Pursuant to Section 305(b) of the Clean Water Act, the Mississippi Department of Environmental Quality reports for USEPA, Congress, and the public the status of the state's surface and ground waters. Mississippi has approximately 84,000 miles of streams and rivers. For the annual 305(b) report, monitored stream miles have been dynamically segmented and rated as fully, partially, or not supporting for their designated uses based on water quality data collected on the segmented waterbody. USEPA has requested that all states have these monitored streams dynamically segmented by 2002.
Mississippi's Water Quality Assessment Section 305(b) Report is required by Congress pursuant to Section to 305(b) of The Clean Water Act (CWA). This section of the Clean Water Act charges the states, territories, and tribal provinces of the United States to provide: "...an analysis of the extent of which all navigable waters of such State provide for the protection and propagation of a balanced population of shellfish, fish, and wildlife, and allow recreational activities in and on the water." In accordance with the above statement, all navigable waters that lie within state boundaries fall under jurisdiction of CWA requirements and are subject to assessment and reporting. Through compilation and analysis of the individual state reports, the United States Environmental Protection Agency (USEPA) transmits summaries of the water quality information to Congress along with an analysis of the status of the quality of the Nation's water resources as a whole. The Mississippi Department of Environmental Quality (MDEQ) is charged with the task of determining and explaining the status of the Mississippi's surface waters for USEPA, Congress and the public. This information is provided to USEPA in the form of written reports and/or the submittal of electronic data files via USEPA's Assessment Database (ADB). The ADB, developed by the Research Triangle Institute (RTI) for USEPA, specifically addresses national 305(b) reporting needs and ensures reporting consistency.
In order to achieve the goals as they are set forth by the CWA, Mississippi must monitor, assess, and report on all of the rivers, streams, coastal shoreline, publicly owned lakes and reservoirs, and estuaries within the state. As a result of the assessment process, the assessed waters in each major drainage basin of the state are rated as having good (fully supporting of its designated uses), fair (partially supporting of its designated uses), or poor (not supporting of its designated uses) water quality. In recent years, one of the most publicly recognized functions of the 305(b) reporting process is identifying the sections or segments of water where the water quality is determined to be poor or not supporting of its designated use and listing these segments on Mississippi's Section 303(d) List of Impaired Waterbodies.
The listing of waters of determined poor quality is required by Section 303(d) of the CWA which states: "The state shall establish a priority ranking for such waters (water not meeting water quality standards), taking into account the severity of the pollution and the uses to be made of such waters." Due to the importance of the 303(d) list and the implications of the resulting Total Maximum Daily Loads (TMDLs) that must be generated to address the water quality impairments, the quality of the assessments as well as the development of accurate and easily understandable graphical representations of 305(b) assessment information and 303(d) listings are of utmost importance. This information is critical for use by decision-makers and the public. Advancements in Geographic Information System (GIS) technology have significantly improved the mapping capabilities for depicting the multiple information datasets to meet these reporting needs.
Mississippi has approximately 84,000 miles of streams and rivers of which 26,454 miles are composed of perennial streams and the remaining waters are intermittent. For the 305(b) report and subsequent 303(d) listings, monitored stream miles are broken down into segments or stream reaches and these segments are rated as fully supporting, partially supporting or not supporting of their designated uses based on water quality data collected on the segmented waterbody. During the process of assessment and reporting, all stations where monitoring data are collected must be geographically represented. This information is then combined, via GIS layers, with any other applicable information (i.e. waters designated in the Water Quality Standards (WQS) document as having special use classifications, existing 303(d) segments, locations of National Pollutant Discharge Elimination System (NPDES) permitted dischargers, land use/land cover data, and any other agency monitoring data). A major goal of 305(b) reporting is for assessment information to be geo-referenced. This geo-referenced information can take many different forms and can be mapped collectively by overlaying the different themes. Also, the ability to geo-reference use support decisions enables agencies to establish and track long-term trends in water quality. USEPA has requested that the states have assessed stream segments geo-referenced by 2002.
