
LIST OF TABLES

1.   Summary of occurrence data sources ..................................................................................1

2.   Summary of non-detect results in the 6YR2 dataset ............................................................2

3.   Summary of non-detect results in the CDPH dataset...........................................................7

4.   Percent of entry points with 75th percentile total chromium concentration above the
            threshold values (6YR2 data, non-detects set to one-quarter of the MRL) ...........18

5.   Correlation of co-occurring contaminants with total chromium in the
            6YR2 dataset ..........................................................................................................20

6.   Estimation of the national CWS entry point inventory......................................................21

7.   Number of CWS entry points above the threshold concentration at various
          threshold concentrations ........................................................................................22

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                      ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
LIST OF FIGURES
1.    Analysis of non-detect MRLs of the 6YR2 dataset (all non-detect samples). .....................3
2.    Cumulative probability distribution of median total chromium concentration by
             entry point of the 6YR2 dataset with non-detects set to zero, one quarter of the
             MRL, half the MRL, and at the MRL ......................................................................4
3.    Cumulative probability distribution of median total chromium concentration by
             entry point of the 6YR2 dataset with non-detects set to zero, and one quarter
             of the MRL, compared to the NCBOS dataset and the Bayesian estimate ..............5
4.    Cumulative probability distribution of total chromium concentration by entry
             point of the 6YR2 dataset (non-detects set to one-quarter of the MRL) .................6
5.    Cumulative probability distribution of chromium in CDPH for raw versus
             treated by sample location (75th percentile concentration per each sample
             location) ...................................................................................................................8
6.    Cumulative probability distribution of total chromium data from the three datasets
             (6YR2 and CDPH are the 75th percentile concentration per sample location) ........9
7.    Cumulative probability distribution of hexavalent chromium NCBOS & CDPH
             raw data (CDPH shows the 75th percentile concentration per sample location,
             non-detects set to zero) ..........................................................................................10
8.    Cumulative probability distributions of total chromium in the 6YR2 dataset by
             source water type (75th percentile concentration per sample location,
             non-detects set to one-quarter of the MRL) ...........................................................11
9.    Cumulative probability distributions of CDPH hexavalent chromium data in ground
             and surface waters divided into raw and treated samples (75th percentile
              concentration per sample location, non-detects set to zero) .................................12
10.   Map of EPA Regions .........................................................................................................13
11.   Cumulative probability distribution of 6YR2 total chromium data by EPA region
             (75th percentile concentration per sample location, non-detects set to
             one-quarter of the MRL) ........................................................................................13
12.   Cumulative probability distributions of 6YR2 total chromium data by system
             size category (75th percentile concentration per sample location, non-detects
             set to one-quarter of the MRL) ..............................................................................14
13.   CDPH paired sample analysis of ground waters and surface waters .................................15
14.   CDPH paired sample analysis of ground waters grouped by raw and treated waters .......15
15.   CDPH paired sample analysis of surface waters grouped by raw and treated waters .......16
16.   National pin map of 75th percentile total chromium concentration from the 6YR2
             dataset by system (for systems with more than two samples) ...............................17
17.   California pin map of 75th percentile hexavalent chromium concentration from
             the CDPH database by system ...............................................................................19
18.   Estimation of total number of CWS entry points with total chromium above
             threshold concentrations ........................................................................................23

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                        ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
1. Introduction

    The purpose of this study is to update the chromium occurrence analysis originally presented
in the Water Research Foundation (WaterRF) Report #2759, “Occurrence Survey of Hexavalent
Chromium and Boron,” (Frey et al., 2004) with new data from the U.S. Environmental
Protection Agency (EPA) and the State of California. New data were obtained from two primary
sources: the California Department of Public Health (CDPH) Water Quality Analysis Database,
and the EPA’s second six year review database. While these occurrence data are already
available, the objective of this assessment was to identify general occurrence trends of chromium
and capture regional occurrence patterns and assess the geographical coverage and extent of
impact of a potential future revision of total chromium or a new hexavalent chromium regulation
on the nation’s water utilities.

2. Description of Data

    Chromium occurrence information was gathered from available sources and the previous
WaterRF survey to guide the development of this updated national occurrence survey. Three
primary datasets are used in this analysis: WaterRF Report #2759, EPA’s second six year review,
and CDPH Water Quality Analysis Database (WQAD). A summary of the datasets is given in
Table 1. Other data sources considered but found not to be useful were: USGS National Water
Information System, National Water-Quality Assessment, and Water Quality Monitoring
Networks databases, US EPA STORET and voluntary water utility monitoring data.

                        Table 1. Summary of occurrence data sources.
  Data Source              WaterRF #2759                 USEPA                      CDPH
  Database                   NCBOS                        6YR2                      WQAD
  Geographic
                               41 States           46 States and 2 Tribes          California
  Coverage
  Period of Record               2003                   1998 - 2005               1995 - 2011
  Source Water                                                              Raw and Treated Drinking
                         Raw Drinking Water       Treated Drinking Water
  Sampled                                                                             Water
                                                    MRL varies from             MRL varies from
  Total Chromium           MRL=0.6 µg/L
                                                     0.02 -100 µg/L                1 -50 µg/L
  Data                    n=342, 39%
2.1. National Chromium and Boron Occurrence Survey (NCBOS)
    The National Chromium and Boron Occurrence Survey (NCBOS) was compiled, analyzed,
and reported in WaterRF Report #2759 (Frey et al., 2004). The survey utilities were recruited on
a voluntary basis, rather than a designed survey to adequately represent regional, system size,
and source water occurrence. Field samples were collected in 2003 and transported to a central
laboratory where the analysis of total and hexavalent chromium was performed. Reporting limits
were 0.6 µg/L for total chromium and 0.2 µg/L for hexavalent chromium. A total of 342 total
chromium and 341 hexavalent chromium raw water samples were reported. The samples were
taken at the same relative location for each of 342 different sources, from 189 different systems
in 41 states. Samples from the treatment system profiles included with the NCBOS were not
included in this analysis.

