Comparison of 2016–17 water years and historical water-quality data, Arkansas 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.” If there is not a "chronic instream standard", the "concern levels" are derived from a "water supply standard", if available. Please take the number of samples available into consideration, when reviewing concern levels.

High concern: The 85th percentile of the data is greater than the instream standard.

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:
Low concern: Half (50th 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 50th percentile of the data was less than 100, it would be listed as low concern.
Medium concern: The 50th percentile of the data is between the instream standard and one-half the standard. This means that 50 percent or more of the data for a given site are nearing or are greater than the standard.
High concern: The 50th percentile of the data is greater than the instream standard.

Dissolved oxygen:
Low Concern: the 15th percentile is greater than the standard
High Concern: the 15th percentile is less than the standard

Escherichia coli:
Low Concern: the geometric mean is less than the standard
High Concern: the geometric mean is greater than the standard

pH:
Low Concern: the 15th or 85th percentile is in the range of 6.5 - 9.0
High Concern: the 15th or 85th percentile is outside the range of 6.5 - 9.0

Water temperature:
Water temperature standards for the Arkansas River are seasonal and the seasons depended upon the reach classification. In the summary tables the first 2 lines of data for temperature provide the statistical comparison of historic and current water temperature data but no comparison to standards. The third through sixth lines provide the historic and current warm and cold season statistics and comparisons to the appropriate standards. The state water temperature standards are based on either the maximum weekly average temperature (MWAT) standard or the daily mean (DM) standard. Since neither of these standards can be determined from periodic water-quality samples, the 85th percentile is used as a comparison to the DM standard.

Concern levels for the seasonal data are:
Low Concern: the 85th percentile is less than the DM standard
High Concern: the 85th percentile is greater than the DM standard

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.

Prior to October 21, 2013, the "concern levels" for filtered and unfiltered (total recoverable) trace metals were based on the 85th percentile. Prior to February 9, 2018, MWAT standard (for water temperature) was used at which time the DM standard was referenced.

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
Attainment of chronic chemical standards, in both streams and rivers, and lakes and reservoir systems, is based upon the 85th percentile of the ranked data, except as otherwise noted below. Percentile values are calculated by ranking individual data points in order of magnitude. Hardness-based metal standards are evaluated by comparing the 85th percentile against the assigned hardness-based equation using the mean hardness. Total recoverable metals are evaluated against the median value, or the 50th percentile. Dissolved metals are evaluated against the 85th percentile. Dissolved oxygen (DO) is evaluated at the 15th percentile for streams. Minima pH is evaluated against the 15th percentile, maxima at the 85th percentile.
Hardness based metals standards may also be evaluated by a detailed assessment where the chronic Table Value Standard is calculated for each paired hardness/concentration data and attainment is determined for each data pair."

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. Censored values for E. coli are assumed to be 1.

"d. Detection Limits
Sample data that are below detection limits will, in general (except E. coli data), be treated as zeroes for assessment of attainment.

e. Escherichia coli (E. coli) Standards
Attainment of the E. coli standard is assessed using the geometric mean of representative stream samples. Notwithstanding the criterion at item d above, E. coli data that are reported as less than detect will be treated as a value of one to allow calculation of a geometric mean."

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.

Hardness:
A number of the trace elements have Table Value Standards (TVS), which are site-specific in-stream standards calculated using stream hardness. TVS equations use a stream hardness value calculated from the “lower 95 percent confidence limit of the mean hardness value at the periodic low-flow criteria determined from a regression analysis of site-specific data” (Colorado Department of Public Health and Environment, 5 CCR 1002-35, Effective March 1, 2017). In this study for stream-sampling sites, the annual low-flow period was defined as October through March. When there were more than five hardness values at a given sampling site during the low-flow period, the lower 95th-percent confidence limit of the mean hardness value was used in the TVS equation. Please take the Laboratory Reporting Level into consideration, when referring to these calculated values, on both the plots and tables.

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.

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.
 (b) Segments may constitute a specified stretch of a river mainstem, a specific tributary, a specific lake or reservoir, or a generally defined grouping of waters within the basin (e.g., a specific mainstem segment and all tributaries flowing into that mainstem segment.
 (c) Segments shall generally be delineated according to the points at which the use, physical characteristics or water quality characteristics of a watercourse are determined to change significantly enough to require a change in use classifications and/or water quality standards. In many cases, such transition points can be specifically identified from available water quality data. In other cases, however, the delineation of segments shall be based upon best judgments of where instream changes in uses, physical characteristics or water quality occur, based upon upstream and downstream data.”

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 32.6(2)(c), Regulation 32). 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:

• Arsenic (chronic) = hybrid, in effect on the following stream segment(s): COARUA03, COARUA04A, COARUA04B, COARUA05, COARMA02, COARMA03, COARLA01B, COARLA09A, and COARFO01A.
• Copper (acute) = Copper BLM - based Fixed Monitoring Benchmark (FMB), copper FMBa = 28.4 micrograms per liter for a subsegment of Monument Creek from immediately above the TriLakes Wastewater Treatment Facility to the North Gate Boulevard Bridge. Copper (chronic) = Copper BLM - based Fixed Monitoring Benchmark (FMB), copper FMBc = 17.8 micrograms per liter for a subsegment of Monument Creek from immediately above the TriLakes Wastewater Treatment Facility to the North Gate Boulevard Bridge, in effect on the following stream segment: COARFO06.
• Selenium (acute/chronic) = existing quality, in effect on the following stream segment: COARLA01A
• Selenium (acute) = TVS:no limit, selenium (chronic) = TVS:0.37 lbs/day, Discharger Specific Variance, in effect on the following stream segment: COARLA01B.
• Sulfate (chronic) = existing quality, in effect on the following stream segment: COARLA01A.
• Temperature (acute/chronic) = current conditions, in effect on the following stream segment: COARMA02.

The temporary modification expiration date for arsenic is 12/31/2021, selenium is 12/31/2018 (COARLA01A), and 12/31/2026 (COARLA01B), sulfate is 12/31/2021, and temperature is 07/01/2021.

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).