CHALLENGES USING THE PENNSYLVANIA IMPAIRED AND NON-IMPAIRED STREAMS GIS COVERAGE

Pennsylvania is ahead of many states in making available GIS data on impaired and non-impaired streams. The Streams Integrated List, based on Clean Water Act stream assessment data, is available from the Pennsylvania Spatial Data Access (PASDA) website. The data sets are the Attaining (not impaired) and Non-Attaining (impaired) streams as assessed for one or more of four uses: aquatic life, recreational, potable water supply, and fish consumption. However, there are problems with the data sets listed by decreasing severity below:

1) 14% of Attaining stream kilometers are identical to segments of Non-Attaining streams. This overlap is contrary to what the metadata indicate.

2) There are 30% more apparent Attaining plus Non-Attaining kilometers than National Hydrography Dataset (NHD, from which most Integrated List segments were copied) kilometers.

3) 25% and 15% of Attaining and Non-Attaining stream kilometers, respectively, are listed for more than one use; e.g., a segment can be Attaining (or Non-Attaining) for both aquatic life and recreational uses.

4) A few Attaining (0.5%) and Non-Attaining (0.7%) stream kilometers extend beyond the PA border.

6) A small number of NHD Flowline segments do not match geographically with Attaining and Non-Attaining segments.

7) After “correction” for the problems listed above, there are a 1.4% more NHD stream kilometers than the combined Attaining and Non-Attaining kilometers. This small percentage of stream kilometers is apparently not assessed.

End users need to be aware of these issues with the data and metadata to avoid over-counting of stream lengths and problems in conclusions drawn regarding stream data.