    2.2. EPA’s Second Six Year Review ICR Contaminants Monitoring Data (6YR2)
    The 6YR2 dataset consists of nationwide entry point samples for total chromium. The EPA
collected the 6YR2 data by a voluntary call-in of compliance data from state and primacy
agencies for samples collected between 1998 and 2005 (EPA, 2010). The samples were
analyzed by many different labs and methods resulting in reporting levels from 0.02 – 100 µg/L.
A total of 162,823 samples were included in the dataset after applying a data quality auditing
procedure. The EPA’s data auditing involved standardizing units of concentration for the
contaminants and ensuring that parameters such as the system type were consistent with data in
federal safe drinking water information system (SDWIS). Systems classified as a groundwater
include only ground water sources. However, systems classified as surface waters may include
groundwater and groundwater under the direct influence (GWUDI) of surface water among its
sources. Some of the data submissions included source water samples. Where possible the
source water data points were removed by the EPA. Also, each sampling point was inferred to
be an entry point to the distribution system.
    Table 2 summarizes the non-detections in the 6YR2 dataset. The results were calculated
considering all the samples in the dataset, then grouping the data by entry point. Thus, a non-
detect result at an entry point means that chromium was never detected in that source.

              Table 2. Summary of non-detect results in the 6YR2 dataset.
                         Total count  Non-detect count Percent non-detects
     All Samples - Cr     162,823          136,826                 84%
     Entry Point - Cr      74,672           58,010                 78%

    Of the 162,823 samples in the 6YR2 database, 136,826 were reported as non-detects with an
associated minimum reporting level (MRL). During the EPA’s data auditing procedure, non-
detects that did not include a MRL were assigned the mode MRL value for that state where other
data in the state were available (EPA, 2010). If no other MRL value was available for that state,
the national mode MRL value was substituted. Figure 1 shows the distribution of the non-detect
results from the 6YR2 dataset. Figure 1 indicates 47% of non-detect results have a MRL of 10
µg/L, which many of these presumably resulting from samples that did not include an MRL.
Another 15% of the non-detects have a MRL of 5 µg/L.

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                   ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Number of Non‐Detects in 6YR2   70,000                                                 100%
                                60,000        A)                                                               B)

                                                                             Cumulative Probability
                                                                                                  80%
                                50,000
                                40,000                                                            60%

                                30,000
                                                                                                  40%
                                20,000
                                                                                                  20%
                                10,000
                                     0                                                                0%
                                         0               10            20                                  0               10              20
                                             Total Chromium MRL (µg/L)                                         Total Chromium MRL (µg/L)

Figure 1.                                    Analysis of non-detect MRLs of the 6YR2 dataset (all non-detect samples).
                                             Panel A shows the histogram of the non-detect MRLs. Panel B shows the
                                             cumulative probability distribution of non-detect MRL concentrations.

    Non-detect samples can have a profound effect on the analysis of the data. Because the
concentration of a non-detect sample can be any value from zero to the MRL, cumulative
probability distributions of the dataset were computed under four conditions to characterize the
effect of the non-detect results:
    1. zero was substituted for a non-detect result,
                                2. one-quarter of the MRL was substituted for the non-detect result,
                                3. half of the MRL was substituted for the non-detect result, and
                                4. the MRL was substituted for the non-detect result.
Substituting zero for the non-detects results in bias towards lower concentrations because some
of the non-detects would have had detectable chromium if it were measured by a method with a
lower MRL. Similarly, substituting the MRL for a non-detect results in bias towards higher
concentrations because the actual sample concentrations were likely lower than the MRL in most
cases. Substituting one-quarter and one-half of the MRL for the non-detect attempts to balance
these biases, but the bias is unknown. See Raucher et al. (1995) for more details of non-detect
handling.
    To create the cumulative probability distributions, the samples were grouped by entry point
to analyze the results on a drinking water source basis (all water source types for both
community water systems (CWS) and non-transient, non-community water systems
(NTNCWS)). Next, the median, 75th percentile, 95th percentile, and maximum value
concentrations were calculated for each of the 74,672 entry points. Figure 2 presents the
cumulative probability curves of median total chromium concentration for the 6YR2 data by the
four non-detect substitution conditions. The median value varies from zero to 10 µg/L as

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                                                   ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
influenced by the non-detects with an MRL of 10 µg/L, again many of which would have been
assigned a value of 10 µg/L because no MRL was reported with the data. The vertical portions
in each curve represent a large number of non-detect results at a particular MRL. For instance,
the vertical lines from about the 40th to 80th percentile are indicative of the non-detects with an
MRL of 10 µg/L.

                     100%

                          80%
 Cumulative Probability

                          60%                                       n.d.=0
                                                                    n.d.=0.25∙MRL
                          40%                                       n.d.=0.5∙MRL
                                                                    n.d.=MRL
                          20%

                          0%
                                0            10          20         30                      40
                                               Total Chromium (µg/L)
Figure 2. Cumulative probability distribution of median total chromium concentration by
          entry point of the 6YR2 dataset with non-detects set to zero, one quarter of the
          MRL, half the MRL, and at the MRL.

     Another method to analyze the impact of the non-detect results is a Bayesian estimate of the
posterior cumulative probability. A Bayesian analysis of the non-detects from total chromium
concentrations between 0.5 and 15 µg/L was prepared using the WinBUGS software at the mean
concentration of each entry point location. The Bayesian analysis uses the detect data and a
defined distribution to estimate the cumulative probability curve.
     Figure 3 shows the cumulative probability of the 6YR2 dataset with non-detects substituted
with zero and one-quarter of the MRL with the cumulative distributions of the NCBOS dataset
and the Bayesian estimate. The NCBOS and Bayesian curves show similar results. This result is
somewhat unexpected as the NCBOS dataset was based upon voluntary participation of water
utilities that were willing to sample their source waters and therefore considered to be biased
towards utilities where chromium was suspected or known to occur. NCBOS was generated on a
sample-by-sample basis because only one sample was taken per sample location, and were raw
water samples. But notably, the NCBOS data shows the smooth, nearly log-normal curve to the

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                                    ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
origin resulting from the low reporting limits used in the study. Raucher et al. (1995) observed
that occurrence data often display a log-normal behavior. Substitution of one-quarter of the
MRL in the 6YR2 data is close to the NCBOS and EPA Bayesian estimates while still being
conservative, particularly above 5 µg/L. Therefore, substitution of one-quarter of the MRL for
non-detects was used for all further analyses of the 6YR2 data, except where noted. Continuing
analyses with the Bayesian approach are not practical because the non-detect values are not re-
assigned, thus each data sub-set would require its own Bayesian analysis.

