March 8, 2002 REVISED DRAFT VI. Recommended Implementation Procedures Dissolved Oxygen Criteria Implementation DETERMINING ATTAINMENT The Chesapeake Bay dissolved oxygen criteria were derived to specifically protect species and communities within the each of the five tidal water designated uses during specific time periods. See Chapter III. Chesapeake Bay Dissolved Oxygen Criteria for detailed descriptions of the designated use specific criteria and time periods for application. Refer to Appendix A. Refined Designated Uses for the Chesapeake Bay and Tidal Tributary Waters for more detailed description of the five designated uses and their boundaries. The Bay dissolved oxygen criteria should not be applied to a designated use or to a time period during the year for which they were not specifically derived (iChapter III. Chesapeake Bay Dissolved Oxygen Criteria. The Chesapeake Bay and tidal tributary dissolved oxygen criteria are summarized below in Table VI-1. Table VI-1. Chesapeake Bay Dissolved Oxygen Criteria Designated Use/Habitat Applicable Time Period Criteria Migratory Spawning and Nursery February 1-May 31 June 1-January 31 6 mg/L 7 day mean 5 mg/L daily minimum 5 mg/L 30 day mean 4 mg/L 7 day mean 3 mg/L daily minimum Shallow Water/Open Water January 1-December 31 5 mg/L 30 day mean 4 mg/L 7 day mean 3 mg/L daily minimum Deep Water June 1-September 30 October 1-May 31 3 mg/L 30 day mean 2.3 mg/L daily mean 1.7 mg/L daily minimum 5 mg/L 30 day mean 4 mg/L 7 day mean 3 mg/L daily minimum Deep Channel June 1-September 30 October 1-May 31 1 mg/L daily minimum 5 mg/L 30 day mean 4 mg/L 7 day mean 3 mg/L daily minimum ------- Magnitude, Duration, Frequency, Space and Time In defining criteria attainment, we need to account for the stressor magnitude, duration, frequency, spatial extent and temporal application period. As described and recommended by a recent National Research Council (2001) review, establishing the magnitude, duration and frequency is crucial for successful development and application of state water quality standards. The magnitude refers to how much of the pollutant can be allowed while still achieving the designated uses. In the case of spring mesohaline chlorophyll a criteria, the magnitude is defined as XZ ug/L. Duration refers to the period of time over which measurements of the pollutant are considered. In the case of the Chesapeake Bay shallow/open water 5 mg/L dissolved oxygen criterion, observations evaluated as a 30 day mean. The allowable frequency at which the criterion can be violated without a loss of the designated use also must be considered. As described further below, the allowable exceedences are defined through the application of a reference curve. For the Chesapeake Bay criteria, two additional dimensions have been added-space and time-for defining criteria attainment. Spatial extent is defined as the spatial assessment unit from which the observation are gathered for comparison to the criterion. The temporal assessment period refers to the time period over which the collective set of observations over the defined durations are gathered to determine criteria attainment. Spatial and Temporal Application of the Criteria Spatial Extent Attainment of dissolved oxygen criteria within the respective designated use habitats should be assessed at the scale of Chesapeake Bay segments (Figure VI-1; Table VI-2), reflecting the scale of data aggregation and reporting for Chesapeake Bay tidal water quality monitoring and physical scale of the designated uses areas. Criteria attainment should be presented in terms of the percentage of the subject designated use habitat within each individual Chesapeake Bay Program segment that meets or exceeds the applicable dissolved oxygen criteria. Temporal Assessment Attainment of the migratory fish spawning and nursery designated use criteria should be assessed and reported for the period February 1 through May 31. Attainment of the shallow/open water criteria should be assessed and reported seasonally, including winter (December, January and February), spring (March, April, and May), summer (June, July, August and September) and fall (October and November). Attainment of the deep water and deep channel designated use criteria should be assessed and reported for the summer period, June 1 through September 30 (Appendix A). The most recent three consecutive years of applicable tidal water quality monitoring data should ------- be used in all determinations of criteria attainment. A three-year period is consistent with the water quality status assessment period used for over a decade by the Bay Program partners (e.g., Alden and Perry ,1997). A three-year period includes some natural year to year variability largely driven by climatic events and also addresses residual effects of one year's condition on the other years. Two years provides no ability to assess central tendency. A period of four or more years delays response to detected problems and is more appropriate for determining trends versus characterizing current water quality conditions. A comparison of criteria attainment across 1-, 3- and 5-year assessment periods confirmed the selection of 3-years as the temporal averaging period (U.S. Environmental Protection Agency 2002). Attainment levels were highly variable using single year periods. The 5-year period smoothed much of the variability and resulted in little difference between one assessment period and the next. Defining Criteria Attainment Through Cumulative Frequency Distribution As the estuarine habitats move towards achievement of the Bay criteria, not only will the concentrations and values increase (dissolved oxygen, water clarity) or decrease (chlorophyll a), but the high short variability concentrations will also dampen. Zero violations of these criteria is a circumstance that is likely to never be observed even within a fully restored Chesapeake Bay ecosystem given natural Bay processes and extreme weather events. However, allowance for a small percentage of criteria exceedences must still provide for full protection of the tidal water designated uses. The cumulative frequency distribution methodology addresses the need to allow for a small percentage of criteria exceedences by effectively integrating all five elements of criteria definition and attainment-magnitude, duration, frequency, space and time. The methodology summarizes the frequency of exceedance of a water quality threshold (e.g., chlorophyll a or dissolved oxygen concentration) as a function of the area or volume affected within a defined time and space domain. Acceptable combinations of frequency and spatial extent of exceedances of the criterion are defined using a reference curve. The reference curve can be defined through a combination of ecological