The two critical components of waterbody segment assessments are the locations of monitoring stations on the segment and the determination of the proper designated use that applies to that waterbody. The locations of monitoring stations are depicted graphically by using latitude and longitude readings recorded from a Global Positioning System (GPS) data recorded during field sampling. The determination of the applicable designated use for that waterbody is found by comparing the waterbody of interest against Mississippi's water quality standards document. It is crucial that the correct designated use for a waterbody be determined before the assessment process begins to ensure that the correct water quality standard, numeric and/or narrative, be applied to the stream segment of interest.
Surface waters in Mississippi are classified and assigned various use classifications based on existing utilization of the waterbody along with any expected future uses. The use classifications used by the State of Mississippi are as follows: Public Water Supply, Recreation, Fish and Wildlife, Shellfish Harvesting Waters, and Ephemeral. Most of Mississippi's waters are classified as Fish and Wildlife but there are several waters that fall under classifications in addition to or other than Fish and Wildlife. For each of the use classifications listed above, there are various water quality criteria or standards that apply to those waterbody uses and which are used in the assessment process. A complete description of the Mississippi's waterbody use classifications and water quality standards can be found in the "State of Mississippi Water Quality Criteria for Intrastate, Interstate, and Coastal Waters" (MDEQ, 1995).
Once the monitoring station locations are verified and the waters with special quality standards are identified, the segmentation process can begin. Waterbodies are dynamically segmented in order to accurately represent the amount of water for which a monitoring station or group of stations is representative. Displaying the locations of NPDES dischargers allows for more accurate segmentation due to the fact that major permitted discharge points indicates changes in projected water quality and helps to determine segment length. With the ability to layer the 303(d) listed segments along with the locations of monitoring stations and WQS waters, previously assessed impaired waters are more easily recognized and are given the appropriate weight when assessment decisions are made.
Dynamic Segmentation is a data model within ArcView that allows the user to associate attributes to any part of a linear feature by using route themes that are based on the route features from an ARC/INFO coverage. Using starting and ending points along an arc, dynamic segmentation allows the user to display, query, and store attributes of a linear feature.
The dynamic segmentation model uses both events and routes. Events store attributes associated with a route or a stream reach. Routes are made up of linear features stored in known measurement units. Attributes of the events include beginning and ending measurements along a route. Line, point, or continuous events can be used. Environmental Systems Research Institute's (Esri) dynamic segmentation model traditionally uses the "F_meas" (from measure), "T_meas" (to measure), and "P_meas" (point measure) fields to geo-reference linear or point features in event tables. The dynamic segmentation model has not been applied to polygonal data. Polygons that need to be tracked with this type of external database can be created as a traditional theme with a key field to associate to the external database.
RTI developed the National Hydrography Dataset Reach Indexing Tool (NHD-RIT) that works with the dynamic segmentation model in ArcView and the National Hydrography Dataset (NHD). The NHD provides a standard unique reach identifier for each surface water feature and a routing network of surface water features. The NHD-RIT was developed to ensure consistency among the fifty states for reporting 303(d) and 305(b) designated waterbodies.
The NHD-RIT can be added to any ArcView project as an extension. The beginning steps for using the NHD-RIT are as follows: extract the compressed NHD files, decompress the extracted files, and create new event/waterbody themes or open existing event/waterbody themes. For more details on this process, the NHD-RIT User Manual can be referenced.
The following is an extraction from the NHD-RIT User Manual outlining the steps for dynamically segmenting:
Editing Events/Waterbodies
1. Make the Event/Waterbody table active and select the entities you wish to modify.
2. Click on the Edit events/waterbodies button, or choose Edit events/waterbodies from the pop-up menu that is activated by right clicking in the View. This activates the Event Editing Tool Box.
3. Use the drop down lists to change the values of any of the attribute fields, or type in a new offset value. Only values that are marked in the check box will be modified when you click Update. If you wish to add new values, click on the ID List button to call the Choose ID dialog.
Changing the Spatial Extent of Linear Events
You can also use the Event Editing Tool Box to modify the spatial extent of existing events. The two spatial modifications that you can perform are moving the end point on an existing event, or dividing one event into two events.