                          100%
 Cumulative Probability

                          75%

                                                             6YR2 (n.d.=0)
                          50%
                                                             NCBOS

                          25%                                EPA Bayesian Estimate
                                                             6YR2 (n.d.=0.25∙MRL)
                           0%
                                 0              5             10        15                   20
                                                    Total Chromium (µg/L)
Figure 3. Cumulative probability distribution of median total chromium concentration by
          entry point of the 6YR2 dataset with non-detects set to zero, and one quarter of
          the MRL, compared to the NCBOS dataset and the Bayesian estimate.

    Figure 4 presents the cumulative probabilities of total chromium from the 6YR2 data by the
median (50th percentile), 75th percentile, 95th percentile, and maximum concentration at each
entry point. Using percentiles to represent the data lessens the impact of non-detect handling
over averaging; therefore, the average at each entry point was not included in this analysis. The
75th percentile, 95th percentile, and maximum value yield very similar curves, with the 95th
percentile and the max value curves being indistinguishable. The median value was eliminated
from consideration because it may not be inclusive enough of systems with variable chromium
concentrations. For instance, a utility that has source water with a variable chromium
concentration, may have a median concentration below a threshold; however, if a regulation
were in place at that threshold the utility would be in violation at times when the chromium
concentration was above the median concentration. The similarity of the 75th, 95th, and

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                                     ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
maximum curves occurs because the sample size of most entry points is low (162,823 samples
divided by 74,672 entry points equals an average of 2.2 samples per entry point). The 75th
percentile has been chosen to be consistent with the USGS analysis of arsenic national
occurrence data (USGS, 2012), and its similarity to the 95th percentile results.
                          100%
 Cumulative Probability

                          75%

                          50%
                                                                median

                          25%                                   75th percentile
                                                                95th percentile & max

                           0%
                                 0              5             10        15                   20
                                                    Total Chromium (µg/L)
Figure 4. Cumulative probability distribution of total chromium concentration by entry
          point of the 6YR2 dataset (non-detects set to one-quarter of the MRL).

    2.3. CDPH Data
    The California Department of Public Health (CDPH) maintains a water quality analysis
database, which includes total and hexavalent chromium in the state of California collected
generally between 1995 and 2011. Data are representative of raw and treated waters from
throughout the state. The database included 136,286 results, 108,779 for total chromium and
27,507 for hexavalent chromium as of November 18, 2011. Because CDPH provided data for
the USEPA 6YR2 database, there is an overlap of up to 23,397 total chromium samples between
the CDPH data and 6YR2 datasets for California (22% of the CDPH database, 14% of the 6YR2
database). CDPH data came classified as raw, treated, and unknown for treatment type.
Samples also came classified as groundwater, surface water, or non-source water.
    Table 3 summarizes the CDPH data and shows a similar percentage of non-detect results on a
sample basis, but a lower percentage of non-detects on a source location basis. The lower
percentage of non-detects on a source location basis can be attributed to two factors. First, the
CDPH data is weighted on more recent data with lower detection limits. Second, from a spatial

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                                     ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
distribution perspective, California has some of the highest levels of chromium (demonstrated in
Section 3).

             Table 3. Summary of non-detect results in the CDPH dataset.
                                  Total count       Non-detect     Percent non-
                                                       count         detects
   All Samples – Total Cr           108,779            90,560          83%
   All Samples - Cr(VI)             27,507             11,387          41%
   Sampling Location – Total Cr     17,846             11,104          62%
   Sampling Location - Cr(VI)        7,880              3,096          39%

    The CDPH total chromium dataset includes 35,968 non-detect samples results at an MRL of
10 µg/L (29% of all non-detects), and another 40,255 had no reported MRLs and were therefore
set to the mode value of the CDPH dataset of 10 µg/L (bringing the total to 73% of all non-
detects). Thus, the CDPH total chromium dataset is similarly influenced by the non-detect
results as the 6YR2 dataset. For comparable analysis with the 6YR2 data, the CDPH chromium
data have been analyzed with non-detects substituted with one-quarter of the MRL and missing
MRL values set to the mode value of 10 µg/L. The MRL for the CDPH hexavalent chromium is
generally 1 µg/L or less. The lower MRL adds very little bias to the analysis of hexavalent
chromium in the range of 1 to 50 µg/L of interest in this study. Therefore, non-detect values
were set to zero for the CDPH dataset for hexavalent chromium.
    Before further analysis can be made, it must be determined whether distribution of the data
differs between raw and treated waters. Figure 5 plots the cumulative probability for raw and
treated waters for both total and hexavalent chromium. The analysis was performed with the
75th percentile concentration on a sample location basis.
    The data shown in Figure 5 indicate more occurrence of total chromium than hexavalent
chromium. Higher total chromium is expected because hexavalent chromium is only a portion of
the total chromium measurement. Treated water data in Figure 5 show a lower occurrence of
both total and hexavalent chromium compared to raw waters. This could be due to treatment
removing some chromium, but a more likely explanation is that sources with high concentrations
of chromium are not used for drinking water except where they cannot be avoided. Also, the
disparity in the number of samples (treated waters only account for about 5% of the data) could
also result in some selection bias. Because the distributions of raw and treated water differ, they
will be analyzed separately throughout the remainder of this report.