information and policy decisions. This approach accommodates rare (in space and time) exceedances of the parameter threshold in a way that explicitly acknowledges that the acceptable frequency of exceedances of a water quality criterion is a function of the spatial extent of those exceedances. To accomplish this approach for defining criteria attainment, the spatial extent (area or volume) of exceedance of a water quality criterion is quantified for each monitoring event. For example, under a monthly monitoring program, the spatial extent of exceedance of a water quality criterion would be estimated for each month. This could be accomplished through interpolation of point or transect data using existing interpolation software, through analysis of remotely sensed data providing continuous spatial coverage, or through statistical estimation from probability-based samples. In that way the criteria measure is estimated at all locations in a given spatial unit. The spatial extent of exceedance for a given monitoring event would be defined as the fraction of the total area or volume (expressed as a percent) that exceeds the criterion. ------- Developing the Cumulative Frequency Distribution To develop the frequency distribution of exceedances, the observations of spatial extent of exceedance collected during the assessment period would be compiled and converted to a cumulative frequency distribution. For example, if a monthly data collection program were conducted over a three year assessment period, there would be 36 observations of the spatial extent of criterion exceedance. To construct a cumulative frequency distribution, the observations of criterion exceedance would be sorted from highest to lowest spatial extent. Cumulative frequencies would be calculated for each observed spatial extent of exceedance by dividing the number of values above that level by 36 (and expressed as a percent by multiplying by 100). In that way every observation of spatial extent of exceedance is associated with a percentage of the total number of observations below it. Each of those data pairs can be plotted on a cumulative frequency distribution graph in which the vertical axis represents cumulative frequency and the horizontal axis represents the spatial extent of criterion exceedance. Attainment can be defined by the extent to which the cumulative frequency distribution curve exceeds a reference curve. Defining the Reference Curve Even under future conditions of restored water quality in the Chesapeake Bay, there will be areas that will exceed the Bay criteria, whether it be due to poor flushing (chlorophyll a), a strong stratification event (dissolved oxygen), a wind resuspension event (water clarity) or some other natural phenomenon. Assessment of water quality criteria attainment should allow for widespread, but rare exceedances of water quality criteria without flagging this condition as non-compliance due to anthropogenic influences. Similarly, frequent exceedances of sufficiently small spatial extent do not equate to an impairment of the Bay's living resources and their habitats. Again, the assessment should allow for these small scale, higher frequency events without flagging this condition as non-attainment of the water quality criterion. Thus, it is only when exceedances are above some spatial extent and frequency that a segment should be judged in non-attainment of the criteria. The development and application of a reference cumulative frequency distribution provides the means for directly determining the amount of exceedance that's acceptable without causing the spatial unit to be considered "out of attainment". At the same time, a reference curve should reflect expect exceedences that occur naturally when the sampled area is not impacted by whatever stress the criteria was designed to limit. There are at least three options for defining a reference curve. Fixed percentages could be selected based on a policy decision or other basis similar to the 10 percent level of acceptable exceedences allowed in 305(b) EPA guidance (U.S. Environmental Protection Agency 19XX). Alternatively, a reference curve could be established, assuming a normal data distribution, using observed or estimated variance (for both time and space) and mean. Finally, field or laboratory-based biological effects information can be used as the empirical data from which a reference curve can be derived. Even this third approach, however, requires technical and/or policy decisions regarding what level of biological effect is unacceptable. Quantifying Degree of Criteria Attainment The actual determination of criteria attainment would be made through a two-step process. First, a statistical test of the significance of difference between the observed data and reference ------- cumulative frequency distribution curves is applied to determine overall attainment/non-attainment of the criteria. A waterbody segment's designated use habitat will be considered in attainment with its respective criteria when the cumulative frequency of space-time based measures of criteria achievement are within the confidence interval of the reference curve. The reference curve confidence interval will be established in recognition of the numerous sources of possible error (e.g., oxygen probes, field sampling techniques, monitoring design) balanced with possible compensations (e.g., biological compensation, conservatism built into criteria derivation). If the two curves are determined to be statistically different, then an estimation of the area between the cumulative frequency distribution and the reference curve. The measure of the magnitude of non-attainment is based on the area between the two curves as a proportion of the total possible area (expressed as a percentage based on multiplying by 100). Factoring in Measurement Error The certainty surrounding the cumulative frequency distribution is highly dependent on sample size. To properly account for uncertainty in the process described above, one must quantify the uncertainty in the interpolated data as a function of the density of points that enter the interpolation procedure. This uncertainty must then be translated into the uncertainty that surrounds the cumulative frequency distribution generated from the interpolated data and addressed as part of statistical test of differences between the cumulative frequency distribution and the reference curve. The cumulative frequency approach to defining attainment is not intended to factor in all anticipated natural ecosystem process which lead to lower dissolved oxygen concentrations. See Factoring in Natural Excursions of Low Dissolved Oxygen Conditions below for more details. Translating Chesapeake Bay Water Quality Monitoring Data into Measures of Attainment ------- |