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| The Beach Report Card Grading Methodology translates complex shoreline bacteria data into a grade format that is meaningful and useable by all California beachgoers |
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In 2005, Heal the Bay modified the Beach Report Card grading methodology to better characterize local beach water quality.
Amendments to the grading methodology include: 1) the inclusion of the geometric mean into the calculation, 2) a firm zero to 100 point scale, 3) greater significance given to the most recent sample(s) relative to past samples, and 4) greater weight for enterococcus and the total to fecal ratio relative to total coliform and fecal coliform.
These modifications stem from comments made by California’s State Water Resources Control Board and the Beach Water Quality Workgroup. With these improvements to the methodology, Heal the Bay’s Beach Report Card grading methodology is now endorsed by the State Water Resources Control Board and the Beach Water Quality Workgroup as an effective way to communicate beach water quality to the public.
Note: The Annual and End of Summer Beach Report Card methodology is modified slightly to accommodate the longer time period. (for example: No greater significance is given to the most recent samples)
The new methodology retains past modifications to the report card, such as the inclusion of new indicator bacteria thresholds, namely the total to fecal ratio, developed by the Santa Monica Bay Restoration Commission in the 1996 health effects studies of Santa Monica Bay beachgoers; and the implementation of standard deviations for each indicator bacteria threshold developed by the Southern California Coastal Waters Research Project and Orange County Sanitation Districts during the 1998 Southern California Bight Study. Each threshold is based on the prescribed standards set in the California Department Health Service’s Beach Bathing Water Standards.
Total Points Available
Table 1
Grade |
Points |
A |
90-100% |
B |
80-89% |
C |
70-79% |
D |
60-69% |
F |
<=59% |
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As seen in Table 1, the new methodology continues to use a standard A through F grading system, and grades are now based on the following formula:
‘Total Points Available’ is derived from adding together two point components (if applicable): the Geometric Mean and the Single Sample Standard. The points for each component are listed in Table 2.
Table 2
“Total Points Available” by Component |
| Geometric Mean |
29 points |
| Single Sample Standard |
71 points |
| Total |
100 points |
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In order for the points in each component to become available, certain criteria must be met. For example, the geometric mean points will be added to the ‘Total Points Available’ only if there are a minimum of 4 dry weather samples collected within the allotted time frame (for the
Annual Report Card, this is April 2005–March 2006). Wet weather data is graded separately from dry weather data, and does not include a geometric mean component. Therefore, it is possible for ‘Total Points Available’ to be less than 100. The new grading methodology allows for a relative grade to be determined based on the actual monitoring completed.
Once the ‘Total Available Points’ has been determined for a specific location, then the ‘Total Points Lost’ can be calculated for the applicable grade components.
Total Points Lost
Separate calculations are used to quantify ‘Total Points Lost’ for each applicable component from the ‘Total Available Points’. The following describes the two calculations.
Geometric Mean
Calculating the ‘Total Points Lost’ for the Geometric Mean component involves using California’s Beach Bathing Standards for the geometric mean. The standards for each of these criteria are presented in Table 3.
Table 3
Calculating the Total Points Lost for the Geometric Mean Component
Indicator Exceeded |
California Beach Bathing Standard* |
% of Total Available Points Lost** Due to Exceedance |
Total Available Points |
| Enterococcus |
35 |
80% |
29 |
| Fecal Coliform |
200 |
40% |
| Total Coliform |
1,000 |
40% |
* Colony forming units per 100 milliliters of ocean water
** Total Percentage Points Lost cannot add up to be > 1 |
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Each geometric mean criterion exceeded for the time frame is assigned a specific percentage of points lost. Non-exceedances are given 0%. The percentage of points lost from each of the three criteria are then added together and multiplied by the ‘Total Available Points’ (any sum of percentages exceeding 100% automatically loses all 29 points available in the geometric mean component).
The following additional procedures apply to the Annual and Summer Beach Report Cards only:
If the number of ‘Total Points Lost’ is less than 29, then the frequency of the sample location’s exceedances of the 30-day geometric mean throughout the timeframe is taken into consideration. If a given location exceeded any state 30-day geometric mean standard more than 20% of sample days, then an additional 10 points are lost for the geometric mean component (up to but not to exceed 29 total points). If the location exceeded any state 30-day geometric mean standard for more than 40% of sample days, then another 10 points are lost for the geometric mean component (up to but not to exceed 29 total points). If the location exceeds any state 30-day geometric mean standard for more than 50% of samples days, then the location automatically loses all 29 points available for the geometric mean component.
Single Sample Standard
Calculating the ‘Total Points Lost’ for the Single Sample Standard component is similar to the calculation used for deriving the points lost for the Geometric Mean. However, the Single Sample Standard component uses a gradient to calculate the ‘Total Points Lost’. The gradient of percentage points lost used in calculating the number of points lost is derived from work completed by the Southern California Coastal Water Research Project and Orange County Sanitation District as part of the 1998 Southern California Coastal Bight Study (see Table 4).
Table 4
Single Sample Gradient Thresholds in cfu/100ml*
| Group |
1
T - 1 s.d.(1) |
2
T + 1 s.d. |
3
>T +
1 s.d. |
4
Very High
Risk |
| Total
Coliform |
6,711-9,999 |
10,000(2)-
14,900 |
>14,900 |
na |
| Fecal
Coliform |
268-399 |
400-596 |
>596 |
na |
| Enterococcus |
70-103 |
104-155 |
>155 |
na |
Total
to Fecal Ratio
(if: Total > 1,000) |
10.1-13 |
7.1-10 |
2.1-7 |
<2.1 |
* Colony forming units per 100 milliliters of ocean water
(1)
s.d. - standard deviation
(2) Red numbers are the CA
State Health Dept. standards for a single sample. |
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‘Percentage of points lost’ is allocated depending upon the threshold exceeded by each of the four criteria. Each single sample criterion exceeded is given a ‘percentage of points lost’. These amounts are presented in Table 5.
Table 5
Calculating the Total Points Lost for the Single Sample Standard Component
Indicator Exceeded |
Slight
% Points Lost |
Moderate
% Points Lost |
High
% Points Lost |
Extreme
% Points Lost |
Total Avail. Points |
| Total Coliform |
10% |
30% |
40% |
na |
71points |
| Fecal Coliform |
10% |
30% |
40% |
na |
| Enterococcus |
20% |
40% |
60% |
na |
| Ratio
(when Total >= 1,000) |
25% |
50% |
75% |
100% |
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Non-exceedances are given zero 0%. The ‘percentage of points lost’ from each of the four criteria for each sample during the time period are added together and divided by the total number of samples. Once this number is calculated (total ‘percentage of points lost’ divided by total number of samples), it is multiplied by the ‘Total Available Points’. In the Single Sample Standard component, more points are lost as the magnitude or frequency of exceedances increases.
Points lost from the Single Sample Standard component are added to the points lost in the Geometric Mean component (if applicable) and this sum becomes ‘Total Points Lost’ Once the ‘Total Points Available’ and the ‘Total Points Lost’ are calculated, a grade for a particular sample site can be determined.
Determining a Grade
Most dry and wet weather Annual grades are calculated with 100 ‘Total Available Points’, although there is no Geometric Mean component for wet-weather grading. Wet weather grades are calculated by the total ‘percentage of points lost’ divided by the total number of samples and then multiplied by 100. This gives the location’s score for wet weather ‘Total Points Lost’. This number is then subtracted from 100 to give the % grade. |