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                    ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Cumulative Probability   100%

                          75%

                          50%                                  Total Cr Raw (n=14,735)

                                                               Total Cr Treated (n=1,003)
                          25%                                  Hex. Cr Raw (n=6,547)
                                                               Hex. Cr Treated (n=419)
                           0%
                                 0              5           10           15                  20
                                                    Concentration (µg/L)
Figure 5. Cumulative probability distribution of chromium in CDPH for raw versus
          treated by sample location (75th percentile concentration per each sample
          location).

3. Analysis of Total and Hexavalent Chromium Occurrence
    3.1. Comparison of Chromium Occurrence between Datasets
    The NCBOS, 6YR2, and CDPH datasets all include total chromium. However, the datasets
differ in the number of samples and sample type (raw versus treated). The 6YR2 and CDPH data
were analyzed with various MRLs. CDPH data include raw and treated waters, where the
NCBOS contain only raw, and 6YR2 contain only data from treated drinking waters. The
datasets are compared by plotting the cumulative probability function of each in Figure 6. The
cumulative probabilities were calculated from the 75th percentile concentration at each sample
location.
    The data in Figure 6 indicates the CDPH raw waters exhibited the most occurrence and the
NCBOS exhibited the least occurrence. The 6YR2 and CDPH treated waters showed roughly
equivalent occurrence. The difference observed between these two curves is due to the 6YR2
dataset having a proportionately higher number of non-detects with an MRL of 20 µg/L (the
vertical step in the black line at 5 µg/L).

                                                                 8
                                     ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Cumulative Probability   100%

                          75%

                          50%                                  NCBOS (n=342)
                                                               CDPH Treated (n=1,003)
                          25%                                  6YR2 (n=74,672)
                                                               CDPH Raw (n=14,735)
                           0%
                                 0              5             10        15                   20
                                                    Total Chromium (µg/L)
Figure 6. Cumulative probability distribution of total chromium data from the three
          datasets (6YR2 and CDPH are the 75th percentile concentration per sample
          location).

   Figure 7 compares the cumulative probability distributions of hexavalent chromium from the
CDPH raw waters and the NCBOS dataset. The analysis was performed with the 75th percentile
concentration of hexavalent chromium on a sample location basis. The California data suggested
higher occurrence than nationally, as expected.

                                                                 9
                                     ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Cumulative Probability   100%

                          75%

                          50%

                          25%                                     NCBOS (n=341)
                                                                  CDPH Raw (n=6,547)
                           0%
                                 0             5          10        15                       20
                                              Hexavalent Chromium (µg/L)
Figure 7. Cumulative probability distribution of hexavalent chromium NCBOS & CDPH
          raw data (CDPH shows the 75th percentile concentration per sample location,
          non-detects set to zero).

    3.2. Analysis of Chromium Occurrence by Source Type, Region, and System Size
    Figure 8 displays the cumulative probability distributions of the 6YR2 total chromium
groundwater data versus surface waters. The concentrations were calculated from the 75th
percentile at each sample location and non-detects were substituted with one-quarter of the MRL.
Results of the analysis indicate that distributions are similar, with a maximum of about 2%
difference between them. As noted previously, some groundwater sources may be labeled as
surface waters if they have any surface water, or GWUDI sources in the same system. Frey et al.
(2004) also found no significant difference in distribution of total chromium for ground waters
versus surface waters. Thus, the analysis of total chromium data will not be separated by source
water type. A similar analysis was performed for water system type. No observable difference
was found between CWS and NTNCWS. Therefore, all data has been included in the analysis
(rather than limit the data to CWS data only).

                                                                10
                                     ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
100%
 Cumulative Probability

                          75%

                          50%                             Ground Water (n=67,269)

                                                          Surface Water (n=7,403)
                          25%

                          0%
                                0             5             10         15                   20
                                                  Total Chromium (µg/L)
Figure 8. Cumulative probability distributions of total chromium in the 6YR2 dataset by
          source water type (75th percentile concentration per sample location, non-
          detects set to one-quarter of the MRL).

    Figure 9 shows the cumulative probability distributions for hexavalent chromium by ground
and surface water from the CDPH database. The occurrence in ground waters is higher than for
surface waters. Frey et al. (2004) found similar results. Figure 9 also separates the raw waters
from the treated waters. Not much difference in distribution is observed in the surface water data
as occurrence was only observed at low levels. The groundwater data; however, indicates higher
occurrences in the treated waters below 6 µg/L and higher occurrences in raw waters above 6
µg/L. An increase in hexavalent chromium could result from oxidation of trivalent chromium
during disinfection. A decrease in hexavalent chromium could be due to treatment provided at
some sites. However, the explanation for the crossover in Figure 9 cannot be determined with
any certainty, and it could also be due to the disparity in the number of samples (1,242 treated
samples vs. 14,448 raw samples).

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                                    ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Cumulative Probability   100%

                          75%

                          50%                                    GW Raw (n=14,448)

                                                                 GW Treated (n=1,242)
                          25%                                    SW Raw (n=628)
                                                                 SW Treated (n=370)
                           0%
                                 0             5          10        15                       20
                                              Hexavalent Chromium (µg/L)
Figure 9. Cumulative probability distributions of CDPH hexavalent chromium data in
          ground and surface waters divided into raw and treated samples (75th percentile
          concentration per sample location, non-detects set to zero).

    The regional distribution of total chromium was investigated using EPA regions (shown in
Figure 10). Figure 11 presents the cumulative probability distributions for total chromium
grouped by EPA region. Some variability based on region was observed, but the uncertainty
surrounding the non-detect data complicates the analysis. Non-detect handling complicates
analysis of Figure 11; however, it can be seen that Region 9 (California, Nevada, and Arizona)
show the highest concentration occurrences. The vertical steps in the curves represent the re-
occurring non-detect values. The Region 9 curve shows the highest concentrations towards the
end of the curves where the influence of non-detects no longer influences the analysis.

                                                                12
                                     ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Figure 10.                         Map of EPA Regions.

