Spatial Pattern 



Copper Concentration 

 ■ High 



Estuary 



Medium 



;•:•;■: Low 



Figure 3.5 Example of a spatial pattern for copper concentrations in 

 sediments found near the head of an estuary. 



In all these examples, the potential cause of a problem is identified 

 through association. Although this kind of circumstantial evidence 

 is valuable, it does not conclusively prove a cause-effect relation- 

 ship. The problem and potential cause could be associated purely 

 by chance; both could be the result of yet another variable. Confir- 

 mation of cause-effect relationships usually requires conducting 

 field or laboratory experiments under carefully controlled condi- 

 tions. 



Cause-Effect Relationship 



Percent 



Vegetated 



Stations 



0.5 1.0 1.5 



Nitrogen Concentration (mg/L) 



2.0 



Figure 3.6 Example of a potential cause-effect relationship: The 



association between declining vegetation and the presence 

 of nitrogen in the water. 



32 



