V.  Critical Loads

A stream's sensitivity to acid deposition is signaled by "indicator organisms" that are selected as representatives of community health.  The sensitivity of each indicator species can be expressed as the critical pH at which half or more of the population experiences acute or chronic effects.  The level of acid deposition that results in the critical pH depends on the watershed's ability to buffer acidity as ions pass through soils. 

In the early 1990s, Maryland Department of Natural Resources conducted a study to estimate critical loads for Maryland streams (Janicki et al. 1991).  For Maryland streams, a critical load can be defined as :

the rate of sulfur and nitrogen deposition that results in stream pH less than the critical pH values for brook trout (5.3), smallmouth bass (5.8), white sucker (6.5), and blueback herring (5.6).

Critical loads values at 73 sites across Maryland were mapped and values were geographically interpolated and smoothed to create a continuous coverage of the whole state.  Figure 5 shows a GIS map of critical loads values across Maryland.

Figure 5.

Click here to see a Detailed discussion of critical loads calculations.

The critical load maps reveal wide differences in the sensitivity of Maryland streams within different provinces:

• The Appalachian Plateau, Coastal Plain and portions of the Blue Ridge are very sensitive (critical load values < 0.5 keq/ha/yr or 24 kg SO₄/ha/yr).
  

• In contrast, the Valley and Ridge, Piedmont, and portions of the Blue Ridge regions exhibit critical loads well over 2.0 keq/ha/year (96 kg SO₄/ha/yr).  These are areas where limestone bedrock and derived soils containing more buffering ability are prevalent.

Return to main page

Continue to Stream Response

Search Maryland DNR

Rivers & Streams 101 | Monitoring & Research | Restoration & Protection
Want Data? | Meetings & Readings | Get Involved!

Return to the Maryland DNR Home Page.
Your opinion counts! Take a survey!
This page was updated on 4/7/2004