              100%
Cumulative Probability

                         75%
                                                                     Region 1 (n=6,677)
                                                                     Region 2 (n=4,808)
                                                                     Region 3 (n=4,601)
                         50%                                         Region 4 (n=10,143)
                                                                     Region 5 (n=14,680)
                                                                     Region 6 (n=8,916)
                         25%                                         Region 7 (n=5,703)
                                                                     Region 8 (n=5,367)
                                                                     Region 9 (n=10,996)
                                                                     Region 10 (n=2,781)
                         0%
                               0                5              10                 15               20
                                                     Total Chromium (µg/L)

Figure 11.                         Cumulative probability distribution of 6YR2 total chromium data by EPA
                                   region (75th percentile concentration per sample location, non-detects set to
                                   one-quarter of the MRL).

                                                                  13
                                       ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
System size was investigated in the same manner as EPA region. Figure 12 presents the
cumulative distribution curves for the 6YR2 data by system size category for all systems
included in the 6YR2 dataset (CWS and NTNCWS). Non-detect handling makes interpretation
of the 6YR2 data based on system size capricious. For instance, the higher concentration
distribution curve for systems serving a population of 1,000,000 results from a significant
number of samples analyzed at an MRL of 50 µg/L.

                           100%

                           75%
  Cumulative Probability

                                                                     Population Range (count)
                                                                      25‐100 (n=18,566)
                                                                      101‐500 (n=21,909)
                           50%                                        501‐1,000 (n=7,423)
                                                                      1,001‐3,300 (n=9,397)
                                                                      3,301‐10,000 (n=6,906)
                           25%                                        10,001‐50,000 (n=6,497)
                                                                      50,001‐100,000 (n=1,631)
                                                                      100,001‐1,000,000 (n=2,053)
                                                                      >1,000,000 (n=290)
                            0%
                                  0                  5               10                   15                 20
                                                           Total Chromium (µg/L)

Figure 12.                        Cumulative probability distributions of 6YR2 total chromium data by system
                                  size category (75th percentile concentration per sample location, non-detects
                                  set to one-quarter of the MRL).

    3.3. Analysis of Paired Total and Hexavalent Chromium Samples
    The CDPH data were also used to study the speciation of chromium. A total of 5,976
samples with both total and hexavalent chromium results were paired by matching sample
location and date. Figures 12-14 display the results by plotting hexavalent chromium versus
total chromium by ranges from 0-100 µg/L and 0-10 µg/L by water type and raw versus treated.
In Figures 12-14 there are a number of samples that indicate a higher hexavalent chromium
concentration than total chromium, which is not truly possible. These points result from
analytical issues that are well described and explained by Parks et al. (2004).

                                                                 14
                                      ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Analysis of the ground versus surface water paired samples shown in Figure 13 indicates
surface waters have generally more trivalent chromium than hexavalent chromium. The ground
waters tend to have much of their total chromium as hexavalent chromium, consistent with Frey
et al. (2004). Despite some scatter in the data the hexavalent chromium concentration is highly
correlated with the total chromium concentration (r=0.98).

                              100                                                                       10

                                                                         Hexavalent Chromium (µg/L)
 Hexavalent Chromium (µg/L)

                               90       GW (n=4,809)                                                     9           GW (n=3,434)
                               80                                                                        8
                                        SW (n=1,167)                                                                 SW (n=1,147)
                               70                                                                        7
                               60                                                                        6
                               50                                                                        5
                               40                                                                        4
                               30                                                                        3
                               20                                                                        2
                               10                                                                        1
                                0                                                                        0
                                    0 10 20 30 40 50 60 70 80 90 100                                         0   1     2     3 4 5 6 7 8           9 10
                                           Total Chromium (µg/L)                                                           Total Chromium (µg/L)

Figure 13. CDPH paired sample analysis of ground waters and surface waters.

    Figure 14 displays groundwater paired samples similarly divided into raw and treated
categories. The data in Figure 13 also indicate treated waters have lower concentrations of
chromium. This may be due to treatment, but could also be due to sampling bias created by high
chromium sources not being used for drinking water. The disparity in sample sizes between raw
and treated waters may also contribute to bias.
    The treated samples visually appear to be more heavily speciated towards hexavalent
chromium, but this is not supported by statistical analysis. Statistically, the central tendency of
the raw waters is more heavily speciated in hexavalent chromium. However, there is not a large
difference between the raw and treated waters.

                              100                                                                       10
 Hexavalent Chromium (µg/L)

                                                                           Hexavalent Chromium (µg/L)

                               90      GW Raw (n=3,943)                                                  9       GW Raw (n=2,798)
                               80                                                                        8
                                       GW Treated (n=646)                                                        GW Treated (n=473)
                               70                                                                        7
                               60                                                                        6
                               50                                                                        5
                               40                                                                        4
                               30                                                                        3
                               20                                                                        2
                               10                                                                        1
                                0                                                                        0
                                    0 10 20 30 40 50 60 70 80 90 100                                         0   1     2     3 4 5 6 7 8           9 10
                                           Total Chromium (µg/L)                                                           Total Chromium (µg/L)

Figure 14.                               CDPH paired sample analysis of ground waters grouped by raw and treated
                                         waters.

                                                                         15
                                              ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Figure 15 indicates surface waters tended to have little hexavalent chromium in raw or
treated waters.

                              100                                                                    10

                                                                        Hexavalent Chromium (µg/L)
 Hexavalent Chromium (µg/L)

                               90      SW Raw (n=676)                                                 9       SW Raw (n=661)
                               80                                                                     8
                               70      SW Treated (n=473)                                             7       SW Treated (n=468)
                               60                                                                     6
                               50                                                                     5
                               40                                                                     4
                               30                                                                     3
                               20                                                                     2
                               10                                                                     1
                                0                                                                     0
                                    0 10 20 30 40 50 60 70 80 90 100                                      0   1   2 3 4 5 6 7 8           9 10
                                          Total Chromium (µg/L)                                                   Total Chromium (µg/L)

Figure 15.                               CDPH paired sample analysis of surface waters grouped by raw and treated
                                         waters.

