Comparison of 2016–17 water years and historical water-quality data, Big Thompson River Basin, Colorado
Definition of terms
Concern levels - The three-tiered concern levels, defined below, are provided to inform watershed users to potential impairments within specific stream segments. The “high concern level” is based on the State of Colorado’s definition of “chronic instream standard.” The "medium and low concern levels" are derived from the “chronic instream standard.” Please take the number of samples available into consideration, when reviewing concern levels.
Medium concern: The 85th percentile of the data is between the instream standard and one-half the standard. This means that 15 percent or more of the data for a given site are nearing or are greater than the standard.
Low concern: The majority (85th percentile) of samples for a given site are less than one-half the instream standard. For example, if a constituent had a standard of 200 and the 85th percentile of the data was less than 100, it would be listed as low concern.
The concern levels as defined above are modified for the following constituents:
Total recoverable metals and total phosphorus:
Concern levels for the seasonal data are:
If there are less than 6 samples in a period, the percentiles (15th and 85th), median, and concern levels are not provided.
If a measured constituent is not listed with a concern level, it can be assumed that it is likely a low concern.
The concern levels are consistent with the methods used by the State of Colorado to assess whether stream water-quality standards are being attained. The following is from the Section 303(d) Listing Methodology - 2018 Listing Cycle, Water Quality Control Division, page 14;
“a. Attainment of Chronic Standards
Some sampling locations may be exempt to particular standards, including but not limited to total phosphorus, chlorophyll or Copper, since these standards are only applicable above/upstream of defined locations. User will need to determine exact location of each individual site to determine whether it is upstream or downstream of the facilities listed in the Regulation, to determine if associated standard is relevant to particular location. When more than one standard was assigned, the lower value is reported.
Censored value: A value reported as less than a laboratory reporting level, for example <0.05. In accordance with methods outlined in Section 303(d) Listing Methodology - 2018 Listing Cycle, Water Quality Control Division, page 14, censored values are assumed to be zero for computation of the statistics shown in the data summary tables.
"d. Detection Limits
Dataset: All data analyzed within the period are typically included in this product. An exception may include constituents that are sampled infrequently or as part of a special project.
Dissolved and total: Constituent concentrations listed in the accompanying figures and tables refer to dissolved concentrations unless specifically stated otherwise. Dissolved constituent concentrations are derived from laboratory analysis of water samples filtered through a 0.45 micrometer filter, whereas total constituent concentrations are determined from laboratory analysis of unfiltered water samples.
HUC: Hydrologic Unit Code--A geographic area representing part or all of a surface drainage basin or distinct hydrologic feature. Each hydrologic unit is identified by an 8-digit number.
Laboratory Reporting Level (LRL) generally is equal to twice the yearly determined long-term method detection level (LT-MDL). The LRL controls false-negative error. The probability of falsely reporting a nondetection for a sample that contained an analyte at a concentration equal to or greater than the LRL is predicted to be less than or equal to 1 percent. The value of the LRL will be reported with a “less than” (<) remark code for samples in which the analyte was not detected. The National Water Quality Laboratory collects quality-control data from selected analytical methods on a continuing basis to determine LT-MDLs and to establish LRLs. These values are reevaluated annually based on the most current quality-control data and therefore may change. (Note: Previously, the LRL has been called the nondetection value or NDV—a term which is no longer used.) The LRL listed in the tables is the LRL associated with the most recent sample for the given site.
Lowess (locally weighted scatterplot smoothing) curves (Helsel and Hirsch, 2002) were provided (smoothing factor of 0.4) when at least five years of record were available and <25% of the data were censored. Lowess curves were not completed for water temperature, dissolved oxygen, and streamflow. Lowess curves may or may not cover the entire period depending on variability in reporting levels (censored values). If sampling frequency was less than quarterly, lowess curves were typically not provided.
Period (water years) is defined in this report as the period of time (in water years) included in the summary tables, and may not include all historic records available.
Regulations - The state regulations referenced in these summaries are the regulations that most recently went through Water Quality Control Commission's rulemaking proceedings. These proceedings routinely occur every 3 to 5 years in each basin. (https://www.colorado.gov/pacific/cdphe/wqcc-rulemaking-proceedings) Minor updates to regulations are made periodically between the rulemaking proceedings, and these may or may not be reflected in the summaries.
Revisions: The following revisions have been completed on the Big Thompson summary. Approximate dates and changes are listed below:
Water temperature standards: August 2016 - The DM standard is now referenced, instead of the MWAT standard.
Total recoverable concern levels: December 2013 - The "concern levels" for unfiltered (total recoverable) trace metals were modified, no longer based on the 85th percentile but are now based on the 50th percentile (median).
Stream segment(ation): (from Colorado Department of Public Health and Environment Water Quality Control Commission Regulation number 31), pg. 9,
“(a) For purposes of adopting site-specific classifications and water quality standards, the streams and other surface water bodies shall be identified according to river basin and/or subbasin and specific water segments.
Standards for tunnel and canal sites are based on receiving waters.
Table Value Standards (TVS) - All Table Value Standards are based on hardness, except the Ammonia TVS. The Ammonia TVS is based on the pH and water temperature of a specific sample.The Ammonia standards provided in the sample statistic table are calculated by taking the average of all samples in that specific period, either historic or current. The Ammonia standards provided on the time-series plots are the average of all samples (historic and current periods).
Temporary modifications - Some stream segments have been granted temporary modifications (section 31.7(3)(a), Regulation 31). For the purpose of this summary, temporary modifications to stream segments were not taken into account when calculating concern levels but exist for some constituents, as listed below:
Expiration date for the arsenic temporary modification is 12/31/2021, selenium is 12/31/2020.
Water Year is defined in this report as the 12-month period October 1 through September 30, designated by the calendar year in which it ends. (http://water.usgs.gov/nwc/explain_data.html).