Moving End Points
1. Make the linear event theme active and select the linear event you would like to modify (only select one event).
2. Click on the Edit events/waterbodies button to activate the Event Editing Tool Box.
3. Click on the Move endpoint button. The NHD-RIT will display the current endpoints of the selected events. (Note: You may need to move the Event Editing Tool Box dialog out of the way to see the endpoints.)
4. Click on the location that you would like to move the end point to. The NHD-RIT will select the endpoint closest to the location you clicked and ask if it is the endpoint you would like to move.
5. If the selected endpoint is the correct one, click Yes and the end point will be moved to the desired location. If the incorrect end point is selected, click No and the NHD-RIT will select the other end point and ask again. Click Yes to move that endpoint to the location indicated.
Splitting One Event into Two
1. Make the linear event theme active and select the event you would like to split into two (select only one).
2. Click on the Edit events/waterbodies button to activate the Event Editing Tool Box.
3. Click on the Split one event into two events button, then click on the location where you would like to divide the event (you may need to move the Event Editing Tool Box out of the way to do this).
4. The NHD-RIT will divide the segment, select one half and ask if you would like to change the attribute values that are currently assigned to the new event. If you click Yes, you will be able to enter/select new values using the Choose ID dialog. If you click No the values will remain unchanged.
5. The NHD-RIT will then select the other half of the split event and ask if you would like to change the attribute values.
6. Even if you do not change the attribute values of either piece, the original event will now exist as two separate events.
Deleting Events/Waterbodies
You can delete events or waterbodies by selecting them and clicking on the Delete events/waterbodies button. Delete events/waterbodies is also available from the pop-up menu that is activated when you right click on the View. Note: It is important to delete events or waterbodies using the delete function available through the NHD-RIT. If you delete events or waterbodies using other methods, the associated metadata and transaction files will be out of synch with the event/waterbody table. The NHD-RIT also offers an Undo Delete function. Undo Delete will restore the events/waterbodies from your most recent delete. Undo Delete also restores the associated metadata and transaction files. Undo Delete is available as a button and from the NHD-RIT menu.
Copying Events
In some cases, there may be a need to assign more than one set of attributes to the same area of a stream. You can represent these areas in linear event tables by creating multiple events on the same NHD reach, and then defining an offset value, so that the copy will display along side of the original reach. There are two ways to create copied events.
1. Make the event table active, select the events you would like to copy, and select Copy Events from the NHD-RIT menu, or from the pop-up menu that is activated by right clicking on the View. Assign the appropriate attribute values and be sure to set an Offset value.
Or
2. Make the NHD Transport Reach Theme (NHDcoverageName _TransportRch) active, select the area you wish to copy and use Add events/waterbodies from the NHD-RIT menu, or from the pop-up menu that is activated by right clicking on the View. Assign the appropriate attribute values and be sure to set an Offset. The Offset value that you set depends on your View projection units. If you wish to display multiple copies of a single reach, simply set different offset values. If you set a negative offset value, it will change the side of the reach the copy displays on. The following values are recommended.
Offset Second Offset Third Offset
Decimal Degrees .001 .002 .003
Meters 100 200 300
Feet 300 600 900
Note: The offset value stored in your event table (eoffset field) is stored in coverage units, but you enter the offset value in View units. If your View is not projected, then coverage units = view units. Due to a known problem with ArcView, offset events do not always display. You will have the best success if you work in the same View projection as your data projection. To force offsets to draw, make the linear event theme active and click the Draw Offsets button, or select Draw Offsets from the NHD-RIT menu. Then click ArcView's Clear Selected Features button and your offsets will display.
The review and update process for Mississippi's 305(b) waters was divided into two separate steps. The first step involved the production of hardcopy maps for review by the MDEQ TMDL Engineering section. The National Resources Conservation Service (NRCS) watershed boundaries were used as map extents for the first map revisions. The data themes, which were included on the first revision, were: the NHD perennial streams, primary roads, county roads, NRCS watershed boundaries, county boundaries, United States Geologic Survey (USGS) 1:100,000 map grid, city boundaries, MDEQ monitoring stations, NPDES locations, WQS waters, and the 1998 303(d) waters. The maps were plotted 24" x 32" with varying scales which maximized the individual watersheds viewable areas. Database tables were also printed on the bottom of the maps, for quick reference and review by the TMDL engineers.