4. Spatial Distribution of Chromium Occurrence

    The 6YR2 dataset was used to calculate the 75th percentile concentration of total chromium
at each entry point. The highest 75th percentile entry point concentration per system was mapped
by latitude and longitude identifiers assigned to each system from SDWIS data. No data were
available in the states of Washington, Pennsylvania, Louisiana, or Mississippi. Figure 16 shows
a national pin map of total chromium concentrations between 5 and 10 µg/L, 10 and 20 µg/L,
and above 20 µg/L. The figure also shows how total chromium is distributed across the country.
The greatest number of high chromium concentrations appears to be in the southwest US from
California to Texas. The region surrounding the Great Lakes also appears to have widespread
occurrence.

                                                                        16
                                             ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Figure 16.     National pin maap of 75th peercentile tootal chromiu
                                                                  um concenttration from
                                                                                       m the
               6YR22 dataset by system (forr systems wiith more than two sam
                                                                           mples).

    Tablee 4 providess a tabular annalysis of th
                                               he spatial disstribution off total chrom
                                                                                      mium occurrrence.
The 6YR R2 (with no on-detects seet to one-qu  uarter of thee MRL) datta was usedd to calculatte the
percent of
         o entry poin nts that excceed total chhromium treeatment goalls of 1, 2, 55, 10, 20, annd 50
µg/L. The
        T results indicate
                     i         thatt the percennt of system
                                                          ms impacted rises steeplyy below 5 µ    µg/L.
Much off this is du ue to non-deetect handlin   ng. For innstance Geoorgia samplees only hadd two
        ns out of 2,358 samples, but becausee the MRLs of the Georggia data werre relatively high,
detection
Table 4 indicates
        i         essentially all entry
                                  e      points exceed 5 µgg/L.

                                               17
                    ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Table 4.     Percent of entry points with 75th percentile total chromium concentration
 above the threshold values (6YR2 data, non-detects set to one-quarter of the MRL).
Threshold           1 µg/L      2 µg/L     5 µg/L     10 µg/L     20 µg/L    50 µg/L
All EPs              84.3%       69.0%     21.6%        7.6%        1.7%      0.2%
Alaska               24.0%       17.9%      7.7%       1.3%        0.4%       0.2%
Alabama             100.0%       99.8%      2.0%        1.0%        0.0%      0.0%
Arkansas            100.0%       99.4%      0.2%       0.2%         0.0%      0.0%
Arizona             95.1%       85.6%      55.9%       29.1%       12.3%      1.2%
California          94.6%        92.4%     15.2%       10.6%        3.2%      0.2%
Colorado            66.9%       56.8%      41.1%       9.5%        2.5%       0.2%
Connecticut          82.4%       74.5%     16.7%       12.4%        1.9%      1.1%
Delaware             53.7%       34.3%      7.8%        1.0%        0.2%      0.0%
Florida             90.6%       84.6%      12.1%       6.6%        1.8%       0.1%
Georgia             100.0%       99.9%     99.9%        0.1%        0.1%      0.0%
Hawaii               34.8%       34.2%      8.9%        2.3%        0.0%      0.0%
Iowa                97.8%        66.3%      8.7%       1.2%         0.8%      0.2%
Idaho               21.0%       20.0%      10.9%       4.6%        0.5%       0.1%
Illinois            99.8%       10.9%       7.6%       2.9%        0.7%       0.1%
Indiana              97.3%       42.3%     28.9%       16.8%        7.0%      0.2%
Kansas               89.5%       62.8%     16.4%        2.1%        0.5%      0.1%
Kentucky             26.5%       19.3%      3.0%        2.7%        0.4%      0.0%
Massachusetts        85.6%       46.4%     10.0%        6.0%        0.1%      0.0%
Maryland            96.5%       94.1%      72.9%       66.1%       2.8%       0.3%
Maine                29.4%       11.4%      1.3%       0.1%        0.1%       0.0%
Michigan            100.0%       93.3%      9.2%       7.6%         1.1%      0.1%
Minnesota           100.0%      100.0%      0.1%       0.1%         0.1%      0.0%
Missouri             12.9%        7.0%      0.4%        0.1%        0.0%      0.0%
Montana             84.6%       83.4%       5.2%       1.4%        0.4%       0.1%
North Carolina      99.2%       93.7%      87.3%       1.4%        0.6%       0.1%
North Dakota        66.9%       52.0%      30.6%       12.9%       2.4%       0.4%
Nebraska             62.1%       46.5%     18.4%        4.6%        0.0%      0.0%
New Hampshire       100.0%       99.9%      1.3%        0.5%        0.3%      0.1%
New Jersey           78.6%       62.9%      9.9%        3.2%        0.8%      0.2%
New Mexico           71.2%       60.9%     28.6%       9.7%        1.2%       0.1%
Nevada               95.2%       63.2%     34.7%       16.7%       4.4%       0.6%
New York             94.4%       14.6%      8.4%       6.1%         0.2%      0.0%
Ohio                87.0%       52.7%       8.5%       6.9%        1.1%       0.0%
Oklahoma            90.7%        89.8%     13.0%       12.7%        8.6%      2.8%
Oregon              11.0%        5.3%       1.7%       0.6%        0.2%       0.0%
Rhode Island        100.0%      100.0%     17.6%       5.3%        0.0%       0.0%
South Carolina      100.0%      100.0%      1.8%       1.7%        0.9%       0.6%
South Dakota        97.8%       90.7%      18.3%       6.5%        0.6%       0.0%
Tennessee            91.0%       86.4%      2.7%        2.2%        0.3%      0.3%
Texas                97.8%       96.6%     22.3%        6.9%        1.9%      0.0%
Utah                 99.1%       25.7%     17.4%        6.5%        1.8%      0.2%
Virginia            99.0%        96.8%      2.4%       1.6%         0.2%      0.0%
Vermont              90.6%       77.0%      3.9%       2.6%         2.1%      0.6%
Wisconsin            37.4%       19.0%      8.7%        4.9%        2.3%      0.3%
West Virginia        98.7%       75.3%      3.8%        1.5%        0.4%      0.0%
Wyoming             99.6%       98.0%      73.8%       72.3%       0.4%       0.2%

                                            18
                 ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Figure 177 shows the CDPH occu    urrence of heexavalent chrromium in C
                                                                      California. B
                                                                                  Because of thhe
lower dettection limitt for hexavallent chromiu
                                              um concentraation ranges of 1 - 5 µg/L
                                                                                   L, 5 - 10 µg//L,
10 - 20 µg/L
        µ     and aboove 20 µg/L are shown. Hexavalent chromium aappears to occcur at locatiions
across the state.

