Background
In general, Water and Wastewater regulatory agencies define how to treat detection limits, reporting limits, and non-detects. Unfortunately, there is not a universal approach on how to treat this type of data so applying these concepts requires flexibility.
The detection limit, often synonymous with the limit of detection (LOD), is the concentration that is statistically greater than the concentration of a method blank with 99% confidence. Essentially it is the lowest concentration that can confidently be reported as “present” when using a particular analytical method. This is a vital threshold for analyte datasets since signal intensity below this threshold cannot be distinguished from a method blank’s baseline noise.
To summarize these concepts:
- Reporting limit (RL): the threshold above which an analyte’s concentration is formally recorded and reported.
- Detection limit (DL or LOD): the lowest concentration that can confidently be reported as “present.”
- Non-detect (ND): When an analyte's concentration is below the detection limit and presence or absence of an analyte cannot be confidently determined due to baseline noise.
In state and federal reports, these are typically represented and treated in a standardized way to ensure clarity and consistency. For example:
- Reporting limits (RL) may be displayed as "<RL" (e.g. "<1" if the RL threshold is 1).
- Non-Detects can also be reported in the same manner as Reporting Limits, since the RL must be higher than the Detection Limit.
- The regulatory agency governing these reports will define what the “effective value” of a data point is when less than the Reporting Limit.
Again, these “standards” are not universally agreed upon so successfully applying these limits for various regulatory interpretations requires a dynamic approach.
RL, ND< and LOD in Waterly
Waterly has diligently developed a methodology to accommodate the varied regulatory guidelines so analyte results are properly handled and reported. This is the ability to define a Waterly metric as one with a detection limit, reporting limit, non-detect limit, and “effective value” so the metric dataset agrees with your specific regulatory agency’s guidelines. This results in a sound dataset and properly displayed reports.
Note: The “effective value” is the value used for aggregate calculations (i.e. min, max, avg) when a value falls below the reporting limit. This value is defined within your regulatory framework and able to be set within Waterly’s Metric Editor.
How to Specify Reporting and Detection Limits
The feature can be accessed by entering Edit Mode. This is done by clicking the three vertical dots ⋮ in the upper right-hand corner of the Waterly page. This will reload the site with the Edit tools. Navigate to the analyte metric to be edited and click on the pencil icon ✎
Note: Edit mode is only accessible by those with Supervisor, (Group) Editor, or (Group) Admin permissions.
This will open the Metric Editor panel as shown below. Locate the "Detection or Reporting Limits" box (in the red circle) and click to enable the feature. This will display the additional fields for "Reporting Limit" "Treat <RL as" and "Detection Limit." We will explore each of these next.
Reporting Limit Field
This field sets the limiting threshold value for reporting. If the value falls below the Reporting Limit, the resulting Effective Value is modified based on the user's defining inputs in the next field.
Note: The Detection limit cannot exceed the Reporting limit.
"Treat <RL as" Field
This field determines the Effective Value when it falls below the Reporting Limit. This can be treated as:
- An exact value based on analyte units: IF <RL THEN Effective Value = 0.025ppt.
- A percentage of the RL: IF <RL THEN Effective Value = 50% * RL = 0.025ppt.
Reference text will be displayed beneath these fields to clarify what Effective Value would be used for reporting and downstream calculations. These Effective Values may be defined by regulatory standards for specific analytes. This is shown in the example below.
Note: The "Treat <RL as" value cannot exceed the Reporting Limit if it is treated as an exact value or 100% if being defined as a percentage. In addition, if Detection and/or Reporting Limits are selected for a metric it cannot also be set for calculation or interpolation.
Detection Limit
This optional field indicates the threshold at which the instrumentation can no longer distinguish the presence of an analyte in a representative sample. If a defining limit is set here, it will be used in an additional step to be discussed later in this article.
Note: The Detection limit cannot exceed the Reporting limit. If the value does exceed the RL a message will display indicating an adjustment is needed.
Once all the Reporting and Detection Limits have been set for that analyte metric, navigate to the bottom right-corner of the Metric Editor panel and click the "Save" button.
How Analyte Results are Affected by RL and DL limits.
The metric entry fields will reflect the set DL and RL parameters for easy reference. If a Detection Limit is set, an additional selection box will display. Here you can see the Detection Limit and the Reporting Limit parameters displayed for reference and the ☐ ND selection is available.
The ☐ ND allows the user to indicate a value below the detectable threshold (Non-Detect) when entering a value. If a value is entered that is less than the DL, Waterly will notify the user and ask if they would like to set this value as a Non-Detect, as shown below.
In this case, we entered 0.003 as an analyte result. Our set Detection Limits are below the method detection level of 0.004. Waterly has automatically determined this is below the ND (Non Detect) limit and allows the user the choice to select the ☐ ND or to clear the data and recheck the entry. If the user confirms the selection by clicking "Set to Non-Detect" the ☐ ND box will be checked and the value removed since values below ND (Non Detect) cannot be distinguished from a method blank’s baseline noise.
Alternately, the user can check the box before entering the result value if they know the analyte result falls below the Detection Limit.
Note: If a Detection Limit is not set, the ☐ ND will not display and the result value can be entered and saved.
The Sparkline Trend will reflect the Effective Value ("Treat <RL as") and the User Entry. These are displayed when hovering over specific days. In this example the Effective Value = 0 and the User Entry = 0.004:
Reporting
The values are reported based on the user defined Reporting Limit (RL) and the optional Detection Limit (DL) if set. If a value is either below the reporting limit or selected as non-detect, the report will reflect that the value is below the Reporting Limit. Here is an example report with values entered for multiple days and meeting several different scenarios.
This method of reporting the values provides mathematical transparency. The values equal to or greater than the Reporting Limit are reported clearly. The results below the Reporting Limit are displayed as "<RL" and the Effective Value used in subsequent calculations is clearly referenced in the footnote.
Dashboard and Trending
The Dashboard will use the Effective Value for trending and aggregate calculations (Min, Max, Avg). This ensures data integrity across Waterly since the Data Entry Metrics, Dashboard, and Regulatory Reporting all match.
Conclusion
The ability to accurately record and appropriately treat analyte results using detection and reporting limits is paramount. The Waterly platform simplifies this process. By seamlessly integrating these reporting requirements into an intuitive and user-friendly interface, Waterly further simplifies water quality data management and compliance reporting. This builds on our commitment to continually improve Water/Wastewater Data Analytics and Reporting by adding more depth to our automated compliance reporting and alerting based on regulatory standards and permits. Waterly's streamlined data management helps environmental professionals quickly identify trends, address potential issues, and make informed decisions. Waterly’s ease of use, combined with accurate and reliable data handling, underscores the importance of maintaining and improving water quality management practices to safeguard public health and optimize process efficiencies. Thank you for trusting Waterly!