As with any map, the symbology for the map themes was an important design element. The symbology for all the themes along with the symbology for the hardcopy edits was decided on before the first map was produced. The stream segment symbology had three layers. The WQS waters were plotted with a line thickness of eight, the 303(d) waters were plotted next with a line thickness of three, and the perennial streams were the top stream segment layer with a thickness of one. This line weight step-down approach allowed one stream reach to show three different attributes. Other symbology was decided upon to provide high contrast to other themes and the hardcopy edits. The annotation symbology needed for each theme was also standardized. Scripts were developed for each theme, which applied the standard font size, type and colors for each ArcView watershed View after the final scale for each watershed was determined.
While a complete 1998 303(d) coverage existed for the entire state, the accuracy of the coverage was in question. These dynamically segmented waters were checked during this process for correct stream reach association and correct lengths. The NHD coverages were found to be accurate for this segmentation process, but did contain some errors. The errors found in the NHD were marked for correction. These corrections were stored at the MDEQ, where a process is being developed for inclusion of these changes into the national coverage. Monitoring stations were checked against survey maps and hand-marked USGS quadrangles obtained over the last several years. Discrepancies were noted on the hard copy maps.
The hardcopy edits were performed using the following rules: red cross-hatching for 303(d) segment deletion, yellow hi-lighting for addition or consolidation of 303(d) segment, red circle around NHD stream "dangles" outside of Hydrologic Unit Code, red cross-hatching through incorrect NHD stream names, red print for added or modified NHD stream names, red tag-line drawn from any change to an editorial comment in the comment table of the map, editorial comments in the margin were clear and concise (e.g., DELETE, ADD, NAME CHANGE, DANGLE, etc.), and red circle around incorrectly located monitoring stations and red arrow drawn pointing to correct location.
As the maps were being edited, the TMDL section initially filled out ADB data entry forms. This entry included the renaming of the 303(d) waterbodies to a new naming convention and completing any location information, beginning upstream and ending downstream. These forms were given to the assessment branch to populate the ADB with waterbody information.
The database tables, which were referenced and reviewed by the TMDL engineers, included data from the 1998 Water Body System, the 1998 MDEQ 303(d) database, and the MDEQ Delisted Waters database. Any discrepancies found were highlighted and handed over to the Assessment Branch for investigation and correction. The edited hardcopy maps were signed by the responsible TMDL engineer and reviewed by the TMDL section-chief before being turned back over to the GIS section.
After the hardcopy edits were completed on the first revision maps, each map was used as a guide for the digital editing of the themes. The monitoring stations, stored in shape file format, were first adjusted to corrected locations. This adjustment was accomplished editing the theme with traditional ArcView editing tools and moving the station to the correct location. The NHD coverages were edited for stream names. The stream names in some cases were misspelled or were incorrect. All of the stream segments from the "route.rch" theme in need of change were selected and the values changed in the theme table. The old Rf3-RIT extension had a useful tool for selecting Rf3 reach segments upstream or downstream from a specified point, which was not found in the new NHD-RIT. To change the stream name, a section of the stream in question was selected from the "route.rch" theme with the ArcView selection tool. The theme table was displayed and the "Name" attribute field was examined for the selected value. The table attribute field "Name" was queried for all values, which matched the previously selected value. The view was examined to check the extent of the selection set. The selection set was modified to add or subtract segments, usually with the feature selection tool. After the selection set was determined to be representative of the segments in need of change, the "Name" field was "calculated" to equal the desired stream name.
The NHD-RIT was used to update the 303(d) dynamically segmented waters. The NHD-RIT proved useful in the editing process where 303(d) waters had to be added, deleted, and redefined in many cases. The NHD-RIT was used in the manner described above in the NHD-RIT section. One important change in the NHD-RIT over the previous Rf3-RIT is the deletion of the dependence on a "Pick List." The redesign made the selection of existing entity-ids considerably faster. The implementation of polygonal support in the NHD and the NHD-RIT helped to realistically represent the State's estuaries, lakes, ponds and reservoirs.