Figure 17.      Califo
                     ornia pin map
                               m of 75thh percentilee hexavalen
                                                              nt chromium
                                                                        m concentration
                from the CDPH database
                               d       by
                                        y system.

5. Co-o
      occurrence with Chromium

    The 6YR2 datab   base was qu  ueried for thhe maximum   m concentraation of eachh of 69 anaalytes
included in the 6YR2  2 dataset for each system
                                              m. As a preliiminary anallysis, the corrrelation bettween
total chro
         omium and thet other 68  8 contaminan nts was calcuulated. Tabble 5 shows the contamiinants
with the highest corrrelations. The
                                  T maximum    m correlationn is seleniumm with a cooefficient of 0.05,
meaning that only 5% 5 of the variation
                                  v         con
                                              ntaminant beeing comparred can be explained by the
variation
        n in total chro
                      omium conccentration. The
                                              T most higghly correlatted compounnds on the list are
naturally
        y occurring, and may bee suspected to  t be solublle under sim milar geochem  mical condittions;
however,, the relationnships are very weak. ForF compariison purposees, the correelation coeffi   ficient
between tetrachloroeethylene and   d trichloroetthylene fromm this samee analysis w  was 0.29. T   These
results are consistennt with the finding of Seidel (20006) which foound no co--occurrence with
chromium m, except beetween total chromium anda iron in suurface waterrs. Iron is noot included iin the
6YR2 daataset and co ould not be investigated
                                   i             further. Inn our opinionn, these resuults do not juustify

                                                19
                     ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
further investigation into co-occurrence using higher statistical techniques such as the Chi
Squared approach.

 Table 5.       Correlation of co-occurring contaminants with total chromium in the 6YR2
                                            dataset.
                Contaminant                            r
                Selenium                             0.05
                Arsenic                              0.04
                Fluoride                             0.04
                Beryllium                            0.03
                Antimony                             0.03
                Cadmium                              0.03
                Thallium                             0.02
                Beta Particles (Gross beta)          0.02
                Tetrachloroethylene (PCE)            0.02
                Nitrate                              0.02
                Barium                               0.02
                Uranium                              0.02
                Trichloroethylene (TCE)              0.02
                Chlordane                            0.02
                1,1,2-Trichloroethane                0.02
                cis-1,2,-Dichloroethylene            0.02
                Alpha Particles                      0.01
                trans-1,2-Dichloroethylene           0.01
                1,1-Dichloroethylene                 0.01
                Ethylene Dibromide (EDB)             0.01
                Combined Radium- 226 & 228           0.01

6. Estimated Extent of Potential Future Regulation

    One of the primary objectives of this occurrence survey is to determine the extent a potential
future regulation may affect drinking water systems. Thus, the number of entry points greater
than a given threshold concentration were estimated for CWSs. Because the 6YR2 database
does not represent a census of all nationwide entry points, the total number of entry points was
estimated. The number of nationwide CWS entry points was estimated by comparing the 6YR2
data to the SDWIS database of CWSs (retrieved in December of 2011), broken down by system
size. Table 6 shows the results of the estimation. Columns 2 and 3 show the number of systems
in the SDWIS and 6YR2 databases, respectively. Table 6 indicates that the 6YR2 database
includes between about 60-90% of the SDWIS totals. To project this to entry points, the number
of entry points in the 6YR2 database is divided by the fraction of systems represented, which is
presented in column 6 of Table 6. Last, the average number of entry points per system is
calculated. The numbers generally increase with system size from about 2 to 14.5 for the largest
systems.

                                              20
                   ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Table 6. Estimation of the national CWS entry point inventory.
         1           2           3           4         5           6            7
                  SDWIS                             6YR2       Projected     Average
     System        CWS        6YR2                   Entry     Number      Number of
   Population     PWSID      PWSID        6YR2 /     Point     of Entry Entry Points
      Served      Census    Inventory SDWIS Inventory           Points    per PWSID
        All       50,936      32,810       64%      74,672     115,925         2.3
      0-100       11,460      8,217        72%      20,116      28,055         2.4
     101-500      16,333      10,509       64%      20,977      32,602         2.0
    500-1,000      5,707       3,432       60%       7,085      11,781         2.1
   1,001-3,300     8,259       4,719       57%       9,189      16,082         1.9
      3,301-
      10,000       4,946       3,092       63%       6,868      10,986         2.2
     10,001-
      50,000       3,254       2,122       65%       6,509       9,981         3.1
     50,001-
     100,000        555         386        70%       1,585       2,279         4.1
    100,001-
    1,000,000       400         313        78%       2,053       2,624         6.6
   >1,000,000        22          20        91%        290         319         14.5

    The number of entry points greater than the threshold values was estimated by multiplying
the total number of entry points by the percent of entry points exceeding the potential regulation.
Table 7 shows these results for total chromium concentrations of 1, 2, 5, 10, 20, and 50 µg/L.
The number of entry points above the threshold concentration have been estimated based on
three conditions:
        1. Using the 6YR2 database with non-detects set to one-quarter of the MRL
        2. Using the 6YR2 database with non-detects set to zero
        3. Using the NCBOS database with non-detects set to zero

