Introduction: The ability of soil and bedrock material to buffer acid derived from atmospheric loading is dependent on the mineral/chemical composition, texture and the flow pathways of water through the material. A first-order control on acidification potential is the bedrock composition, so we will use bedrock coverages to examine watersheds in the western Adirondacks and Tug Hill Plateau. We will then assess the impact of bedrock composition on stream water chemistry in these watersheds. The approach that we will take is to extract the appropriate bedrock coverage area of interest, assign buffering capacity values based on the mineral/chemical composition of the bedrock types, and produce a new coverage which depicts the spatial distribution of buffering capacity.

Map files and Data: The shape file Adir_bed.shp and the Watertown 1 to 100 .tif file are located in the Lab 7 folder at the network address. You will need to copy both to your Zip disk, along with the Excel files Lab 7 Water and Lab 7 Rock. You will also want to copy the text file ‘bedrock key’, which contains the descriptions of the bedrock map units in the study area. The file ‘aoiclip’ is a shapefile that defines the area of interest for this lab. The file ‘watersamples.shp’ is a point coverage which locates the site of each of the water samples.

Open the bedrock shape file and clip to the area of interest: Follow the procedure used in lab #6 to clip the bedrock coverage to the area of interest defined by the clipping file provided. Save the clipped bedrock coverage as a new shape file (e.g. arearock).

Delete the original bedrock shape file and the clipping file. Examine the clipped bedrock coverage and identify the bedrock units present in the area of interest.

Add the topographic map coverage and point coverage to locate the following streams and sampling sites:

Moose River

Black River

Sugar River

Whetstone Creek

Fish Creek

Alder Creek

Assign a buffering potential value to each of the bedrock units: Make a list of the bedrock units in the area of interest as a new Excel file (you can copy this from the Rock data file); use the exact material ID used in ArcView to define each unit. Using the Lab 7 Rock data, assign a ‘buffering potential’ value to each unit, using a scale of 1-10, with 1 the least buffering potential, and 10 the greatest. We will discuss this in lab. When you have completed the buffering potential assignment, use ‘save as’ in Excel to save the file as a .dbf file. You will use this file as a table in ArcView.

Open a ‘buffering potential’ table in ArcView and join to the bedrock coverage table: With the clipped bedrock coverage file opened in ArcView, go the ‘project’ menu and select ‘add table’. Navigate to the .dbf file you have just saved, and open it as an ArcView table. Note that the table should have only two columns – "Material" and the attribute name you assigned for buffering potential. Open the table for the clipped bedrock coverage, such that both tables are now open. Use the ‘Join’ command under the Table menu, and follow the procedures to join the tables. Examine the clipped bedrock coverage to be sure that the new buffer attribute column is properly assigned.

Prepare a map depicting buffering capacity: Use the clipped shapefile with joined table to prepare a map and layout depicting buffering potential based upon the bedrock characteristics you have just assigned. This is done using the legend editor; the "Graduated Color" legend type works well for this sort of display. You may wish to overlay this coverage on the topographic base map.

Water chemistry and bedrock buffering capacity: Use the topographic map coverage to locate the six streams . You may wish to make a new theme (a line coverage) that locates the streams on your buffering potential map, or use the watersamples.shp point coverage. Alternatively, you can locate the streams on the view or layout with the graphic editor.

Examine the watershed of each stream, and assess the overall buffering capacity of the various rock units.

1. Based on the analysis above, what is the correlation between the bedrock buffering potentials that you assigned and the water chemistry of the five streams of interest?

2. What other factors are not accounted for this analysis? What other information would be useful to predict watershed buffering capacity more fully?