The second step of the review process involved the production of hardcopy maps for review by the MDEQ Assessment section. The NRCS watershed boundaries were again used as map extents for the second revision maps. The data themes included on the second revision were: the NHD perennial streams, primary roads, county roads, NRCS watershed boundaries, county boundaries, USGS 1:100,000 map grid, city boundaries, MDEQ monitoring stations, NPDES locations, WQS waters, and the corrected 303(d) waters. The second revision maps were plotted at 24" x 32" with varying scales which matched the first revisions. Database tables, different from the ones produced for the first revision maps, were also printed on the bottom of the maps, for quick reference and review by the Assessment section. The tables showed all pertinent NPDES information for permit locations represented in the individual watersheds.
The symbology for the second revision maps was again an important component. The stream segment symbology had three layers. The WQS waters were plotted with a line thickness of eight, the corrected 303(d) waters were plotted next with a line thickness of three, and the perennial streams were the top stream segment layer with a thickness of one. The perennial streams were drawn using the drainage theme from the NHD. The drainage theme differentiates between perennial and intermittent streams. All later segmentation was done with the undisplayed routing theme. Like the first revision maps, other symbology was decided upon to provide high contrast to other themes and the hardcopy edits. Similar scripts were used for each theme, which applied the standard font size, type and colors for each ArcView watershed View.
The hardcopy editing process for the second revision maps centered around the creation of the 305(b) segments. Stream segments were marked with beginning and ending crossbars. The length of the segments were shaded with red ink ad a leader line was draw to a letter code, which began with "A" for each new watershed. This tedious process, performed by the Assessment section, utilized many monitoring databases, analysis routines, and hardcopy reports.
As the maps were being edited, the Assessment section completed the ADB data entry forms. This entry included the assignment of waterbody and segment names following the new naming conventions and completing any location information, beginning upstream and ending down stream. The assessments of the waterbody segments were also filled in on these forms. The forms were used to finish populating the ADB.
After the hardcopy edits were completed on the second revision maps, each map was used as a guide for the digital editing of the themes. The NHD-RIT was again used to create the 305(b) dynamically segmented waters and update the 303(d) waters with any changes made by the Assessment section. The NHD-RIT proved useful in this editing process. The NHD-RIT was used in the manner described above in the NHD-RIT section. The NHD and the NHD-RIT not only handled the dynamic segmentation of the linear stream events, but was also useful in the segmentation of the polygonal waterbodies. The NHD occasionally represented, what the assessment section viewed as perennial streams, as center flow paths for large waterbodies. This did not slow down the segmentation process, but played an important role later while totaling linear stream mileage. There were several instances where the Assessment section wished to segment a linear waterbody, which did not exist in the NHD. These segments were digitized from USGS 7.5-minute quads and stored as simple shape files with corresponding waterbody segment identifiers. The NHD-RIT was used initially to create and segment polygonal events. These events are simple polygons stored in a shape file format with corresponding waterbody identifiers. Later editions and edits were done using the traditional ArcView polygon editing tools. The Assessment section wanted to segment and label the remaining perennial and intermittent stream in each watershed with general identifiers. Because of 305(b) segments sometimes ended partially through an NHD reach, this step should be done first, before any other segmentation is attempted. The process outlined here segmented these waters after the bulk of the individual segments had been entered. Special consideration was always given to segments that ended partially through an NHD reach. The complete reach was first classified with the unique 305(b) segment identifier and then split, renaming the remainder of the segment with the watershed perennial or intermittent segment identifier.
Mississippi Department of Environmental Quality, State of Mississippi Water Quality Criteria for Intrastate, Interstate, and Coastal Waters, 1995.
Research Triangle Institute, The Reach Indexing Tool for the National Hydrography Dataset (NHD-RIT) User Guide, 2000.
The Clean Water Act of 1972, Public Law 92-500.