                                               21
                    ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Table 7. Number of CWS entry points above the threshold concentration at various
                                 threshold concentrations.
 System
                   Threshold
Population                             1 µg/L    2 µg/L    5 µg/L   10 µg/L 20 µg/L 50 µg/L
                  Concentration
 Served
              6YR2 n.d.=0.25·MRL       97,693    79,953    25,044    8,829     1,976      217
 All Entry
                 6YR2 n.d.=0           23,447    18,581    10,417    6,013     1,826      217
  Points
                   NCBOS               50,543    29,793    10,897    1,971     1,043        0
              6YR2 n.d.=0.25·MRL       23,022    19,135     5,957    2,178      432        42
   0-100         6YR2 n.d.=0            5,245     4,048     2,332    1,408      400        42
                   NCBOS               12,232    7,210     2,637      477       252         0
              6YR2 n.d.=0.25·MRL       27,254    22,268     7,248    2,539      516        71
 101-500         6YR2 n.d.=0            6,655     5,242     2,827    1,680      461        71
                   NCBOS               14,215    8,379     3,065      554       293         0
              6YR2 n.d.=0.25·MRL        9,969     7,817     2,554     825       197        27
500-1,000        6YR2 n.d.=0            2,325     1,827     1,032     544       181        27
                   NCBOS               5,137     3,028     1,107      200       106         0
              6YR2 n.d.=0.25·MRL       13,553    10,811     3,101     881       181        15
  1,001-
                 6YR2 n.d.=0            3,262     2,552     1,289     589       164        15
  3,300
                   NCBOS               7,012     4,133     1,512      273       145         0
              6YR2 n.d.=0.25·MRL        9,424     7,733     2,144     724       208        17
  3,301-
                 6YR2 n.d.=0            2,319     1,839      966      550       192        17
  10,000
                   NCBOS               4,790     2,823     1,033      187        99         0
              6YR2 n.d.=0.25·MRL        8,769     7,136     2,231     800       221        15
  10,001-
                 6YR2 n.d.=0            2,089     1,742     1,072     605       218        15
  50,000
                   NCBOS               4,352     2,565       938      170        90         0
              6YR2 n.d.=0.25·MRL        2,055     1,787      476      245        88        14
 50,001-
                 6YR2 n.d.=0             489       422       303      210        87        14
 100,000
                   NCBOS                 994       586       214       39        21         0
              6YR2 n.d.=0.25·MRL        2,276     2,001      749      299        77         6
100,001-
                 6YR2 n.d.=0             705       602       394      272        69         6
1,000,000
                   NCBOS                1,144      674       247       45        24         0
              6YR2 n.d.=0.25·MRL         304       289       149      132        13         2
>1,000,000       6YR2 n.d.=0              76        68        37       24        13         2
                   NCBOS                 139       82         30       5          3         0

    The results of Table 7 indicate the number of entry points impacted increases steeply below
treatment goals of 10 µg/L. Again this is largely due to the uncertainty associated with non-
detect handling. Figure 18 shows this trend graphically aggregating all system sizes. For
instance, the estimated number of entry points with concentrations above 10 µg/L range from
1,971 (NCBOS) to 8,829 (6YR2 with non-detects set to one-quarter of the MRL). Thus, even at
a concentration above the influence of the non-detect results, over a four-fold difference is
estimated depending on which data set is selected.

                                              22
                   ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
100,000
                                                                      6YR2 n.d.=0.25∙MRL
 Number of Entry Points

                           80,000
   Above Threshold

                                                                      NCBOS
                           60,000
                                                                      6YR2 n.d.=0

                           40,000

                           20,000

                               0
                                    0           10      20        30       40         50
                                        Total Chromium Threshold Concentration (µg/L)
Figure 18. Estimation of national number of CWS entry points with total chromium above
             threshold concentrations.

7. Conclusions

    The purpose of this study was to update the NCBOS chromium occurrence analysis
originally presented in the WaterRF Report #2759 with new data from the EPA 6YR2 and
CDPH WQAD datasets. The comparisons also validate key findings already presented in
WaterRF Report #2759, including:
        1. no significant difference in distribution of total chromium for ground waters versus
            surface waters;
                    2. ground waters tend to have much of their total chromium as hexavalent chromium;
                    3. total chromium occurrence is distributed across the country; and
                    4. the highest chromium concentration occurrences were observed in EPA Region 9
                       (California, Nevada, and Arizona).
    The total chromium data is limited by MRLs, making predictions of systems impacted
subject to a high degree of uncertainty below 10 µg/L. A suitable dataset is not available for
predicting hexavalent chromium occurrence on a national basis. Preliminary analysis of systems
impacted at different chromium concentrations indicates the potential for a very large number of
systems to be impacted by regulation.

                                                           23
                                ©2012 Water Research Foundation. ALL RIGHTS RESERVED.
8. References

EPA. (2010) “The Analysis of Regulated Contaminant Occurrence Data from Public Water
   Systems in the Support of the Second Six-Year review of national Primary Drinking Water
   Regulations,” US Environmental Protection Agency report EPA-815-B-09-006.
Frey, M.M., Seidel, C.J., Edwards, M., Parks, J.L., McNeill, L. (2004) Occurrence Survey of
   Boron and Hexavalent Chromium, Water Research Foundation, Water Research Foundation
   Report #2759 Denver, CO.
Parks, J. L., McNeill, L., Frey, M., Eaton, A. D., Haghani, A., Ramirez, L., Edwards, M. (2004)
   “Determination of total chromium in environmental water samples,” Water Research, 38(12):
   2827-2838.
Raucher, R.S., Castillo, E.T., Dixon, A., Breffle, W., Waldman, D., and Drago, J.A. (1995)
   Estimating the Cost of Compliance with Drinking Water Standards: A User’s Guide. Water
   Research Foundation, Denver, CO.
Seidel, C.J. (2006) “Investigation of Hexavalent Chromium as a Future Regulated Drinking
   Water Contaminant,” Doctoral Thesis, University of Colorado at Boulder.
USGS.        (2012)      Arsenic      in     Groundwater       of    the      United     States.
   http://water.usgs.gov/nawqa/trace/arsenic/ accessed March 1, 2012.

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